EFFECT OF STRAIN RATES ON TENSILE PROPERTIES AND FRACTURE
TOUGHNESS DETERMINATION OF EXTRUDED Mg-Al-Zn ALLOYS
NORADILA BINTI ABDUL LATIF
THESIS SUBMITTED IN FULFILMENT FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
FACULTY OF ENGINEERING AND BUILT ENVIRONMENT
UNIVERSITI KEBANGSAAN MALAYSIA
BANGI
2015
KESAN KADAR TERIKAN TERHADAP SIFAT-SIFAT TEGANGAN DAN
PENENTUAN KELIATAN PATAH BAGI ALOI Mg-Al-Zn TERSEMPRIT
NORADILA BINTI ABDUL LATIF
TESIS YANG DIKEMUKAKAN UNTUK MEMPEROLEH IJAZAH
DOKTOR FALSAFAH
FAKULTI KEJURUTERAAN DAN ALAM BINA
UNIVERSITI KEBANGSAAN MALAYSIA
BANGI
2015
v
ABSTRACT
Extruded Mg-Al-Zn alloy is a lightweight and high strength magnesium alloy that is
becoming a preferred material to be used as a structural component in automobiles.
During a crash event, an automobile structure is subjected to dynamic loading. The
magnesium alloy structures must be able to maintain its integrity and provide adequate
protection in survivable crashes. Besides static tensile properties, tensile properties at
high strain rates of extruded magnesium alloys and their fracture behaviour are some of
the important parameters to be considered in design in ensuring the durability and
reliability of automobile structures. In this study, the effect of strain rates on tensile
properties and work hardening behaviour were evaluated for extruded Mg-Al-Zn alloys.
Further, the fracture behaviour at different loading rates and the effect of temperature on
fracture toughness of Mg-Al-Zn alloys were investigated. The extruded Mg-Al-Zn alloys
used in this study were AZ61 and AZ31 magnesium alloys. Tensile tests under low and
high strain rates were carried out using a universal testing machine and high strain rate
tensile tester, respectively. The high strain rate tensile tester was designed and fabricated
in-house to fulfil the requirement of tensile test under high strain rate ranging from 100 to
600 s-1. Work hardening behaviour for low strain rate tensile specimen was determined
by referring to the ASTM E646. To obtain the fracture behaviour of both alloys at
different loading rates, three-point bending fracture test was conducted on pre-cracked
specimens. Standard test methods i.e. ASTM E1820 and JSME S001 were referred to
determine the elastic-plastic fracture toughness JIC value of AZ31 and AZ61 alloys. The
JIC value obtained were then used as a standard reference value to identify a proper
groove depth of a single side-grooved specimen. The side-groove depths evaluated were
25%, 35% and 50%. The proper depth of the side-grooves is confirmed after the J value
obtained from the side-grooved specimen test method is identical to the JIC value that of
the standard test method. The side-grooved specimen with proper groove depth was then
used to determine the JIC value of AZ61 alloy at high temperature. From the results, the
tensile strengths were gradually increased with increasing strain rates. However, at above
200 s-1, the tensile strength increased significantly to more than 600 to 800 MPa. In
addition, the work hardening rate for AZ61 was found higher compared to that of AZ31.
Both alloys exhibited significant elastic-plastic fracture behaviour at different loading
rates. It was found that 50% side-grooves depth is appropriate enough to produce valid
JIC value using a single specimen. This finding is very useful especially in determining
JIC value in a condition where standard multiple specimen test method is difficult to be
conducted such as in high temperature environment. The JIC values of AZ31 and AZ61 at
room temperature were 19 and 25 kJ/m2, respectively. Meanwhile, the JIC value of AZ61
at 150 °C was found twice higher than the JIC value at room temperature.
vi
KESAN KADAR TERIKAN TERHADAP SIFAT-SIFAT TEGANGAN DAN
PENENTUAN KELIATAN PATAH BAGI ALOI Mg-Al-Zn TERSEMPRIT
ABSTRAK
Aloi Mg-Al-Zn tersemperit merupakan aloi magnesium yang ringan dan berkekuatan
tinggi yang makin menjadi bahan pilihan untuk digunakan sebagai komponen struktur
dalam kenderaan. Semasa kemalangan, struktur kenderaan dikenakan dengan beban
dinamik. Struktur daripada aloi magnesium mestilah berupaya dalam mengekalkan
integriti dan memberi perlindungan yang secukupnya semasa kemalangan. Selain
sifat-sifat tegangan statik, sifat-sifat tegangan pada kadar terikan yang tinggi aloi
magnesium tersemperit dan sifat patahnya adalah parameter penting yang perlu
dipertimbangkan dalam rekabentuk bagi memastikan ketahanan dan kebolehpercayaan
pada struktur kenderaan. Dalam kajian ini, kesan kadar terikan terhadap sifat-sifat
tegangan dan sifat pengerasan kerja telah dinilai bagi aloi Mg-Al-Zn tersemprit.
Seterusnya, sifat patah pada kadar pembebanan berbeza dan kesan suhu terhadap
keliatan patah bagi aloi Mg-Al-Zn telah dikaji. Aloi Mg-Al-Zn tersemprit yang
digunakan dalam kajian ini adalah aloi magnesium AZ31 dan AZ61. Ujian tegangan
pada kadar terikan rendah dan tinggi telah dijalankan dengan masing-masing
menggunakan mesin ujian semesta dan mesin ujian kadar terikan tinggi. Mesin ujian
kadar terikan tinggi telah direka dan dibangunkan sendiri untuk memenuhi keperluan
ujian tegangan pada kadar terikan tinggi di antara 100 sehingga 600 s-1. Sifat
pengerasan kerja untuk spesimen tegangan pada kadar terikan rendah telah ditentukan
dengan merujuk kepada ASTM E646. Bagi memperolehi sifat patah kedua-dua aloi
pada kadar pembebanan yang berbeza, ujian patah tiga-titik lenturan telah dijalankan
pada spesimen pra-retak. Kaedah ujian piawai iatu ASTM E1820 dan JSME S001
telah dirujuk untuk menentukan nilai JIC keliatan patah elastik-plastik bagi aloi AZ31
dan AZ61. Nilai JIC yang diperoleh kemudiannya digunakan sebagai nilai rujukan
piawai untuk mengenalpasti kedalaman alur yang wajar pada satu spesimen sisi
beralur. Kedalaman sisi beralur yang dinilai adalah 25%, 35% dan 50%. Kedalaman
wajar bagi sisi alur disahkan selepas nilai J yang diperolehi daripada kaedah ujian
spesimen sisi beralur adalah sama dengan nilai JIC daripada kaedah ujian piawai.
Spesimen sisi beralur dengan kedalaman alur yang wajar kemudiannya digunakan
untuk menentukan nilai JIC bagi aloi AZ61 pada suhu tinggi. Daripada keputusan,
kekuatan tegangan meningkat secara perlahan dengan peningkatan kadar terikan.
Bagaimanapun, pada kadar melebihi 200 s-1, kekuatan tegangan meningkat dengan
ketara melebihi dari 600 hingga 800 MPa. Di samping itu, kadar pengerasan kerja
untuk AZ61 didapati tinggi berbanding AZ31. Kedua-dua aloi mempamerkan sifat
patah elastik-plastik yang ketara pada kadar pembebanan yang berbeza. Didapati 50%
kedalaman sisi alur adalah cukup sesuai untuk mendapatkan nilai JIC yang sah
menggunakan hanya satu spesimen. Penemuan ini amat berguna terutamanya bagi
menentukan nilai JIC dalam keadaan di mana kaedah ujian piawai berbilang spesimen
adalah sukar untuk dilakukan seperti dalam persekitaran suhu yang tinggi. Nilai-nilai
JIC bagi AZ31 and AZ61 pada suhu bilik adalah masing-masing 19 dan 25 kJ/m2.
