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Thermal Processing of Polycrystalline NiTi Shape Memory Alloys

Published online by Cambridge University Press:  01 February 2011

Carl P. Frick ,
Alicia M. Ortega ,
Jeff Tyber ,
Ken Gall ,
Hans J. Maier ,
A. El.M.  and
Yinong Liu
Carl P. Frick
Affiliation:
Mechanical Engineering, 427 UCB, University of Colorado, Boulder, Colorado, USA
Alicia M. Ortega
Affiliation:
Mechanical Engineering, 427 UCB, University of Colorado, Boulder, Colorado, USA
Jeff Tyber
Affiliation:
Mechanical Engineering, 427 UCB, University of Colorado, Boulder, Colorado, USA
Ken Gall
Affiliation:
Mechanical Engineering, 427 UCB, University of Colorado, Boulder, Colorado, USA
Hans J. Maier
Affiliation:
Lehrstuhl für Werkstoffkunde Universität Paderborn, Pohlweg 47–49, D-33098 Paderborn, Germany
Yinong Liu
Affiliation:
School of Mechanical Engineering, M050 University of Western Australia, 35 Stirling Highway, Crawley WA6009, Australia
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Abstract

The objective of this study is to examine the effect of heat treatment on polycrystalline Ti-50.9 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.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 855: Symposium W – Mechanically Active Materials , 2004 , W1.9
Copyright
Copyright © Materials Research Society 2005

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References

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