Thermal processing of polycrystalline NiTi shape memory alloys
The objective of this study is to examine the effect of heat treatment on polycrystalline Ti-50.9 at.%Ni in hot-rolled and cold-drawn states. 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 the mechanical properties. Deformation processing of the NiTi consisted of hot-rolling, as well as, hot-rolling then cold-drawing. The high temperature of the hot-rolling process caused recrystallization, recovery, and hindered precipitate formation, essentially solutionizing the NiTi. The subsequent cold-drawing induced a high density of dislocations and martensite. Heat treatments were carried out on both materials 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 Ti 3Ni4 precipitates can be used to elicit a desired isothermal stress-strain behavior in polycrystalline NiTi. Copyright© 2005 ASM International.
Frick, C; Ortega, A; Tyber, J; Gall, K; Maier, HJ; Maksound, AEM; Liu, Y
Medical Device Materials Ii Proceedings of the Materials and Processes for Medical Devices Conference 2004
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International Standard Book Number 10 (ISBN-10)