Severe Plastic Deformation of Nickel Aluminium Bronze Alloys for Marine Applications

329 Pages·2015·21.52 MB·English

by  Cameron Barr

Severe Plastic Deformation of
Nickel Aluminium Bronze Alloys for
Marine Applications

By Cameron James Barr

Submitted in total fulfilment of the requirements of the degree of
Doctor of Philosophy

January 2015

Department of Mechanical Engineering
The University of Melbourne
(Produced on Archival Quality Paper)

Abstract
Nickel aluminium bronze (NAB) is a high strength copper based alloy normally used
in seawater applications due to its strong resistance to general corrosion and biofouling.
Unfortunately, the formation of a nickel rich lamellar phase leads to major issues with
selective phase corrosion during service, requiring costly replacement of NAB components at
regular intervals. This study aims to refine these intermetallic phases as well as the matrix
grains through severe plastic deformation (SPD) with a view to eliminating the detrimental
lamellar morphology and significantly enhancing the mechanical properties.
SPD was conducted using equal channel angular pressing (ECAP) at various
temperatures. In addition to NAB, Fe-Al-bronze (FAB without nickel) and Al-bronze
(without nickel and iron) were included to investigate potential alternatives (with the nickel
based lamellae completely removed) to NAB, as well as the contributions of composition and
microstructure to the strength of copper based alloys.
Microstructures were characterised by a combination of SEM and EBSD, with
particular focuses on the evolution of the lamellar phase and the grain refinement process. A
detailed geometric model was developed to enable a quantitative and systematic study of the
effect of ECAP on lamellar transformation, supported by detailed image analysis of the size
and morphology of the refined lamellae. Route B was found most effective for
A
transforming the lamellae into very fine fragments, and the effects of ECAP routes were
discussed using a combination of the fibre loading and dislocation pileup models. The
morphologies of the transformed lamellae were influenced by temperature, changing from
fragments at 400°C to spheroids at 600°C. Grain refinement was also dependent on
temperature, and occurred through dynamic recrystallisation along deformed twin boundaries.
This process was slower in NAB due to interference from fine precipitates, and was effective
in all the routes except C. The grain sizes following ECAP and post ECAP heat treatment
have been modelled using a variety of techniques.
The large amount of microstructural data has allowed a systematic study of
strengthening contributions in both NAB and Al-bronze. The combination of fine grains and
highly refined lamellae in NAB results in significant increases in yield strength (as high as
950 MPa) while retaining good ductility, leading to the potential of the ECAP processed
NAB to compete with high strength steels currently used in similar applications.
i
Declaration

This is to certify that

(i) The thesis comprises only my original work towards the PhD except where indicated in
the Preface,

(ii) Due acknowledgement has been made in the text to all other material used,

(iii) The thesis is less than 100,000 words in length, exclusive of tables, maps, bibliographies
and appendices.

_______________________________
Cameron James Barr

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Acknowledgements
I would like to express my deepest and sincere thanks to all those who have made this work
possible. I particularly want to thank Prof. Kenong Xia for giving me this incredible
opportunity to pursue my fascination with materials, and for his ongoing support and
guidance through my PhD study. I also want to express my deepest gratitude to the Defence
Materials Technology Centre (DMTC), Mark Hodge the CEO and everyone involved in
project 2.2, for funding and supporting this study into Nickel Aluminium Bronze. I would
also like to thank Dr. Gang Sha and Dr. Patrick Trimby form the University of Sydney for the
extensive and high quality EBSD characterisation which has made up an integral part of this
work.

A special thanks to the members of our research group, both past and present, for enriching
my experience in learning, for long discussions about various research problems, and for all
the tips and tricks taught along the way. I would like to extend my thanks to Dr. Daniel
McDonald, Dr. Ryan Cottham, Dr. Wei Xu and Dr. Xiaolin Wu for their expertise and advice,
as well as to Edward, Shouqie, Morgan, Mattieu, Ben and Ahmad for their friendship and for
making the office an enjoyable place to be.

Finally, I would like to thank my dearest wife Jiebao, whose love and understanding has
supported me through every step of my Ph.D. It has been an amazing journey, and I look
forward to all that our future holds.

