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Microstructural evolution and mechanical properties of an aluminum alloy processed by equal-channel angular pressing and high-pressure torsion

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Microstructural evolution and mechanical properties of an aluminum alloy processed by equal-channel angular pressing and high-pressure torsion

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MICROSTRUCTURAL EVOLUTION AND MECHANICAL
PROPERTIES OF AN ALUMINUM ALLOY PROCESSED BY EQUAL-
CHANNEL ANGULAR PRESSING AND HIGH-PRESSURE TORSION

by

Shima Sabbaghianrad

A Dissertation Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(CHEMICAL ENGINEERING)

August 2014

Copyright 2014 Shima Sabbaghianrad

ii
Acknowledgements

I would like to express my sincere gratitude to my advisor, Professor Terence G.
Langdon, for his encouragement, support and guidance during the entire course of this
program. His support towards my career and invaluable advice made my achievement
possible and his enlightening guidance and earnest attitude will also benefit me in my
future path. My special thanks also extend to his wife, Mady Langdon, for her concern
and kindness.
I am grateful to my dissertation committee, Professors Edward Goo and
Satwindar S. Sadhal for their agreement to serve on my dissertation committee, for their
feedback and evaluating my Ph.D. study. I would like to thank Professor Megumi
Kawasaki in the Division of Materials Science and Engineering at Hanyang University,
Seoul, Korea and Professor Jittraporn Wongsa-Ngam in Faculty of Engineering, King
Monkut University, Thailand for providing beneficial advice.
It is my pleasure to acknowledge the significant help and beneficial discussions
provided by Dr. Chuan Ting Wang, Dr. Mahmood Shirooyeh, and Dr. Yi Huang. I would
like to acknowledge Professor Roberto Figueiredo and Professor Cheng Xu for their help
and encouragement.
Most importantly, none of this would have been possible without the love of my
family. I would like to present this achievement as a gift to my grandmother, mother and
father. It is their everlasting love, unconditional support and understanding that
strengthened my determination in pursuing this degree. I would also like to express my
iii
heart-felt gratitude to my sisters.
This work was supported by the National Science Foundation of the United States
under Grant No. DMR-1160966.

iv
Table of Contents
Acknowledgments ii
List of Tables vi
List of Figures vii
Abstract xii
1 Introduction 1
2 Literature Review 3
2.1 Severe plastic deformation (SPD) 3
2.2 Equal-channel angular pressing (ECAP) 3
2.2.1 Hardness homogeneity after processing by ECAP 8
2.2.2 Microstructural evolution after processing by ECAP 9
2.2.3 Superplasticity after processing by ECAP 13
2.2.4 Effects of ECAP condition on superplasticity 17
2.2.5 Double-shear creep properties after processing by ECAP 18
2.3 High-pressure torsion (HPT) 20
2.3.1 Hardness homogeneity after processing by HPT 25
2.3.2 Microstructural evolution after processing by HPT 28
2.3.3 Superplasticity after processing by HPT 30
2.4 Combination of SPD processes 33
2.5 Application of the materials processed by SPD techniques 34

3 Experimental Materials and Procedures 35
3.1 Experimental material 35
3.2 Methods of severe plastic deformation 35
3.2.1 Processing by ECAP 35
3.2.2 Processing by HPT 38
3.2.3 ECAP followed by HPT processing 39
3.3 Microstructural analysis 40
3.3.1 Electron-backscatter diffraction 40
3.3.2 Transmission electron microscopy (TEM) 41
3.4 Mechanical experiments 41
3.4.1 Microhardness measurements 41
3.4.2 Tensile measurements 43
3.4.3 Double shear creep 45

4 Experimental results 48
4.1 Microhardness measurements 48
4.1.1 Microhardness values after HPT 48
4.1.2 Microhardness values after processing by a combination of
ECAP and HPT 52
v
4.1.2.1 Microhardness values of samples processed by
ECAP for 4 passes and HPT 52
4.1.2.2 Microhardness values after ECAP for 8 passes
and ECAP + HPT 55
4.1.3 A comparison of Vickers microhardness for samples
processed by SPD techniques 58
4.2 Microstructural analysis 60
4.2.1 Microstructural behavior of the samples before and
after HPT 60
4.2.2 Misorientation angles before and after HPT 62
4.2.3 Microstructural behavior of the samples after a combination
of SPD processes 64
4.2.4 Misorientation angles after processing by a combination of
SPD processes 67
4.3 Mechanical behavior 70
4.3.1 Tensile properties of the samples processed by HPT 70
4.3.2 Double-shear creep of the samples processed by ECAP for
8 passes 73
4.3.3 Tensile properties of the samples processed by a combination
of SPD processes 75
4.3.3.1 Tensile properties after ECAP for 4 passes and HPT 75
4.3.3.2 Tensile properties after ECAP for 8 passes and HPT 79

5 Discussion 81
5.1 The effect of HPT 81
5.2 The effect of the combination of ECAP and HPT 85
5.3 Hardness saturation after the combination of SPD processes 88

6 Summary and conclusions 92
References 95
Alphabetized Bibliography 105

vi
List of Tables
Table 2.1 Elongations to failure for various materials processed by HPT 28

vii
List of Figures
Figure 2.1. The principle of ECAP through a die, showing the two angles,
Φ and Ψ [2] 4

Figure 2.2. The four processing routes in ECAP [35] 5
Figure 2.3. Shearing characteristics for six different processing routes [36] 6
Figure 2.4. Illustration of the X, Y and Z planes denote the transverse plane,
the flow plane and the longitudinal plane, respectively [37] 7

Figure 2.5. Variation of the equivalent strain, ε, with the channel angle, Φ,
for a single pass where N = 1 [38] 7

Figure 2.6. Hardness homogeneity along the Y-plane of a
Cu-0.1 wt% Zr alloy after processing by ECAP for (a) 1,
(b) 2, (c) 4, and (d) 8 passes [39] 8

Figure 2.7. A model for grain refinement in the central region of the billet
in ECAP [40] 10

Figure 2.8. The appearance of the microstructures on the Y plane after four
passes using routes A, B
C
and C [42] 10

Figure 2.9. Microstructures in high purity Al in (a) the unprocessed condition
and (b)–(g) after processing by ECAP through increasing numbers of passes:
the unit triangle denotes the crystallographic orientations [9] 11

Figure 2.10. The increase in 0.2% proof stress with increasing numbers of ECAP
passes for a range of commercial aluminum alloys [47] 13

Figure 2.11. The first report of superplasticity achieved in a sample
of Al-Mg-Li-Zr after processing by ECAP at a temperature of 623 K [48] 15

Figure 2.12. Temperature and grain size compensated strain rate against
normalized stress for many aluminum alloys [60] 16

Figure 2.13. Plot of shear strain against time for an Al-7034 alloy at473K
under a shear stress of 24.2 MPa: curves shown for specimens in the as-received
condition and after processing by ECAP through 6 passes at 473 K [71] 19

Figure 2.14. Schematic illustration of HPT processing [72] 20

Figure 2.15. Schematic illustration of HPT for (a) unconstrained,
(b) constrained, and (c) quasi-constrained conditions [72] 21
viii

Figure 2.16. Parameter used in estimating the total strain in HPT [16] 22
Figure 2.17. Variation of equivalent strain as a function of number of turns
in a disk with a thickness of 0.1 mm processed by HPT [72] 24

Figure 2.18. Variation of grain size in a bulk sample of an Al-Mg-Sc alloy
processed by HPT [84] 25

Figure 2.19. Values of Vickers microhardness for a material with fast recovery
rate (upper curve), and a material with slow recovery rate (lower curve) [88] 26

Figure 2.20. Microhardness measurements across the disk for samples of
pure aluminum processed by m-HPT through totals of (a) 1, (b) 2, (c) 4 turns,
or by c-HPT through total numbers of (d) 1A + 1B, (e) 2A + 2B,
(f) 1A + 1B + 1A + 1B turns [96]. 27

Figure 2.21. Schematic illustration of microstructural evolution with straining
along in pure Al. Thin double lines depicted in region II represent low angle
boundaries with some extension of boundary width and thick lines in region III
represent high angle boundaries [94] 29

Figure 2.22. Temperature and grain size compensated strain rate against the
normalized stress for various aluminum alloys processed by HPT [108] 31

Figure 2.23. Crystallize size distributions for nickel samples obtained by
combinations of different methods: (a) ECAP, (b) ECAP + cold rolling,
(c) ECAP + HPT, and (d) ECAP + cold rolling + HPT [116] 33

Figure 2.24. The innovation potential as a function of material strength [117,118] 34
Figure 3.1. Drawings of the die and plunger of ECAP 36

Figure 3.2. Schematic of ECAP facility 37
Figure 3.3. Schemiatic of the combination of ECAP and HPT 39
Figure 3.4. Schematic of the positions of hardness indentation over a quarter of
the disk processed by SPD [96] 43

Figure 3.5. Schematic of the miniature tensile samples cut by EDM [123] 44

Figure 3.6. Geometry and dimensions of a specimen used for
double-shear creep testing 45

Figure 3.7. Schematic of the double-shear creep machine [127] 46
ix

Figure 4.1. Vickers microhardness against distance from the center of
the disk processed by HPT though totals of 1/8 to 10 turns 49

Figure 4.2. Color-coded map of Vickers micohardness values over a quarter of
the surface of disks processed by HPT through (a) ½ turn, (b) 1 turn, (c) 2 turns,
and (d) 5 turns 51

Figure 4.3. Vickers microhardness values along diameter of the disks
processed by ECAP for 4 passes and HPT through totals of 1/8 to 20 turns 53

Figure 4.4. Average values of the Vickers microhardness across diameters
of the disks against the number of turns of HPT processing for the samples
processed by HPT or by a combination of ECAP and HPT 54

Figure 4.5. The Vickers microhardness values against equivalent
strain after processing by HPT ECAP for 4 passes and HPT through
totals of 1/8 to 20 turns 54

Figure 4.6. Vickers microhardness values along diameter of the disks
processed by ECAP for 4 passes and HPT through totals of 1/8 to 20 turns 55

Figure 4.7. Vickers microhardness values against equivalent strain for samples
processed by ECAP for 8 passes and HPT through totals of 1/8 to 20 turns 57

Figure 4.8. A comparison of the Vickers microhardness values of the samples
processed by HPT, ECAP for 4 passes and HPT, or ECAP for 8 passes and HPT 58

Figure 4.9. A schematic of the Vickers microhardness values against equivalent
strain for the samples processed by HPT, ECAP for 4 passes and HPT, or ECAP
for 8 passes and HPT 59

Figure 4.10. OIM images of the samples in the (a) annealed condition, and after
processing by HPT for 5 turns (b) in the center and (c) around the peripheral
region of the disk 61

Figure 4.11. Misorientation angles of samples (a) in the annealed condition,
and after processing by HPT through total of 5 turns (b) in the center and
(c) around the peripheral region of the disk 63

Figure 4.12. OIM image of Al-7075 after (a) processing by ECAP for 4 passes,
(b) a combination of ECAP for 4 passes and HPT through total number of 20 turns,
a combination of ECAP for 8 passes and HPT through total numbers of 20 turns
(c) in the center, and (d) around the peripheral region of the sample 66

x

Figure 4.13. TEM images of Al-7075 alloy processed by ECAP for 8 passes and
HPT through total number of 20 turns: (a) bright field image and (b) dark field
image of the same position 67

Figure 4.14. Misorientation angle of an Al-7075 alloy after processing by
ECAP for 4 passes, (b) ECAP for 4 passes and HPT through total number of
20 turns, (c) ECAP for 8 passes in the center, and (d) ECAP for 8 passes
around the peripheral region of the disk 69

Figure 4.15. Plots of engineering stress versus engineering strain for the
Al-7075 alloy in the annealed condition or processed by HPT through total
of 1, 5, or 10 turns at initial strain rates of (a) 1.0 × 10
-2
, (b) 1.0 × 10
-3
, and
(c) 1.0 × 10
-4
s
-1
70

Figure 4.16. Plot of flow stress versus strain rate for the Al-7075 alloy pulled in
tension in the annealed condition or after processing by HPT through total of 1,
5, or 10 turns 71

Figure 4.17. Plot of elongation to failure versus strain rate for the Al-7075 alloy
pulled in tension in the annealed condition or after processing by HPT through
total number of 1, 5, or 10 turns 72

Figure 4.18. Plots of shear strain against time for the Al-7075 alloy processed by
ECAP for 8 passes under constant stresses of 9.9, 14.1, and 18.3 MPa 74

Figure 4.19. Plots of shear strain against time for the Al-7075 alloy in the annealed
condition and processed by ECAP for 8 passes under constant stress of 14.1 MPa 75

Figure 4.20. Plots of engineering stress versus engineering strain for the Al-7075
alloy processed by ECAP or ECAP + HPT and then pulled in tension to failure at
623 K at initial strain rates of (a) 1.0 × 10
-1
, (b) 1.0 × 10
-2
and (c) 1.0 × 10
-3
s
-1
77

Figure 4.21. Flow stress versus strain rate at 623 K for the Al-7075 alloy after
processing by ECAP for 4 passes or a combination of ECAP and HPT 78