Sementara itu, nilai JIC bagi AZ61 pada 150 °C didapati dua kali lebih tinggi
berbanding nilai JIC pada suhu bilik.
vii
CONTENTS
Page
DECLARATION iii
ACKNOWLEDGEMENTS iv
ABSTRACT v
ABSTRAK vi
CONTENTS vii
LIST OF FIGURES xi
LIST OF TABLES xvi
NOMENCLATURE xviii
CHAPTER I INTRODUCTION
1.1 Research Background 1
1.2 Problem Statement 3
1.3 Hypothesis 6
1.4 Research Objectives 7
1.5 Research Scope 7
1.6 Thesis Outline 9
CHAPTER II LITERATURE REVIEW
2.1 Magnesium 11
2.2 Magnesium Alloys
2.2.1 Classification of Magnesium Alloys
2.2.2 Alloying Element Composition
2.2.3 Plastic Deformation and Slip Systems of
Magnesium Alloys
2.2.4 Strengthening Mechanisms of Magnesium Alloys
12
13
14
16
18
2.3 Applications of Magnesium Alloys in Automotive
Industry
2.3.1 Major Application of Magnesium Alloys in
Vehicle
2.3.2 Manufacturing Process of Magnesium Alloys in
Automotive Applications
24
24
29
2.4 Mechanical Properties
2.4.1 Tensile Properties
31
34
viii
2.4.2 High Strain Rate Tensile Properties
2.4.3 Work Hardening Rate, Strain Hardening Exponent
(n) and Strength Coefficient (Ks)
36
38
2.5 Fracture Toughness
2.5.1 Linear Elastic Fracture Mechanics (LEFM) and
Fracture Toughness KIC
2.5.2 Elastic-Plastic Fracture Mechanics (EPFM) and
Fracture Toughness JIC
2.5.3 Determinations of Fracture Toughness KIC and
Fracture Toughness JIC
2.5.4 Fracture Toughness KIC and Fracture Toughness
JIC
40
44
47
51
56
2.6 Extreme Test Condition
2.6.1 Effect of Strain Rate on Mechanical Properties
2.6.2 Effect of Temperature on Mechanical Properties
59
60
63
2.7 Summary of Review 65
CHAPTER III METHODOLOGY
3.1 Experimental Procedure 67
3.2 Experimental Material 71
3.3 Metallography 71
3.4 Vickers Hardness 73
3.5 Tensile Test 74
3.6 Tensile Test Specimen 77
3.7 Development of The High Strain Rate Tensile Tester
3.7.1 Design Structure and Parts of The High Strain
Rate Tensile Tester
3.7.2 Test Procedure of The High Strain Rate Tensile
Tester
3.7.3 Calibration
79
82
89
90
3.8 Work Hardening Rate, Strain Hardening Exponent (n)
and Strength Coefficient (Ks)
94
3.9 Pre-Cracked Single Edge Notched Bend (SENB)
Specimen
3.9.1 Preparation of Pre-Cracked Single Edge Notched
Bend (SENB) Specimen
3.9.2 Side-Grooved Specimen
97
97
102
ix
3.10 Three-Point Bending Fracture Test 104
3.11 Fracture Toughness JIC Test
3.11.1 Standard Multiple Specimens JIC Test
3.11.2 Side-Grooved Specimen JIC Test
3.11.3 Compliance Test of Testing Jigs
3.11.4 JIC Test of Side-Grooved Specimen Test Method at
Temperature
106
106
110
112
114
3.12 Fractograph Observation 115
3.13 Summary of Chapter III 117
CHAPTER IV RESULTS AND DISCUSSION
4.1 Microstructure of Mg-Al-Zn Alloys 118
4.2 Hardness 120
4.3 Effect of Low Strain Rate on Tensile Properties
4.3.1 Effect of Low Strain Rate on Tensile Fracture
Surface
4.3.2 Effect of Low Strain Rate on Work Hardening
Rate
4.3.3 Effect of Low Strain Rate on Strain Hardening
Exponent (n) and Strength Coefficient (Ks)
122
126
128
130
4.4 Effect of High Strain Rate on Tensile Properties 134
4.5 Fracture Surface of Fatigue Pre-Cracked Single Edge
Notched Bend (SENB) Specimen
143
4.6 Effect of Loading Rate on Fracture Behaviour of Pre-
Cracked Single Edge Notched Bend (SENB) Specimen
146
4.7 Fracture Toughness JIC
4.7.1 Determination of Fracture Toughness JIC Using
Standard Multiple Specimens JIC Test
4.7.2 Determination of Fracture Toughness JIC Using
Side-Grooved Specimen JIC Test
4.7.3 Determination of Fracture Toughness JIC Using
Side-Grooved Specimen JIC Test at Room
Temperature and High Temperature Conditions
154
155
164
170
4.8 Summary of Chapter IV 179
CHAPTER V CONCLUSIONS AND RECOMMENDATIONS
5.1 Conclusions 180
x
5.2 Contribution to Knowledge 181
5.3 Recommendations 182
REFERENCES 183
APPENDICES
A Drawing of High Strain Rate Tensile Tester 196
B Specification of Magnetic Lock 198
C Specification of Load Cell 202
D Specification of Linear Variable Differential Transformer
(LVDT)
204
E Data of Low Tensile Test 205
F Data of Side-Grooved Specimen JIC Test at Room
Temperature
213
G Data of Side-Grooved Specimen JIC Test at 150 °C 215
H List of Publications and Conferences 216
xi
LIST OF FIGURES
Figure No. Page
2.1 Major slip systems of HCP structure for magnesium 12
2.2 Formation of twinning in extruded AZ31 at room temperature 17
2.3 Effect of grain size on (a) tensile strength and (b) average
hardness for Mg-Al-Zn alloys
19
2.4 Effect of manganese content on average grain size for
Mg-Al-Zn alloys
20
2.5 Effect of temperature on (a) tensile strength and (b) 0.2%
proof stress for pure magnesium and magnesium-calcium
21
2.6 Stress-strain curves for different hardening behaviours 22
2.7 The -Mg17Al12 precipitation in AZ91 24
2.8 The utilization of several materials for vehicle in 2000 25
2.9 Application of magnesium alloys in motor vehicle 26
2.10 (a) Cast wheel, (b) die cast crankcase and (c) die cast inner
door
27
2.11 (a) Extruded window frame and (b) bonnet inner (magnesium
sheet)
28
2.12 Forged wheel 29
2.13 Yield stress and strain of magnesium wrought alloys and
casting alloys
35
2.14 Schematic diagram of tensile split Hopkinson bar apparatus 36
2.15 Instron/Dynatup instrumented drop weight impact machine 37
2.16 Illustration of stress fields in cracked specimens. (a) linear
elastic behaviour (b) linear elastic behaviour with small scale
yielding (c) elastic-plastic behaviour with large scale yielding
40
2.17 Standard specimens of (a) three-point bend (single edge
notched bend, SENB) specimen and (b) compact tension (CT)
specimen
42
2.18 A sinusoidal load with constant amplitude and frequency 43
2.19 Illustration of cyclic slip by fatigue loading 43
2.20 Types of load-displacement curves for different material
behaviour
45
2.21 Effect of thickness on fracture toughness KIC 46
2.22 Formation of plastic zone at crack tip 46
xii
2.23
Illustration of crack growth behaviour for elastic-plastic
material
47
2.24 Formation of stretch zone for elastic-plastic material 48
2.25 A path independent J-integral contour at crack surface 49
2.26 Load-load line displacement curve that used to determine the
J-integral
50
2.27 Resistance curve (R-curve) 51
2.28 (a) Single edge notched bending (SENB) specimen, (b) side-
grooved SENB specimen, (c) detail of electrical discharge
machining (EDM) V-notch and (d) detail of side-groove
54
2.29 Side-grooves compact tension specimen 55
2.30 Effect of temperature on fracture toughness JIC for fracture
toughness KIC and fracture toughness JIC of tungsten and
