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Title:An advanced perspective on twin growth in nickel-titanium
Author(s):Ezaz, Tawhid
Director of Research:Sehitoglu, Huseyin
Doctoral Committee Chair(s):Sehitoglu, Huseyin
Doctoral Committee Member(s):Beaudoin, Armand J.; Johnson, Harley T.; Robertson, Ian M.
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Shape memory NiTi Austenite Martensite Ab-Initio Twin
Abstract:This work presents an energetic approach to investigate the attributes of twinning in Nickel-Titanium (NiTi) shape memory alloy. In particular, atomistic mechanisms of twin growth in two different phases of NiTi are characterized with barrier energy in this thesis. In addition, energetics of dislocation twin boundary interactions in a face centered cubic (fcc) metal (Copper) that contributes to strengthening is investigated. Density Functional Theory based ab initio calculation and Molecular dynamics (MD) correspond to the right length and time scale for these events, and utilized in this study. The first part of the work elucidates the details of twinning in martensite (B19’) and austenite (B2) as the major deformation mode in NiTi undergoing thermoelastic phase transformation. Combined shears, shuffles and interface shifts are operative in a complicated way to generate different twin modes in these two phases. Along with the growth mechanism, we report on generalized planar fault energy and generalized stacking fault energy barriers in NiTi shape memory alloys in the monoclinic martensite state and cubic austenitic state. Specifically, in martensitic regime, we report energy barriers for (001), (100) and type twin nucleation and growth with atomistic details of combined shear and shuffle. Our energetic approach successfully predicts the evolution of deformation twins in martensite that are observed in experimental studies. In addition, we investigate the (112) and (114) deformation twin formation mechanism in austenitic NiTi that provides enhanced ductility in this intermetallic compound. The entire potential energy surface (PES) and mean energy path (MEP) during twinning is an outcome of our simulations, providing the needed insight to the atomistic processes. We suggest that the results provide a quantitative methodology in development of new shape memory alloys where twinning can occur at stress levels far below that corresponding to plastic deformation. Further, an outstanding issue, detwinning in NiTi, has been addressed via first principles energetics calculations of twin nucleation and growth. Shape memory behavior of NiTi alloys directly depends on the detwinning or growth of twin variants in martensite. Our results based on ion relaxation and valence charge distribution point to a distinct energy barrier during detwinning process and the mechanism is mediated by a complex conjunction of shear and shuffle. In the second part of the work, the energetics of slip-coherent twin boundary (CTB) interactions are established under tensile deformation in fcc copper with molecular dynamics simulations, exploring the entire stereographic triangle. The CTBs serve as effective barriers in some crystal orientations, more than others, consistent with experimental observations. The resulting dislocation structures upon slip-twin reactions are identified in terms of Burgers vector analysis. Visualization of the dislocation transmission; lock formation; dislocation incorporation to twin boundaries; dislocation multiplication at the matrix-twin interface; and twin translation, growth, and contraction behaviors cover the most significant reactions that can physically occur providing a deeper understanding of the mechanical behavior of fcc alloys in the presence of twin boundaries. The results show a considerable contribution of shear stresses and the residual dislocation at the twin boundaries and point to its effect in strengthening of fcc metals.
Issue Date:2012-02-06
Rights Information:Copyright 2011 Tawhid Ezaz
Date Available in IDEALS:2012-02-06
Date Deposited:2011-12

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