Figure 4.22. Elongation to failure versus strain rate at 623 K for the Al-7075
alloy after processing by ECAP for 4 passes or a combination of ECAP and HPT 78

Figure 4.23. Plots of engineering stress versus engineering strain for the Al-7075
alloy processed by ECAP for 8 passes or a combination of ECAP + HPT and then
pulled in tension to failure at 623 K at initial strain rates of (a) 1.0 × 10
-1
,
(b) 1.0 × 10
-2
and (c) 1.0 × 10
-3
s
-1
80

xi
Figure 5.1. Accumulated shear strain versus distance from the center of the disk
processed by HPT derived from a simulation [130] 82

Figure 5.2 The variation of Vickers microhardness values against equivalent strain
for pure metals processed by HPT at different homologous temperatures [142] 83

xii
Abstract
A commercial Al-7075 alloy was processed by severe plastic deformation (SPD)
techniques, namely equal-channel angular pressing (ECAP) and high-pressure torsion
(HPT) or a combination of the two processes. After processing, the microstructural
properties were examined, microhardness measurements were recorded across the disk
diameters, and miniature tensile specimens were pulled to failure at a temperature of 623
K.
Using TEM and EBSD techniques, it is demonstrated that the three SPD
processing techniques have a potential for producing an ultrafine-grain structure
containing reasonably equiaxed grains with high-angle grain boundary misorientations.
However, microstructures were refined to different levels depending on the processing
operation. It is shown that the most refined grain structure was achieved after processing
by a combination of ECAP for 8 passes and HPT. The grain refinement mechanisms are
primarily governed by dislocation activities.
It is shown that the maximum saturation hardness achieved at high equivalent
strains by different processing techniques increases with increasing amounts of
deformation and it is the highest after processing by a combination of ECAP for 8 passes
and HPT. The saturation hardness values were ~231 after processing by HPT, ~249 after
processing by ECAP for 4 passes + HPT and ~273 after processing by ECAP for 8 passes
+ HPT.
Tensile testings show that the elongations to failure increase by increasing the
amount of deformation. It is shown that after ECAP for 8 passes + HPT samples of the
Al-7075 alloy have lower flow stresses and superplastic elongations up to >1000% when
xiii
pulling to failure at 623 K. Superplastic elongations were not achieved after processing
only by ECAP because of the non-uniform grain size and the presence of many larger
grains.

1
1. Introduction
The processing of metallic materials through the application of severe plastic
deformation (SPD) is now recognized as an effective way for producing ultrafine-grained
(UFG) structures [1]. Typically, the grain sizes produced by SPD processing are in the
submicrometer or even the nanometer range [2] and this leads to improved mechanical
properties including high strength and a potential for achieving superplastic elongations
at elevated temperatures. Several methods of SPD processing have been developed over
the past two decades [3] but the most attractive procedures are equal-channel angular
pressing (ECAP) [4] and high-pressure torsion (HPT) [5].
In processing by ECAP, the material is in the form of a rod or bar and it is pressed
repetitively through a bent channel with an abrupt angle. This processing method is
simple to establish and it has been used effectively to give grain refinement and improved
mechanical properties in a wide range of metals and metallic alloys [6-10]. In processing
by HPT, the material is generally in the form of a thin disk and it is subjected to a high
applied pressure and concurrent torsional straining. This technique has also been used to
achieve substantial grain refinement and improved properties in many different metals
[11-14]. Experimental results show generally that processing by HPT is preferable to
ECAP because it leads to materials having smaller grain sizes and with higher fractions
of grain boundaries having high-angles of misorientation [15-17]. However, a significant
disadvantage with conventional HPT processing is that the samples are in the form of
very thin disks, typically having thicknesses of ~0.8 mm. To overcome this problem,
numerous experiments have been conducted recently to extend HPT processing to much
larger cylindrical samples [18-21].
2
The present investigation was conducted to evaluate the microstructural properties
and the mechanical characteristics after processing through a combination of ECAP and
HPT. This type of experiment was first conducted by Stolyarov et al. [22] using
commercial purity Ti and it was reported that processing by HPT after ECAP leads to
additional grain refinement and a consequent increase in the hardness. Experiments on
other materials also confirmed the occurrence of additional grain refinement after a
combination of ECAP and HPT [23-25]. Recent studies suggest that very similar
microstructures were obtained when processing Nb single crystals by HPT or by a
combination of ECAP and HPT [26,27] and this suggests that the saturation condition is
not affected by the initial state of the material.
Based on this information, the objective of the present study was to compare the
mechanical properties and microstructural behavior of samples of an Al-7075 alloy
processed by a combination of ECAP and HPT using either 4 or 8 passes of ECAP and
different numbers of revolutions in HPT. Earlier reports described some results obtained
on this alloy when processing by HPT [28] or by ECAP through 4 passes followed by
HPT [29]. Al-7075 alloy has a stacking fault energy of ~125 mJ/m
2
[30] and is used in
aerospace applications.

3
Chapter 2. Literature Review
2.1 Severe Plastic Deformation (SPD)
SPD is a processing technique in which a very high strain is applied to a sample
via extensive hydrostatic pressure to refine grain structure into ultrafine-grains.
Ultrafine-grained materials (UFG) are materials with homogeneous and equiaxed
microstructure having average grain size of less than 1 µm with a large fraction of high-
angle grain boundaries [5]. The principles of SPD are traced back to ~500 BC [31];
however the scientific concepts of it were presented by Bridgman in 1930s [32].
Samples processed by SPD have improved mechanical properties and a uniform
ultrafine-grain structure within the whole volume of the sample. Samples should not
have any mechanical damage or cracks after processing by SPD [2]. Several SPD
processes have been applied to materials among which equal-channel angular pressing
(ECAP) and high-pressure torsion (HPT) received the most attention from researchers
recently.
2.2 Equal-Channel Angular Pressing (ECAP)
The process of ECAP was first developed by Segal [33] in the beginning of the
1980s. This process introduces intense plastic strain into the material via pure shear
without changing the dimension of sample. The principle of this process is illustrated
schematically in Fig. 2.1. In this process a billet shaped sample with a round or square
cross section is pressed through a die with identical cross section. The die contains two
channels that intersect with an inner angle
Φ
and an outer angle Ψ. Strain value
imposed on the billet during pressing for N passes can be calculated using the following
equation [34]:
4

where ε
N
is the strain imposed after multiple pressings and N is the number of ECAP
passes.

Repetitive processing is possible with ECAP method and four major routes have
been practically applied to materials as shown in Fig. 2.2 [35]. As described the billet is
repetitively pressed without rotation in route A, while in route B
A
the billet is rotated by
90º in alternative directions at each individual pass. The billet is rotated by 90º in the
same direction at each separate pass in route B
C
and it is rotated by 180º in route C.
Experimental results show a change in microstructural properties with changing routes.
Figure 2.1. The principle of ECAP through a die, showing the two angles Φ and Ψ [2]
⎥
⎦
⎤
⎢
⎣
⎡
⎟
⎠
⎞
⎜
⎝
⎛ Ψ
+
Φ
Ψ +
⎟
⎠
⎞
⎜
⎝
⎛ Ψ
+
Φ
=
2 2
cosec
2 2
cot 2
3
N
N
ε
(2.1)
5
Figure 2.3 shows the detailed shearing characteristics after various processing routes
using a square channel ECAP die with the inner angle of
Φ
= 90º [36] where the X, Y
and Z planes as shown in Fig. 2.4 [37] represent the planes perpendicular to the pressing
direction, parallel to the side face of the sample and parallel to the top face of the
sample, respectively. It can be seen that after four passes by the pressing routes B
C
or C
the cubic elements in all four faces are restored and therefore these pressing routes can
be reasonable for processing with ECAP. Figure 2.5 illustrates the equivalent strain for
a channel with an outer angle of
Φ
over the range of 45 to 180° for a single pass where
N = 1 [38]. It can be seen in Fig. 2.5 that the equivalent strain is ~0.8 for a channel with
an outer angle of
Φ = 110° and an inner angle of Ψ = 20°.

Figure 2.2. The four processing routes in ECAP [35]
6

Figure 2.3. Shearing characteristics for six different processing routes [36]
7

Figure 2.4. Illustration of the X, Y and Z planes denote the transverse plane, the flow
plane and the longitudinal plane, respectively [37]
Figure 2.5. Variation of the equivalent strain, ε, with the channel angle, Φ, for a single
pass where N = 1 [38]
8
2.2.1 Hardness homogeneity after processing by ECAP
Various methods were occupied to investigate the hardness homogeneity
of the samples processed by ECAP. A convenient method for investigating the degree
of homogeneity achieved in a material after processing by ECAP is to evaluate the
evolution of hardness with additional straining. Figure 2.6 shows the hardness
homogeneity along the Y-plane of a Cu-0.1 wt% Zr alloy after processing by ECAP for
(a) 1, (b) 2, (c) 4, and (d) 8 passes [39]. These maps were constructed with the Z
direction as the vertical axis where Z = 0 denotes the centerline of each billet, and the X
direction as the horizontal axis where X = 0 and 40 mm denote the back and front
positions of each billet, respectively.
Figure 2.6. Hardness homogeneity along the Y-plane of a Cu-0.1 wt% Zr alloy after
processing by ECAP for (a) 1, (b) 2, (c) 4, and (d) 8 passes [39]
9

It is apparent from Fig. 2.6 that the microhardness values increase with increasing
numbers of passes up to 4 passes and the hardness is essentially saturated after 8 passes.

2.2.2 Microstructural evolution after processing by ECAP
Processing by ECAP can introduce remarkable grain refinement in the coarse-
grained materials. As shown in Fig. 2.7, a feasible model of grain refinement in pure Al
during ECAP using route B
C
has been developed [40]. This model is based on the
mechanical shearing of grains. In this model the ultimate equiaxed grain size and the
fraction of boundaries having high-angle misorientation in each substructure based on
theoretical models are denoted by d and ξ, respectively [41]. The equiaxed grains are
formed when the elongated arrays of grains are subsequently sheared in other
directions. An important feature of this model is that the ultimate size of the equiaxed
grains in equilibrium is dictated by the width of the subgrain bands produced in the
initial pass. Figure 2.8 shows the microstructural behavior of the Y plane after four
passes using different routes A, B
C
, and C [42]. It can be observed that an equiaxed
microstructure is achieved most rapidly in ECAP when using route B
C
but it is rarely
developed when using route A and C.
10

Figure 2.7. A model for grain refinement in the central region of the billet in ECAP [40]
Figure 2.8. The appearance of the microstructures on the Y plane after four passes
using routes A, B
C
and C [42]
11
Figure 2.9 shows the microstructure of a high purity (99.99%) aluminium after
processing by ECAP for up to a maximum of 12 passes [9]. These samples were
processed at room temperature using route B
C
with an ECAP die having an internal
angle of Φ= 90° and an outer angle of Ψ= 20°.

Figure 2.9. Microstructures in high purity Al in (a) the unprocessed condition and (b)–(g) after
processing by ECAP through increasing numbers of passes: the unit triangle denotes the
crystallographic orientations [9]
12
An array of elongated subgrains are visible after the first pass of processing by
ECAP in Fig. 2.9, but thereafter the microstructure gradually evolves into equiaxed
grains separated by high-angle grain boundaries. Measurements showed an average
grain size of ~1.2 µm and a fraction of high-angle boundaries of ~74% after processing
by ECAP for 12 passes.
A theoretical dislocation model describing the quantitative dependence of
minimum grain size on several physical parameters has been developed for samples
processed by ECAP [43]. This model suggests that for the minimum grain size obtained
by ECAP agrees well with the model for the minimum grain size obtained by ball
milling and they both follow the equation [43]:
!
!"#
!
= 𝐴
!
exp −
!"
!!"
!
!"
!!
!
!
!
!"
!.!"
!
!"
!.!
(
!
!
)
!.!"
(2.2)
where b is the value of Burgers vector, A
3

is a dimensionless constant, β is a constant
(∼0.04), Q is the self-diffusion activation energy, R is the gas constant, T is the absolute
temperature, D
PO

is the frequency factor for pipe diffusion, G is the shear modulus, ν
o
is
the initial dislocation velocity, k is Boltzmann’s constant, γ is the stacking fault energy
and σ is the normal yield stress.
Plotting
!
!"#
!
against melting temperature (T
m
) or Bb
3
for various metals results in
straight lines and therefore the above equation can be simplified to:
!
!"#
!
= 2600 exp (−0.0006 𝑇
!
) (2.3)
!
!"#
!
= 1600 exp (−0.2 𝑏
!
𝐵) (2.4)
13
where B is the bulk modulus.
2.2.3 Superplasticity after processing by ECAP
Superplasticity is defined as the ability of a polycrystalline material to exhibit
very high elongations prior to failure in a generally isotropic manner. Having a stable and
small grain size (less than 10 µm) and a diffusion-controlled process at elevated
temperature (T > 0.5 T
m
) is required for obtaining superplastic flow [44]. The measured
elongations in superplasticity are generally at least 400% and the measured strain rate
sensitivities are close to ~0.5 [45]. It is important to note that the small grain size should
be retained in the high temperature condition in order to achieve superplastic behavior in
processing by ECAP at elevated temperature. The constitutive equation for conventional
superplasticity is given by [46]:
(2.5)
where is the steady strain rate, A is a dimensionless constant, D is the appropriate
diffusivity (lattice or grain boundary), d is the grain size, p is the grain size exponent
and n is the exponent of the stress (normally, in the superplasticity flow region p = 2,
n = 2 and D = D
gb
where D
gb
is the coefficient for grain boundary diffusion).

n p
G d kT
DG
A
⎟
⎠
⎞
⎜
⎝
⎛
⎟
⎠
⎞
⎜
⎝
⎛
=
σ
ε
b b
!
ε !
14
Grain refinement introduced by SPD processing results in high strength in
materials. An example is shown in Fig. 2.10 where the 0.2% proof stress is plotted
against the equivalent strain for a number of different commercial alloys processed by
ECAP at room temperature using route B
C
. The ECAP die used in this study has a
channel angle of Φ = 90°; therefore the equivalent strain is equal to the number of
ECAP passes based on eq. 2.1, and an equivalent strain of zero corresponds to the
initial annealed condition [47]. It is evident that the 0.2% proof stress increases sharply
for all alloys in the first pass but thereafter there is only a minor increase up to a
maximum of 8 passes.