tungsten-rhenium ( is tungsten, is tungsten- 5% rhenium
and is tungsten- 10% rhenium)
58
2.31 Effect of temperature on fracture toughness JIC for binary and
chromium titanium aluminides and niobium titanium
aluminides
59
2.32 Effect of strain rate on yield stress and tensile strength for (a)
maraging and (b) austenitic steels
61
2.33 Effect of temperature on elongation for pure magnesium and
magnesium-calcium
64
3.1 Flow chart of initial study for determining the fracture
toughness of Mg-Al-Zn alloys
68
3.2 Flow chart of research study 70
3.3 Specimen in epoxy cold mount 72
3.4 Zwick Roell Indentec micro hardness tester 74
3.5 Square-based pyramidal-shaped diamond indenter 74
3.6 Universal testing machine (UTM) with capacity of 100 kN 75
3.7 Extensometer 76
3.8 Tensile test specimen for tensile test 78
3.9 Tensile test specimen for high strain rate tensile test 79
3.10 Illustration of the high strain rate tensile tester 81
3.11 High strain rate tensile tester 82
3.12 Illustration of the top-bottom fixtures, drop weight and impact
platform
84
xiii
3.13
Position of linear variable differential transformer (LVDT)
and (a) installation of specimen and (b) gripped specimen in
the top-bottom testing fixtures
85
3.14 Sensor Interface of PCD-300B 88
3.15 Application of DCS-100A software 88
3.16 Calibration curve of load cell under influence of weight 91
3.17 Calibration curve of load cell under influence of high strain
rate
91
3.18 Illustration of calibration system for evaluating actual strain
rate
92
3.19 Calibration devices for evaluating actual strain rate 93
3.20 Calibration curve of strain rate under different drop weight
heights
94
3.21 Single edge notched bend (SENB) specimen 98
3.22 (a) Schematic illustration and (b) real fatigue pre-cracking
process
99
3.23 Fatigue crack growth rate against the stress intensity factor
range
101
3.24 Fatigue crack growth rate against the crack length 101
3.25 Fatigue pre-crack in SENB specimen 102
3.26 Geometry of side-groove specimen (in unit mm) 104
3.27 Three-point bending fracture test configuration 105
3.28 Procedure of the standard multiple specimens JIC test 107
3.29 Load-load line displacement curve of pre-cracked SENB
specimen for AZ61 magnesium alloy
108
3.30 Pre-cracked SENB specimen after conducting the standard JIC
test
108
3.31 Subsequent fatigue loading to break-open the tested specimen 109
3.32 Area under the curve up to the maximum load (Pmax) point 111
3.33 Compliance test of testing jigs 113
3.34 Load-load line displacement curve of testing jigs 113
3.35 Side-grooved specimen JIC test at 150 °C using the universal
testing machine (UTM) with high temperature furnace
115
3.36 Stereo microscope 116
3.37 Scanning electron microscope (SEM) 116
xiv
4.1 Microstructures of Mg-Al-Zn alloys 119
4.2 Grain sizes of Mg-Al-Zn alloys 120
4.3 The -Mg17Al12 precipitation of AZ61 120
4.4 Vickers hardnesses of Mg-Al-Zn alloys 121
4.5 Effect of low strain rate on nominal stress-strain curve for
Mg-Al-Zn alloys
123
4.6 Effect of low strain rate on tensile properties for Mg-Al-Zn
alloys
124
4.7 Tensile fracture surface of different strain rates for AZ31 127
4.8 Tensile fracture surface of different strain rates for AZ61 128
4.9 Effect of strain rate on work hardening rate for Mg-Al-Zn
alloys
129
4.10 The log T - log p curve of Mg-Al-Zn alloys 131
4.11 Effect of strain rate on strain hardening exponent (n) and
strength coefficient (Ks) for Mg-Al-Zn alloys
133
4.12 Effect of high strain rate on load-time curve for Mg-Al-Zn
alloys
135
4.13 Effect of high strain rate on average yield stress and tensile
strength for AZ31
137
4.14 Effect of high strain rate on average yield stress and tensile
strength for AZ61
138
4.15 Effect of high strain rate on yield stress and tensile strength
for Mg-Al-Zn alloys
139
4.16 Effect of strain rate on flow stress for Mg-Al-Zn alloys 140
4.17 Effect of strain rate on elongation for Mg-Al-Zn alloys 141
4.18 Effect of high strain rate on fractograph for AZ31 142
4.19 Effect of high strain rate on fractograph for AZ61 143
4.20 Fracture surfaces of fatigue pre-cracked for Mg-Al-Zn alloys 145
4.21 Cyclic slip produced by fatigue loading 146
4.22 Effect of loading rate on load-load line displacement curve for
Mg-Al-Zn alloys
148
4.23 Effect of loading rate on J-integral for Mg-Al-Zn alloys 149
4.24 Overview fracture surface of AZ31 and AZ61 magnesium
alloys at different loading rates
151
4.25 Fracture surfaces of Mg-Al-Zn alloys 152
4.26 Fracture surfaces of crack growths at three different loading
rates for Mg-Al-Zn alloys
153
xv
4.27
Load-load line displacement curve of Mg-Al-Zn alloys at
different interrupted displacement
156
4.28 Overview fractograph of extruded AZ31 at different
interrupted displacement and detail SEM micrographs of
ductile crack growth (a)
158
4.29 Overview fractograph of extruded AZ31 at different
interrupted displacement and detail SEM micrographs of
ductile crack growth (a)
159
4.30 Stretch zone region of Mg-Al-Zn alloys 160
4.31 Ductile crack growth (a) region of Mg-Al-Zn alloys 161
4.32 R-curve of Mg-Al-Zn alloys 163
4.33 Load-load line displacement curve of AZ61 magnesium alloy
at different side-grooved specimens
165
4.34 Trend of J value to the side-grooves depth 167
4.35 Effect of grooves depth on plastic constraint of side-grooved
specimens
169
4.36 Fracture surface of extruded AZ61 magnesium alloy at fatigue
crack growth, stretch zone and ductile crack growth regions
170
4.37 Load-load line displacement curve of AZ61 magnesium alloy
at room temperature and 150 °C
172
4.38 Overview fracture surface after tensile test at different
temperatures
173
4.39 Ductile fracture patterns after tensile test at different
temperatures
174
4.40 Overview fracture surface of 50% side-grooved specimen
after JIC test at different temperatures
176
4.41 Fracture surface of 50% side-grooved specimen after JIC test
at different temperatures
177
4.42 Ductile fracture patterns after JIC test at different temperatures 178
xvi
LIST OF TABLES
Table No. Page
2.1 ASTM designation system of magnesium alloys 13
2.2 Specific compositions 14
2.3 Chemical compositions of AZ, AM, ZK and WE magnesium
alloys series
16
2.4 Manufacturing processes for magnesium alloys 30
2.5 Automotive components that made from magnesium alloys 31
2.6 Mechanical properties of extruded pure magnesium and
magnesium alloys
33
2.7 Tensile properties of several Mg-Al-Zn alloys 35
