A Diffraction Based Study of the Deformation Mechanisms In

$A Diffraction Based Study of the Deformation Mechanisms In$ A DIFFRACTION BASED STUDY OF THE DEFORMATION MECHANISMS IN ANOMALOUSLY DUCTILE B2 INTERMETALLICS A Dissertation Presented to The faculty of the School of Engineering and Applied Science University of Virginia In Partial Fulfillment of the requirements for the Degree Doctor of Philosophy (Materials Science and Engineering) by Rupalee Prashant Mulay December 2011 APPROVAL SHEET This dissertation is submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Materials Science and Engineering) This dissertation has been read and approved by the examining Committee: ~San~ R. Agn .. ~ uJZ d(i ~) &tb~ William A. Soffa on) Richard P. Gangloff J~~ . ~ Accepted for the School of Engineering and Applied Science: an, School of En Applied Science December, 2011 i Abstract For many decades, the brittle nature of most intermetallic compounds (e.g. NiAl) has been the limiting factor in their practical application. Many B2 (CsCl prototypical structure) intermetallics are known to exhibit slip on the <001>{110} slip mode, which provides only 3 independent slip systems and, hence, is unable to satisfy the von Mises (a.k.a. Taylor ) criterion for polycrystalline ductility. As a result, inherent polycrystalline ductility is unexpected. Recent discovery of a number of ductile B2 intermetallics has raised questions about possible violation of the von Mises criterion by these alloys. These ductile intermetallic compounds are MR (metal (M) combined with a rare earth metal or group IV refractory metal (R)) alloys and are stoichiometric, ordered compounds. Single crystal slip trace analyses have only identified the presence of <100>{011} or <100>{010} slip systems. More than 100 other B2 MR compounds are known to exist and many of them have already been shown to be ductile (e.g., CuY, AgY, CuDy, CoZr, CoTi, etc.). Furthermore, these alloys exhibit a large Bauschinger effect. The present work uses several diffraction based techniques including electron back scattered diffraction (EBSD), X-ray diffraction (XRD) and in-situ neutron diffraction; in conjunction with scanning electron microscopy (SEM), transmission electron microscopy (TEM), mechanical testing, and crystal plasticity modeling, to elucidate the reason for ductility in select B2 alloys, explore the spread of this ductility over the B2 family, and understand the Bauschinger effect in these alloys. Several possible explanations (e.g., slip of <111> dislocations, strong texture, phase transformations and twinning) for the anomalous ductility were explored. An X-ray ii diffraction based analysis ruled out texture, phase purity and departure from order as explanations for the anomalous ductility in MR alloys. In-situ neutron diffraction and post deformation SEM, EBSD, and TEM were unable to detect any evidence for phase transformations in CoTi and CoZr. Also, post deformation characterization did not reveal any evidence of twinning. However, TEM based g·b analysis and EBSD based in-grain misorientation axis (IGMA) analysis showed that beyond a transition in the strain hardening behavior in CoTi, slip modes involving dislocations with <110> and <111> Burgers vectors are activated. The slip of such dislocations can reduce stress concentrations that could otherwise lead to premature fracture, thus providing a satisfying explanation for the anomalous ductility of CoTi and related compounds, like CoZr. Dislocation self-energy calculations accounting for elastic anisotropy suggest that the choice of slip direction in these alloys is mobility-, rather than source-, limited. The reach of this “ductilizing effect” over B2 alloys was explored by producing, characterizing, and testing a number of simple metal-rare earth metal compounds, namely MgY, MgNd and MgCe. MgR intermetallics with the B2 structure were found to be brittle and exhibit a cleavage type fracture indicating that the ductilizing effect is not as widespread as was initially thought. MgY and MgNd were found to primarily cleave along the {100} planes, while MgCe was found to cleave on the {111} planes. A large Bauschinger effect was observed in several of the anomalously ductile B2 compounds, such that the material actually begins to yield in the reverse direction on unloading. When only the primary slip mode <100>{011} is active in CoZr (prior to a transition in strain hardening), the buildup of intergranular stresses is large and is chiefly iii responsible for the observed Bauschinger effect. However, past the aforementioned transition in strain hardening, the effect of intergranular stresses diminishes. The results demonstrate that the activation of hard, secondary slip modes causes the internal strains to develop more uniformly among the grains, thus reducing the intergranular stresses which cause the Bauschinger effect. Crystal plasticity modeling, which accounts for the initial paucity of independent slip modes and allows for the activation of complementary hard slip modes, reproduces these trends in the Bauschinger effect and provides additional evidence that the experimental observations have correctly identified the cause of the anomalous ductility. iv Collaborative Contribution Each chapter from 2-5 constitutes an individual journal paper. Chapters 2 and 5 contain authors other than the dissertation author R.P. Mulay and advisor S.R. Agnew. Chapter 2 has four additional authors. J.A. Wollmershauser helped with the polycrystal modeling, M.A. Heisel was an undergraduate student in the group who carried out some of the X-ray diffraction scans, H. Bei provided figure 2.5 and A.M. Russell provided some of the alloys used. Chapter 5 has one additional author, B. Clausen, who was the instrument scientist at Los Alamos National Laboratory (LANL), where the in-situ neutron diffraction experiments were performed. v Acknowledgements I would like to express my sincere thanks to my advisor Prof. Sean Agnew for his guidance and support over the past four years. His zest for science and teaching is infectious and he has always helped me put forth my best effort. I would also like to thank Prof. William Soffa for many helpful discussions. I am grateful to my colleagues Drs. James Wollmershauser, Vipul Gupta and YunJo Ro for many constructive discussions and assistance with using equipment. I am also thankful to John Polesak, Dr. Jason Hadorn, Chris Amaro and Dr. Babak Raeisinia. My gratitude also goes out to my friends and colleagues in the department for a great working atmosphere. I would also like to thank my committee members, Professors Sean Agnew, William Soffa, Richard Gangloff, Jerry Floro and Marek-Jerzy Pindera for their constructive comments and help. I am grateful to Richard White (nano-scale materials characterization facility) for training me to use various microscopes and equipment and NSF CAREER Grant DMR- 0547981 for funding this research. Finally, a big thank you to my family and friends, particularly my parents and my brother. I can always count on them for their support and guidance. vi Table of Contents Abstract ............................................................................................................................... i Collaborative Contribution ............................................................................................. iv Acknowledgements ........................................................................................................... v Table of Contents ............................................................................................................. vi List of Figures ................................................................................................................... ix List of Tables ................................................................................................................... xii List of Symbols ............................................................................................................... xiii 1.Introduction .................................................................................................................... 1 1.1 Driving force for research .................................................................................... 1 1.2 Background .......................................................................................................... 2 1.2.1 Archetypal behavior in the B2 family at room temperature ....................... 2 1.2.2 Anomalously ductile B2 intermetallics ....................................................... 3 1.2.3 The Bauschinger effect ............................................................................... 8 1.3 Dissertation Objectives ...................................................................................... 12 1.4 Dissertation format ............................................................................................. 12 1.5 References .............................................................................................................. 13 2. X-ray diffraction study of the phase purity, order, and texture of ductile B2 intermetallics ................................................................................................................... 18 2.1 Abstract ................................................................................................................... 18 2.2. Introduction ............................................................................................................ 19 2.3 Experimental Procedures .......................................................................................

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