Figure 2.10. The increase in 0.2% proof stress with increasing numbers of ECAP passes for a
range of commercial aluminum alloys [47].

15
The first report of occurrence of high strain rate superplasticity in commercial cast
Al-based alloys after ECAP was reported in 1997 [48]. As shown in Fig. 2.11, Al-Mg-
Li-Zr showed the high elongation of ~1180% without failure at 1.0 × 10
-2
s
-1
and
~910% at failure using 1.0 × 10
-1
s
-1
at a testing temperature of 623 K [48].

Subsequently, there have been numerous reports demonstrating the occurrence of
superplastic flow after ECAP with high tensile elongations of > 500% in a range of
aluminum, copper, magnesium and zinc alloys which was tabulated in a review [49].
The temperature and grain size compensated strain rate is plotted against the
normalized stress for many aluminum alloys [48,54-59] in Fig. 2.12 [60]. The following
values for aluminum alloys were used to achieve the experimental data points for Fig.
2.12:
b =2.86 × 10
-10
m [61], G (MPa) = (3.022 × 10
4
) – 16T where the temperature is in
degrees Kelvin [61], D
gb
= D
o(gb)
exp (-Q
gb
/RT), D
o(gb)
= 1.86 × 10
-4
m
2
s
-1
, Q
gb
= 86 kJ
Figure 2.11. The first report of superplasticity achieved in a sample of Al-Mg-Li-
Zr after processing by ECAP at a temperature of 623 K [48]
16
mol
-1
[61], and converting from the reported mean linear intercept grain size, 𝐿, to the
spatial grain size, d, using the expression d = 1.74 𝐿 [62].
This plot shows an excellent agreement between the results of UFG aluminum
alloys processed by ECAP and the strain rates predicted for conventional superplasticity
where n = 2.
It is demonstrated in Fig. 2.12 that very high elongations to failure are achieved in
aluminum alloys at strain rates at and above 10
−2
s
−1
. These results confirm the
requirement for high strain rate superplasticity.
Aluminum and its alloys have been subjected to ECAP processing in many
studies [54-59]. However, studies on Al-Zn-Mg-(Cu) alloys, or 7000 series Al alloys
which are used for high strength structural applications are limited [50-53]. Due to the
Figure 2.12. Temperature and grain size compensated strain rate against
normalized stress for many aluminum alloys [60]
17
limited deformability of these alloys at room temperature, it is almost impossible to
process Al-7000 series alloys by ECAP at room temperature. However processing Al-
7000 series alloys by ECAP at elevated temperature was investigated in some studies
and it results in reasonable superplasticity [63-65].
2.2.4 Effects of ECAP condition on superplasticity
Grain boundary sliding is an important process leading to superplastic flow of
materials. Therefore, the existence of a number of grain boundaries with high angle
misorientation is necessary for the grain boundary sliding and, thus, superplastic flow in
materials. It is known that the use of ECAP has a potential to produce ultrafine grains
having a higher fraction of high angle grain boundaries with increasing number of passes
[9]. Accordingly, there is possibility to improve superplasticity properties by increasing
the number of passes in ECAP.
This behavior has been investigated in many studies. The tensile superplasticity
testing was performed in Al-Mg-Li-Zr alloy at testing temperatures from 573 to 723 K
[66]. Samples were pressed by ECAP using route B
C
for a total of 4 passes, a total
imposed strain of ~4, at 673 K and the other samples were presses using route B
C
for 8
passes at 673K and an additional 4 passes at 473 K lead to a total of 8 passes, a total
imposed strain of ~12 in the latter samples. The results showed the peak elongations are
shifted to faster strain rate of 1.0 × 10
-2
to 1.0 × 10
-1
s
-1
when the samples are processed to
larger numbers of passes of 4 to 12 passes with elongation to failure >1000%. The
displacement to even faster strain rates in the superplasticity process with high elongation
18
was also demonstrated in the Al-7037 alloy where the samples were passed by ECAP for
6 passes and 8 passes [67].
2.2.5 Double-shear creep properties after processing by ECAP
Creep flow refers to a plastic deformation in the materials subjected to a constant
stress or a constant load. Creep is a diffusion-controlled process; therefore it is very
important when materials are tested at elevated temperatures.
A double-shear configuration is a well-established technique in creep testing
[68,69]. It has been used extensively in many investigations of creep behavior [62,70]
and it has the advantage that testing under a constant load is equivalent to testing with a
constant stress.
A study investigating the effect of ECAP on creep behavior of an aluminum alloy
was conducted and the result is demonstrated in Fig. 2.13 [71]. This figure plots the
shear strain against time for an Al-7034 alloy at 473K under a shear stress of 24.2 MPa:
curves shown for specimens in the as-received condition and after processing by ECAP
through 6 passes at 473 K [71]. This plot exhibits the characteristic behavior usually
observed in high-temperature creep testing: there is an initial instantaneous strain, a
primary stage of creep in which the creep rate decreases, and a secondary stage follows
the primary stage and the creep rate reaches a reasonably steady-state value in this
region. It is apparent that the processed sample deforms under creep conditions at a rate
which is much faster than the unprocessed sample.
19

The steady-state shear strain rate, γ ̇, is related to the shear stress with an equation
of the form:
γ ̇ ~ τ
n
(2.6)
where n is the stress exponent. For the Al-7034 alloy, both sets of specimens give a
stress exponent of n ≈ 7. This high value of n is not consistent with the occurrence of a
grain boundary mechanism such as grain boundary sliding or diffusion creep. This
value of n suggests that an intragranular deformation mechanism such as dislocation
climb is the rate controlling process [71].

Figure 2.13. Plot of shear strain against time for an Al-7034 alloy estedat473K under a shear stress
of 24.2 MPa: curves shown for specimens in the as-received condition and after processing by
ECAP through 6 passes at 473 K [71]
20
2.3 High-Pressure Torsion (HPT)
The other SPD method that has attracted many researchers is HPT. The first
principles of HPT are based on a work by Prof. Bridgman in 1943 where a thin disk was
subjected to compression and torsional strain [31]. However, the technique has attracted
considerable attention only within the last decade because it is now recognized that it
provides the ability to produce materials having exceptionally small grain sizes,
generally smaller than those produced using ECAP [15,23].
As shown in Fig 2.14, HPT is a processing facility made of two anvils and the
sample in the form of a disk is located between them. The sample is subjected to a
compressive applied pressure at room temperature or at an elevated temperature and it
is simultaneously subjected to a torsional strain, which is imposed by rotation of one of
the anvils.
Three types of HPT processing have been used namely unconstrained,
constrained, and quasi-constrained HPT [72]. As shown in Fig 2.15 (a), in unconstrained
HPT, the sample is placed on the lower anvil and it is subjected to an applied pressure
Figure 2.14. Schematic illustration of HPT processing [72]
21
and torsional straining, therefore the material can flow outward under the applied
pressure. Due to the frictional forces acting between the sample and the anvil, a minor
back-pressure is introduced into the sample. To avoid this disadvantage, constrained and
quasi-constrained HPT were introduced in which the sample is placed in a depression in
the lower anvil and there is minimal outflow of material after applying the compressive
pressure and torsional straining in Fig. 2.15 (b),(c).

In modern HPT processing, a sample is subjected to a compressive applied
pressure, P, of several GPa at room temperature or at an elevated temperature and it is
subjected to a torsional straining through the rotation of the lower anvil simultaneously.
Deformation of the disk occurs due to the shear caused by the surface frictional forces
under a quasi-hydrostatic pressure. For an infinitely small rotation, dθ, and a
displacement, dl, as shown in Fig 2.16, the incremental shear strain, dγ is calculated by
the following equation [16]:

(2.7)
h
r
h
l θ
γ
d d
d = =
Figure 2.15. Schematic illustration of HPT for (a) unconstrained, (b) constrained, and
(c) quasi-constrained conditions [72]
22
where r and h are the radius and thickness of the disk respectively and dl = rdθ.

By further assuming that the thickness of the disk is independent of the rotation
angle, θ, it follows from formal integration that, since θ = 2πN, the shear strain, γ, is
given by [73-75]:
(2.8)
where N is the number of revolutions. Finally, in many investigations the von Mises
equivalent strain is then calculated using the relationship:
(2.9)
The use of Eq. 2.9 is correct for small imposed shear strains, but for large strains
where , the equivalent strain is given by [76]:
]
(2.10)

The equations suggest an inhomogeneity in the imposed strain across the sample,
which leads to an inhomogeneous microstructure across the sample. This variation was
h
r N ⋅
=
π
γ
2
3 / γ ε =
8 . 0 ≥ γ
2 / ) 4 / 1 ln[( ) 3 / 2 (
2 / 1 2
γ γ ε + + =
Figure 2.16. Parameter used in estimating the total strain in HPT [16]
23
investigated in many studies [72,77-82] and despite an inhomogeneous microstructure
after low number of turns, the microstructure became homogeneous with increasing
numbers of turns in many materials.
An alternative relationship is developed for the true strain, which incorporates the
change in thickness of the disk as a result of the applied pressure. The true strain for this
condition is given by [83]
𝜀
!"#$
= ln[ 1+
!"
!
!
]
!
!
+𝑙𝑛
!
!
!
(2.11)
where θ = 2πN, h
0
and h are the initial and final thicknesses, respectively.
The true accumulated strain, ε, was estimated from the following equation [83]:
𝜀= ln
!.!
!
= ln (
!!".!
!
) (2.12)
Figure 2.17 illustrates the variation of equivalent strain as a function of the
number of revolutions for different distance from the center of the disk processed by HPT
using equations 2.8, 2.10 [72]. The thickness of the disk for this figure is fixed at 0.1 mm.
24
It is apparent from Fig. 2.17 that a quasi-saturation is achieved after total of ~2 turns of
processing by HPT and the variation of accumulated strain is not very large after about 2
revolutions.
Several studies were conducted to investigate the feasibility of processing by HPT
on bulk samples with a thickness higher than 0.1 mm [84-86]. A study on a sample of Al-
Mg-Sc with a thickness of ~8 mm confirmed the potential for using HPT with small
cylinders. However, variations in the Vickers microhardness were observed through the
sample after processing by HPT [84]. This variation is visible in the microstructure as
shown in Fig. 2.18.
Figure 2.17. Variation of equivalent strain as a function of number of turns in a
disk with a thickness of 0.1 mm processed by HPT [72]
25

2.3.1 Hardness homogeneity after processing by HPT
Microhardness measurements and microstructural observations are two methods
that are used in evaluating homogeneity across the disks processed by HPT. The
experimental results indicate a possible dichotomy. The results obtained from austenitic
steel [77], Cu [78] and high-purity Ni [79] show significant variations in the values of the
microhardness across the diameters of disks, with lower hardness values in the center of
the disks and higher values in the peripheral regions; while the results on commercially
purity Al [72], an Al-Mg-Sc alloy [80], Cu [81] and high purity Ni [82] show a
reasonable homogeneity in microstructure across the disks after several numbers of turns
when torsional straining is sufficiently high under a high imposed pressure.
Experimental results obtained for pure aluminum [87-89] show a gradual
evolution to hardness homogeneity by increasing numbers of revolutions and the
microhardness values become saturated at a constant value after 20 turns. Plotting the
Figure 2.18. Variation of grain size in a bulk sample of an Al-Mg-Sc alloy processed by
HPT [84]
26
individual datum points against the equivalent strain correlates the variation of
microhardness values. This correlation was shown in many studies [90-95].
It is well known that the high-purity aluminum has a high stacking fault energy;
therefore, the partial dislocations are not widely separated. Nickel and aluminum-based
alloys have lower stacking fault energies and the rate of recovery is significantly slower
for these materials. It is anticipated that the hardness values are higher in the peripheral
region for materials with slower recover rate and they are lower in the peripheral region
for the materials with faster recover rate. The schematic of the Vickers microhardness
values against distance from center of the disk processed by HPT in the early stages of
processing is illustrated in Fig. 2.19, and it shows the behavior of the materials with
different stacking fault energies [88].

