The Influence of Alloying Additions on Diffusion and Strengthening of Magnesium

312 Pages·2015·9.59 MB·English

by  Catherine Carlisle Kammerer

THE INFLUENCE OF ALLOYING ADDITIONS ON DIFFUSION AND STRENGTHENING OF
MAGNESIUM

by

CATHERINE C. KAMMERER
B.S. Missouri University of Science and Technology, 1994
M.S. University of Central Florida, 2013

A dissertation submitted in partial fulfillment of the requirements
for the degree of Doctor of Philosophy
in the Department of Materials Science and Engineering
in the College of Engineering and Computer Science
at the University of Central Florida
Orlando, Florida

Summer Term
2015

Major Professor: Yongho Sohn
©2015 Catherine C. Kammerer
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ABSTRACT
Magnesium alloys are being developed as advanced materials for structural
applications where reduced weight is a primary motivator. Alloying can enhance the
properties of magnesium without significantly affecting its density. Essential to alloy
development, inclusive of processing parameters, is knowledge of thermodynamic, kinetic,
and mechanical behavior of the alloy and its constituents. Appreciable progress has been
made through conventional development processes, but to accelerate development of
suitable wrought Mg alloys, an integrated Materials Genomic approach must be taken
where thermodynamics and diffusion kinetic parameters form the basis of alloy design,
process development, and properties-driven applications.
The objective of this research effort is twofold: first, to codify the relationship
between diffusion behavior, crystal structure, and mechanical properties; second, to
provide fundamental data for the purpose of wrought Mg alloy development. Together, the
principal deliverable of this work is an advanced understanding of Mg systems. To that end,
the objective is accomplished through an aggregate of studies. The solid-to-solid diffusion
bonding technique is used to fabricate combinatorial samples of Mg-Al-Zn ternary and Mg-
Al, Mg-Zn, Mg-Y, Mg-Gd, and Mg-Nd binary systems. The combinatorial samples are
subjected to structural and compositional characterization via Scanning Electron
Microscopy with X-ray Energy Dispersive Spectroscopy, Electron Probe Microanalysis, and
analytical Transmission Electron Microscopy. Interdiffusion in binary Mg systems is
determined by Sauer-Freise and Boltzmann-Matano methods. Kirkaldy’s extension of the
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Boltzmann-Matano method, on the basis of Onsager’s formalism, is employed to quantify
the main- and cross-interdiffusion coefficients in ternary Mg solid solutions. Impurity
diffusion coefficients are determined by way of the Hall method. The intermetallic
compounds and solid solutions formed during diffusion bonding of the combinatorial
samples are subjected to nanoindentation tests, and the nominal and compositionally
dependent mechanical properties are extracted by the Oliver-Pharr method.
In addition to bolstering the scantly available experimental data and first-principles
computations, this work delivers several original contributions to the state of Mg alloy
knowledge. The influence of Zn concentration on Al impurity diffusion in binary Mg(Zn)
solid solution is quantified to impact both the pre-exponential factor and activation energy.
The main- and cross-interdiffusion coefficients in the ternary Mg solid solution of Mg-Al-Zn
are reported wherein the interdiffusion of Zn is shown to strongly influence the
interdiffusion of Mg and Al. A critical examination of rare earth element additions to Mg is
reported, and a new phase in thermodynamic equilibrium with Mg-solid solution is
identified in the Mg-Gd binary system. It is also demonstrated that Mg atoms move faster
than Y atoms. For the first time the mechanical properties of intermetallic compounds in
several binary Mg systems are quantified in terms of hardness and elastic modulus, and the
influence of solute concentration on solid solution strengthening in binary Mg alloys is
reported. The most significant and efficient solid solution strengthening is achieved by
alloying Mg with Gd. The Mg-Nd and Mg-Gd intermetallic compounds exhibited better room
temperature creep resistance than intermetallic compounds of Mg-Al. The correlation
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between the concentration dependence of mechanical properties and atomic diffusion is
deliberated in terms of electronic nature of the atomic structure.

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ACKNOWLEDGMENTS
I write this acknowledgement with the deepest gratitude for the many people who
have given me the opportunity to pursue this personal goal of earning my Ph.D. in Materials
Science and Engineering. First and foremost, I give thanks to my family and friends who
have endured my absence and given me never-ending support. It has been a long and
arduous journey, and I recognize the sacrifices they have made for me.
I am grateful for the guidance, generosity, and scholarship provided by Dr. Yongho
Sohn and my committee, Dr. K. Coffey, Dr. C. Suryanarayana, Dr. Y. Bai, and Dr. R. Blair.
Many thanks to each of you! Many thanks are owed to my research cohorts for their
comradery and friendship. It has gotten me through some of the biggest challenges, and
their sustained support has made my work possible. Special thanks are given to Le Zhou for
extensive TEM services, and to Felipe Betancor for enthusiastic metallographic work. Deep
appreciation for the friendship and support of Karen Glidewell is offered.
I also express my gratitude to Kyu Cho and Anit Giri at the U.S. Army Research
Laboratory (ARL). This work has been made possible by financial support of ARL under the
Advanced Materials Design and Innovative Processing Development project,
W911NF1120020.

