Thesis-Deformation Behaviours of Coarse-grained and Nanocrystalline Mg-5wt Al Alloys

Deformation Behaviours of Coarse-Grained and

Nanocrystalline Mg-5wt% Al Alloys

Hui Diao

Submitted for the Degree of Doctor of Philosophy in

The Faculty of Built Environment and Engineering,

Queensland University of Technology

Brisbane, Australia

2011

I

Keywords

nanocrystalline Mg alloy, texture, dislocation, twinning, strength, strain rate

sensitivity, strain hardening, strain softening, grain boundary sliding, Hall-Petch

relationship, inverse Hall-Petch, size effect

II

Abstract

Magnesium alloys have been of growing interest to various engineering

applications, such as the automobile, aerospace, communication and computer

industries due to their low density, high specific strength, good machineability

and availability as compared with other structural materials. However, most Mg

alloys suffer from poor plasticity due to their Hexagonal Close Packed structure.

Grain refinement has been proved to be an effective method to enhance the

strength and alter the ductility of the materials. Several methods have been

proposed to produce materials with nanocrystalline grain structures. So far, most

of the research work on nanocrystalline materials has been carried out on Face-

Centered Cubic and Body-Centered Cubic metals. However, there has been little

investigation of nanocrystalline Mg alloys.

In this study, bulk coarse-grained and nanocrystalline Mg alloys were

fabricated by a mechanical alloying method. The mixed powder of Mg chips and

Al powder was mechanically milled under argon atmosphere for different

durations of 0 hours (MA0), 10 hours (MA10), 20 hours (MA20), 30 hours

(MA30) and 40 hours (MA40), followed by compaction and sintering. Then the

sintered billets were hot-extruded into metallic rods with a 7 mm diameter. The

obtained Mg alloys have a nominal composition of Mg5wt% Al, with grain

sizes ranging from 13 m down to 50 nm, depending on the milling durations.

The microstructure characterization and evolution after deformation were

carried out by means of Optical microscopy, X-Ray Diffraction, Scanning

Electron Microscopy, Transmission Electron Microscopy, Scanning Probe

Microscopy and Neutron Diffraction techniques.

Nanoindentaion, compression and micro-compression tests on micro-pillars

were used to study the size effects on the mechanical behaviour of the Mg alloys.

Two kinds of size effects on the mechanical behaviours and deformation

mechanisms were investigated: grain size effect and sample size effect.

III

The nanoindentation tests were composed of constant strain rate, constant

loading rate and indentation creep tests. The normally reported indentation size

effect in single crystal and coarse-grained crystals was observed in both the

coarse-grained and nanocrystalline Mg alloys. Since the indentation size effect

is correlated to the Geometrically Necessary Dislocations under the indenter to

accommodate the plastic deformation, the good agreement between the

experimental results and the Indentation Size Effect model indicated that, in the

current nanocrystalline MA20 and MA30, the dislocation plasticity was still the

dominant deformation mechanism. Significant hardness enhancement with

decreasing grain size, down to 58 nm, was found in the nanocrystalline Mg

alloys. Further reduction of grain size would lead to a drop in the hardness

values. The failure of grain refinement strengthening with the relatively high

strain rate sensitivity of nanocrystalline Mg alloys suggested a change in the

deformation mechanism. Indentation creep tests showed that the stress exponent

was dependent on the loading rate during the loading section of the indentation,

which was related to the dislocation structures before the creep starts.

The influence of grain size on the mechanical behaviour and strength of

extruded coarse-grained and nanocrystalline Mg alloys were investigated using

uniaxial compression tests. The macroscopic response of the Mg alloys transited

from strain hardening to strain softening behaviour, with grain size reduced

from 13 m to 50 nm. The strain hardening was related to the twinning induced

hardening and dislocation hardening effect, while the strain softening was

attributed to the localized deformation in the nanocrystalline grains. The

tensioncompression yield asymmetry was noticed in the nanocrystalline region,

demonstrating the twinning effect in the ultra-fine-grained and nanocrystalline

region. The relationship < failed in the nanocrystalline Mg alloys; this was attributed to the twofold effect of grain size on twinning.

The nanocrystalline Mg alloys were found to exhibit increased strain rate

sensitivity with decreasing grain size, with strain rate ranging from 0.0001/s to

0.01/s. Strain rate sensitivity of coarse-grained MA0 was increased by more than

10 times in MA40. The Hall-Petch relationship broke down at a critical grain

IV

size in the nanocrystalline region. The breakdown of the Hall-Petch relationship

and the increased strain rate sensitivity were due to the localized dislocation

activities (generalization and annihilation at grain boundaries) and the more

significant contribution from grain boundary mediated mechanisms.

In the micro-compression tests, the sample size effects on the mechanical

behaviours were studied on MA0, MA20 and MA40 micro-pillars. In contrast to

the bulk samples under compression, the stress-strain curves of MA0 and MA20

micro-pillars were characterized with a number of discrete strain burst events

separated by nearly elastic strain segments. Unlike MA0 and MA20, the stress-

strain curves of MA40 micro-pillars were smooth, without obvious strain bursts.

The deformation mechanisms of the MA0 and MA20 micro-pillars under micro-

compression tests were considered to be initially dominated by deformation

twinning, followed by dislocation mechanisms. For MA40 pillars, the

deformation mechanisms were believed to be localized dislocation activities and

grain boundary related mechanisms. The strain hardening behaviours of the

micro-pillars suggested that the grain boundaries in the nanocrystalline micro-

pillars would reduce the source (nucleation sources for twins/dislocations)

starvation hardening effect. The power law relationship of the yield strength on

pillar dimensions in MA0, MA20 supported the fact that the twinning

mechanism was correlated to the pre-existing defects, which can promote the

nucleation of the twins.

Then, we provided a latitudinal comparison of the results and conclusions

derived from the different techniques used for testing the coarse-grained and

nanocrystalline Mg alloy; this helps to better understand the deformation

mechanisms of the Mg alloys as a whole.

At the end, we summarized the thesis and highlighted the conclusions,

contributions, innovations and outcomes of the research. Finally, it outlined

recommendations for future work.

V

Table of Contents

KEYWORDS ...........................................................................................................................I

ABSTRACT ........................................................................................................................... II

TABLE OF CONTENTS...................................................................................................... V

LIST OF TABLES ............................................................................................................... IX

LIST OF FIGURES .............................................................................................................. X

LIST OF ABBREVIATIONS ........................................................................................... XV

LIST OF PUBLICATIONS.............................................................................................. XVI

AUTHORSHIP ............................................................................................................... XVIII

ACKNOWLEDGEMENTS.............................................................................................. XIX

1 INTRODUCTION ............................................................................................................... 1

1.1 MOTIVATION .................................................................................................................. 1 1.2 RESEARCH PROBLEM ..................................................................................................... 3 1.3 RESEARCH METHODOLOGY ........................................................................................... 4

1.3.1 Experimental Testing ............................................................................................. 4 1.3.2 Theoretical Analysis .............................................................................................. 4

1.4 SCOPE OF THE THESIS ..................................................................................................... 5

2 LITERATURE REVIEW................................................................................................... 8

2.1 MG AND MG ALLOYS ..................................................................................................... 8 2.1.1 Texture Formation ................................................................................................. 8 2.1.2 Deformation Systems in Mg .................................................................................. 9

2.1.2.1 Dislocation Slip ............................................................................................................ 9 2.1.2.2 Twinning .................................................................................................................... 12

2.1.3 Texture Effect on Plastic Deformation ................................................................ 15 2.1.3.1 Texture Effect on Deformation Mechanism ............................................................... 15 2.1.3.2 TensionCompression Yield Asymmetry ................................................................... 15

2.1.4 Twinning-Detwinning In Cyclic Loading ........................................................... 19 2.1.5 Properties of NC Mg-5Al Alloys ......................................................................... 21

2.2 NANOCRYSTALLINE MATERIALS .................................................................................. 22 2.2.1 Categories and Synthesis ..................................................................................... 22 2.2.2 Mechanical Testing on NC Materials .................................................................. 25

2.2.2.1 Tensile Tests ............................................................................................................... 25