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Table of Contents
Abstract ................................................................................................ ii
Acknowledgements .............................................................................. iv
Table of Contents .................................................................................. v
List of Figures ........................................................................................v
List of Tables ..........................................................................................v
Chapter 1: Motivation ......................................................................... 1
Chapter 2: Critical Literature Review ................................................. 3
2.1 Al-Bronze Alloys ................................................................................... 3
2.1.1 Composition, Microstructure and Strength ................................................................. 3
2.1.2 Corrosion Performance ................................................................................................. 6
2.1.3 Improving the Corrosion Resistance of NAB................................................................ 8
2.1.4 Al-bronze Alloys – Summary ........................................................................................ 9
2.2 Ultrafine grained materials through
Severe Plastic Deformation ................................................................ 10
2.2.1 Synthesis of UFG an NS materials .............................................................................. 10
2.2.2 High Pressure Torsion ................................................................................................. 13
2.2.3 Equal Channel Angular Pressing ............................................................................... 15
2.2.4 ECAP + HPT ................................................................................................................ 17
2.2.5 Processing Parameters of ECAP ................................................................................. 18
2.2.5.1 Temperature and Strain Rate .......................................................................... 18
2.2.5.2 Processing Route ............................................................................................ 18
2.2.5.3 Backpressure ................................................................................................... 21
2.2.5.2 Lubricant ......................................................................................................... 21
2.2.6 Ultrafine grained materials through Severe Plastic Deformation – Summary ......... 22
2.3 Grain Refinement and Strengthening Mechanisms in
SPD Processed Single Phase Alloys .................................................... 22
2.3.1 Grain Refinement through Plastic Deformation ........................................................ 22
2.3.1.1 Dislocation Nucleation ................................................................................... 23
2.3.1.2 Formation of Low Energy Cell Structures ...................................................... 24
2.3.1.3 Dislocation Saturation and the Formation of new HAGBs ............................ 25
2.3.2 Effect of Stacking Fault Energy .................................................................................. 26
2.3.2.1 Twin Formation and Growth .......................................................................... 27
2.3.2.2 Grain Refinement in Low SFE Alloys ............................................................ 30
2.3.3 Effect of ECAP routes on Grain Refinement in FCC Materials ............................... 32
2.3.2.1 Texture Evolution after Large Strains through Simple Shear ........................ 33
2.3.2.2 Texture Evolution after 1 Pass of ECAP......................................................... 33
2.3.2.3 Texture Evolution after Multiple ECAP Passes .............................................. 33
2.3.2.4 Correlation between Texture and Grain Refinement ...................................... 34
2.3.4 Effect of Temperature on Grain Refinement .............................................................. 35
2.3.4.1 Influence of Temperature on SFE and Twinning ............................................ 35
2.3.4.2 Recovery and Dynamic Recrystallisation during SPD ................................... 36
2.3.4.3 Static Recrystallisation in SPD Processed Materials ..................................... 37
2.3.4.3 Summary ......................................................................................................... 37