“It’s good to want; it gives you goals and objectives.”
“It’s all relative.”
-- Catherine Carlisle Kammerer
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TABLE OF CONTENTS
LIST OF FIGURES........................................................................................................................................................ xii
LIST OF TABLES ........................................................................................................................................................ xix
LIST OF ACRONYMS............................................................................................................................................... xxvi
CHAPTER 1: INTRODUCTION ................................................................................................................................. 1
1.1 General Background ....................................................................................................................................... 1
1.2 Motivation ........................................................................................................................................................... 5
1.3 Objectives ............................................................................................................................................................ 7
CHAPTER 2: EXPERIMENTAL METHODS ......................................................................................................... 9
2.1 Combinatorial Sample Fabrication ......................................................................................................... 9
2.2 Interdiffusion Zone Characterization ................................................................................................. 14
2.3 Nanomechanical Testing ........................................................................................................................... 15
CHAPTER 3: ANALYTICAL FORMULAE ........................................................................................................... 18
3.1 Parabolic Growth and the Integrated Interdiffusion Coefficient ........................................... 18
3.2 Boltzmann-Matano Method ..................................................................................................................... 19
3.3 Sauer-Freise Method ................................................................................................................................... 20
3.4 Effective Interdiffusion .............................................................................................................................. 21
3.5 Hall Method ..................................................................................................................................................... 22
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3.6 Kirkaldy’s Extension ................................................................................................................................... 23
3.7 Ternary Interdiffusion Coefficient Transformation .................................................................... 26
3.8 Arrhenius Relationship .............................................................................................................................. 27
3.9 Oliver-Pharr Method ................................................................................................................................... 29
CHAPTER 4: INFLUENCE OF SOLUTE CONCENTRATION ON IMPURITY DIFFUSION ............. 33
4.1 Background ..................................................................................................................................................... 33
4.2 Experimental Parameters......................................................................................................................... 34
4.3 Results and Discussion .............................................................................................................................. 35
4.4 Summary ........................................................................................................................................................... 43
CHAPTER 5: INTERDIFFUSION IN TERNARY MAGNESIUM SOLID SOLUTIONS OF
ALUMINUM AND ZINC .......................................................................................................................... 45
5.1 Background ..................................................................................................................................................... 45
5.2 Experimental Parameters......................................................................................................................... 46
5.3 Results................................................................................................................................................................ 46
5.4 Discussion ........................................................................................................................................................ 56
5.5 Summary ........................................................................................................................................................... 58
CHAPTER 6: DIFFUSION KINETICS AND CRYSTALLOGRAPHIC CHARACTERIZATION OF
INTERMETALLIC COMPOUNDS IN THE MAGNESIUM-ZINC BINARY SYSTEM ........ 60
6.1 Background ..................................................................................................................................................... 60
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6.2 Experimental Parameters......................................................................................................................... 62
6.3 Phase Constituents and Crystallography .......................................................................................... 63
6.4 Diffusion Kinetics Characteristics ........................................................................................................ 71
6.5 Summary ........................................................................................................................................................... 74
CHAPTER 7: PHASE EQUILIBRIUM AND DIFFUSION IN MAGNESIUM-RARE EARTH BINARY
SYSTEMS...................................................................................................................................................... 76
7.1 Background ..................................................................................................................................................... 76
7.2 Experimental Parameters......................................................................................................................... 78
7.3 Mg-Y System .................................................................................................................................................... 79
7.3.1 Diffusion Zone Composition and Morphology......................................................... 79
7.3.2 Parabolic Growth Constants and Interdiffusion ..................................................... 83
7.3.3 Y Impurity Diffusion in Mg Solid Solution ................................................................. 90
7.4 Mg-Gd System ................................................................................................................................................. 93
7.4.1 Phase Diagram Examination ............................................................................................ 93
7.4.2 Crystal Structure .................................................................................................................... 98
7.4.3 Diffusion Kinetics ............................................................................................................... 104
7.5 Discussion ..................................................................................................................................................... 115
7.6 Summary ........................................................................................................................................................ 118
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CHAPTER 8: MECHANICAL PROPERTIES OF BINARY MAGNESIUM BASED INTERMETALLIC
COMPOUNDS AND MAGNESIUM SOLID SOLUTIONS ......................................................... 120
8.1 Introduction ................................................................................................................................................. 120
8.2 Hardness and Reduced Modulus in Mg-X (X = Al, Gd, Nd, Y, Zn) Intermetallic
Compounds ........................................................................................................................................................... 122
8.2.1 Composition Dependence of Mechanical Properties in Mg-X Intermetallic
Compounds (X = Gd, Nd, Y, Zn)............................................................................................................... 136
8.3 Solid Solution Strengthening in Mg(X) (X = Al, Gd, Y, Zn) ...................................................... 139
8.4 Summary ........................................................................................................................................................ 153
CHAPTER 9: COMPREHENSIVE SUMMARY ............................................................................................... 156
APPENDIX A: RECOMMENDED FUTURE STUDIES ................................................................................. 164
APPENDIX B: FUNDAMENTAL DATASETS ................................................................................................. 169
B.1 Mg-Al Binary System ............................................................................................................................... 170
B.2 Mg-Gd Binary System .............................................................................................................................. 176
B.3 Mg-Nd Binary System .............................................................................................................................. 211
B.4 Mg-Y Binary System ................................................................................................................................. 215
B.5 Mg-Zn Binary System .............................................................................................................................. 222
B.6 Mg-Al-Zn Ternary System ..................................................................................................................... 246
APPENDIX C: LIST OF PUBLICATIONS AND PRESENTATIONS ........................................................ 269
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