VI

2.2.2.2 Compression Tests ......................................................................................................26 2.2.2.3 Indentation Tests.........................................................................................................27

2.2.3 Mechanical Properties of NC Material ................................................................ 27 2.2.3.1 Strength ......................................................................................................................27 2.2.3.2 Inverse Hall-Patch Effect ............................................................................................31 2.2.3.3 Strain Hardening/Strain Softening ..............................................................................34 2.2.3.4 Strain Rate Sensitivity (SRS) ......................................................................................37

2.2.4 Deformation Mechanism of NC Material ............................................................ 41 2.2.4.1 Pile-Up Breakdown ....................................................................................................41 2.2.4.2 Grain Boundary as Sources and Sinks for Dislocations ..............................................43

2.2.4.2.1 Molecular Dynamics Simulations.......................................................................44 2.2.4.2.2 TEM Observations .............................................................................................45

2.2.4.3 Deformation Twinning ...............................................................................................46 2.2.4.4 Grain Boundary Sliding ..............................................................................................49

2.2.4.4.1 Grain Boundary Sliding Model ..........................................................................49 2.2.4.4.2 Experimental Observations and Molecular Dynamics Simulations ....................49

2.3 SUMMARY .................................................................................................................... 54

3 MICROSTRUCTURE CHARACTERIZATION OF MG-5WT% AL ALLOYS ...... 55

3.1 MATERIALS .................................................................................................................. 55 3.2 METHODOLOGY OF MICROANALYSIS ........................................................................... 55

3.2.1 Sample Preparation .............................................................................................. 55 3.2.2 X-Ray Diffraction (XRD) .................................................................................... 56 3.2.3 Scanning Electron Microscopy (SEM) ................................................................ 56 3.2.4 Transmission Electron Microscopy (TEM) ......................................................... 57 3.2.5 Neutron Diffraction ............................................................................................. 57

3.3 RESULTS AND DISCUSSIONS ......................................................................................... 58 3.3.1 X-Ray Diffraction Spectrum................................................................................ 58 3.3.2 SEM ..................................................................................................................... 60 3.3.3 Optical and TEM Observation ............................................................................. 61 3.3.4 Neutron Diffraction ............................................................................................. 65

3.4 SUMMARY .................................................................................................................... 67

4 INDENTATION TESTS ................................................................................................... 68

4.1 INTRODUCTION ............................................................................................................. 68 4.2 THEORY........................................................................................................................ 68

4.2.1 Hardness .............................................................................................................. 68 4.2.2 Modulus ............................................................................................................... 69 4.2.3 Strain Rate ........................................................................................................... 69

4.3 SPECIMEN PREPARATION .............................................................................................. 70

VII

4.4 EXPERIMENT ................................................................................................................ 70 4.5 RESULTS AND DISCUSSION ........................................................................................... 72

4.5.1 Indentation Size Effect ........................................................................................ 72 4.5.2 Strain Rate Effect ................................................................................................ 82 4.5.3 Hall-Petch Relationship ....................................................................................... 87 4.5.4 Time Dependent Deformation ............................................................................. 88

4.6 SUMMARY .................................................................................................................... 98

5 COMPRESSION TESTS ............................................................................................... 100

5.1 INTRODUCTION........................................................................................................... 100 5.2 EXPERIMENT .............................................................................................................. 101

5.2.1 Sample Preparation ............................................................................................ 101 5.2.2 Compression Tests............................................................................................. 101

5.3 EXPERIMENTAL RESULTS ........................................................................................... 102 5.3.1 Mechanical Properties ....................................................................................... 102 5.3.2 Fracture and Microstructure Evolution .............................................................. 104

5.4 DISCUSSION ............................................................................................................... 107 5.4.1 Macroscopic Yielding ....................................................................................... 107 5.4.2 Tensile/Compression Asymmetry ..................................................................... 115 5.4.3 Hall-Petch Relationship ..................................................................................... 117 5.4.4 Strain Rate Sensitivity ....................................................................................... 120

5.5 SUMMARY .................................................................................................................. 126

6 MICRO-PILLAR TESTS............................................................................................... 129

6.1 INTRODUCTION........................................................................................................... 129 6.2 EXPERIMENT .............................................................................................................. 134

6.2.1 Sample Preparation ............................................................................................ 134 6.2.2 Micro-Compression Tests.................................................................................. 137

6.3 RESULTS AND DISCUSSION ......................................................................................... 137 6.3.1 MA0: Coarse-Grained Micro-Pillars ................................................................. 139 6.3.2 MA20: Nanocrystallline Micro-Pillars .............................................................. 146 6.3.3 MA40: Nanocrystalline Micro-Pillars ............................................................... 150 6.3.4 Sample Size Effect on Strength ......................................................................... 153

6.4 SUMMARY .................................................................................................................. 156

7 DEFORMATION MECHANISMS OF MG-5WT% AL ALLOYS ........................... 158

7.1 LATTICE ORIENTATION .............................................................................................. 158 7.2 MECHANICAL PROPERTIES AND DEFORMATION MECHANISMS .................................. 159

7.2.1 Mechanical Properties ....................................................................................... 159

VIII

7.2.1.1 H-P Relationship.......................................................................................................159 7.2.1.2 Strain Rate Sensitivity ..............................................................................................160

7.2.2 Deformation Mechanisms .................................................................................. 162 7.2.2.1 Loading Conditions and Grain Size Effect ...............................................................162

7.2.2.1.1 Nanoindentation ...............................................................................................163 7.2.2.1.2 Compression Tests ...........................................................................................163

7.2.2.2 Sample Size Effect ....................................................................................................164

8 CONCLUSIONS ............................................................................................................. 167

8.1 SUMMARY .................................................................................................................. 167 8.2 INNOVATION OF THIS RESEARCH ............................................................................... 167 8.3 MAIN CONCLUSIONS .................................................................................................. 168 8.4 RECOMMENDATIONS FOR FUTURE WORK .................................................................. 173

REFERENCE ..................................................................................................................... 175

IX

List of Tables

Table 2-1 Independent modes of slip systems in HCP materials [28] ................ 11

Table 2-2 Compressive yield strength of nc Cu and Pd synthesized by IGC [64] ............................................................................................................................ 30

Table 2-3 Comparison of HallPetch slopes for slip and twinning [147] .......... 47

Table 3-1 Average grain size of the unmilled and nc Mg alloys ........................ 64

Table 4-1 Characteristic values of MA0, MA20 and MA30 .............................. 78Table 4-2 CRSS for some deformation modes in magnesium and its alloys .... 79

Table 4-3 Parameters of indentation creep tests of MA20 and MA40 ............... 96Table 4-4 Steady state stress exponent of indentation creep of MA20 and MA40 at different loading rate ...................................................................................... 98

Table 5-1 Yield Strength (MPa) of coarse-grained and nc Mg-5%Al Alloys in compression tests .............................................................................................. 104

Table 5-2 Schmid factor of the various slip and twinning systems in Mg for uniaxial compression along < 1120 > and < 2113 > direction [201] .......... 109Table 5-3 H-P slopes of wrought Mg-xAl-yZn Alloys tests in tension and compression ...................................................................................................... 118

Table 6-1 Fitting parameters of the power law relationship between the strength and the diameter of the micro-pillars ............................................................... 154

Table 7-1 SRS and activation volume values obtained from Indentation and Compression tests ............................................................................................. 161

X

List of Figures

Figure 2-1 Schematic diagram showing the formation of basal texture in (a) extruded magnesium and (b) ECAE process [25] ................................................ 9

Figure 2-2 Deformation systems of magnesium, (a) basal slip and (b) prismatic slip, (c) pyramidal ( 2) slip and (d) {1012} twinning [27] ............................... 10Figure 2-3 {10 1 1} twins observed in AZ31 rolled sheets during tensile deformation at room temperature [30] ............................................................... 13

Figure 2-4 EBSD map of AM30 after 8% straining [37] ................................... 14

Figure 2-5 True strain vs. true stress curves for specimens subjected to uniaxial compression [27] ................................................................................................ 16