2.8 Low and high strain rates tensile properties of mild steel and
stainless steel
38
2.9 Strain hardening exponent, n and strength coefficient, Ks for
several materials
39
2.10 Determination of fracture toughness JIC by using the side-
grooved specimen test method
55
2.11 Fracture toughness KIC and fracture toughness JIC for several
metallic materials
56
2.12 Tensile and compression tests under low and high strain rates
for magnesium alloys
62
2.13 Tensile and compression tests under low temperature and high
temperature for magnesium alloys
65
3.1 Chemical compositions of the AZ31 and AZ61 magnesium
alloys
71
3.2 Travelling system of the high strain rate tensile tester 86
3.3 Safety parts of the high strain rate tensile tester 87
3.4 Measurement devices at the high strain rate tensile tester 89
4.1 Effect of low strain rate on tensile properties for Mg-Al-Zn
alloys
124
4.2 Effect of strain rate on strain hardening exponent (n) and
strength coefficient (Ks) for Mg-Al-Zn alloys
132
4.3 Effect of high strain rate on tensile properties for Mg-Al-Zn
alloys
136
xvii
4.4 Effect of loading rate on fracture properties for Mg-Al-Zn
alloys
149
4.5 Shear lips ratio of Mg-Al-Zn alloys 152
4.6 J value, ductile crack growth (a) and critical stretch zone
width (SZWC) of Mg-Al-Zn alloys at interrupted displacement
161
4.7 J value of ungrooved and side-grooved specimen for extruded
AZ61
166
xviii
NOMENCLATURE
Symbols
a crack length
a crack growth
dN
da
fatigue crack growth rate
A area
A original cross section area of test specimen
Ai final cross section area of test specimen
AT area of cross section at force
Amax area under the curve up to maximum load (Pmax) point
Aspecimen actual area under the curve of testing specimen
Atotal total area under the curve of test
Ajig area under the curve of testing jigs
b remaining crack ligament
B specimen thickness
BN net specimen thickness
Be effective thickness
Be1 effective thickness 1
Be2 effective thickness 2
Be3 effective thickness 3
second phase
d displacement
d1 length of long diagonal
ds side-grooves depth
xix
ds increment of the countour path
E Young’s modulus
F force
f(α) geometry factor
g acceleration of gravity (9.81 m/s2)
h drop weight height
J J-integral
JC critical J-integral
JIC fracture toughness JIC
K stress intensity factor
Ks strength coefficient
KQ critical stress intensity factor
KI mode I stress intensity factor
KId dynamic fracture toughness
KIC fracture toughness KIC
K stress intensity factor range
Km mean stress intensity factor
Kmin minimum stress intensity factor
Kmax maximum stress intensity factor
L original gage length
Lf final gage length
dL increment of elongation
m mass
Mg17Al12 precipitation
xx
n strain hardening exponent
P load
PQ critical load
P5 load at intersection of 95% slope on load-displacement curve
Pmax maximum load
Py load point at 0.2% offset from the linear slope on load-
displacement curve
R stress ratio
S span length
s displacement of loading points
T outward traction vector on ds
u displacement vector at ds, x, y, z of the rectangular coordinates
v impact velocity or high strain rate
w loading work per unit volume
W specimen width
WP work done
Y geometry factor
elongation
T true strain
e true elstic strain
P true plastic strain
Td increment of true plastic strain
y yield stress
UTS tensile strength
xxi
f flow stress
C critical stress
T true stress
Td increment of true stress
path of the integral around the crack tip
dsx
uT
work input rate from the stress filed into the area enclosed by
xxii
Abbreviations
HCP hexagonal close packed
CRSS critical resolved shear stress
UTM universal testing machine
LVDT linear variable differential transformer
MSDS material safety data sheet
CT compact tension
SENB single edge notched bending
EDM electrical discharge machining
ASTM American society for testing and materials
JSME Japan society of mechanical engineers
LEFM linear elastic fracture mechanics
EPFM elastic-plastic fracture mechanics
SEM scanning electron microscope
SHPB split Hopkinson pressure bar
Chapter I
INTRODUCTION
1.1 RESEARCH BACKGROUND
Magnesium alloys have been attractive to engineers due to their lightweight and high
specific strength properties as compared to aluminium and medium-strength steel
alloys (Smith 1998). The specific strength of alloys is measured based on the strength
to the weight ratio, while the lightweight property refers to the density of alloys. In
this regard, magnesium alloys have been used in electric and electronic appliances as
well as automotive applications. In automotive applications, these parameters are
beneficial to reduce fuel consumption, enhance the energy efficiency of engines and
reduce emissions. In addition, magnesium alloys are also excellent in machinability,
castability, recyclability and high damping capacity. Hence, magnesium alloys attract
the automobile manufacturers to use these alloys as components and structures for
replacing the conventional materials such as steel, cast iron and aluminium (Gaines
1995; Mordike & Ebert 2001; Kainer 2003; Watarai 2006). Magnesium has been used
since the 1930s in the Volkswagen (VW) Beetle. Nowadays, automobile
manufacturers such as Volkswagen, Audi, DaimlerChrysler (Mercedes-Benz), Toyota,
Ford, BMW, Jaguar, Fiat, Hyundai, and Kia Motors Corporation generally use
magnesium alloys as components and structures (Gupta & Sharon 2011). However,
utilisation of magnesium alloys in automotive applications is still limited compared to
that of conventional materials (Luo 2002).
The most popular magnesium alloys used as automobile components and
structures are AZ (Mg-Al-Zn), AM (Mg-Al-Mn), ZK (Mg-Zn-Zr) and WE (Mg-Y-
RE) series magnesium alloys. This is due to the best properties of magnesium alloys
2
for certain applications are governed by the combination of alloying element contents.
In the present study, Mg-Al-Zn series magnesium alloys have been used. The main
elemental contents of Mg-Al-Zn alloys are aluminium and zinc. These main elements
are beneficial for enhancing the strength of alloys (Kainer 2003). In addition, Mg-Al-
Zn alloys are characterised as low cost, a good strength and good ductility alloy. Due
to its ductile property, Mg-Al-Zn alloys are easy to produce in the shape of extruded
and simple forged products. Therefore, extruded and simple forged products of Mg-
Al-Zn alloys are used as components and structures in automobile applications
(Becker & Fischer 2004).