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2.3.4 Strengthening and Mechanical Properties in Single Phase Alloys............................ 37
2.3.5.1 Solid Solution Strengthening........................................................................... 37
2.3.5.2 Solute Drag and the Effect of Temperature .................................................... 39
2.3.5.3 Grain Boundary Strengthening ....................................................................... 40
2.3.5.4 Solid Solution and the Hall Petch Relationship .............................................. 41
2.4 Multiphase Materials .......................................................................... 43
2.4.1 Secondary Phases ......................................................................................................... 43
2.4.1.1 The Limits of Solid Solution ............................................................................ 43
2.4.1.2 Particle Shape out of Precipitation................................................................. 45
2.4.2 Influence of Intermetallic Particles on Grain Refinement......................................... 46
2.4.2.1 Non-Deforming Particles ................................................................................ 46
2.4.2.2 Deforming Particles ........................................................................................ 47
2.4.2.3 Effect of Particles on Recrystallisation........................................................... 48
2.4.2.4 Effect of Particles on Recrystallisation........................................................... 48
2.4.3 Morphological Changes of Second Phase Particles during Heat Treatment ............ 49
2.4.3.1 Spheroidisation of Lamellar Structures .......................................................... 49
2.4.3.2 Accelerated Spheroidisation through Plastic Deformation ............................ 52
2.4.3.3 Coarsening of Spherical Precipitates ............................................................. 53
2.4.4 Refinement of Second Phase Particles through Plastic Deformation ....................... 53
2.4.4.1 Particle Size and Interfacial Strength ............................................................. 53
2.4.4.2 Precipitate Morphology and Orientation ....................................................... 54
2.4.4.3 Distribution of Second Phase Particles .......................................................... 56
2.4.4.4 Particle Dissolution during Plastic Deformation ........................................... 57
2.4.4.5 Rotation of Particles during Deformation ...................................................... 58
2.4.4.6 Lamellar Structures undergoing Plastic Deformation ................................... 61
2.4.4.7 Summary ......................................................................................................... 64
2.4.5 Strength Contribution from Second Phase Particles .................................................. 64
2.4.5.1 Rule of Mixtures .............................................................................................. 65
2.4.5.2 Discontinuous Fibre Models ........................................................................... 65
2.4.5.3 Dislocation Pileup Models .............................................................................. 66
2.4.5.3 Orowan Strengthening .................................................................................... 66
2.4.5 Multiphase Materials – Summary ............................................................................... 68
2.5 Concluding Remarks .......................................................................... 69
Chapter 3: Research Problems and Objectives ................................ 71
Chapter 4: Experimental Materials and Procedures ....................... 73
4.1 Starting Materials ............................................................................... 73
4.1.1 Aluminium Bronze (C61000) ....................................................................................... 73
4.1.2 Iron Aluminium Bronze ............................................................................................... 75
4.1.3 Nickel Aluminium Bronze ........................................................................................... 75
4.2 Processing ............................................................................................ 76
4.2.1 Closed Die Forging ...................................................................................................... 77
4.2.2 Equal Channel Angular Pressing (ECAP) ................................................................. 77
4.2.3 Heat Treatment ............................................................................................................. 80
4.3 Characterisation Techniques .............................................................. 80
4.3.1 Sample Preparation ...................................................................................................... 80
4.3.2 Optical Microscopy (OM)............................................................................................. 82
4.3.3 Scanning Electron Microscopy (SEM) ....................................................................... 82
4.3.4 Energy Dispersive X-Ray Spectroscopy (EDS) ........................................................... 82
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4.3.5 Electron Back-Scatter Diffraction (EBSD) ................................................................. 82
4.3.6 Electron Back-Scatter Diffraction (EBSD) ................................................................. 82
4.4 Mechanical Testing .............................................................................. 83
4.3.1 Hardness Measurements .............................................................................................. 83
4.3.2 Tensile Testing ............................................................................................................. 83
4.5 Quantitative Microstructural Analysis ................................................ 84
4.5.1 Characterisation of Precipitate Phase Evolution ........................................................ 84
4.5.1.1 Distribution of Particle Length and Aspect Ratio........................................... 84
4.5.1.2 Evolution of Precipitate Morphology ............................................................. 87
4.5.2 EBSD Measurements ................................................................................................... 88
4.5.1.1 Grain Sizes ...................................................................................................... 88