Figure 2-6 Tension and compression yield stresses and CYS/TYS at different lo ad an gles for grain size o f 8 . 9 m ( is th e angle between loading axes, i.e. longitudinal axis of the specimen, and extrusion direction) [46] ....................... 17

Figure 2-7 Tensile and compressive yield strength vs. inverse square root of average grain size for extruded bars from ZM21 [8] ......................................... 18

Figure 2-8 EBSD map of in situ observation (a) compressive strain ~0.5% (b) unloading from compression (c) loading reverses from compression to tension at tensile strain ~0.7% (d) unloading from tension [50] ......................................... 20

Figure 2-9 Classification schema for nanostructured materials according to their chemical composition and the dimensionality (shape) of the crystallites (structural elements) [54] ................................................................................... 22

Figure 2-10 Inverse H-P trend for Cu and Pd as shown by Chokshi et al. [108] 32

Figure 2-11 H-P plot for bulk nickel electrodeposits [68] .................................. 32

Figure 2-12 H-P plot of hardness H vs d for laser-deposited nc Zn [112] ......... 33Figure 2-13 (a) Tensile test specimens are machine cut from the nc metal, prepared by powder metallurgy (b) Comparison of stress and strain for nc and coarse-grained copper [114] ............................................................................... 34

Figure 2-14 True stressstrain curves obtained by (a) tensile tests and (b) compressive tests [115] ...................................................................................... 35

Figure 2-15 True stresstrue plastic strain curves after room temperature compression tests of both UFG-Al and mc-Al (coarse-grained) at a strain rate of 210-4 s [116] ..................................................................................................... 36

Figure 2-16 Typical stress-strain curves obtained for the consolidated iron under quasistatic (14 10-4 s-1) and high-strain rate (36 103 s-1) uniaxial compression for all of the grain sizes [117] ....................................................... 37

Figure 2-17 SRS of nc and coarse-grained Cu from various literature data [119-125] ..................................................................................................................... 38

Figure 2-18 Double logarithmic plot of normalized stress vs. strain rate to

XI

determine m for consolidated Fe with different grain sizes (80 nm20 m) [117] ............................................................................................................................ 39

Figure 2-19 Plot of normalized flow stress vs. strain rate to determine m for ECAP Fe, ECAP Ta, and consolidated nc V, in comparison with their annealed, coarse-grained counterparts [126] ...................................................................... 40

Figure 2-20 Pile-up model with a locked dislocation of strength mb [136] ....... 42

Figure 2-21 Exact and approximate comparison of the Pande et al. model with H-P [136] ............................................................................................................ 43

Figure 2-22 Full dislocation in nc-Al nucleated near a triple junction [137] ..... 44Figure 2-23 The percentage of grains containing at least one obvious twin (using Optical Microscopy) plotted against the second differential of stress with strain (measured at strains between 0.05 and 0.15) [40] .............................................. 48

Figure 2-24 A section of the grain boundary between Grains 1 and 14 Displacement vectors are shown indicating the change in position between two levels of strain during plastic deformation. Atomic shuffling between the grains can be observed [150] ......................................................................................... 50

Figure 2-25 TEM observations of deformation processes in nc Ni3Al during in-situ tensile test at 750 C. Images are individual frames extracted from a real-time video: (a)(c) a continuous change in contrast of separated grains (d) rapid change in contrast of group of grains with visible formation of sliding surfaces (marked by arrows) (e) same image as in (d) with highlighting of recognizable grain boundaries, forming the cooperative sliding surfaces [151] ..................... 52

Figure 2-26 High resolution TEM micrographs of nc Pd showing a mesoscopic shear plane extending over several grain boundaries (arrowed) [152] .............. 53

Figure 3-1 Representative XRD patterns of sintered and extruded a) MA0 b) MA10 c) MA20 d) MA30 and e) MA40 ............................................................ 59

Figure 3-2 SEM pictures of the polished samples showing smooth surface at (a) low magnification and (b) high magnification ................................................... 60

Figure 3-3 (a) Optical microstructures of MA0 and TEM images of (b) MA10 (c) MA20 (d) MA30 and (e) MA40 ......................................................................... 62

Figure 3-4 Histograms showing the grain size distribution for (a) MA10 (b) MA20 (c) MA30 and (d) MA40 ......................................................................... 64

Figure 3-5 Pole figures and inverse pole figures of a) coarse-grained MA0 b) MA10 c) MA20 d) MA30 and e) MA40 ............................................................ 66

Figure 3-6 Schematic diagram showing basal texture in the extruded bar, with basal plane (Brown) parallel to the extrusion direction (longitudinal axis of samples) - Z axis ................................................................................................. 67Figure 4-1 Indentation samples ......................................................................... 70

Figure 4-2 One Indent imprint of MA30 after indentation tests ........................ 73

Figure 4-3 Loadingunloading curves of MA0, MA20 and MA30 under the

XII

constant strain rate of 0.1/s ................................................................................. 73

Figure 4-4 Hardness vs Displacement diagram showing the decline trend of the hardness value in the initial stage of the indentation tests ................................. 74

Figure 4-5 GNDs created by a rigid conical indentation. The dislocation structure is idealized as circular dislocation loops [170] ................................... 75

Figure 4-6 Depth dependence of hardness of MA0, MA20 and MA30 under constant strain rate of 0.1/s ................................................................................. 78

Figure 4-7 (a) Scanning Probe Microscopy image of an imprint of MA30 with peak load of 200 mN, (b) the cross-section view of the indent of the imprint ... 80

Figure 4-8 Relative position between the indenter and the basal pole of the textured grains .................................................................................................... 81

Figure 4-9 Optical microscopy pictures of imprints of a) MA0 b) MA10 c) MA20 d) MA30 and e) MA40 at various loading rates ..................................... 82

Figure 4-10 Load vs displacement curves of (a) MA0 (b) MA10 (c) MA20 (d) MA30 (e) MA40 under various loading rates .................................................... 84

Figure 4-11 Hardness vs Loading rate behaviour for the Mg-5wt% Al Alloy under maximum load of 200 mN ....................................................................... 85

Figure 4-12 Youngs Modulus vs displacement chart showing the growing tendency of the E values with the increasing milling hours ............................... 86

Figure 4-13 Activation Volume and m values of Mg alloys with various grain sizes .................................................................................................................... 86

Figure 4-14 Hardness of coarse-grained and nc Mg-5wt% Al as a function of d-1/2 ...................................................................................................................... 87Figure 4-15 Loading schedules for the creep tests under different loading rates 89

Figure 4-16 Load versus Displacement curves for indentation creep tests on (a) MA20 (b) MA40 ................................................................................................ 90

Figure 4-17 Topographical (top) and gradient (bottom) in-situ Scanning Probe Microscopy images of the surface of sample MA20 before and after indenting with loading rate of a) 0.05 mN/s b) 1 mN/s and c) 20 mN/s ............................ 92

Figure 4-18 Topographical (top) and gradient (bottom) in-situ Scanning Probe Microscopy images of the surface of sample MA40 before and after indenting with loading rate of a) 0.05 mN/s b) 1 mN/s and c) 20 mN/s ............................ 93

Figure 4-19 Creep displacement during load hold in (a) MA20 and (b) MA40 (The starting points were aligned to facilitate comparison.) .............................. 94

Figure 4-20 Experimental and fitted creep curves in (a) MA20 and (b) MA40 . 96

Figure 4-21 Ln(Strain Rate)-Ln(Stress) plot of the indentation Creep of (a) MA20 and (b) MA40 .......................................................................................... 97

Figure 5-1 Cylinder compression samples with 5mm diameter and 10mm length .......................................................................................................................... 101

XIII

Figure 5-2 True Stress-True Strain curves of coarse-grained and nc Mg-5wt% Al alloys at strain rate of (a) 0.0001/s (b) 0.0005/s (c) 0.001/s and (d) 0.01/s ...... 103

Figure 5-3 Fractured samples after compression tests at strain rate of 0.001/s of (a) MA0 (b) MA10 (c) MA20 (d) MA30 and (e) MA40 ................................. 105