Mechanical properties are referred to when choosing a good potential material
especially for application in automobile concerned with the safety parameter and
quality assurance. Mechanical properties comprise hardness, tensile properties,
flexural strength, fracture toughness etc. Excellent mechanical properties refer to the
potential material related to the durability and reliability of materials. In the present
study, the potential materials such as Mg-Al-Zn alloys are beneficial for use in
automobile applications to prevent or reduce the impact of critical damage in the case
of accident. Mg-Al-Zn alloys are also ductile material (Becker & Fischer 2004)
beneficial in automobile applications to prevent or reduce the critical crashes due to
high energy absorption capability. However, application of Mg-Al-Zn alloys in
extreme conditions is believed to give influenced on the mechanical properties. In
general, extreme conditions are referred to the condition such as high loading rate,
impact loading response, elevated temperature conditions etc. Tensile strength is
significantly strain rate dependent for many materials. At the same time, tensile
strength is also temperature dependent for pure and magnesium alloys (Chino 2002;
Feng et al. 2014; Kim & Chang 2011; Ulacia et al. 2011). In case of accident, a high
loading rate with impact response is significantly applied to the vehicle. Elevated
temperature is commonly subjected to automobile components and structures since the
powertrain applications such as transmission cases are operated up to 175 °C, while
the engine blocks and engine pistons up to 200 °C and 300 °C, respectively (Luo
2002; Luo 2004; Gupta & Sharon 2011). Therefore, engine surrounding temperatures
3
could rise up to more than 100 °C. Hence, knowing the effect of extreme conditions
on mechanical properties is for Mg-Al-Zn alloys are important.
1.2 PROBLEM STATEMENT
As mentioned, magnesium alloys have several advantages when used as components
and structures in automobile applications. However, the utilisation of magnesium
alloys in automobile applications is still limited compared to that of conventional
materials (Luo 2002). This is because the conventional used are of significantly high
strength materials beneficial for reducing or preventing critical crashes during
accidents. Consequently, studies in obtaining the mechanical properties of those
materials are also widely reported (Mutoh 1987; Yi & Xiao-Wei 1988; Kobayashi et
al. 1997; Zhang & Shi 1992; Kong et al 2011). Thus, many automotive manufacturers
are still using conventional materials in automotive applications.
Several series magnesium alloys are used in automotive application. For
example, car body parts, crankcase, seat frame and wheel are made from AZ31, AZ91,
AM60 and ZK30 magnesium alloys, respectively (Fink 2003; Friedrich & Mordike
2006; Becker & Fischer 2003). Commonly, these components and structures are
subjected to high and different velocities, impact loads and then critical crash during
accident. Hence, mechanical properties of magnesium alloys under impact response
are required to refer prior to applying the potential magnesium alloys in automotive
applications. In addition, it is important to understand these impact fracture properties
to understand the material response after being subjected to similar actual situations
during accident. However, in previous studies, most mechanical properties are
reported under testing at static response and fatigue loads in the case of vibration in
service applications (Chamos et al. 2008; Somekawa et al. 2008; Khan et al. 2006).
Moreover, these fracture properties are not appropriate for crash design and impact
fracture properties are preferable.
For magnesium alloys, mechanical properties of these alloys such as hardness
and tensile properties are well known. In previous studies, many researchers reported
4
the tensile properties of magnesium alloys tested at high strain rates using the Split
Hopkinson pressure bar (SHPB) tester (Kurukuri et al. 2012; Kurukuri et al. 2014;
Ahmad & Wei 2010; Hasenpouth et al. 2009; Yokoyama 2003; Ulacia et al. 2010;
Ulacia et al. 2011; Feng et al. 2014). However, high strain rate tensile tests using free
fall principle with drop weight event are rarely investigated. Only Hasenpouth et al.
(2009), Kong et al. (2011) and Zabotkin et al. (2003) conducted such studies.
Additionally, this experimental technique has never been performed to extruded
magnesium alloys to obtain the high strain rate tensile properties. It is very important
to simulate the impact loading during collision and its effect to the material of the
structures and components.
Sajuri (2005) reported that brittle magnesium alloys are referred to cast
magnesium alloys, while ductile magnesium alloys refer to wrought magnesium
alloys. However, the brittleness and ductility of magnesium alloys are also depending
on the alloying element contents (Kainer 2003). Many of cast and wrought
magnesium alloys that have been developed are generally used in automotive
applications. For example, automotive parts that are made by the casting process
include gearbox housing, crankcase and cylinder head cover. Examples of automotive
parts that are made by the wrought process include the wheel, bumper support beam
and car body parts (Fink 2003; Friedrich & Mordike 2006; Becker & Fischer 2003;
Gaines et al. 1995). Generally, wrought magnesium alloys have good formability and
high ductility for forging, extrusion and rolling processes.
In determining the KIC and JIC values, different fracture toughness test methods
are conducted. In comparison, KIC test method is easier than that of JIC. In addition,
the JIC test includes several test methods such as multiple specimen method, unloading
compliance method and side-grooved specimen method. Among these test methods,
the most complicated test is multiple specimen method while the simplest test method
is side-grooved specimen test method (Mutoh 1987; Yi & Xiao-Wei 1988). Multiple
specimens method requires at least four pre-cracked specimens, while elastic
unloading compliance test requires only a single pre-cracked specimen. However, the
elastic unloading compliance method is still complicated because it requires numerous
5
unloading steps during testing. Thus, single side-grooved specimen test method is the
simplest and convenient test method, especially in conducting tests at high
temperature (Mutoh 1987; Gnanamoorthy et al. 1995) or in high loading rate
conditions. In addition, the study of JIC for ductile magnesium alloys is rarely
reported, being only reported by Somekawa et al. (2008) and Bargabagallo & Cerri
(2004). Subsequently, determination of fracture toughness JIC of extruded magnesium
alloys at high temperature using simple side-groove specimen method has not been
done in previous studies. Nevertheless, all methods obtained fracture toughness
complying with plane strain conditions.
Considering the application of magnesium alloys in engine compartment
where temperatures can rise up to more than 100 °C during operation, it is also
essential to determine fracture toughness at high temperature conditions. However,
multiple specimen method and unloading compliance method are difficult to conduct
under such conditions. Therefore, side-grooved specimen test method is preferable
and can be easily performed in high temperature conditions. In high temperature
condition, tensile strength could significantly decreases and ductility increases, and
consequently the temperature the brittle magnesium alloys could fracture in ductile
manner at high temperature. Therefore, the KIC test becomes invalid and the JIC test is
preferable.
6
1.3 HYPOTHESIS
Based on the problem statement, the main research hypotheses of the current study
have been summarised based on expectation result of tensile properties and fracture
toughness JIC under extreme test conditions for Mg-Al-Zn alloys. The result
expectations of hardness and work hardening properties are also included. Thus, the
research hypotheses have been listed as follows:
(i) Strain rate is dependent on tensile properties, work hardening rate, strain
hardening exponent (n) and strength coefficient (Ks). Subsequently, loading
rate is dependent on fracture behaviour of Mg-Al-Zn alloys.
(ii) Elastic-plastic fracture mechanics (EPFM) approach is considered to obtain the
fracture toughness JIC since the ductile fracture behaviour of Mg-Al-Zn alloys.
(iii) Hardness, tensile properties and fracture toughness JIC of AZ61 magnesium
alloy are higher compared to that of AZ31 magnesium alloy.
(iv) Temperature is dependent on fracture toughness JIC of AZ61 magnesium alloy.
7
1.4 RESEARCH OBJECTIVES
The main objective of this research is to obtain the tensile properties and fracture
toughness JIC of ductile Mg-Al-Zn alloys under several test methods and conditions as
listed in the following sub-objectives:
(i) To determine the effect of low and high strain rates on tensile properties of
Mg-Al-Zn alloys.
(ii) To determine the effect of strain rate on work hardening rate, strain hardening
exponent (n) and strength coefficient (Ks). Consequently, to investigate the
effect of loading rate on fracture behaviour of Mg-Al-Zn alloys.
(iii) To determine the fracture toughness (JIC value) of Mg-Al-Zn alloys.