Figure 2.19. Values of Vickers microhardness for a material with fast recovery rate (upper
curve), and a material with slow recovery rate (lower curve) [88]
27
Continuous straining in processing by HPT is possible in a forward direction or
monotonic HPT (m-HPT), or by reversing the direction of straining in cyclic HPT (c-
HPT) [96]. Microhardness measurements of pure aluminum samples processed by m-
HPT and c-HPT revealed that homogeneity is achieved after earlier number of turns
when processing by m-HPT than processing by c-HPT.
The variation of microhardness values from the center of the disk to the peripheral
region is higher after processing by c-HPT as shown in Fig. 2.20.
Figure 2.20. Microhardness measurements across the disk for samples of pure
aluminum processed by m-HPT through totals of (a) 1, (b) 2, (c) 4 turns, or by c-HPT through
total numbers of (d) 1A + 1B, (e) 2A + 2B, (f) 1A + 1B + 1A + 1B turns [96].
28
2.3.2 Microstructural evolution after processing by HPT
The first model for grain refinement was developed in Armco iron using HPT
[64]. This model shows that increasing strain leads to a gradually increasing refinement
in the microstructure.
The grain refinement was investigated in pure Al after processing by HPT based
on the change in the hardness with imposed strain and a subsequent evolution of
microstructure [94]. This model focused on dislocation behavior and the formation of
high angle boundaries as a function of equivalent strain. As shown in Fig 2.21, the
hardness variation with equivalent strain and corresponding microstructure evolution was
divided into three regions. In the first region (I), the hardness increases with strain, the
accumulation of dislocations and the formation of subgrain boundaries occurs. In the
second region (II), the hardness increases, some dislocations are annihilated at subgrain
boundary leads to increasing of the misorientation angles. In the third region (III), the
hardness remains constant, and there is a balance between an increase in dislocation and
absorption of dislocation at grain boundaries.
Mohamed and Dheda [95] suggested that the minimum grain size after processing
by HPT follows the same model as Eq. 2.2. Plotting the minimum grain size after
processing by HPT against normalized stacking fault energy, melting temperature (T
m
),
and bB
3
results in straight lines and therefore the minimum grain size after processing by
HPT follows equations:
!
!"#
!
=𝐶 (
!
!"
)
!.!
(2.13)
29
!
!"#
!
= 3818 exp (−0.00056 𝑇
!
) (2.14)
!
!"#
!
= 3528exp −0.3 𝑏
!
𝐵 (2.15)
Figure 2.21. Schematic illustration of microstructural evolution with straining along in
pure Al. Thin double lines depicted in region II represent low angle boundaries with
some extension of boundary width and thick lines in region III represent high angle
boundaries [94]
30
2.3.3 Superplasticity after processing by HPT
A limited number of reports presented the occurrence of high ductilities in alloys
after processing by HPT. The first report introducing superplasticity flow for Al-4% Cu-
0.5%Zr alloy after HPT processing was presented by Valiev et al. [97]. This material
exhibited an elongation of ~800% at 773 K using strain rate of 3 × 10
-4
s
-1
. Subsequently,
many studies reported elongations of more than 400% in various materials as tabulated in
Table 2.1 [81,98-103].
Material
Strain rate
(𝜺) s
-1

Processing
Temperature
Testing
Temperature
Elongation to
failure
Ti-6% Al-4% V [83] 10
-3
293 K 923 K ~575%
Al-1420 [84] 10
-1
Ambient 573 K ~750%
Al 2024 [85] 1.7 × 10
-3
293 K 673 K ~570%
AZ61 [86] 3.3 × 10
-3
423 K 473 K ~620%
Mg-9%Al alloy [87] 5.0 × 10
-4
423 K 473 K ~810%
Al-3%Mg-0.2%Sc [70] 3.3 × 10
-2
s
-1
Ambient 673 K ~500%
Al-7034 [88] 1.0 × 10
-2
s
-1
473 K 703 K ~750%
Table 2.1. Elongations to failure for various materials processed by HPT
31

Nevertheless, It has been reported that the occurrence of high ductilities after HPT
often does not satisfy the conditions for superplastic flow because elongations to failure
are < 500%. This report showed an elongation of ~300% in Pb-62% Sn eutectic alloy
after HPT having an tensile testing at room temperature with strain rate 1×10
-4
s
-1
[104].
By comparison with materials processed by ECAP, despite the larger grain size
introduced by ECAP, the elongations to failure in ECAP are often significantly higher
and often exceed 1000% [49].
These lower elongations in HPT may be explained by the very small thicknesses
of the gauge lengths of samples. It is well established in conventional superplasticity that
the measured elongations to failure are related to the geometry of the test specimens
[105]. Recent bulk samples of an Al-3%Mg-0.2%Sc alloy, in the form of small cylinder
with diameter of 10 mm and height of 8.6 mm, were processed by HPT at room
temperature leading to grain size of ~130 nm at the periphery [86]. The measured
elongations depended upon the position of the specimens within the cylinder. The tensile
elongation of >1000% and ~1600% were recorded in the vertical and horizontal
specimens, respectively. These results demonstrate that the elongations to failure will be
larger as the gauge widths of the tensile specimens increase.
32
A plot of temperature and grain size compensated strain rate against the
normalized stress for various aluminum alloys processed by HPT
[28,80,99,100,103,106,107] shows superplastic behavior. This plot is shown in Fig. 2.22
[108] and the solid line illustrates the theoretical prediction for superplastic flow in
conventional metals.

This plot which uses the same approach as Fig. 2.12, is in agreement with the
theoretical model with a slope of n = 2. At any imposed strain rate, the measured
normalized stress is about one half order of magnitude larger than the theoretical value
and this discrepancy is due to the small gauge length in miniature tensile specimens.

Figure 2.22. Temperature and grain size compensated strain rate against the
normalized stress for various aluminum alloys processed by HPT [108]
33
2.4. Combination of SPD processes
Various SPD processing techniques have been applied to metals and alloys to
achieve grain refinement, improved mechanical properties, and microstructural behavior.
Recently, the combination of various SPD processes has been investigated on metals. The
combination of accumulative roll bonding (ARB) and friction stir processing (FSP) [109],
ECAP followed by HPT [110-115], ECAP followed by cold rolling (CR), and a
combination of ECAP, CR, and HPT [116] are examples of the combination of SPD
processes that have been studied recently. A comparison between different combinations
of SPD processes on pure Ni is shown in Fig. 2.23 [116]. This figure illustrates further
grain refinement after each additional SPD processing and an increase in the dislocation
density is observed after the combination of ECAP and HPT [116].

Figure 2.23. Crystallize size distributions for nickel samples obtained by
combinations of different methods: (a) ECAP, (b) ECAP + cold rolling,
(c) ECAP + HPT, and (d) ECAP + cold rolling + HPT [116]
34
2.5. Applications of the materials processed by SPD techniques
The preceding discussion shows a potential for using SPD processing to acheive
significant grain refinement. SPD processing techniques result in equiaxed grains having
sizes within the submicrometer range and a large fraction of high angle grain boundaries.
Using either the ECAP or HPT processing techniques results in a reasonable
homogeneity. A comprehensive review describes the basic characteristics of this type of
processing [2].
The commercialization of these materials is now attracting much attention and the
innovation potential for these materials is high [117]. The innovation probability of a
material is plotted schematically as a function of the specific strength of that material in
Fig. 2.24 [117,118]. This diagram demonstrates that there is a high probability for these
UFG materials to find major applications under extreme environmental coditions.
Examples include biomedical applications, aeronautical systems, and high-performance
sports applications.
Figure 2.24. The innovation potential as a function of material strength [117,118].
35
Chapter 3. Experimental Materials and Procedures
3.1 Experimental material
The experiments were conducted using a commercial Al-7075 alloy having a
composition (in wt%) of 5.6% Zn, 2.5% Mg, and 1.6% Cu. The alloy was received in the
form of extruded rods with a diameter of 10 mm. These rods were cut into billets having
length of ~65mm. The billets were machined into disks with a thickness of ~1.3 mm and
were polished to final thicknesses of ~0.83 mm for processing by HPT.
3.2 Methods of severe plastic deformation
The material was processed using severe plastic deformation techniques: ECAP,
HPT, and a combination of ECAP and HPT.
3.2.1 Processing by ECAP
An ECAP facility was composed of a die having two intersecting channels, a
plunger and a hydraulic press. The two channels have an internal angle of Φ = 110° and
an outer angle at the intersection of the two channels of Ψ = 20°. The die and the plunger
were made of a tool steel and were heat treated to obtain a Rockwell hardness of ~55.
The tolerance of the plunger was kept extremely low to prevent materials from flowing in
between the walls of the channel and the plunger. The drawings of the ECAP die and
plunger, and the ECAP facility are shown in Fig 3.1 and Fig 3.2, respectively.
36
Figure 3.1. Drawings of the die and plunger of ECAP
37

Figure 3.2. Schematic of ECAP facility
Before pressing, the billets and plunger were well lubricated. The billets were
annealed at a temperature of 753 K for 1 h and then cooled to room temperature before
processing by ECAP. The ECAP processing was conducted at a temperature of 473 K.
This geometry and these angles lead to an imposed strain of ~0.8 on each separate
passage through the die [38]. The billets were processed by ECAP for 4 and 8 passes
which correspond to the total imposed strain up to ~6.4. All samples were processed
using route B
C
in which the sample is rotated by 90° in the same direction between each
consecutive pass [35]. This processing route was selected because it leads most
expeditiously to an array of equiaxed grains separated by an array of boundaries having
high-angles of misorientation [119-121].
38
3.2.2 Processing by HPT
The billets of Al-7075 were cut and polished into disks with a thickness of ~0.83
mm. These polished disks were processed by HPT at room temperature through total
numbers, N, of 1/8, 1/4, 1/2, 3/4, 1, 2, 5, 10 and 20 turns.
The HPT facility imposed a pressure of 6.0 GPa and a concurrent torsional straining
through two massive anvils. Both the upper and lower anvils were made from high-
strength YXR3 tool steel having nitride surfaces and were machined with spherical
depressions at the centers of the adjacent surfaces with diameters of 10 mm and depths of
0.25 mm. Each disk was placed in the depression on the lower anvil and the anvils were
then brought together to impose a pressure on the disk.. Torsional straining was achieved
by rotation of the lower anvil at a constant speed of 1 rpm. The processing was conducted
under quasi-constrained conditions in which there is a very small outflow of material
around the perimeter of each disk during the processing operation [122].
The imposed pressure was achieved by an applied force of 470 kN and was determined
using the following calculation:
𝐹=
!
!
× 𝑑
!
!
× 𝑃
!
(3.1)
where F is the applied force, P
h
is the hydraulic pressure, d
h
is the diameter of the
hydraulic cylinder (250 mm in this facility).
This force then could be transferred to the pressure applied on the 10 mm-diameter disk,
P
d
using:
39
𝑃
!
=
𝐹
(𝜋 × 5)
!
(3.2)
3.2.3 ECAP followed by HPT processing
ECAP billets after pressing through 4 or 8 passes were sliced in the direction
perpendicular to the longitudinal axis using a diamond blade sawing with a thickness of
1.5 mm and grinded to final thickness of ~0.83 mm. Disks are then subjected to HPT for
1/8 to 20 turns under an applied pressure of 6.0 GPa at a speed of 1 rpm. The schematic
of the combination of ECAP and HPT is illustrated in Fig. 3.3.

Figure 3.3. Schemiatic of the combination of ECAP and HPT
40
3.3 Microstructural analysis
3.3.1 Electron-backscatter diffraction (EBSD)
Microstructures of samples after ECAP and HPT processing were also observed using
electron backscatter diffraction (EBSD) technique. The EBSD patterns were observed on
the screen with a charge coupled device (CCD) camera. Due to the combination of good
spatial and angular resolution in this technique, it provides unique and quantitative
information on the local preferred orientation of crystallite and the grain boundary
misorientations.
For the billet after ECAP, a disk was cut from the central part of the billet in the
transverse plane or X-plane perpendicular to the pressing direction. Then ECAP and HPT
disks were mounted for grinding and polishing. Grinding was started with 240 grit SiC
paper and continued through 2400 grit SiC. Polishing was used to remove any
deformation introduced during grinding with 9, 6, 3 and 1 µm diamond suspension, and
ending with 0.04 µm colloidal silica suspension using the vibratory polisher. The EBSD
analysis was taken using a JSM 7001F scanning electron microscope (SEM) operating at
an accelerating voltage of 15 kV, a working distance of 15 mm, and a sample tilt of 70º.
The EBSD patterns were collected with TSL OIM software with the step sizes varies
from 0.04 µm to 0.10 µm, depending on the grain size. The grain boundaries were
analyzed using this software in which high-angle grain boundaries (HAGBs) are defined
as misorientations between adjacent measured points of more than 15º. The clean-up
procedure was performed in an OIM-TSL analyzer and included grain dilation (GD) and
grain confidence index standardization. The total number of modified points was not
41
higher than 10% of the total points measured in the experiments.
3.3.2 Transmission electron microscopy (TEM)
A sample processed by ECAP for 8 passes and HPT through total of 20 was prepared by
focused ion beam (FIB) for examination by transmission electron microscopy (TEM).
The TEM thin foil was sectioned from the cross section (X plane) in the center of the
disk. Specimens were processed by FIB under 30 kV using Gallium ion to a final
thickness of ~70 nm. TEM was performed using a Philips CM12 microscope operating at
200 kV for the sample processed by a combination of ECAP and HPT. Grain size
measurements were conducted using an Olympus analySIS FIVE software.
3.4 Mechanical experiments
3.4.1 Microhardness measurements
Microhardness measurements were performed using a digital microhardness tester FM-1e
equipped with a Vickers diamond indenter using a load of 200 gf and a dwell time of 10
s. The minimum distance between adjacent indentations and distance from indentation to
the edge of the specimen was designed based on ASTM E384 in which at least 2.5 times
the indentation size to be considered in order to avoid interaction between the work-
hardened regions and effects of the edge. The values of the Vickers microhardness, Hv,
were measured through different procedures for processing by ECAP and HPT.