Figure 5-4 (a) Optical microscopy and (b) (c) TEM images of MA0 after deformation (d) before and (e) after deformation of MA20 and (f) before and (g) after deformation of MA40, respectively ......................................................... 107

Figure 5-5 (a) Schematic diagram of the compression tests on textured Mg alloys, in which the loading direction is parallel to the z-axis, and the c-axis of the grains lies in the x-y plane (b) Schematic diagram of Schmid Factor Calculation ........................................................................................................ 108

Figure 5-6 Strain hardening rates of MA0 at different strain rates .................. 110

Figure 5-7 Consideres criterion of MA0, MA10, MA20, MA30 and MA40 at strain rate of (a) 0.0001/s (b) 0.0005/s (c) 0.001/s and (d) 0.01/s, respectively 113

Figure 5-8 Critical radius for twin embryo, rc, as a function of local shear stress 130 .................................................................................................................... 115 Figure 5-9 Grain size dependence of the yield strength of Mg alloys under compression and tensile tests ........................................................................... 116

Figure 5-10 Deformed MA40 sample showing good ductility at strain rate of 0.0001/s ............................................................................................................ 120

Figure 5-11 Critical grain sizes for the transition to inverse H-P relationship . 120

Figure 5-12 True stress-true strain curves of (a) MA0 (b) MA10 (c) MA20 (d) MA30 and (e) MA40 at different strain rates ................................................... 123

Figure 5-13 Relationship between log and log , where the SRS (m) value can be obtained from the slope of the curves .......................................................... 124

Figure 5-14 SRS and activation volume depending on grain size ................... 125

Figure 6-1 Micro-pillars of MA0 with diameter of (a) 3.5 m (b) 12 m and (c) 16 m ................................................................................................................ 135

Figure 6-2 Micro-pillars of MA20 with diameter of (a) 3.5 m (b) 7.5 m and (c) 10 m .......................................................................................................... 135

Figure 6-3 Micro-pillars of MA40 with diameter of (a) 3.5 m (b) 7.5 m and (c) 15 m .......................................................................................................... 136

Figure 6-4 Force-Displacement curves of (a) MA0 (b) MA20 and (c) MA40 pillars ................................................................................................................ 138

Figure 6-5 Compression true stress-true strain curves of MA0 pillars and bulk sample ............................................................................................................... 139

Figure 6-6 Enlarged partial view of true stress-true strain curves of MA0 pillars (a) 3.5 m (b) 12 m and (c) 16 m ..................................................... 140

Figure 6-7 Strain hardening rate values of MA0 micro-pillars and bulk sample

XIV

.......................................................................................................................... 142

Figure 6-8 MA0 pillars with diameter of (a) 3.5m (b) 12m and (c) 16 m after micro-compression tests ........................................................................... 145


Figure 6-10 Strain bursts in the stress-strain curves of MA20 pillars of (a) 3.5 m and (b) 10 m diameter .............................................................................. 147

Figure 6-11 Strain hardening rate values of MA20 micro-pillars and bulk sample .......................................................................................................................... 148

Figure 6-12 MA20 Micro-pillars with diameter of (a) 3.5m (b) 7.5m and (c) 10m after micro-compression tests ................................................................ 150


Figure 6-14 Strain hardening rate values of MA40 micro-pillars and bulk sample .......................................................................................................................... 151

Figure 6-15 MA40 Micro-pillars with diameter of (a) 3.5 m (b) 7.5 m and (c) 15 m after micro-compression tests ............................................................... 153

Figure 6-16 Logarithmic plot of true stress at 3% true strain for all pillars tested. The solid lines represent best fit power law functions. .................................... 154

Figure 7-1 Schematic diagram showing the relative position of the sample longitudinal axis and the basal planes of the HCP lattice ................................. 159

Figure 7-2 Hardness and strength of coarse-grained and nc Mg-5% Al as a function of under compression and indentation tests ................................. 160Figure 7-3 Loading conditions of samples under (a) indentation test and (b) uniaxial compression and micro-pillar compression test ................................. 162

XV

List of Abbreviations

BCC Body-Centered Cubic

CRSS Critical Resolved Shear Stress

ECAE Equal Channel Angular Extrusion

FCC Face-Centered Cubic

FIB Focused Ion Beam

GND(s) Geometrically Necessary Dislocation(s)

HCP Hexagonal Close Packed

H-P Hall-Petch

ISE Indentation Size Effect

nc Nanocrystalline

SEM Scanning Electron Microscopy

SPM Scanning Probe Microscopy

SRS Strain Rate Sensitivity

TEM Transmission Electron Microscopy

UFG(Ms) Ultra-Fine-Grained (Materials)

XRD X-Ray Diffraction

CYS Compression Yield Strength

TYS Tensile Yield Strength

XVI

List of Publications

Journal Publications

1. H. Diao, C. Yan, J.M. Bell. Grain and Sample Size Effects on

Deformation of Coarse-Grained and Nanocrystalline Mg Alloy Micro-

Pillars. (In Preparation)

2. H. Diao, C. Yan, J.M. Bell, L. Lu. Deformation Behaviour of Ultra Fine

Grained and Nanocrystalline Mg- 5wt%Al Alloy under Compression.

(Submitted to Materials Science & Engineering)

3. H. Diao, C. Yan, J.M. Bell, L. Lu, G.P. Zhang, S. Kabra, K.-D. Liss,

M.W. Chen. Compressive Behaviour of Nanocrystallilne Mg-5wt%Al

Alloys. Modern Physics Letters B, In Press. Accepted 20 October, 2010

4. S.T. Deng, H. Diao, Y.L. Chen, C. Yan, H.F. Zhang, A.M. Wang and Z.Q.

Hu. Metallic Glass Fibers Reinforced Zr-based Bulk Metallic Glass.

Scripta Materialia. 64 (2011) 8588.

5. H. Diao, C.Q. Wang, L. Wang. Bonding of aluminum alloy by hot-

dipping tin coating. Advanced Materials Research. 32 (2008) 93-98.

Conference Publications

6. H. Diao, C. Yan, J.M. Bell and L. Lu. Hall-Petch Relationship and

Strain Rate Sensitivity of Nanocrystalline Mg- 5wt% Al Alloy.

Conference on Engineering, Designing and Developing the Built

Environment for Sustainable Wellbeing. 27th-29th April 2011, Brisbane,

Australia.

7. H. Diao, C. Yan, J. Bell, L. Lu, and G.P. Zhang. Indentation Size Effect

and Strain Rate Sensitivity of Nanocrystalline Mg-Al Alloys. 5th World

Congress on Engineering Asset Management. 25th-27th October 2010,

Brisbane, Australia

8. H. Diao, C. Yan, J. Bell, L. Lu, and G.P. Zhang. Deformation Behaviour

XVII

of Nanocrystalline Mg-Al Alloys during Nanoindentation. International

Nanoelectronics Conference 2010. 3rd- 8th January 2010, Hong Kong,

China.

9. H. Diao, C. Yan, J. Bell. Compressive Behaviour of Nanocrystalline Mg-

Al Alloy. International Conference on Materials and AustCeram 2009,

1st-3rd July, 2009, Gold Coast, Australia.

XVIII

Authorship

The work contained in this thesis has not been submitted for a degree or

diploma in any higher education institution. To the best of my knowledge and

understanding, the thesis contains no material previously published or written by

any other person except where due reference is made.

Hui Diao

XIX

Acknowledgements

I would like to take this opportunity to express my thanks to those without

whose help and support this research and thesis would not have been completed

successfully.

First and foremost, I would like to express my sincere gratitude and deep

appreciation to my supervisors, Professor Cheng Yan and Professor John Bell,

for their motivation, guidance, inspiration and support throughout the project.

They have always guided me in the right direction with constructive suggestions,

and have provided assistance in many ways. Their patience and encouragement

which helped me through the hard times are highly appreciated.

As a recipient of a Queensland University of Technology (QUT) Tuition Fee

Waiver Scholarship and a Built Environment and Engineering (BEE) Living

Allowance Scholarship, I would like to thank QUT, BEE and my supervisor,

Professor Cheng Yan, for their financial support to the project.