(iv) To determine a proper side-grooves depth for validate JIC value of a single
side-grooved specimen test method by comparing to JIC value of standard
fracture toughness test method.
(v) To determine the fracture toughness (JIC value) of AZ61 magnesium alloy at
room and high temperatures.
1.5 RESEARCH SCOPE
The mechanical properties of Mg-Al-Zn alloys are crucial for choosing the potential
alloys to be used in automobile application. In the present study, the important
mechanical properties of tensile strength and fracture toughness for AZ31 and AZ61
magnesium alloys are obtained using the tensile test and fracture toughness test. The
tensile and fracture toughness tests are also conducted under extreme test conditions to
understanding the effect of extreme test conditions on mechanical properties of alloys.
In addition, Vickers hardness test is also conducted to determine the material’s
resistance to localized plastic deformation of AZ31 and AZ61 alloys.
In the current study, most tests are performed by referring to the American
Society for Testing and Materials (ASTM) and Japan Society of Mechanical
Engineers (JSME) recommendations. Standard ASTM E8 is referred to when
8
conducting the tensile test at two different strain rates of low and high loading
responses in lab air condition. The tensile test under low strain rate is performed under
four different strain rates of 1×10-4, 1×10-3, 1×10-2 and 1×10-1 s-1 using the universal
testing machine (UTM) of 100 kN load capacity. Meanwhile, the tensile test under
high strain rate is carried out under strain rates of 100, 200, 400 and 600 s-1 using the
high strain rate tensile tester of 125 kN load capacity. The high strain rate tensile tester
is designed and fabricated in-house to fulfil the requirement of tensile test at high
strain rate. This is because the UTM is unavailable to perform the tensile test at high
strain rate condition. Hence, the tensile properties of AZ31 and AZ61 alloys under
low and high strain rates could be determined. In addition, the effect of low strain rate
on work hardening rate, strain hardening exponent (n) and strength coefficient (Ks) are
also obtained in the present study. The standard ASTM E646 is referred to evaluate
the n and Ks of AZ31 and AZ61 alloys.
For fracture toughness, generally refer to the two approaches of LEFM and
EPFM. LEFM related to the brittle fracture behaviour with small-scale yielding at the
crack tip. EPFM is relevant for ductile fracture behaviour that exhibits a large-scale
yielding at the crack tip. However, the EPFM approach is used due to result of three-
point bending fracture test under different loading rates of 5, 50 and 500 mm/min
confirm the elastic-plastic fracture behaviour of the AZ31 and AZ61 alloys. In this
study, recommendations by ASTM E1820 and JSME S001 standards are followed to
determine the fracture toughness JIC of ductile AZ31 and AZ61 alloys. Further, the
specimen used for determining the JIC value was a pre-cracked single edge notched
bending (SENB) specimen.
The fatigue pre-crack on SENB specimen was done following ASTM E399
standard. Fracture toughness test was conducted based on ASTM E1820 and JSME
S001 standards. To identify the effect of temperature for Mg-Al-Zn alloy on JIC value
of AZ61 magnesium alloy, fracture toughness test was conducted at 150 °C using a
single side-grooved specimen test method. The fractographs of all tested specimens
were captured using the stereo microscope and scanning electron microscope (SEM).
9
1.6 THESIS OUTLINE
This thesis is divided into five chapters. The first chapter describes the research gap
highlighting the importance of investigating the mechanical properties of Mg-Al-Zn
alloys. Good mechanical properties of Mg-Al-Zn alloys are then promoted for a wider
application in the automobile industry.
The second chapter reviews the relevant literature on tensile properties and
fracture toughness for magnesium alloys and conventional materials determined under
extreme test conditions. Results of the work hardening rate, strain hardening exponent
(n) and strength coefficient (Ks) for magnesium alloys and several metallic materials
are also discussed. The simplicity and convenience of side-grooved specimen test
method to obtain the fracture toughness is also discussed.
Chapter 3 explains the experimental procedure of tensile and fracture
toughness tests under extreme test conditions. The work hardening rate, strain
hardening exponent (n) and strength coefficient (Ks) are then analysed to understand
the tensile ductility of materials. In-house development of high strain rate tensile tester
is also described to test tensile strength under high strain rates. This includes a
discussion on the proper method for obtaining the fracture toughness using the side-
grooved specimen test method. Procedures for fractography, metallography and
hardness tests are also included in this chapter.
Chapter 4 exhibits the results and present the analysis of the results of tensile
and fracture toughness tests of Mg-Al-Zn alloys under extreme test conditions. Work
hardening rate, n and Ks of Mg-Al-Zn alloys are included. The fracture behaviour and
fracture mechanisms of these alloys are also discussed. Next, fracture toughness for
ductile Mg-Al-Zn alloys is determined based on the elastic-plastic fracture mechanics
(EPFM) approach. Other than that, the average grain size and Vickers hardness of
Mg-Al-Zn alloys are reported in this chapter.
10
The research findings have been summarised in the chapter 5 including the
contribution of the research and suggestion for future studies. In this study, side-
grooved specimen test method is validated to determine the fracture toughness of Mg-
Al-Zn alloys with simple and convenient test method especially under extreme test
conditions.
Chapter II
LITERATURE REVIEW
2.1 MAGNESIUM
Magnesium (Mg), an alkaline earth metal, is the sixth most abundant element in the
earth crust and third most abundant dissolved mineral in seawater. Magnesium has a
glossy silver colour, is ductile, and easily reacts with chemical substances such as
oxygen, nitrogen, carbon dioxide or water (Golabczak 2011). Magnesium is classified
as a light material compared to other engineering metals with two-thirds of the density
of aluminium and one-quarter of iron. The density of magnesium, aluminium and iron
at 20 °C are 1.74 g/cm3, 2.70 g/cm3 and 7.86 g/cm3, respectively (Gaines 1995; Joksch
2003; Kainer 2003). However, the density of magnesium is reduced to 1.65 g/cm3 at
melting temperature of 650 °C. The crystal structure of magnesium is a hexagonal
close packed (HCP) structure with c/a ratio of 1.624 at room temperature (Friedrich &
Mordike 2006). Major slip systems of HCP structure for magnesium are shown in
Figure 2.1.
12
Figure 2.1 Major slip systems of HCP structure for magnesium
Source: Barnett 2013
2.2 MAGNESIUM ALLOYS
Magnesium has several advantages such as the lowest density among all conventional
materials, high specific strength, and good castability and machinability. However,
several disadvantages of magnesium include low mechanical strength, low formability
at low temperature, poor creep resistance at elevated temperature and low corrosion
resistance. For these reasons, magnesium fails to meet the requirements of many
technical applications. Thus, alloying development is required to improve the
properties of magnesium by adding other alloying elements (Mordike & Ebert 2001;
Kulekci 2008). Many alloying elements are used with sufficient composition in order
to obtain good properties of magnesium alloys.
13
2.2.1 Classification of Magnesium Alloys
The American Society for Testing and Materials (ASTM) has listed the abbreviation
letters of alloying elements generally designated for magnesium alloys (see Table
2.1). Each alloying element represented by letters for the alloy indicating the main
element content in magnesium. This is followed by the weight percentage (wt%) of
these elements. The last letter refers to the specific compositions as summarised in
Table 2.2. For example, in the AZ series magnesium alloy of AZ91C (Mg-9%Al-
1%Zn), A and Z represent the element content of aluminium and zinc. The following
numbers 9 and 1 are the rounded numbers for the weight percentages of aluminium
and zinc. Lastly, C indicates the third specific compositions of alloy (Kainer 2003;
Gupta & Sharon 2011).