42
3.4.1.2 Microhardness measurement of disks processed by HPT or a
combination of SPD processes
Following HPT, each processed disk was mounted and polished to have a mirror-like
surface quality and microhardness measurements were taken using two different
procedures as shown in Figure 3.4. First, the average microhardness values were
measured along a randomly selected diameter of each disk. These measurements were
taken at intervals of 0.3 mm and at every point the local value of Hv was taken as the
average of four separated hardness values recorded at uniformly separated points
displaced from the selected point by a distance of 0.15 mm. Using this procedure, it was
possible to achieve a high accuracy in the individual values of Hv and for every position
the error bars were estimated corresponding to the 95% confidence limits. Second, the
distribution of local hardness was measured over a quarter of the total surface of each
individual disk by recording the individual values of Hv following a rectilinear grid
pattern with an incremental spacing of 0.3 mm. These individual datum points were then
used to construct color-coded contour maps to provide a simple visual display of the
hardness distributions within each disk.
43

3.4.2 Tensile Measurements
In order to investigate the mechanical behavior at elevated temperature, tensile
experiments were conducted on the samples in the as-received condition, and after
processing. For ECAP samples, the tensile testing were carried out on X plane where
disks with diameter of 10 mm and thickness of ~0.75-0.8 mm, similar to the dimensions
of sample after HPT processing, were cut from the central part of billets. Thereafter,
tensile specimens were prepared from the disks in each condition. The dimension of
tensile specimens is presented in Fig. 3.5.
This drawing was designed in order to avoid any potential problems associated
with microstructural inhomogeneities near the centers of the disks in the early stage of
HPT processing. In Fig. 3.5, two miniature tensile specimens were cut from two off-
Figure 3.4. Schematic of the positions of hardness indentation over a quarter of the
disk processed by SPD [96]
44
center positions on either side of the disks using electro-discharge machining (EDM)
[123]. Earlier research showed that higher elongations are generally achieved in
specimens machined from the off-center positions in HPT processing [124].

These tensile specimens were pulled to failure at temperature of 623 K using an
Instron testing machine operating at constant rate of cross-head displacement and with
initial strain rates in range from 1 × 10
-4
to 1 × 10
-2
s
-1
. At least two specimens were used
for each condition to obtain consistent stress-strain curves.
The elevated temperature was achieved using a single-zone split furnace with a
vertical height of 60 cm and a proportional temperature controller. The temperature was
measured using a K-type thermocouple attached near to the surface of the specimen.
Usually it took approximately 30 minutes for the specimen in the furnace to achieve the
desired testing temperature and then held at that temperature for ~10 min in order to
reach thermal equilibrium prior to starting the test.
Figure 3.5. Schematic of the miniature tensile samples cut by EDM [123]
45
3.4.3 Double Shear Creep
The creep testing used a double-shear specimen geometry with a round cross-
section. The dimensions of the specimen are indicated in Figure 3.6. The double shear
geometry has been chosen for the following reasons [125,126]:
(1) Equivalence of constant stress and constant load creep data, (2) Uniformity of
deformation over the gauge section, i.e. necking is avoided, and (3) Achieving large
strains in comparison to tensile specimens.
The double-shear creep specimens were machined such that the loading direction
is normal to the extrusion direction. During the creep tests the shear displacement of the
gauge section was measured using the Linear Variable Differential Transducer (LVDT)
extensometer system.

Figure 3.6. Geometry and dimensions of a specimen used for double-shear creep testing
46

The specimen temperature was monitored using alumel-chromel thermocouple
attached to the thermocouple wells at the free ends of the shoulder sections of the
Figure 3.7. Schematic of the double-shear creep machine [127]
47
specimen. The temperature, after stabilization during the artificial aging interval, was
then maintained within an allowance range of ±2K. The double-shear configuration of the
creep machine used in the present testing procedure is indicated in Fig. 3.7.
The creep test was left to run until a well-defined secondary creep region was
established. After the completion of the test, the specimen was air-quenched under load
to maintain the creep substructure at room temperature. The creep data obtained from
double-shear specimens were related to the shear strain and shear stress. These quantities
were then converted to their tensile counterparts, according to the following equations:
𝜎= 2𝜏=
!
!
!
(3.3)
𝜀=
!
!
𝛾=
!
!
!
!
!
(3.4)
where σ is the tensile stress, τ is the shear stress, P is the applied load, A
o
initial cross-
sectional area, of the gauge section, 𝜀 is the tensile strain rate, 𝛾 is the shear strain rate,
𝛿 is the rate of shear displacement of the gauge section, l
o
is the initial gauge length.

48
Chapter 4. Experimental Results
4.1 Microhardness measurements
4.1.1 Microhardness values after HPT
The individual values of the Vickers microhardness, Hv, were measured across
the diameter of each disk and these data are plotted against distance from the center of the
disk in Fig. 4.1. The microhardness value for each indentation was obtained from the
average of four separate hardness measurements recorded at uniformly separated points
displaced around the selected position. The distance between each indentation was 0.3
mm and this was effectively reduced to 0.15 mm due to the averaging procedure. The
lower dashed line in Fig. 4.1 denotes the value of hardness for the as-annealed sample
prior to HPT processing.
It is apparent from Fig. 4.1 that the hardness generally increases with increasing
numbers of turns in the HPT facility. Initially, after up to 1 or 2 turns, the hardness
increases significantly around the periphery of the disk but it increases less in the central
region. It is important to note that there is a significant increase in hardness around the
edge of the disk even after only 1/8 turn: thus the hardness of Hv ≈ 102 in the as-annealed
condition is increased to Hv ≈ 150 around the edge and to Hv ≈ 110 in the center after 1/8
turn. Thereafter, and up to 2 turns, all hardness values increase gradually at essentially
similar rates so that there is no obvious homogeneity within these 2 turns.
49

The situation becomes different after 5 and 10 turns. For both of these conditions
the hardness values are high, up to two or more times larger than in the as-annealed
condition, but the values of Hv are reasonably constant across the diameters of the disks.
However, in the Al-7075 alloy the homogeneity fails to reach a saturation condition even
after 5 turns and instead there is a measurable increase in hardness between 5 and 10
turns. Thus, the average hardness values are Hv ≈ 210 after N = 5 turns but Hv ≈ 225
after N = 10 turns.
Figure 4.2 provides pictorial displays of the variations in microhardness over one-
quarter incremental areas of the surfaces of disks processed by HPT through totals of 1/2,
1, 2 and 5 turns. These color-coded maps provide a useful display of the hardness
Figure 4.1. Vickers microhardness against distance from the center of the disk
processed by HPT though totals of 1/8 to 10 turns
50
distributions with the colors corresponding to the color-scale shown on the right where
the hardness values increase through increments of 20: the X and Y coordinates shown in
Fig. 4.2 represent arbitrarily-selected orthogonal axes that lie in the plane of each disk
and intersect at the centers of the disks at the position having coordinates of (0, 0).
These displays confirm both the increase in hardness with increasing straining and
the gradual transition towards higher values of hardness with increasing torsional
straining. Thus, close inspection shows there is a relatively large area around the center
of the disk processed by HPT through a total of 1/2 turn where the hardness has a
significantly lower value by comparison with the edge of the disk.
As the numbers of turns increase, this central area with lower microhardness
becomes increasingly reduced in size so that there is a gradual evolution towards greater
homogeneity with increasing strain but at the same time the average hardness value also
increases. These color-coded maps are consistent with, but provide more detailed
information on, the variations in hardness recorded along randomly-selected disk
diameters in Fig. 4.1.

51

Figure 4.2. Color-coded map of Vickers micohardness values over a quarter of the surface of disks
processed by HPT through (a) ½ turn, (b) 1 turn, (c) 2 turns, and (d) 5 turns
52
4.1.2 Microhardness values after processing by a combination of ECAP
and HPT
The Vickers microhardness value, Hv, of individual points across diameter of each disk
were measured by averaging the microhardness values of the four separate hardness
measurements of uniformly separated points displaced from the selected position.
4.1.2.1 Microhardness values of samples processed by ECAP for 4
passes and HPT
The data are plotted against distance from center of the disks processed by ECAP
for 4 passes and by a combination of ECAP for 4 passes and HPT through total numbers
of 1/8 to 20 turns in Fig. 4.3. It is apparent that microhardness values are nearly similar
across the diameter after processing by ECAP for 4 passes and they generally increase
after combination of ECAP and HPT. The Vickers microhardness values increase by
increasing total number of turns in the HPT processing in the combination of SPD
processes.
The average of microhardness values over the diameter of disks after processing
with the combination of ECAP for 4 passes and HPT through various numbers of turns is
larger than the values after processing by only HPT for the same number of turns [28].
This comparison is demonstrated in Fig. 4.4 and is consistent with an earlier report for
high purity Cu where the hardness values were also higher after processing by ECAP +
HPT but the results on Cu were different because recrystallization occurred during
processing and this produced a bimodal grain size distribution [128].
53
After processing by ECAP for 4 passes, the Al-7075 alloy demonstrates a
reasonably constant Vickers microhardness value of Hv ≈
130. Thereafter, The hardness
values increase significantly around the peripheral regions of the samples by increasing
the total number of turns processed by HPT. The increase in hardness values is less
significant around the center of the samples processed by the combination of SPD
processes up to the total number of 1 turn processing by HPT.
Figure 4.3. Vickers microhardness values along diameter of the disks processed by ECAP
for 4 passes and HPT through totals of 1/8 to 20 turns

As-annealed
54

Figure 4.4. Average values of the Vickers microhardness across diameters of
the disks against the number of turns of HPT processing for the samples
processed by HPT or by a combination of ECAP and HPT
Figure 4.5. The Vickers microhardness values against equivalent strain after
processing by HPT ECAP for 4 passes and HPT through totals of 1/8 to 20 turns
55
The difference between hardness values in the center and around the peripheral
region of one specimen decreases as the total number of turns processed by HPT
increases and this difference becomes negligible after 20 turns of HPT processing and the
sample reaches hardness homogeneity with a hardness value of Hv ≈
250.
The hardness values are replotted against equivalent strain in Fig. 4.5. The
equivalent strain imposed on each disk is calculated using Eq. 2.8. The results show that
the hardness values saturate at Hv ≈
250.
4.1.2.2 Microhardness values after ECAP for 8 passes and ECAP + HPT

The values of the Vickers microhardness, Hv, after processing by ECAP for 8
passes (shown as the line labeled ECAP) and after ECAP + HPT with HPT from 1/8 to
20 turns are plotted against the positions on the disk in Fig. 4.6: the lower dashed line
shows the hardness value in the initial annealed condition.
Figure 4.6. Vickers microhardness values along diameter of the disks
processed by ECAP for 4 passes and HPT through totals of 1/8 to 20 turns
As-annealed
56

It is readily apparent that the microhardness values are essentially identical
across the diameter of the disk after processing by ECAP for 8 passes. However, the
microhardness values increase for the samples processed with ECAP + HPT and the
increase becomes larger with increasing numbers of turns in the HPT processing.
Initially, this increase occurs preferentially around the edge of the disk but the hardness
values at the edges tend to stabilize after about 10 turns.
The Vickers microhardness values are initially relatively low in the centers of the
disks but they gradually increase with increasing numbers of turns so that ultimately,
after 20 turns, there is a reasonable level of homogeneity across the disk. The
development of reasonable homogeneity after large numbers of turns in HPT is consistent
with numerous earlier results when processing aluminum alloys by HPT without an initial
step of ECAP [11,129] and it is consistent with theoretical predictions based on strain
gradient plasticity modeling [130]. Nevertheless, very recent results on the NiTi shape
memory alloy show that a hardness homogeneity is not attained in this material even after
40 turns of HPT processing [131].
It was shown in a very early investigation of HPT processing that it is generally
possible to correlate all hardness values so that they fall on or about a single curve by
plotting the values of Hv against the equivalent strain as estimated using eq. 2.9 [77].
This type of approach has been used extensively in investigations of materials processed
by HPT [29,107,132-136] and it is appropriate to use the same procedure in analyzing the
present data. Accordingly, Fig. 4.7 shows a plot of the Vickers microhardness against the
equivalent strain for samples processed by ECAP for 8 passes and then by HPT for up to
57
20 turns. For simplification, the error bars corresponding to the 95% confidence limits
are included in Fig. 4.7 only for the points at the highest equivalent strains. Inspection of
Fig. 4 leads to two important conclusions. First, all datum points fall reasonably about a
single curve. Second, it is apparent that the Vickers microhardness values increase
rapidly at lower values of equivalent strain and then they cluster around a well-defined
saturation line at equivalent strains higher than ~200.