4.5.1.2 Misorientations ............................................................................................... 88
4.5.1.3 Texture Measurements .................................................................................... 89
Chapter 5: Experimental Results ...................................................... 91
5.1 Microstructures .................................................................................... 91
5.1.1 Al-Bronze ...................................................................................................................... 91
5.1.2 Fe-Al-Bronze (FAB) .................................................................................................... 93
5.1.3 Nickel Aluminium Bronze (NAB) ............................................................................... 95
5.1.3.1 As Received + Heat Treatment ....................................................................... 95
5.1.3.2 Forged NAB .................................................................................................... 97
5.1.3.3 After ECAP for One Pass at 400°C ................................................................ 98
5.1.3.4 After ECAP via Route A at 400°C ................................................................ 100
5.1.3.5 After ECAP via Route B and B at 400°C .................................................. 101
A C
5.1.3.6 After ECAP via Route C at 400°C ................................................................ 103
5.1.3.7 After ECAP at Temperatures > 400°C ......................................................... 105
5.1.3.8 After ECAP and Subsequent Heat Treatment ............................................... 111
5.2 EBSD Analysis on NAB..................................................................... 113
5.2.1 As-Received................................................................................................................. 113
5.2.2 After One Pass at 400°C ............................................................................................ 115
5.2.3 After 2 Passes via Route B at 400°C ....................................................................... 117
A
5.2.4 After 4 Passes via Routes A and B at 400°C ........................................................... 119
A
5.2.5 After 8 Passes via Route C at 400°C .......................................................................... 120
5.2.6 After 4 Passes via Route B at 450°C ....................................................................... 121
A
5.2.7 After 4 Passes via Routes B and B at 500°C ......................................................... 121
A C
5.2.8 After ECAP and Heat Treatment .............................................................................. 123
5.3 Mechanical Properties ....................................................................... 124
5.3.1 Hardness Testing ........................................................................................................ 124
5.3.2 Tensile Testing ........................................................................................................... 126
5.3.3.1 Al-Bronze ...................................................................................................... 126
5.3.3.2 Fe-Al Bronze ................................................................................................. 127
5.3.3.3 Nickel Aluminium Bronze ............................................................................. 128
5.3.3.4 Comparison between Materials .................................................................... 130
5.4 Fracture Surfaces .............................................................................. 131
Chapter 6: Quantitative Analyses ................................................... 135
6.1 Microstructural Analysis ................................................................... 135
6.1.1 Aspect Ratio Distribution of κ Fragments ............................................................. 135
III
6.1.2 Length Distribution .................................................................................................... 136
6.1.3 Evolution of the κ Morphology .............................................................................. 137
III
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6.1.4 Combined κ-phase size distribution ........................................................................... 138
6.2 EBSD Analyses .................................................................................. 140
6.2.1 Grain Size Analysis .................................................................................................... 141
6.2.1.1 Al-Bronze ...................................................................................................... 141
6.2.1.2 Fe-Al-Bronze ................................................................................................. 142
6.2.1.3 Nickel Aluminium Bronze ............................................................................. 144
6.2.2 Misorientation Distributions ...................................................................................... 150
6.2.2.1 Al-Bronze ...................................................................................................... 150
6.2.2.2 Fe-Al-Bronze ................................................................................................. 150
6.2.2.3 Fe-Al-Bronze ................................................................................................. 151
6.2.3 Texture Analysis ......................................................................................................... 155
6.2.3.1 Al-Bronze ...................................................................................................... 155
6.2.3.2 Fe-Al-Bronze ................................................................................................. 156
6.2.3.3 Nickel Aluminium Bronze ............................................................................. 156
Chapter 7: Quantitative Analyses ................................................... 161
7.1 Geometric Model for the Transformation
of the Lamellae during ECAP ........................................................... 161
7.2 Transformation of the Lamellar κ Phase ...................................... 166
III
7.3 Effects of ECAP routes on Lamellar Transformation ..................... 167
7.3.1 Route C ....................................................................................................................... 170
7.3.2 Route A ....................................................................................................................... 170