I am grateful to Professor Lu Li from the National University of Singapore

for fabrication of the bulk Mg-Al alloys for the project. I would also like to

thank Dr Klaus Dieter Liss and Dr Suarabh Kabra from the Australian Nuclear

Science and Technology Organization, Donald McAuley from Struers Company,

Professor Mingwei Chen from Tohoku University and Professor Guangping

Zhang from the Institute of Metal Research, Chinese Academy of Sciences for

their help with some of the experiments.

My sincere thanks and gratitude go to all the staff at BEEs mechanical

testing laboratory, as well as everybody at the BEE Workshop for their technical

support. Also, a big thank you goes to all the staff members at the Analytical

Electron Microscopy Facility, Faculty of Science for their instrumental

assistance, and to all the fellow members in our research group for their

friendship and support.

Finally, my special thanks go to my parents for their unconditional love,

support and belief in me always. I would not have been here without their

XX

always being there for me. A huge thank you goes to my extremely supportive

partner, Philip, for his understanding, encouragement and confidence in me

through the PhD journey. I would also like to thank my dearest friends, Hao

Wang and Lidi Gong for their precious and valuable friendship.

Chapter 1 Introduction

1

1 Introduction

1.1 Motivation

Magnesium alloys have been of growing interest as light-weight structural

materials due to their low density and high specific strength. However, most Mg

alloys suffer from poor plasticity due to their Hexagonal Closed Packed (HCP)

structure. Recent study shows that grain refinement would help to improve the

strength/ductility of Mg alloys. Yamashita et al. [1] reported that the effects of

grain size on elongation-to-failure of Mg0.9% Al alloy increased up to 15%

when grain size was reduced from 400 to 17 m. Mukai et al. [2] also

demonstrated that the ductility in WE43 magnesium alloy (1.5 m) could be

enhanced by the grain refinement even at a dynamic strain rate of 1.1103 s1.

Kubota et al. [3] reported a good combination of high strength and high ductility

of fine grained Mg alloys at room temperature as a result of grain refinement.

Even superplastic behaviour was observed at high strain rates ( 101s1) or low temperatures ( 473 K) [3].

Several methods, such as mechanical alloying and severe plastic deformation,

have shown the effectiveness of grain size refinement down to nanometer level

[4-6]. Since nanocrystalline (nc) alloys possess significantly large volume

fraction of grain boundaries, they have been demonstrated to manifest very

different properties in comparison to their traditional coarse-grained

counterparts [7].

While most of the investigations on the deformation behaviour of nc metals

have been focused on Face-Centered Cubic (FCC) and Body-Centered Cubic

(BCC) metals, little work has been done on HCP metals. Up to date, the research

has been mainly focused on the deformation behaviour of coarse-grained Mg

alloys [8-10]. A number of studies on nc Mg alloys have also been reported, but

the main concern is with the hydrogen storage capability of the material [11].


2

The application of grain refinement as a powerful tool to design

microstructures with superior properties and performance has long been the

focus of metallurgical research [12]. Nanostructured materials have attracted

large scientific interest in the past decade [13]. The relatively smaller grain size

and higher volume fraction of grain boundaries contribute to the unique

mechanical properties of the nc materials, namely, increased strength/hardness

and reduced ductility. This has been the incentive for widespread research in this

area, but the underlying correlation between the microstructural characteristics

of nc materials and their mechanical properties are not well established. Precise

characterization of the mechanical properties of the nc materials is required for

proper design and structure miniaturization.

In the last two decades, Microelectromechanical Systems research has

received tremendous attention and registered prodigious growth. Rapid

advancement in microfabrication technologies, coupled with the drive toward

miniaturization of existing systems, has resulted in the development of

innovative Microelectromechanical Systems and Microfluidic Systems for use

in novel applications spread across diverse technological disciplines [14]. The

better understanding of the deformation mechanism is helpful for the design and

fabrication of the miniaturized structures with dimensions on the micrometre

scale.

Due to the lack of understanding of material behaviour at small scales, the

engineering industries are facing difficulty in materials selection and optimal

design. The applications of structures are limited by the fact that the deformation

and failure mechanisms and associated reliability problems have not been well

understood and are, to a large extent, still unknown. The metallic materials are

often subject to two kinds of constraints: microstructural constraint as a result of

reduced grain size to nanometer scale, and/or sample size constraint introduced

by physical dimensions. However, the effects of these constraints on

deformation and failure mechanisms in nc alloys are still not well understood.

Further advances in system design and structure miniaturization require a more


3

complete understanding of the physics underlying performance and reliability

for optimal design and reliability evaluation [15].

1.2 Research Problem

The research aims to investigate the deformation and failure behaviours of

extruded coarse-grained and nc Mg-5wt% Al alloys under compression load. By

detecting the mechanical properties of the nc materials under a series of

different loading conditions, the proposed research project analyzes the two

kinds of size effects, namely, grain size effect and sample size effect on

mechanical properties and deformation mechanisms of the coarse-grained and

nc Mg alloys. Based on experiment and microanalysis, further discussion will

explore the deformation and failure mechanisms of nc materials.

Innovative approaches in both concepts and methodologies will be used in

this study, including:

a. Mechanical testing on bulk nc alloys

Due to the difficulty of fabricating bulk nc materials, they are normally

produced in the form of thin films, synthesized by inert gas condensation or

electrodeposition technology. To some extent, the fact that the nc materials are

fabricated in the form of thin films greatly limits our understanding of the

deformation and failure mechanisms in these materials as only limited

mechanical tests can be carried out. In this thesis, bulk nc Mg alloys are adopted,

which allow the conventional mechanical testing to be performed.

b. Grain size and sample size effects

For coarse-grained and nc specimens under different physical dimension

constraints, the overall influence of grain refinement and sample size constraint

from the interfaces on mechanical behaviour will be investigated. In this project,

we will study the grain size effect and sample size effect separately, through

systematically changing the sample geometry dimensions and the grain sizes.

Possible interaction between the grain size and sample size effects, either by

competition or cooperation, will be investigated.


4

c. The strain rate effect

The nc materials are more rate sensitive than their coarse-grained

counterparts. A change in the strain rate sensitivity suggests a change in the

deformation mechanism; thus, the strain rate effect on the mechanical behaviour

of Mg-5wt% Al alloys with a grain size spectrum ranging from micrometer to

nanometer will be examined.

1.3 Research Methodology

1.3.1 Experimental Testing

Nanoindentation, compression and micro-compression tests are adopted as

the main testing methods to investigate the grain size effect and geometry

constrain effect on the mechanical behaviours and deformation mechanisms of

coarse-grained and nc Mg-5wt% Al alloys. All the experimental data will be

carefully collected with the help of the data acquisition instruments connected to

various testing machines.

X-Ray Diffraction (XRD) analysis will be carried out to detect the phase

composition and distribution. Scanning Electron Microscopy (SEM) and

Scanning Probe Microscopy (SPM) will be used to examine the deformation

around the indents. Optical Microscopy, Transmission Electron Microscopy

(TEM), and Focused Ion Beam (FIB) cross-sectional examination will enable

the examination of microstructural evolution and deformation mechanisms at the

nano- and micro-scale.

1.3.2 Theoretical Analysis

Geometrical constraint plays a major role in determining deformation and

failure behaviour of materials. Miniaturized and bulk samples will behave

differently due to the difference in geometric dimensions. If materials are

tailored to small components, the physical mechanism may begin to feel the

presence of the surface or an interface. Due to the lack of the three-dimensional

constraint and a high volume fraction of the surface area, layered and


5

miniaturized samples may not represent the mechanical behaviour of bulk form

samples. Detailed analysis will be conducted to gain a better understanding of

the effects of dimensional constraint on deformation mechanisms in these

miniaturized and bulk samples.

The effect of microstructural constraint induced by small grain size in nc Mg-

5wt% Al on mechanical properties and deformation mechanisms will be

evaluated. The dependence of yield strength, ductility and strain rate sensitivity

on grain size, and grain size effect on dislocation/twinning activities will be

discussed. In addition, the possible interactions with sample size effects, either

by competition or cooperation will be explored.