Figure 4.7. Vickers microhardness values against equivalent strain for samples
processed by ECAP for 8 passes and HPT through totals of 1/8 to 20 turns
58
4.1.3 A comparison of Vickers microhardness for samples processed by
SPD techniques
In order to obtain a reasonable comparison between the Vickers microhardness
values obtained in HPT and ECAP + HPT, Fig. 4.8 shows the average hardness values
plotted against the numbers of turns in HPT for three different conditions: (i) a sample
processed only by HPT without any ECAP, (ii) a sample processed by ECAP for 4 passes
and then HPT and (iii) a sample processed by ECAP for 8 passes and then HPT. For
these plots, the average hardness values simply represent the average value obtained from
all microhardness measurements taken at equal spacings of 0.3 mm along the diameter of
each disk. Since each disk provided the same numbers of individual measurements, this
averaging provides a direct and simple comparison between the different processing
routes.
Figure 4.8. A comparison of the Vickers microhardness values of the samples processed by
HPT, ECAP for 4 passes and HPT, or ECAP for 8 passes and HPT
59
It is apparent from Fig. 4.8 that processing by HPT alone leads to the lowest
hardness values, processing by ECAP for 4 passes plus HPT leads to intermediate values
and the highest values are recorded on the samples processed by 8 ECAP passes plus
HPT. It is also apparent from Fig. 4.8 that all the samples reach a fairly constant
hardness value after processing by HPT through a total of 10 turns.
A comparison between the plots of Vickers microhardness against equivalent
strain is schematically illustrated in Fig. 4.9. This figure directly shows the significance
of the saturation hardness values obtained under different conditions.
Figure 4.9. A schematic of the Vickers microhardness values against
equivalent strain for the samples processed by HPT, ECAP for 4 passes
and HPT, or ECAP for 8 passes and HPT
60
The results in Fig. 4.9 are shown for the microhardness values of samples
processed by HPT without any ECAP, by ECAP for 4 passes followed by HPT and by
ECAP for 8 passes followed by HPT: the latter line is taken from Fig. 4.7, the line in the
middle is taken from Fig. 4.5, and the other line is taken from earlier measurements [28].
The experimental points are not included in Fig. 4.9 for simplicity in presentation,
because they are readily available in Fig. 4.5 and 4.7.
There are two important conclusions from Fig. 4.9. First, the curves for all three
conditions are essentially similar and they show a leveling off at equivalent strains above
~200. Second, the saturation values at the highest strains increase with the imposition of
additional straining on the samples. Specifically, the saturation hardness values are
represented by the horizontal dashed lines and they correspond to ~231 for HPT alone,
~249 for ECAP for 4 passes plus HPT and ~273 for ECAP for 8 passes plus HPT.
4.2 Microstructural analysis
4.2.1 Microstructural behavior of the samples before and after HPT
Figure 4.10 shows OIM images of the Al-7075 alloy in (a) the as-annealed
condition and after HPT through 5 turns at (b) the central region and (c) the periphery of
the disk: following conventional practice, high-angle grain boundaries (HAGB) are
defined as misorientations across the interfaces of 15º or more and the measurements
denote low-angle grain boundaries (LAGB) as having measured misorientations from 2º
to 15º. The markedly elongated nature of the grain structure in the as-annealed condition
is readily apparent in Fig. 4.10 (a) but after 5 turns of HPT the grains are much smaller
and the configurations are reasonably equiaxed in both the central part of the disk in Fig.
4.10 (b) and near the edge in Fig.4.10 (c).
61

The average grain size in the as-annealed sample, measured perpendicular to the
elongated structure, was ~8 µm but the grain sizes were very much reduced by processing
through 5 turns of HPT. The average grain sizes after 5 turns were measured as ~500 nm
in the center of the disk and ~250 nm near the edge of the disk. These grain sizes are
Figure 4.10. OIM images of the samples in the (a) annealed condition, and
after processing by HPT for 5 turns (b) in the center and (c) around the peripheral
region of the disk
62
very small by comparison with the grain size prior to HPT but nevertheless the grain
sizes at the center and the edge of the disk differ by a factor of two and this indicates that
microstructural homogeneity is not yet fully attained after processing through 5 turns.
4.2.2 Misorientation angles before and after HPT
An investigation of the misorientation angles of the samples before and after
processing by HPT is shown in Fig. 4.11. It is evident in Fig. 4.11, that an annealed
sample has a large number fraction of low-angle grain boundaries of ~66 %. This value is
reduced significantly after processing by HPT through total of 5 turns to ~22 % in the
center and ~19 % around the peripheral region. The solid curves represent the predicted
theoretical distributions for a random array of misorientation angles [137,138]. It is
observed in Fig. 4.11 that the number fraction of misorientation angles has a significant
variation from the random distribution of misorientation angles before processing by
HPT. This variation is reduced significantly after processing by HPT through total of 5
turns. There is an excess number fraction of low angle grain boundaries in all three plots,
which is consistent with other studies on UFG materials. This excess in the amount of
LAGB is related to dislocations produced after every turn of processing by HPT. This
variation is suggested to be in parts due to the disorientation distribution function (DDF)
used in EBSD OIM software [139].
63

(a)
(b) (c)
Figure 4.11. Misorientation angles of samples (a) in the annealed condition, and after processing by
HPT through total of 5 turns (b) in the center and (c) around the peripheral region of the disk
64
4.2.3 Microstructural behavior of the samples after a combination of
SPD processes
Figure 4.12 shows the OIM image of Al-7075 after (a) processing by ECAP for 4 passes,
(b) a combination of ECAP for 4 passes and HPT through total number of 20 turns, a
combination of ECAP for 8 passes and HPT through total numbers of 20 turns (c) in the
center, and (d) around the peripheral region of the sample.
The average grain size in this image is ~310 nm around the center of the sample
processes by a combination of ECAP for 4 passes and HPT through total of 20 turns
which is significantly smaller compared to ~680 nm for the samples of Al-7075
processed by ECAP for 4 passes and no HPT processing. The average grain size of the
Al-7075 disk processed by HPT through total number of 5 turns is ~500 nm in the center
and ~250 nm around the peripheral regions as reported earlier.
Inspection shows that the microstructures of the sample processed by ECAP for 8
passes are not uniform and they consist of grains of various sizes in both the central and
edge regions of the billet. Nevertheless, there was a measured average grain size of ~600
nm and this is smaller than the grain size of ~680 nm reported earlier for a specimen
processed by ECAP for only 4 passes. Thus, it is apparent that there is continued but
relatively minor grain refinement when processing by ECAP through 4 to 8 passes.
Earlier measurements showed that processing by HPT introduced significant grain
refinement in the alloy. The average grain size in this alloy was ~500 nm in the center of
an HPT disk after processing by HPT for 5 turns but a significantly smaller grain size of
~310 nm was recorded after processing by ECAP for 4 passes and then conducting
additional processing by HPT through a total of 20 turns.
65
TEM images of the sample processed by ECAP for 8 passes and HPT through
total number of 20 turns reveal an ultrafine equiaxed microstructure with an average
grain size of < 200 nm. TEM images of the sample are illustrated in Fig. 4.13: (a) bright
field image and (b) dark field image at the same position on the sample.
Microstructural images of the samples processed by a combination of SPD
processes demonstrate homogeneous and equiaxed grains that are in good agreement with
the results obtained from microhardness measurements.
These results demonstrate that further grain refinement is achieved when the
processing is continued to higher strains. Taken together, the present results and the
earlier results show that additional grain refinement may be achieved in SPD processing
either by adding more passes in ECAP or by cutting the ECAP billets into disks and then
conducting further straining by HPT. Thus, these results appear to question the concept
of the development in SPD processing of a true and constant saturation microstructure.

66

(a)
(b)
(c) (d)
Figure 4.12. OIM image of Al-7075 after (a) processing by ECAP for 4 passes, (b) a
combination of ECAP for 4 passes and HPT through total number of 20 turns, a combination of
ECAP for 8 passes and HPT through total numbers of 20 turns (c) in the center, and (d) around
the peripheral region of the sample
67

4.2.4 Misorientation angles after processing by a combination of SPD
processes
Number fraction of misorienation angles after processing by (a) ECAP for 4
passes , (b) ECAP for 4 passes and HPT through total number of 20 turns, (c) ECAP for 8
passes in the center, and (d) ECAP for 8 passes around the peripheral region of the disk is
illustrated in Fig. 4.14. The solid curves are the same as the ones in Fig. 4.11, and they
represent the predicted theoretical distributions for a random array of misorientation
angles by Mackenzie [137,138]
(a) (b)
Figure 4.13. TEM images of Al-7075 alloy processed by ECAP for 8
passes and HPT through total number of 20 turns: (a) bright field image
and (b) dark field image of the same position
68
It is readily apparent from Fig. 4.14 that the number fractions of HAGBs increase
with increasing numbers of ECAP passes. Specifically, the fraction is ~59% after 4
passes in the center of the billet but this increases to ~72% in the center of the billet after
8 passes. These fractions are similar to those reported earlier for high purity Al processed
by ECAP through different numbers of turns [9].
Inspection of the misorientation distributions after processing by ECAP and after
the combination of ECAP and HPT shows there is a significant increase in the fraction of
HAGBs when additionally processed by HPT. Thus, the fraction of HAGBs is ~59%
after ECAP for 4 passes and ~71% after ECAP for 4 passes and HPT through total
numbers of 20 turns. For both conditions, there is an excess of LAGBs by comparison
with the theoertical distribution and this is consistent with the results from all SPD
processing where each pass of ECAP or turn of HPT introduces additional dislocations
into the material.

69

(Center)
(Edge)
(a) (b)
(d) (c)
Figure 4.14. Misorientation angle of an Al-7075 alloy after processing by (a) ECAP for 4 passes,
(b) ECAP for 4 passes and HPT through total number of 20 turns, (c) ECAP for 8 passes in the
center, and (d) ECAP for 8 passes around the peripheral region of the disk
70
4.3 Mechanical behavior
4.3.1 Tensile properties of the samples processed by HPT
Figure 4.15 shows representative stress-strain curves obtained from the alloy at
623 K after testing at initial strain rates of (a) 1.0 × 10
-2
, (b) 1.0 × 10
-3
, and (c) 1.0 × 10
-4

s
-1
: separate curves are shown for the as-annealed condition and for samples processed
through 1, 5 and 10 turns.

Figure 4.15. Plots of engineering stress versus engineering strain for the Al-7075 alloy in the
annealed condition or processed by HPT through total of 1, 5, or 10 turns at initial strain rates
of (a) 1.0 × 10
-2
, (b) 1.0 × 10
-3
, and (c) 1.0 × 10
-4
s
-1

71
These curves are typical of those generally observed in ultrafine-grained materials
at elevated temperatures including little or no strain hardening and reasonable ductilities.
As anticipated at elevated temperatures, the ultrafine grain sizes introduced by HPT lead
to lower flow stresses and significantly higher elongations to failure. The measured flow
stresses are plotted in Fig. 4.16, where the data are recorded using logarithmic
coordinates so that the slopes of these lines give a strain rate sensitivity of the order of
~0.3.

Figure 4.16. Plot of flow stress versus strain rate for the Al-7075 alloy pulled in
tension in the annealed condition or after processing by HPT through total of 1, 5, or
10 turns.
72

The increased ductility associated with HPT processing is demonstrated in
Fig.4.17, where the elongations to failure for each sample are recorded against the initial
strain rate for the as-annealed condition and after 1, 5 and 10 turns.