7.3.3 Route B and B ........................................................................................................ 171
A C
7.3.4 Experimental Variation from the Geometric Model ................................................. 173
7.4 Microstructural Implications ............................................................ 174
7.5 Effects of Different Lamellar Configurations .................................. 175
7.6 Summary ............................................................................................. 176
Chapter 8: Influence of Temperature on the Fragmentation,
Spheroidisation and Distribution of the Lamellae ...... 177
8.1 Influence of ECAP Temperature on the NAB Microstructure ....... 177
8.2 Modelling the Fragmentation of Lamellar Structures ..................... 180
8.2.1 The Fibre Loading Model – Theory .......................................................................... 181
8.2.2 The Fibre Loading Model – Implementation ............................................................ 182
8.2.3 Dislocation Pileup Model – Theory ........................................................................... 186
8.2.4 Dislocation Pileup Model – Implementation............................................................. 188
8.2.5 Combined Fibre Loading and Pileup Model ............................................................. 190
8.2.6 Relating Simulation Parameters to Processing Conditions ..................................... 192
8.2.6 Modelling the Fragmentation of Lamellar Structures – Summary ........................ 194
8.3 Enhanced Spheroidisation and Coarsening through ECAP ........... 194
8.3.1 Enhanced Matrix Contributions to Spheroidisation ................................................ 196
8.3.2 Effect of Deformed Lamellar Morphologies on Spheroidisation ............................. 196
8.3.3 Effect of Increased Fragment Size of Spheroid Morphology ................................... 199
8.3.4 Recrystallisation and Particle Migration .................................................................. 200
8.3.5 Enhanced Spheroidisation and Coarsening through ECAP – Summary ................ 203
8.4 Influence of Temperature and Strain on Homogeneity ................... 203
8.4.1 Temperature and Coarsening .................................................................................... 204
8.4.2 Total Strain and Strain Path ...................................................................................... 204
8.4.3 Recrystallisation and Grain Growth .......................................................................... 205
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8.3 Summary ............................................................................................. 206
Chapter 9: Grain Refinement in Al-Bronze and NAB during
ECAP at Elevated Temperatures ................................. 207
9.1 Grain Refinement Mechanisms in Al-bronze at
Elevated Temperatures ..................................................................... 207
9.1.1 Key Features in the Al-bronze Microstructure following ECAP ............................. 208
9.1.2 Combined Roles of Twinning and Recrystallisation in Grain Refinement ............. 210
9.1.3 Influence of Temperature on Grain Refinement ...................................................... 213
9.1.4 Grain Refinement Mechanisms in Al-bronze at
elevated temperatures – Summary ............................................................................ 214
9.2 Influence of Precipitate Phases on Grain Refinement in NAB ....... 215
9.2.1 Key Features in the NAB Microstructure following ECAP ..................................... 215
9.2.2 Effect of Precipitate Size on Grain Refinement Mechanisms .................................. 220
9.2.3 Conversion from Cell to Twin Formation ................................................................. 222
9.2.4 Grain Refinement in Lamellar Microstructures ....................................................... 223
9.2.5 Influence of Temperature on Grain Refinement in NAB ........................................ 225
9.2.6 Influence of Precipitate Phases on Grain Refinement in NAB -- Summary ........... 226
9.3 Effect of Route on Grain Refinement via Twin/Recrystallisation ... 227
9.3.1 Differences in Grain Structure between the ECAP Routes ...................................... 227
9.3.2 Effect of Changing Shear Plane on Twin Formation .............................................. 229
9.3.3 Effect of Changing Shear Planes on Detwinning and
Incoherent Twin Boundaries ..................................................................................... 232
9.3.4 Effect of Redundant Strain on Recrystallisation ...................................................... 232
9.3.5 Effect of Route on Grain Refinement via Twin/Recrystallisation – Summary ....... 232
9.4 Summary ............................................................................................. 234
Chapter 10: Modelling the Grain Size in NAB following ECAP
and in Subsequent Heat Treatment ........................... 235
10.1 Predicting Equilibrium Grain Size in ECAP at
Elevated Temperatures ...................................................................... 235
10.1.1 Recrystallised Grain Sizes in ECAP processed NAB ................................................ 236
10.1.2 Prediction of Grain Size through the Zener-Hollomon Parameter ......................... 238
10.2 Modelling Grain Growth in a System of Evolving Precipitates ....... 241
10.2.1 Observed Microstructure and Softening Behaviour ................................................. 241
10.2.2 Proposed Evolution of the Microstructure ................................................................ 242
10.2.3 Modelling the Limiting Grain Size in a Field of Mobile Precipitates ...................... 244
10.2.3 Modelling the Time Dependency of Grain Size due to Precipitate Coarsening ...... 248
10.3 Summary ............................................................................................. 254
Chapter 11: Correlation between Mechanical Properties and
Microstructure in Al-bronze and NAB ...................... 255
11.1 Theoretical Strength Contributions based on the
Hall Petch Relationship ..................................................................... 257
11.2 Decomposing the Individual Strengthening Contributions
in Al - bronze ...................................................................................... 258
11.3 Decomposing the Individual Strengthening Contributions
in NAB ................................................................................................ 259
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