Based upon the analyses above, deformation and failure mechanisms of nc

material will be discussed. A better understanding of the effects of physical

dimensional constraint, grain size, strain rate and stress state can be gained

through these experimental and theoretical analyses. This understanding will

benefit materials fabrication, reliability evaluation and optimal design in

relevant industries.

1.4 Scope of the thesis

Chapter 2 provides a detailed review that is related to the aims and research

background of the project. This chapter covers two major topics: coarse-grained

Mg alloys and nc materials. For coarse-grained Mg alloys, the deformation

mechanisms of Mg alloys are discussed based on the twinning and slip systems

in Mg. Considering the fabrication process used to produce the Mg-5wt% Al,

the texture formation and its effect on deformation behaviours of Mg are

examined. Following the reviews of Mg, a detailed investigation of synthesis,

mechanical properties and deformation mechanisms of previous reported nc

materials is made.

Chapter 3 introduces the method used to fabricate the coarse-grained and nc

Mg-5wt% Al alloys, using mechanical alloying. Through milling for different

hours in the chamber, followed by the hot extrusion process, the grain size of the


6

materials is gradually decreased from the micrometre to nanometre region. The

microstructure characterizations of the Mg alloys are performed using Optical

Microscopy, XRD, SEM, TEM, Scanning Probe Microscopy and the neutron

diffraction technique. The Optical Microscopy and TEM observations reveal

that the grain size of the materials reduces from 13 m for MA0 to 50 nm after

being milled for 40 hours. XRD patterns confirm that the additional Al atoms

are homogeneously distributed and diffused into the Mg matrix, forming a solid

solution. The texture measurement from the neutron diffraction manifests the

basal texture, in which the (0002) basal planes of the grains are parallel to the

extrusion direction of the Mg rods.

Chapter 4 describes the nanoindentation tests on the Mg alloys. The existence

of Indentation Size Effect in nc Mg alloys, normally reported in single crystal

and coarse-grained crystals, is confirmed. The Indentation Size Effect in the nc

MA20 and MA30 suggests that the dislocation mechanisms are the dominant

mechanisms during nanoindentation test. The validity of Strain Rate Sensitivity

(SRS) and strength enhancement with grain refinement in the nc Mg alloys are

evaluated. The breakdown of the Hall-Petch (H-P) relationship in hardness in

MA40, together with the obviously increased SRS values, indicates more

significant grain boundary mediated mechanisms in nc grains. The rate

sensitivity of the indentation creep behaviour is related to the loading rate during

nanoindentation before the constant creep load holding, resulting in stress

exponent dependence on the loading rate. The possible underlying deformation

mechanisms for the coarse-grained and nc Mg alloys during nanoindentation

tests and indentation creep tests are discussed.

Chapter 5 details the uniaxial compression tests on Mg alloys. The influence

of grain size on mechanical behaviour and strength are investigated. For

materials with a grain size above 78 nm, the macroscopic yielding behaviour is

plastic deformation with significant strain hardening. With the grain size

reduced to below 58 nm, the strain hardening is replaced by strain softening in

the stress-strain curves. The strain hardening is attributed to dislocation

hardening effect and twinning induced hardening effect, which is ascribed to the


7

influence of twins on slip rather than to the stress of twinning activation. The

nucleation of twinning is suppressed with further decrease of grain size, and

pyramidal < c+a > slip becomes the dominant mechanism. Also, the small grain

size leads to more contribution from grain boundary sliding towards the total

plastic strain. Dislocation generation and annihilations at grain boundaries and

more significant grain boundary sliding mechanisms result in the strain

softening, the breakdown of H-P relationship and the increased SRS values.

Chapter 6 discusses the micro-compression tests on the micro-pillars that deal

with the twofold size effects from both the grain size effect and sample size

effect on the Mg alloy. The micro-compression stress-strain curves of the MA0

and MA20 micro-pillars are characterized with strain bursts connected by

sections of nearly elastic loading. The magnitude of the strain appears to be

random; no direct relationship is observed between strain bursts length and

dimensions of the micro-pillars. In contrast to the significant strain bursts in the

stress-strain curves that have been observed in the MA0 and MA20 micro-pillars,

the micro-compression curves of MA40 micro-pillars appear to be smooth,

without obvious strain bursts. The deformation mechanisms of the MA0 and

MA20 micro-pillars under micro-compression tests are considered to be initially

dominated by deformation twinning, followed by dislocation mechanisms. For

MA40 pillars, the deformation mechanism is believed to be localized dislocation

activities and grain boundary related mechanism. The grain boundaries reduce

the source starvation hardening effect in the nc micro-pillars. The power law

relationship of the yield strength on pillar dimensions in MA0, MA20 supports

the theory that the twinning mechanism is related to the pre-existing defects,

which can promote the nucleation of the twins.

Chapter 7 provides a comparison of the results and conclusions derived from

different testing technique of the coarse-grained and nc Mg alloy, which will

help to better understand the deformation mechanisms of the Mg alloys as a

whole.

Chapter 8 summarises the results and main conclusions of the thesis. It then

outlines further opportunities for future research in this area.

Chapter 2 Literature Review

8

2 Literature Review

This chapter presents the background research (to date) which is relevant to

the present research project. It explores two major topics:

1. Mg and Mg alloys - properties and mechanisms (if not specifically

specified as nc Mg, all Mg (alloys) in the following sections refer to

coarse-grained Mg ) and

2. nc materials - synthesis, mechanical properties and deformation

mechanisms.

2.1 Mg and Mg alloys

Weight-saving possibilities from the application of Mg in structural parts

have prompted intensive research, especially in the automotive and aerospace

industries. Mg has a density of 1.74 g cm-3, which is 35.6% and 61.3% lower

than that of Al and Ti, respectively [16]. Due to Mgs low density, high

strength-to-weight ratio and high specific stiffness at both ambient and elevated

temperatures, Mg and Mg alloys are attractive choices among these three light

weight metals.

To overcome the limitations of Mg such as low strength, low ductility and

poor corrosion resistance, it is often alloyed with various alloying elements such

as Al, Zn, Mn, Li, Y and rare earth materials [17]. Currently, most commercially

available Mg alloys are either die cast or hot worked to form parts and

components. Of all the Mg alloys, the AZ (MgAlZn) and AM (MgAl) series

are the most common. The addition of Al in these alloys serves principally as a

solid solution strengthening element to further improve their strength [18].

2.1.1 Texture Formation

The limited number of active deformation systems in HCP metals results in

the formation of a strong crystallographic texture upon mechanical processing,

such as extrusion, Equal Channel Angular Extrusion (ECAE) and rolling [19-23].


9

The prevalent textures in rolled sheets and extruded rods of MgAl alloys

were already recognised in the 1930s [24]. The texture formations from direct

extrusion and the ECAE process are shown in Figure 2-1.

Figure 2-1 Schematic diagram showing the formation of basal texture in (a) extruded

magnesium and (b) ECAE process [25]

2.1.2 Deformation Systems in Mg

2.1.2.1 Dislocation Slip

In polycrystalline metals, the requirement of five independence slip systems

is necessary to be able to sustain a general homogeneous deformation without

generation of cracks; this is known as the von Mises-Taylor criterion [26]. In

Mg, three possible Burgers vectors can be active on various slip planes, basal

slip, prismatic slip and pyramidal slip [27], as shown in Figure 2-2. The main

deformation mode in magnesium and magnesium alloys is basal slip, i.e., slip on

the (0 0 0 1) plane with a Burgers vector. Prismatic slip {1100} and pyramidal slip (1) {1011} have also been observed; however, their critical resolved shear stress (CRSS) at room temperature is roughly a 100-

fold greater than for basal slip [19].

(b) (a)


10

Figure 2-2 Deformation systems of magnesium, (a) basal slip and (b) prismatic

slip, (c) pyramidal ( 2) slip and (d) {1012} twinning [27]

Groves and Kelly investigated the independence modes of the slip systems in

HCP materials [28], and the results are shown in Table 2-1. Since pyramidal slip

systems with < a > Burgers vector is crystallographic equivalent to the

combination of basal and prismatic slip, a total number of four independent

modes are provided with < a > Burgers vector under the condition of cross-slip.