These results show the elongations to failure tend to increase with decreasing
strain rate and, in addition, the elongations are similar for the samples processed through
5 and 10 turns. The elongations recorded in the as-annealed condition are close to
~200% and this is not within the superplastic regime, which requires elongations of at
Figure 4.17. Plot of elongation to failure versus strain rate for the Al-7075
alloy pulled in tension in the annealed condition or after processing by HPT
through total number of 1, 5, or 10 turns.
73
least 400% [45]. However, the Al-7075 alloy exhibits superplastic flow at 623 K after
processing by HPT with elongations up to 700% at the lowest strain rate of 10
-4
s
-1
. It is
important to note also that the elongations to failure are also essentially identical after 5
and 10 turns, which is consistent with attaining a reasonable homogeneity at least on
either side of the disk centers.
4.3.2. Double-shear creep of the samples processed by ECAP for 8
passes
Samples of the Al-7075 alloy processed by ECAP for 8 passes were machined to
the geometry of double shear specimens and were tested by a double-shear creep facility
at a temperature of T = 573 K. Double shear creep tests are conducted under a constant
load. Constant loads of 10, 15, and 20 pounds were used in this experiment, which are
equivalent to constant stresses of 9.9, 14.1, and 18.3 MPa.
Plots of shear strain against time for the Al-7075 alloy processed by ECAP for 8
passes under constant stresses of 9.9, 14.1, and 18.3 MPa are demonstrated in Fig. 4.18.
The primary and secondary stages of creep are visible in the three plots and it can be seen
that the secondary stage has a larger slope at higher stresses.
Figure 4.19 demonstrates plots of the shear strain against time for an Al-7075
alloy in the annealed condition and after processing by ECAP for 8 passes. It is evident
that at a constant stress, the sample in the annealed condition has a significantly lower
strain compared to the sample processed by ECAP for 8 passes.
74
This behavior is expected by Eq 2.5, where strain rate (𝜀) has an inverse
relationship with grain size. Processing by ECAP for 8 passes results in a smaller average
grain size. Thus at elevated temperature samples processed under this condition are
expected to have higher strain rates.
Figure 4.18. Plots of shear strain against time for the Al-7075 alloy processed by
ECAP for 8 passes under constant stresses of 9.9, 14.1, and 18.3 MPa
75

4.3.3 Tensile properties of the samples processed by a combination of
SPD processes
4.3.3.1 Tensile properties after ECAP for 4 passes and HPT
Miniature tensile specimens were cut from disks of the Al-7075 alloy after
processing by ECAP and by ECAP + HPT. The tensile tests were performed at a
temperature of 623 K at initial strain rates of 1.0 × 10
-1
, 1.0 × 10
-2
and 1.0 × 10
-3
s
-1
and
the results for these three strain rates are shown in Fig. 4.20 (a), (b) and (c), respectively.
Separate curves are shown for samples processed only with ECAP for 4 passes
and for samples processed by a combination of ECAP for 4 passes and HPT through 5,
10 and 20 turns. These curves are generally typical of the behavior anticipated for
Figure 4.19. Plots of shear strain against time for the Al-7075 alloy in the annealed
condition and processed by ECAP for 8 passes under constant stress of 14.1 MPa

76
ultrafine-grained materials at high temperatures. For all strain rates, samples processed
using only ECAP for 4 passes show the highest values for the flow stresses and the
lowest stresses are recorded consistently after processing by ECAP + HPT.
Significantly higher elongations to failure are also attained consistently after
ECAP + HPT. All of these results confirm the significant reduction in grain size, by a
factor of up to more than 2, when using the combined ECAP + HPT processing.
Nevertheless, it is apparent that there is some scatter in the results after combining ECAP
and HPT so that there is a lack of consistency concerning the effect of increasing the
numbers of HPT turns on either the flow stresses or the elongations to failure.
This can be seen by plotting the flow stresses against strain rate as shown in Fig.
4.21, or the elongations to failure against strain rate as shown in Fig. 4.22. This scatter is
attributed to the relatively small variations occurring in the final grain sizes after
additional processing through 5, 10 or 20 turns of HPT since it was demonstrated earlier,
in experiments conducted on this alloy using only HPT, that the grain sizes and the
mechanical properties became reasonably stabilized after processing through a minimum
of 5 turns [28].

77

(a
)
(b
)
(c)
Figure 4.20. Plots of engineering stress versus engineering strain for the Al-
7075 alloy processed by ECAP or ECAP + HPT and then pulled in tension
to failure at 623 K at initial strain rates of (a) 1.0 × 10
-1
, (b) 1.0 × 10
-2
and
(c) 1.0 × 10
-3
s
-1
.

78

Figure 4.21. Flow stress versus strain rate at 623 K for the Al-7075 alloy after
processing by ECAP for 4 passes or a combination of ECAP and HPT.
Figure 4.22. Elongation to failure versus strain rate at 623 K for the Al-7075 alloy
after processing by ECAP for 4 passes or a combination of ECAP and HPT.
79
It is also apparent from Fig. 4.22, that the elongations to failure increase with
decreasing strain rate in the samples processed by ECAP + HPT. The elongations
recorded in the ECAP + HPT specimens provide excellent examples of superplastic flow
with a maximum elongation of ~800% in the sample processed by HPT through 10 turns
at the lowest strain rate of 1.0 × 10
-3
s
-1
. Inspection of Fig. 4.21 shows that the datum
points for this material lie close to a line having a slope, and therefore a strain rate
sensitivity, of ~0.5 although the other samples tend to have lower strain rate sensitivities.
It is also apparent from Fig. 4.22 that the highest elongation recorded in the sample
processed only by ECAP was ~300% at the lowest strain rate and this does not fulfill the
requirements for superplasticity.
4.3.3.2 Tensile properties after ECAP for 8 passes and HPT
Tensile tests were performed at a temperature of 623 K at initial strain rates of 1.0 × 10
-1
,
1.0 × 10
-2
and 1.0 × 10
-3
s
-1
and the results for these three strain rates are shown in Fig.
4.23 (a), (b), and (c), respectively. For all strain rates, the samples processed only by
ECAP for 8 passes show the highest values for the flow stresses whereas much lower
stresses are consistently recorded after processing by ECAP for 8 passes plus HPT. The
elongations to failure after processing by a combination of ECAP and HPT reaches a
maximum of ~1100% in the sample processed by HPT through 20 turns at the strain rate
of 1.0 × 10
-2
s
-1
and this demonstrates excellent superplastic properties since
superplasticity requires tensile elongations of at least 400% [45]. By contrast, the
samples processed only by ECAP for 8 passes show maximum elongations of ~400%
which are on the limit of the true superplastic regime.
80

(a)
(b)
(c)
Figure 4.23. Plots of engineering stress versus engineering strain for the Al-7075 alloy
processed by ECAP for 8 passes or a combination of ECAP + HPT and then pulled in tension
to failure at 623 K at initial strain rates of (a) 1.0 × 10
-1
, (b) 1.0 × 10
-2
and (c) 1.0 × 10
-3
s
-1
.

81
Chapter 5. Discussion
5.1 The effect of HPT
The present results on the Al-7075 alloy processed by HPT demonstrate the
ability to develop a reasonable hardness homogeneity after higher numbers of turns of
HPT. When a disk is processed by HPT, the equivalent von Mises strain imposed on the
disk, ε
eq
, shown in eq. 2.8 suggests that the equilibrium strain should be zero at the center
of the disk and it is a maximum around the peripheral region.
In practice, due to the presence of a compressive strain as well as a torsional strain
in the HPT disk, the total imposed strain is not zero at the center of the disk so that
differences arise between experimental measurements and the simple predictions of eq.
2.8. There are also additional features of HPT processing under quasi-constrained
conditions that deviate from the ideal condition such as a small outflow of material
around the periphery of the disk during torsional straining. This material outflow was
first reported in very early HPT experiments [77] and has been subsequently documented
[140] and predicted directly using finite element modeling [141].

82
The accumulated shear strain distribution calculated by a first order gradient
model is in agreement with the results obtained for the Al-7075 alloy. The simulated
accumulated shear strain versus distance from the center of the disk is shown in Fig. 5.1
[130]. This simulation was conducted for pure copper processed by a constrained HPT
under a pressure of 5.0 GPa and the thickness of the disk processed by HPT is assumed
constant. It is apparent in Fig. 5.1 that the accumulated shear strain becomes reasonably
uniform after processing by HPT through 5 turns [130].

The results in Fig. 5.1 show that the hardness is initially lower at the center than
in the periphery but there is an evolution towards hardness homogeneity with increasing
numbers of turns in HPT processing. However, the results at present suggest that
additional straining is required to fully establish a saturation condition and a fully
homogeneous structure. These results are generally consistent with other investigations
on a range of different materials [14,88,128,129] and they also match the model
Figure 5.1. Accumulated shear strain versus distance from the center of the disk
processed by HPT derived from a simulation [130]
83
described earlier for microstructural evolution in HPT, which was developed using strain
gradient plasticity theory [130].
The variation of microhradness values against equivalent strain was plotted in Fig. 4.5.
This figure shows a significant increase in the Vickers microhardness values in the early
stage of straining and steady state behavior with more straining. This behavior is
observed in other metals and alloys as shown in Fig. 5.2 [142].