Table 2.1 ASTM designation system of magnesium alloys
Alloying element Abbreviation letter Alloying element Abbreviation letter
Aluminium A Nickel N
Bismuth B Lead P
Copper C Silver Q
Cadmium D Chromium R
Rare earth metals E Silicon S
Iron F Tin T
Thorium H Yttrium W
Zirconium K Antimony Y
Lithium L Zinc Z
Manganese M
Source: Kainer 2003, Gupta & Sharon 2011
14
Table 2.2 Specific compositions
Abbreviation letter Description
A First compositions, registered with ASTM
B Second compositions, registered with ASTM
C Third compositions, registered with ASTM
D High purity, registered with ASTM
E High corrosion resistance, registered with ASTM
X Experimental alloy, not registered with ASTM
Source: Gupta & Sharon 2011
2.2.2 Alloying Element Composition
One commercial magnesium alloy is the AZ (Mg-Al-Zn) series. The main element
contents of this alloy are aluminium and zinc. These alloying elements are used to
modify the significant microstructure for producing better properties of magnesium
alloys. The best combination of aluminium and zinc is to improve the mechanical
properties, ductility, corrosion resistance and castability of magnesium based alloy.
The mechanisms that improve these properties are solid-solution strengthening and
precipitation hardening (Kainer 2003). Solid solution strengthening refers to alloying
with impurity atoms while, precipitation hardening refers to second phase
strengthening in materials to restrict the dislocation mobility for material
strengthening and hardening (Callister 2007).
Aluminium (Al) is a common alloying element used in magnesium. The
addition of aluminium with sufficient composition corresponds to improved strength,
hardness and corrosion resistance. The addition of aluminium between 3 wt% and 9
wt% produces good mechanical properties and high corrosion resistance of
magnesium based alloys. Aluminium content increases the tensile strength and
hardness of magnesium alloys due to the formation of -Mg17Al12 precipitation.
However, high aluminium content tends to form a high -Mg12Al17 precipitation to
15
give low strength and low ductility (Kainer 2003; Friedrich & Mordike 2006). Beck
(1940) reported increased tensile strength and elongation with the addition of
aluminium content up to the 6 wt%, whereas the tensile strength and elongation
decreased with aluminium content up to the 12 wt% in magnesium. Apart from that,
aluminium content corresponds to reduce grain size (StJohn et al. 2005; Dargusch et
al. 2006; Sillekens & Bormann 2012) to improve the tensile strength of magnesium. In
previous studies, Sajuri (2005) reported the tensile strength of extruded and as-cast
billet AZ91D was higher than the tensile strength of extruded and as-cast billet AZ61.
Meanwhile, Chamos et al. (2008) reported that the tensile strength of rolled AZ61 was
higher compared to the tensile strength of rolled AZ31 due to the high aluminium
content in magnesium alloys.
Other than aluminium, zinc (Zn), manganese (Mn), zirconium (Zr), yttrium
(Y) and rare earth (RE) elements are also common alloying elements used in
magnesium. Zinc is similar to aluminium element that corresponds to strengthening.
However, increasing zinc content corresponds to microporosity and reduced ductility
(Kainer 2003). Zinc content above 3 wt% has a tendency to undergo hot cracking
(Baghni et al. 2003). Manganese is beneficial to improve the strength, ductility,
corrosion resistance and creep resistance of magnesium. Rare earth elements enhance
the high temperature strength, creep resistance, and corrosion resistance. Yttrium is
usually incorporated with rare earth elements to increase high temperature strength
and creep performance. Zirconium improves strength, ductility and high-temperature
strength (Gupta & Sharon 2011; Sillekens & Bormann 2012). The best combination of
these alloying elements would be promoted to other commercial magnesium alloys
systems such as AM (Mg-Al-Mn), ZK (Mg-Zn-Zr) and WE (Mg-Y-RE) series. Due to
the good properties, these commercial magnesium alloys are widely used in
automotive application. Table 2.3 summarises the example of the chemical
compositions for AZ, AM, ZK and WE magnesium alloys.
16
Table 2.3 Chemical compositions of AZ, AM, ZK and WE magnesium alloys
series
Alloy designation Al Zn Mn Zr RE Y Mg
AZ31B, C, D 3 1 0.2 - - - Balance
AZ61A 6 1 0.2 - - - Balance
AZ63A, B, C, D 6 3 0.2 - - - Balance
AZ80A 8 0.5 0.2 - - - Balance
AZ91B, C, D, E 9 1 0.2 - - - Balance
AM50A 5 - 0.4 - - - Balance
AM60A, B 6 - 0.2 - - - Balance
ZK40A - 4 - 0.6 - - Balance
ZK60A - 6 - 0.6 - - Balance
WE43A, B - - - 0.6 3 4 Balance
WE54A - - - 0.6 4 5 Balance
Source: Friedrich & Mordike 2006
2.2.3 Plastic Deformation and Slip Systems of Magnesium Alloys
Plastic deformation is permanent deformation after applying shear stress and relates to
the slips involved with the movement of a large number of dislocations. The motion of
dislocation corresponds to the slip plane and slip direction. The combination of slip
plane and slip direction is called the slip system. Motion of dislocation begins when
there is large shear stress on the slip system and is referred to as critical resolved shear
stress (CRSS) (Roesler et al. 2006; Callister 2007).
The slip systems for the HCP structure for magnesium are shown in Figure
2.1. According to the von Mises criteria, more than five independent slip systems must
be operated in polycrystalline materials for high plastic deformation. The reduced
plastic deformation of magnesium in the HCP structure means slip system is fewer.
Moreover, the number of independent basal slip systems is fewer than required. Thus,
activation of prismatic or pyramidal slip planes (non-basal slip planes) and twinning
17
are necessary for material to deform. At low temperatures, magnesium is hard to
deform due to restricted independence causing the slip system to occur easily. In this
regard, high deformation of magnesium is relying on the activation energy of the non-
basal slip system (Friedrich & Mordike 2006; Faramarz & Stephen 2011).
Magnesium alloys is difficult to deform at room temperature because the
activation of slip planes at room temperature is limited and only involved the basal
planes. However, CRSS of non-basal slip systems at room temperature is 100 times
greater than CRSS of the basal slip system. When temperature increases, the
formability of magnesium is increased due to the activation of non-basal slip system,
while the CRSS of non-basal slip planes tends to decrease rapidly. However, CRSS of
the basal is temperature independent. Other than that, twinning is also considered a
deformation mechanism. Twinning is commonly generated at room temperature.
Figure 2.2 shows the example of twinning for extruded AZ31 at room temperature.
However, it decreased at increasing temperature due to the activation of non-basal slip
systems (Friedrich & Mordike 2006; Faramarz & Stephen 2011). Ulacia et al. (2010)
reported the tensile yield stress decreased with temperature at high strain rates and low
strain rates for AZ31 magnesium alloy because the CRSS of the non-basal slip
decreased with increasing temperatures.