(b) (a)

(c) (d)


11

Meanwhile, a slip system with < c+a > Burgers vector alone could offer five

independent modes that fulfil the Von Mises-Taylor criterion.

Table 2-1 Independent modes of slip systems in HCP materials [28]

Slip Systems Crystallographic Elements No. of Independent

modes

< a >

Basal Slip (0002) < 1120 > 2 Prismatic Slip (1100) < 1120 > 2

Pyramidal Slip ( 1) (110) < 1120 > 4 < c > (0)[0001]

< c+a > Pyramidal Slip ( 2) () < 1123 > 5

The CRSS for the basal slip in pure magnesium is very low, approximately

0.600.7 MPa which is also independent of temperature. In contrast, the critical

shear stress for the non-basal slip is over 40 MPa at low temperature; this is two

orders of magnitude higher than that for the basal slip, and drastically decreases

to 2-3 MPa with increasing temperature [29].

In single crystal Mg, basal slip is the dominant deformation mechanism at

room temperature. However, when it comes to the polycrystalline Mg, the

situation is different since the grains are constrained by the surrounding grain

boundaries. If only the basal slip is activated, it can only offer two independent

slip systems, which is less than the required five. This will cause strain

incompatibility at the grain boundaries. When the grain boundaries are strong

enough, stress concentration occurs to maintain strain compatibility at the grain

boundaries [30]. This stress concentration would lead to the activation of non-

basal slip, as well as twining, in Mg.

There are a few reports confirming the activities of < c+a > slip in Mg alloys

at room temperature. Kokie et al. [31] observed 40% of pyramidal slip ( 2)

segment of dislocations of the total dislocation density, in an ECAE AZ31 alloy

with an average grain size of 8 m. The occurrence of < c+a > dislocations,

which could arise from the cross-slip from basal plane to non-basal plane,


12

agreed with the large yield anisotropy factor from the tests. Agnew et al. [32]

also found direct evidence of extensive pyramidal slip ( 2) of deformed AZ31B

alloys in TEM observation, at room temperature. Clausen et al. [33] investigated

the mechanical response of Mg AZ31 alloy dominated by twinning, in

combination with an elastic-plastic self-consistent modelling. They found that

the relative activity of pyramidal slip ( 2) increased to ~0.5 after 2% of strain,

when the activity of < a > slip decreased rapidly after plastic yielding.

2.1.2.2 Twinning

Since the Burgers vector lies in the basal plane, no plastic strain parallel to

the c-axis can be accommodated by any of these < a > slip systems. However,

such strain can be produced by twinning. Due to the limited number of

independent slip systems in Mg, twinning has been theoretically and

experimentally proven to be of crucial importance in plastic deformation.

Three types of deformation twins are frequently observed in Mg Alloys:

extension twins, contraction twins and double twins [9, 10, 34-36]. Among those

twining systems, the {10 1 2} extension twins is the only type that allows extension strain parallel to the c-axis [9]. Thus, the {10 1 2} twins form prevalently when tensile stress is applied in the parallel direction to the c-axis.

The {1012} extension twins have the second smallest CRSS following basal slip; therefore, the {1012} twins could be activated in the initial deformation stage and influences yielding behaviour. Contraction twinning, including {10 1 1} twins and {1013} twins, could accommodate the compression strain along c-axis [10]. It is activated when there is a contraction strain component parallel to

the c-axis or when macroscopic extension is being applied perpendicular to the

c-axis. In addition to primary twinning, secondary twinning can take place

within the reoriented primary twins [37]. Generally reported double twinning are

{1011} - {1012} and {1013} - {1012} twinning. Generally {1011} or {1013} contraction twins form first, after which {1012} extension twins are propagated within the original contraction twins. Both types of double twins lead to a

contraction along the c-axis [38].


13

Gharghouri et al. [39] studied the mechanical properties in an extruded Mg-

7.7 at. % Al alloy by means of in-situ neutron diffraction. The occurrence of

twinning was easily detected by neutron diffraction, considering it would lead to

an abrupt reorientation of the crystal lattice, reflected by a change in the peak

density. The testing results was consistent with the activation {1012} twinning. The {1012} twins in extruded Mg-3Al-Zn alloys under compression tests led to distinctive flow curves characterized by high levels of working hardening and

concave shape [40]. The hardening effect was attributed to the effect of twins

on slip, rather than to the stress required activating it. With decreasing grain size

and increasing temperature, the deformation mechanism transferred from

twinning to slip dominant flow.

Koike et al. [30] reported {1011} twins in the rolled Mg alloys sheets had a strong basal texture deformed in tension. The c-axis contraction {1011} twins were observed and characterized with narrow banded structures, as shown in

Figure 2-3. The formation of the c-axis contraction twins was found to induce a

large shear deformation within the twin volume, showing an obvious height

change accompanying the narrow twins on the deformed sample surface.

Figure 2-3 {1011} twins observed in AZ31 rolled sheets during tensile deformation at room temperature [30]


14

Profusely formed {1011} - {1012} double twins (highlighted in purple) were observed in polycrystalline AM30 Mg alloy [37] with tensile axes parallel to the

extrusion direction, as shown in Figure 2-4. Some primary {1011} twins (green) and {1013} - {1012} double twins (blue) were also present, but fewer in number.

Figure 2-4 EBSD map of AM30 after 8% straining [37]

Once the twins are formed, the twin boundaries act as obstacles for

dislocation movement, hence becoming a source of strain hardening. Yoo et al.

[41] pointed out that the a {1012} twin is repulsive to prismatic or basal slip dislocations, leading to an increase in the flow stress. This is further confirmed

by Serra et al. [42], using atomic-scale simulation. They found that the twins

created barriers to the motion of crystal dislocations gliding on the basal plane,

and the strength of the barrier depends on crystallographic parameters and the

atomic structures of the interfaces.


15

2.1.3 Texture Effect on Plastic Deformation

Due to the formation of texture and the different CRSS values of different

deformation mechanisms, the mechanical properties of the materials depend not

only on the stress direction but also on the stress sign (tension or compression)

[19]. Consequently, qualitative explanations for the observed mechanical

anisotropy of textured wrought magnesium alloys could be given.

2.1.3.1 Texture Effect on Deformation Mechanism

Yi et al. [43] described the anisotropic mechanical behaviour of extruded

AZ31 magnesium alloy in relation to the crystallographic texture, by means of

in-situ texture measurement and viscoplastic self-consistent simulation. The

activity of the basal < a > slip and the extension twinning exert a significant

effect on the mechanical anisotropy during tension while the importance of the <

c+a > slip increases during compression. Cylindrical specimens of a hot-rolled

AZ31 Mg alloy under uniaxial compression tests, in both the rolling and the

normal direction, revealed that texture evolution, work hardening and

macroscopic anisotropy are strongly dependent on the loading direction [27].

The relative position of the rolling direction and loading axes is shown in Figure

2-5. An analysis of texture evolution using the viscoplastic self-consistent

polycrystal model revealed that the tensile twin largely accommodates the

deformation at a low strain level for CA/RD samples. However, as the true

strain increases, the activation of the pyramidal < c+a > slip accommodates the

deformation. On the other hand, the pyramidal < c+a > slip as a primary mode

and the basal < a > slip as a secondary mode accommodate the deformation for

CA/ND direction.

2.1.3.2 TensionCompression Yield Asymmetry

The presence of deformation texture in magnesium alloys often results in

relatively low compression yield stress (only 1/2 or 3/4 of the tension yield

stress) along extrusion or rolling direction [44]; this is referred to as tension-

compression yield asymmetry.


16

The ratio between Compression Yield Strength (CYS) and Tensile Yield

Strength (TYS), CYS/TYS, is used to measure the extent of tension-

compression yield asymmetry. With different tilting angles between loading axis

relative to the extrusion direction, deformation anisotropy in extruded Mg-6%

Al- 1% Zn alloy with basal texture has been observed [45]. Basal slip and {1012} twins were inhibited when the extrusion direction was parallel to the stress axes,

resulting in high tensile (207 MPa) and compression (120 MPa) yield strength.

