Thermal Processing of Polycrystalline NiTi Shape Memory Alloys
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Thermal Processing of Polycrystalline NiTi Shape Memory Alloys Carl P. Frick, Alicia M. Ortega, Jeff Tyber, Ken Gall, Hans J. Maier1, A.El.M. Maksound1, and Yinong Liu2 Mechanical Engineering, 427 UCB University of Colorado, Boulder, Colorado, USA 1 Lehrstuhl für Werkstoffkunde Universität Paderborn, Pohlweg 47-49, D-33098 Paderborn, Germany 2 School of Mechanical Engineering, M050 University of Western Australia, 35 Stirling Highway, Crawley WA6009, Australia ABSTRACT The objective of this study is to examine the effect of heat treatment on polycrystalline Ti50.9 at.%Ni subsequent to hot-rolling. In particular we examine microstructure, transformation temperatures and mechanical behavior of deformation processed NiTi. The results constitute a fundamental understanding of the effect of heat treatment on thermal/stress induced martensite, which is critical for optimizing mechanical properties. The high temperature of the hot-rolling process caused recrystallization, recovery, and hindered precipitate formation, essentially solutionizing the NiTi. Subsequent heat treatments were carried out at various temperatures for 1.5 hours. Transmission Electron Microscopy (TEM) observations revealed that Ti3Ni4 precipitates progressively increased in size and changed their interface with the matrix from being coherent to incoherent with increasing heat treatment temperature. Accompanying the changes in precipitate size and interface coherency, transformation temperatures were observed to systematically shift, leading to the occurrence of the R-phase and multiple-stage transformations. Room temperature stress-strain tests illustrated a variety of mechanical responses for the various heat treatments, from pseudoelasticity to shape memory. The changes in stress-strain behavior are interpreted in terms of shifts in the primary martensite transformation temperatures, rather then the occurrence of the R-phase transformation. The results confirm that Ti3Ni4 precipitates can be used to elicit a desired isothermal stress-strain behavior in polycrystalline NiTi. INTRODUCTION Shape memory alloys, such as Nickel-Titanium (NiTi), demonstrate a unique ability to recover their initial shape after deformation through a reversible thermo-elastic phase transformation. Specifically, NiTi can restructure itself from a B2 austenite phase (A), to a B19’ martensite phase (M) through a decrease in temperature or an increase in applied stress. It is this solid-state phase transformation that allows NiTi to regain large strains, either spontaneously (pseudoelasticity) or through an increase in temperature (shape memory effect). The purpose of this study is to investigate the effect of various heat treatments on hot-rolled NiTi. Since this deformation process is typical for NiTi, understanding its influence on the structure, transformation temperatures and mechanical behavior is critical. Consequently, isothermal stress-strain and differential scanning calorimeter (DSC) curves were analyzed for all representative heat treatments of the hot-rolled
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