Figure 2.2 Formation of twinning in extruded AZ31 at room temperature
Source: Barnet 2007
18
2.2.4 Strengthening Mechanisms of Magnesium Alloys
Plastic deformation is mainly related to the dislocation movement. If motion of
dislocation is piled up and blocked at the grain boundary, this will harden and
strengthen the material. In this regard, strengthening mechanisms can possibly impede
the dislocation movement. Strengthening mechanisms in metals are commonly
divided into grain size reduction, solid-solution alloying, work hardening and
precipitation hardening.
a. Grain size reduction
Strengthening by grain size reduction refers to when the greater total grain boundaries
area act as an obstacle to dislocation motion. In this regard, many slip planes are
stopped and require changing the slip directions to move to the next grains due to the
different orientations of grains. This indicates that the fine grain size is harder and
stronger than the coarse grain size which, due to the greater total grain boundaries
area, impedes dislocation motion. Grain size reduction not only improves the strength
and hardness, but also improves the toughness of the material (Callister 2007).
Somekawa and Mukai (2005) reported yield stress, tensile strength, elongation
to fracture and fracture toughness of extruded pure magnesium increased with grain
refinement. The grain size of the extruded pure magnesium decreased due to
decreasing the extrusion temperature. Similarly, Somekawa and Mukai (2006)
reported the fracture toughness of AZ31 improved by grain refinement. The grain size
of AZ31 was reduced due to the equal-channel-angular extrusion. As seen in Figure
2.3, Khan et al. (2006) reported that tensile strength and hardness of AZ31 and AZ10
increased by decreasing the grain size. In this finding, the reduction in grain size was
due to increasing the manganese content up to 0.4 wt%, as shown in Figure 2.4.
Subsequently, twinning formation also decreases in smaller grain size. Barnett
(2007) and Li et al. (2009) mentioned the twinning formation decreases at finer grain
structure of magnesium alloys. Li et al. (2009) reported the minor twinning formation
19
(a)
(b)
Figure 2.3 Effect of grain size on (a) tensile strength and (b) average hardness for
Mg-Al-Zn alloys
Source: Khan et al. 2006
20
in small grain size of ZK60 where the grain size of alloy is only ~0.8 µm. The strength
and ductility of magnesium alloys are improved by reducing twinning in finer grain
structure.
Figure 2.4 Effect of manganese content on average grain size for Mg-Al-Zn alloys
Source: Khan et al. 2006
b. Solid solution alloying
In general, pure metals are softer and weaker than alloys composed by same base
metal. Thus, alloying with impurity atoms into a solid is a technique to strengthen and
harden the material by restricting the dislocation movement. In this case, resistance to
slip is increased with the presence of impurity atom in the material. These factors
increase the strength and hardness of solid solution alloys (Callister 2007).
As referred to in Figure 2.5(a), Chino et al. (2002) reported that the tensile
strength of magnesium-calcium (Mg-Ca) alloy was higher compared to the tensile
strength of pure magnesium at increasing temperatures. However, the tensile strength
of Mg-Ca alloy at room temperature was almost the same to the tensile strength of
21
(a)
(b)
Figure 2.5 Effect of temperature on (a) tensile strength and (b) 0.2% proof stress
for pure magnesium and magnesium-calcium
Source: Chino et.al 2002
pure magnesium. A 0.2% proof stress of Mg-Ca alloy was found higher than that of
pure magnesium at all tested temperatures, as seen in Figure 2.5(b). Increasing tensile
strength and yield stress of this alloy was due to the addition of calcium in
magnesium. The addition of calcium element is beneficial to grain size refining and
increases creep resistance (Kainer 2003). In such a way, alloying is one common
22
effective method to improve the mechanical properties and microstructure of
materials, as discussed in section 2.2.4(a).
c. Work hardening
Work hardening refers to the ductile material that becomes harder and stronger due to
plastic deformation at low temperature. Dislocation density is generally increased
during plastic deformation where the yield stress and tensile strength of material are
increased. This is caused by higher resistance to dislocation motion. However, high
dislocation density decreases the ductility of the material (Callister 2007). Figure 2.6
shows the stress-strain curve of different hardening behaviours. Figure 2.6(b) indicates
the stress is increased after yielding for strain hardens, while Figure 2.6(a) indicates
material is perfectly plastic with no hardening after yielding (Roesler et al. 2006).
Apart from that, work hardening tends to decrease at increasing temperatures
(Callister 2007).
(a) perfectly plastic (b) strain hardening
Figure 2.6 Stress-strain curves for different hardening behaviours
Source: Roesler et al. 2006
Noda et al. (2011) reported the maximum stress and work hardening level of
extruded AZ31 magnesium alloy decreased when increasing the temperature from 250
°C to 350 °C. They also reported strain hardening exponent of extruded AZ31
decreased at increasing temperatures, probably due to the dynamic recovery and
recrystallization. In this thesis, further explanation of work hardening related to flow
23
equation was discussed in section 2.4.3. Hence, the findings of work hardening
properties such as strain hardening exponent (n) and strength coefficient (Ks) are
discussed in detail in terms of plastic characteristics for several materials.
d. Precipitation hardening
Precipitation hardening is the development of second phase fine particles in the
material through heat treatment. It mostly develops at grain boundaries, as seen in
Figure 2.7. The presence of precipitation hardening impedes the dislocation motion
thus increasing the hardness and strength of the alloy (Roesler et al. 2006). In
magnesium, most alloying elements tend to form precipitation hardening. For
instance, aluminium content forms -Mg17Al12 precipitation in magnesium. An
increase of aluminium content tends to increase the -phase to enhance the strength of
magnesium alloys (Kainer 2003).
Sajuri (2005) reported the distribution and volume fraction of -Mg17Al12
precipitation was higher in AZ91D than that of AZ61 due to the high aluminium
content in AZ91D compared to that of AZ61. Thus, tensile strength of AZ91D was
slightly higher than that of AZ61. Other than strength, Somekawa et al. (2007)
mentioned that the precipitate dispersion is an effective means to enhance the fracture
toughness of magnesium alloy. They found that the fracture toughness of ZK60 was
higher than that of pure magnesium and AZ31 due to lower volume fraction of
precipitates in pure magnesium and AZ31 compared to ZK60.
24
Figure 2.7 The -Mg17Al12 precipitation in AZ91
Source: Kaya 2013
2.3 APPLICATIONS OF MAGNESIUM ALLOYS IN AUTOMOTIVE
INDUSTRY
In the early 1930s, magnesium was used in the Volkswagen (VW) Beetle. Over the
past decade, magnesium-based materials have been increasingly used in commercial
vehicles by automobile manufacturers such as Volkswagen, Audi, DaimlerChrysler
(Mercedes-Benz), Toyota, Ford, BMW, Jaguar, Fiat, Hyundai, and Kia Motors
Corporation. To date, magnesium alloys have been used as structural components in
automotive applications due to the combination of low density, high specific strength
properties, machinability, castability, excellent damping capacity and high recycling
potential of magnesium based alloys. These good properties of magnesium alloys
attract automobile manufacturer to choose these materials for replacing conventional
materials such as steel and aluminium (Gupta & Sharon 2011).
2.3.1 Major Application of Magnesium Alloys in Vehicle
Figure 2.8 shows the use of several materials for vehicles in 2000. Steel is the major
material applied in vehicles (Luo 2002). The main interest in applying lightweight
magnesium alloys in automotive applications pertain to energy saving, increased
engine efficiency and reduced fuel consumption. Indirectly, it is also helps in lowering
harmful emissions. In the case of high specific strength magnesium alloys, these
183
REFERENCES
ASTM E8M. 2004. Standard Test Methods for Tension Testing of Metallic Materials.
ASTM E384. 2000. Standard Test Method for Microindentation Hardness of
Materials.
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