Furthermore, both the yield strengths reduced to ~95 MPa when the tilt angles

of extrusion direction relative to the loading direction increased from 0 to 90.

The orientation dependent deformation behaviour was much more significant in

tension than in compression.

Figure 2-5 True strain vs. true stress curves for specimens subjected to uniaxial

compression [27]

Uniaxial tensile and compression tests on extruded Mg3Al1Zn alloys at

room temperature indicated CYS/TYS dependence on the deviation angle, ,

between the loading direction and the extrusion direction [46]. As shown in

Figure 2-6, the climb of the CYS/TYS values from below 1 to 1.28 with


17

increasing from 0 to 90, shows remarkable dependence of macro-yield

properties on the load directions. Considering the fibre basal texture in this

extruded AZ31 Mg alloy, the geometry condition for twinning and the polarity

of twinning is fundamentally responsible for the tension-compression yield

asymmetry.

Figure 2-6 Tension and compression yield stresses and CYS/TYS at different load

angles for grain size of 8.9m ( is the angle between loading axes, i.e. longitudinal

axis of the specimen, and extrusion direction) [46]

Also, grain size has been shown to have certain influence on twinning

behaviour of magnesium alloys [40, 47, 48]. By conducting the uniaxial tension

and compression tests on Mg3Al1Zn alloy with various grain sizes prepared

by equal channel angular pressing, conventional extrusion and annealing, Wang

et al. [47] revealed that compression-tensile yielding asymmetry originated from

different number fractions of twinned grains. Barnett et al. [40] found the

percentage of grains in extruded AZ31 Mg alloy under compression tests

decreased with decreasing grain size. Therefore, with smaller grain sizes, we

should expect lower values of CYS/TYS. Then, the load direction and the grain


18

size were two important factors that determine the tension-compression yield

asymmetry of Mg alloys.

As seen in Figure 2-7, the yield asymmetry decreases with decreasing grain

size for extruded ZM21 Mg alloys, from ~ 0.8 to ~ 2 with grain size increased

from 8 m to 25 m [8]. The decrease of CYS/TYS was explained by the fact

that {1012} twinning is geometrically more favoured in compression than in tension, and by the fact that the activity of twinning decreases with decreasing

grain size.

Figure 2-7 Tensile and compressive yield strength vs. inverse square root of average

grain size for extruded bars from ZM21 [8]

It has been recently reported that alloys containing rare earth elements and/or

yttrium (RE/Y) can develop more random textures [49]. Soluble Zn and RE

additions (Ce or Y) were considered to be responsible for the modification of the

rolling texture of alloys ZK10, ZE10, ZEK100, ZEK410 and ZW41 from the

conventional texture observed in alloys AZ31 and ZM21. The basal pole

intensity aligned with the sheet normal direction was low, rather than high, in all


19

the high-Zn solute alloys. The addition of RE elements led to a weakening of the

texture. As a result of the weakened texture, a reduced tensioncompression

yield asymmetry was observed, in contrast to conventional alloys which had a

CYS/TYS value of ~2.

2.1.4 Twinning-Detwinning In Cyclic Loading

A compressive plastic pre-deformation along extrusion on AZ61 Mg alloys

up to pre-strain values of 1, 2 and 3% permanent compressive yields of 0.45,

1.40 and 2.35% before stopped. Then, those samples were subjected to tensile

tests following the pre-compressive deformation. It has been found that these

twinned regions (favourably formed {1012} twins) are capable of detwinning during reloading in tension [45]. The mechanical responses have concluded that

all the strain caused by twinning during compressive plastic pre-deformation

was regained by untwinning during tensile testing.

The intensity variations obtained in cyclic tension of extruded Mg alloys by

in-situ neutron measurement suggested the twinning-detwinning behaviour of

{101 1} twins [39], which yielded a contribution to the grain strain tensor consistent with the direction of straining. The reason that such twinning has not

been observed in polycrystalline samples by previous workers is that it appears

to be essentially elastic, in the sense that it disappears almost entirely upon

removal of the applied stress.

Twinning and detwinning activities were also detected in the cyclic

deformation process of extruded magnesium alloy AZ31 Mg alloys with a

strong basal fibre texture under plastic strain amplitude and stress amplitude-

control at room temperature [50]. During the cyclic loading, a large number of

twins formed under compressive loading from in-situ EBSD measurement of the

texture (see Figure 2-8 a). With the compression load decreasing, some twins

became narrower and shorter as indicated by the white and red arrows in Figure

2-8 b while the majority of twins remained almost unchanged at that stage. With

the transition from a compressive load to a tensile load, a significant amount of

twins became narrower and shorter or even disappeared, as indicated by arrows


20

in Figure 2-8 c. During unloading from tension, no obvious change in the

microstructure was observed, as shown in Figure 2-8 d. Both twinning and

detwinning were found to play an important role in the cyclic deformation

response of this material.

Wu et al. [51] confirmed the occurrence of alternative twinning and

detwinning behaviour in wrought ZK60A Mg alloys with the cyclic loading

using in situ neutron scattering; that is, most twins formed during compression

are removed via detwinning when the load is reversed.

Figure 2-8 EBSD map of in situ observation (a) compressive strain ~0.5% (b)

unloading from compression (c) loading reverses from compression to tension at tensile

strain ~0.7% (d) unloading from tension [50]


21

2.1.5 Properties of NC Mg-5Al Alloys

Light-weight materials such as Mg alloys have recently become of interest in

various engineering applications due to their high strength to weight ratio.

However, most Mg alloys suffer from poor plasticity due to their hcp structure.

There are reports of Mg-5Al alloys displaying interesting properties which are

mechanically alloyed at various mechanical milling durations.

Chua et al. [18] carried out tensile tests on bulk Mg-5Al alloys consolidated

from powders mechanically alloyed for different durations, with grain size

ranging from 13 m down to 45 nm. During the first 5 h (160 nm) and 10 h (93

nm) of milling, high yield strength was obtained due to possible strengthening

mechanisms such as solid solution and grain refinement, and particle dispersion

strengthening. For samples underwent 20 h (65 nm) and 30 h (45 nm) milling

durations, a low yield strength was observed while the previous strengthening

mechanisms appeared to be no longer effective during the plastic deformation.

Enhanced ductility with no work hardening behaviour was observed in the 30 h

milled sample.

Grain size effects on tensile behaviours were investigated on coarse-grained

and mechanical alloyed ultra-fine grained and nc Mg-5Al alloys [52]. The

results indicated that the yield strength and elongation of the coarse-grained Mg-

5Al alloys were independent of strain rate within the range of strain rates

between 3.33x10-5 s-1and 3.33x10-3 s-1. However, yield stress and elongation of

the mechanically alloyed nanostructured samples were strain rate dependent.

Lu et al. [53] studied the mechanical properties of bulk Mg-5Al alloys

synthesized by mechanical alloying (MA) and equal channel angular pressing

(ECAP) techniques. For mechanically alloyed samples, a dramatic increase in

yield strength to 498 MPa was achieved after 5 h milling. Strain softening

accompanied by increased ductility of the longer milled samples was observed

through the gradual decrease in yield strength. The lowest yield strength of 200

MPa was observed after 30 h milling while the elongation at fracture was

dramatically increased to 45%. The average grain size was about 75 nm for


22

specimens milled for 30 h. In ECAP, the accumulation of redundant shear strain

and dynamic recrystallisation refined the microstructure while increasing the

materials yield strength. Consequently, ductility was reduced.

2.2 Nanocrystalline Materials

The subject of nc materials has attracted both in-depth and broad-brush

research activities over the last couple of decades, with significant achievements

in their understandings [54-56]. The following sections summarize the literature

related to the topic of categories and synthesis of nc materials, mechanical

properties and discussions of deformation mechanisms of nc materials.

2.2.1 Categories and Synthesis

Nc materials can be classified depending on the number of dimensions in

which the material has nanometer modulations (Se

Thesis-Deformation Behaviours of Coarse-grained and Nanocrystalline Mg-5wt Al Alloys

Documents

Transcript of Thesis-Deformation Behaviours of Coarse-grained and Nanocrystalline Mg-5wt Al Alloys