Figure 5.2 shows the variation of Vickers microhardness against equivalent strain for
pure metals processed by HPT. This figure shows that three distinct types of hardness
behavior can be seen in pure metals at various homologous temperatures.
In the first type, the hardness increases with an increase in the equivalent strain at
an early stage of straining but it becomes steady and enters into a steady state region
where the hardness remains unchanged with further straining. Second, at moderate
Figure 5.2 The variation of Vickers microhardness values against equivalent strain for pure
metals processed by HPT at different homologous temperatures [142]
84
homologous temperatures, the hardness initially increases with increasing strain and, after
reaching a maximum, the hardness decreases to a steady level. Third, at high homologous
temperatures, the hardness remains almost constant or it slightly decreases with an
increase in the equivalent strain at an early stage of straining and then it enters into a
steady state at large strains.
The increase in hardness homogeneity is confirmed also by the color-coded
contour maps shown in Fig. 4.2. Thus, with increasing numbers of HPT turns the average
value of the microhardness increases and the gradient of hardness between the center and
the edge of the disk decreases. It is apparent from Fig. 4.2(d) that this gradient is close to
zero after 5 turns of torsional straining.
The average grain size is reduced significantly from elongated grains with an
average width of ~8 µm to equiaxed grains with average grain size of ~500 nm in the
center and ~250 nm around the peripheral region of the Al-7075 alloy after processing by
HPT through total number of 5 turns. This variation between the average grain size in the
center and around the peripheral region of the disk suggests a minimal inhomogeneity in
the sample. Perfect homogeneity can be reached after processing by HPT through higher
number of turns.
It is well known that the grain size is the major microstructural parameter that
affects the properties of a polycrystalline material. The precise significance of the grain
size at low temperature is based on the Hall-Petch relationship, which is shown in the
following equation [143,144]:
𝜎
!
= 𝜎
!
+ 𝑘
!
𝑑
!
!
!
(5.1)
85
where 𝜎
!
is the yield stress, 𝜎
!
is the lattice friction stress, 𝑘
!
is constant of yielding and
𝑑 is the grain size. Thus, the strength of the material increases when the grain size is
decreased.
The tensile testing of the Al-7075 alloy after processing by HPT shows improved
ductility. The elongations to failure decrease with increasing strain rate and little or no
strain hardening is observed in the tensile testing at elevated temperature. Lower flow
stress is observed, which is due to the ultrafine grained structure achieved after
processing by HPT.
The results from these experiments show that HPT processing leads to
improvements in the Al-7075 alloy both in the microhardness values at room temperature
and in the mechanical properties at elevated temperature. This is consistent with the
behavior anticipated from grain refinement. Furthermore, the results demonstrate a
potential for achieving exceptional grain refinement in a commercial aluminum alloy, and
arrays of essentially equiaxed grains, despite the very highly-elongated microstructure
that was present in the alloy prior to HPT processing.
5.2 The effect of the combination of ECAP and HPT
Severe plastic deformation techniques produce exceptional grain refinement but
HPT is preferable because, by comparison with ECAP, it leads to both smaller grains and
a higher fraction of boundaries having high angles of misorientation. Processing of the
Al-7075 alloy by 4 passes produced a grain size of ~680 nm as shown in Fig. 4.11 (a) but
additional processing by HPT through 20 turns reduced the grain size in the center of the
disk to ~310 nm as shown in Fig. 4.11 (b). This marked reduction in grain size produced
superplastic elongations in the ECAP + HPT material but superplasticity was not
86
achieved in the alloy processed only by ECAP. Furthermore, the superplastic elongations
after ECAP + HPT were consistently higher than the elongations achieved in the same
alloy after processing only by HPT to 10 turns with a pressure of 6.0 GPa and then testing
at 623 K [28]. Thus, the earlier results for the HPT samples gave elongations of ~400%
at a strain rate of 1.0 × 10
-2
s
-1
and ~560% at 1.0 × 10
-3
s
-1
but with an increase to ~680%
at 1.0 × 10
-4
s
-1
: these elongations are consistently lower than in the ECAP + HPT
samples shown in Fig. 4.21.
An important requirement in processing by ECAP + HPT is to make a direct
comparison with data obtained when the same alloy is processed only by HPT. However,
this tends to be difficult because there may be inhomogeneities in the as-processed
materials and it is reasonable to anticipate that processing by HPT may produce small
variations in the sizes of the grains across the diameters of the HPT disks. In addition, it
is well known that the measured grain sizes may differ when using different experimental
techniques such as EBSD or transmission electron microsciopy [53,145,146].
Accordingly, the optimum procedure for comparing HPT and ECAP + HPT is to make
use of the earlier suggestion that the Vickers microhardness, Hv, may be plotted against
the equivalent strain to reveal directly the evolution into a saturated condition [77].
It was shown in Fig. 4.5 that the datum points for ECAP + HPT level off at
equivalent strains above ~200 and there is a well-defined saturation at a microhardness
value of Hv ≈ 250. The same type of plot may be constructed for samples of the Al-7075
alloy processed by only HPT or ECAP for 8 passes and HPT using data reported earlier.
The applied pressure in all of the experiments is 6.0 GPa. All points scatter around a
single line for each individual condition of SPD processing, there is a leveling off above
87
an equivalent strain of ~180 and the saturation hardness is Hv ≈ 230 for HPT processing.
This saturation condition occurs at a value for Hv which is significantly lower than for
the samples processed by ECAP + HPT as shown in Fig. 4.4 and inspection shows that
the difference in the saturation hardness values between the two procedures is outside of
the experimental scatter. Thus, the present results demonstrate unambiguously that
processing by ECAP + HPT produces a grain size which is both smaller than when
processing by ECAP and also smaller than when processing only by HPT. It is
concluded, therefore, that processing through a combination of SPD processes, as in
ECAP + HPT, provides samples with smaller grains, higher hardness at the saturation
level and improved mechanical properties.
The grain size is smaller after processing by ECAP for 8 passes by comparison
with processing by ECAP for 4 passes and there are also further reductions in grain size
when the samples are processed by a combination of the two SPD techniques. These
results lead to two important conclusions, which require further discussion.
First, it is apparent from Fig. 4.22 that the alloy processed by ECAP for 8 passes
and then HPT through 5 to 20 turns exhibits excellent superplastic properties with
elongations up to and exceeding 1000%. By contrast, superplastic elongations were not
achieved in the alloy after ECAP through 8 passes without additional HPT even though
the average grain size in this condition was measured as ~600 nm. Tensile specimens are
usually cut from ECAP billets along the longitudinal axes corresponding to the X
direction but in this research the tensile specimens lay in the X or cross-sectional plane.
However, it is unlikely that this is difference has any effect on the mechanical properties
because it was established in earlier experiments that the mechanical behavior is
88
essentially identical [147], including the superplastic properties [148], for specimens cut
from the ECAP billets along each of the three basic orthogonal axes.
The result also cannot be attributed to significant size differences between the
ECAP and the ECAP + HPT specimens [149] because special care was taken to ensure
that both sets of specimens were essentially identical in size. Therefore, the lack of
superplastic behavior in the specimens processed only by ECAP, and the corresponding
high levels of stress recorded in the stress-strain curves, must be due to the markedly non-
uniform nature of the microstructure and the presence of many relatively large grains as
visible in Fig. 4.11 (c) and (d).
It is apparent that the microstructures after processing by ECAP for 8 passes are
not uniform and they consist of grains of various sizes in both the central and edge
regions of the billet. This is in sharp contrast with the very uniform and equiaxed grains
which are visible after processing by ECAP + HPT [29]. It is important to note also that
the superplastic elongations after processing by ECAP for 8 passes + HPT are
consistently higher than the elongations achieved in the same alloy after processing by
ECAP for 4 passes + HPT or after processing only by HPT to 10 turns [29].
5.3 Hardness saturation after the combination of SPD processes
The saturation in hardness corresponds to the condition where the grain size has
attained and has saturated at a minimum value, d
min
. There has been much speculation
concerning the appropriate value of d
min
in materials processed by HPT [19, 150,151] but
the most definitive approach has been through the development of a model for HPT
which is based on the reasonable assumption that the grain size stabilizes when there is a
balance between the formation of a new dislocation structure because of the introduction
89
of dislocations during processing and the concurrent rate of increase in grain size due to
recovery [95]. This model was developed initially to describe the attrition to a stable and
minimum grain size during milling [152,153] and subsequently a similar approach was
used to predict the minimum grain sizes in HPT [95] and in ECAP [43]. The general
conclusions from these analyses was that several SPD processes may be modeled in a
similar manner including ECAP, HPT, accumulative roll bonding and ball milling [154].
According to the model, the normalized minimum grain size in HPT is given by
an expression of the form [95]
(5.2)
Inspection of Eq. 5.2 shows that, neglecting the minor variation of shear modulus
with temperature, all terms are constant for any selected material except only for the
temperature T and the yield stress σ. In the present investigation, the HPT processing
and hardness measurements were all undertaken at room temperature so that the only
variable in Eq. 5.2 is the yield stress σ which is equal to H/3 where H is the hardness.
Thus, Eq. 5.2 predicts that an increase in hardness will lead to a decrease in d
min
and this
is consistent with early conclusions from experiments on ball milling [155]. It is readily
apparent that the initial hardness values prior to HPT processing were different for the
disks processed only by HPT and those processed by a combination of ECAP + HPT.
For the disks processed earlier by HPT without ECAP, the initial Vickers
microhardness was equal to the as-annealed hardness as shown in Fig. 4.1 which is given
by Hv ≈ 102. By contrast, the ECAP + HPT disks were processed by HPT after
processing by ECAP and this means the initial condition, immediately prior to HPT, was
25 . 1 5 . 0
25 . 0
3
min
4
exp ⎟
⎠
⎞
⎜
⎝
⎛
⎟
⎠
⎞
⎜
⎝
⎛
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
⎟
⎠
⎞
⎜
⎝
⎛−
=
σ
γ
ν
β G
G kT
G D
RT
Q
A
d
o
po
b
b
b
2
90
equal to the hardness after 4 passes of ECAP which is given, based on Fig. 4.3, as a
hardness value of Hv ≈ 135.
The expected difference in grain size between these two processing conditions
can be estimated by substituting hardness values of 102 and 130 into Eq. 5.2 so that it is
anticipated the grain sizes will differ by a factor of ~1.4. In practice, this small difference
is difficult to detect unambiguously using EBSD when the grain sizes are typically <500
nm. Therefore, this calculation confirms that there is a significant advantage in
comparing the relative merits of processing by HPT and ECAP + HPT by making a direct
comparison between the values of the saturation hardness in the two conditions.
Second, an important additional requirement is to compare directly the
significance of the saturation hardness values obtained under different conditions. This is
illustrated schematically in Fig. 4.9 where results are shown for the microhardness values
of samples processed by HPT without any ECAP, by ECAP for 4 passes followed by
HPT and by ECAP for 8 passes followed by HPT: the latter line is taken from Fig. 4, the
other lines are taken from earlier measurements [28,29] and, for simplicity of
presentation, the experimental points are not included in Fig. 4.9.
There are two important conclusions from Fig. 4.9. First, the curves for all three
conditions are essentially similar and they show a leveling off at equivalent strains above
~200. Second, the saturation values at the highest strains increase with the imposition of
additional straining on the samples.
Specifically, the saturation hardness values are represented by the horizontal
dashed lines and they correspond to ~231 for HPT alone, ~249 for ECAP for 4 passes
plus HPT and ~273 for ECAP for 8 passes plus HPT. These results are therefore
91
consistent with the early date by Stolyarov et al. [110] on commercial purity Ti showing
there is additional grain refinement when samples processed by ECAP are further
processed by HPT and they are not consistent with the more recent result on commercial
purity Nb where the same grain size was achieved both after HPT and ECAP + HPT
[114]. Thus, processing through ECAP for 8 passes + HPT provides samples with a
measured high hardness at the saturation level, excellent superplastic properties when
testing in tension at 623 K and properties that are superior to those achieved either by
ECAP for 4 passes + HPT or by processing alone either by ECAP or HPT.

92
Summary and Conclusions

The Al-Zn-Mg-Cu alloys or 7000 series Al alloys are increasingly replacing the
2000 series Al alloys in aerospace applications due to their outstanding strength. The
potential of further strengthening the 7000 series Al alloys using SPD methods validates
conducting ECAP as well as HPT on these alloys.
A commercial Al-7075 alloy was processed by individual SPD methods or by a
combination of them. The microstructural behavior and mechanical properties of this
alloy was investigated under various conditions.

1. Al-7075 was processed by HPT through totals of 1/8, 1/4, 1/2, 1, 2, 5 and 10
turns. Increasing numbers of torsional straining resulted in hardness homogeneity
over the tested disks with higher average hardness values.
2. Mechanical testing on this material after processing by HPT showed an increase
in the flow stress and a decrease in elongation to failure with increasing strain
rate. It was also observed that this material has superplastic behavior at 10
-4
– 10
-2

s
-1
at 623 K after totals of 5 or more turns of torsional straining.
3. Microstructural imaging showed a decrease in grain size after processing by HPT.
It was also observed that the grain size is smaller in the peripheral region, which
is in total agreement with higher microhardness values in that region.
4. The Al-7075 alloy was also processed by ECAP for 4 passes and then by HPT
through various totals up to a maximum of 20 turns. Orientation imaging
microscopy shows the grain size is significantly reduced both after ECAP and
93
after a combination of ECAP and HPT. Thus, the initial elongated grains with
lengths of ~450 µm and widths of ~8 µm were refined to grain sizes of ~680 nm
after ECAP and ~310 nm in the center of the disk after ECAP + HPT.
5. The microhardness values increase after ECAP and then further increase after
ECAP + HPT. In HPT processing, the hardness values are initially lower in the
centers of the disk but there is a gradual evolution to produce reasonable
homogeneity after processing through 20 turns.
6. Mechanical testing at a temperature of 623 K revealed lower flow stresses and
significantly higher elongations to failure after processing by ECAP + HPT. No
superplasticity was observed in the samples processed by ECAP but superplastic
elongations of up to ~800% were achieved at a strain rate of 1.0 × 10
-3
s
-1
after
processing by ECAP + HPT.
7. By plotting the values of the Vickers microhardness against the equivalent strain
after ECAP for 4 passes + HPT, it is shown that there is a saturation hardness at
Hv ≈ 250. By contrast, for samples tested only by HPT without a preceding step
of ECAP, the saturation hardness occurs at a lower value of Hv ≈ 230. These
results demonstrate that processing by ECAP + HPT produces higher hardness
and greater grain refinement than processing only by HPT.
8. The commercial Al-7075 alloy was processed by ECAP for 8 passes and then by
HPT through various totals up to a maximum of 20 turns. Orientation imaging
microscopy showed a significant reduction in grain size both after ECAP and after
a combination of ECAP and HPT.
94
9. The microhardness values increase after processing by ECAP for 8 passes + HPT
until they reach a saturation at Hv ≈ 273. This is higher than the saturation
hardness values attained either after ECAP for 4 passes + HPT where Hv ≈ 249 or
after HPT alone where Hv ≈ 231.
10. Lower flow stresses and significantly higher elongations to failure were observed
in tensile testing after processing by ECAP for 8 passes + HPT. The elongations
to failure were in the superplastic regime and exceeded 1000% at a testing
temperature of 623 K. Superplasticity was not achieved after processing only by
ECAP because of the very non-uniform grain size.
11. Double shear creep of the samples processed by ECAP for 8 passes revealed that
the deformation occurs faster by increasing the shear stress with a maximum in
σ = 18.3 MPa in this study. It was also evident that the samples processed by
ECAP for 8 passes undergo a faster deformation creep than the annealed sample
under the same conditions. This behavior is explained by Eq. 2.5, where strain
rate has an inverse relationship with grain size.

95
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104
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105
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Abstract (if available)

Abstract A commercial Al-7075 alloy was processed by severe plastic deformation (SPD) techniques, namely equal‐channel angular pressing (ECAP) and high‐pressure torsion (HPT) or a combination of the two processes. After processing, the microstructural properties were examined, microhardness measurements were recorded across the disk diameters, and miniature tensile specimens were pulled to failure at a temperature of 623 K. ❧ Using TEM and EBSD techniques, it is demonstrated that the three SPD processing techniques have a potential for producing an ultrafine‐grain structure containing reasonably equiaxed grains with high‐angle grain boundary misorientations. However, microstructures were refined to different levels depending on the processing operation. It is shown that the most refined grain structure was achieved after processing by a combination of ECAP for 8 passes and HPT. The grain refinement mechanisms are primarily governed by dislocation activities. ❧ It is shown that the maximum saturation hardness achieved at high equivalent strains by different processing techniques increases with increasing amounts of deformation and it is the highest after processing by a combination of ECAP for 8 passes and HPT. The saturation hardness values were ~231 after processing by HPT, ~249 after processing by ECAP for 4 passes + HPT and ~273 after processing by ECAP for 8 passes + HPT. ❧ Tensile testings show that the elongations to failure increase by increasing the amount of deformation. It is shown that after ECAP for 8 passes + HPT samples of the Al-7075 alloy have lower flow stresses and superplastic elongations up to >1000% when pulling to failure at 623 K. Superplastic elongations were not achieved after processing only by ECAP because of the non‐uniform grain size and the presence of many larger grains.

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Asset Metadata

Creator Sabbaghianrad, Shima (author)

Core Title Microstructural evolution and mechanical properties of an aluminum alloy processed by equal-channel angular pressing and high-pressure torsion

School Viterbi School of Engineering

Degree Doctor of Philosophy

Degree Program Chemical Engineering

Publication Date 07/07/2014

Defense Date 05/19/2014

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag aluminum 7075 alloy,equal‐channel angular pressing,hardness,high‐pressure torsion,OAI-PMH Harvest,severe plastic deformation,tensile testing

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Langdon, Terence G. (committee chair), Goo, Edward K. (committee member), Sadhal, Satwindar S. (committee member)

Creator Email ssabbagh@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c3-432197

Unique identifier UC11287123

Identifier etd-Sabbaghian-2625.pdf (filename),usctheses-c3-432197 (legacy record id)

Legacy Identifier etd-Sabbaghian-2625.pdf

Dmrecord 432197

Document Type Dissertation

Format application/pdf (imt)

Rights Sabbaghianrad, Shima

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA