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The Influence of Hafnium Content, Cold Work, and Heat Treatment on the R-Phase Transformation of Niti Based Shape Memory Alloys

Published online by Cambridge University Press:  10 February 2011

Chen Zhang ,
Paul E. Thoma  and
Ralph H. Zee
Chen Zhang
Affiliation:
ME Dept., MTL Program, Auburn University, AL 36849
Paul E. Thoma
Affiliation:
Johnson Controls, Inc., Central Research, Milwaukee, WI 53201–0591
Ralph H. Zee
Affiliation:
ME Dept., MTL Program, Auburn University, AL 36849
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Abstract

The R-phase transformation of a Ti-rich NiTi shape memory alloy (SMA) and two ternary SMAs having the compositions Ni49Ti51-XHfx with lat% and 3at% Hf, has been investigated. The influence of cold work (CW) and heat treatment (HT) on the R-phase transformation is analyzed thermally using Differential Scanning Calorimetry (DSC). Results show that the R-phase transformation depends on the SMA composition as well as the CW and HT conditions in a complex manner. For example, the formation of R-phase upon cooling from austenite (A) is increasingly suppressed with the substitution of Hf for Ti. For the ternary SMA with 3at% Hf, the A→R and R→A transformations are observed only at relatively large amounts of CW (above approximately 40%) and at a high HT temperature (450°C). DSC results also show that for the Ti-rich NiTi and the ternary SMA containing lat% Hf, the A→R and R→A transformation temperatures (TTs) are insensitive to cold work at a HT temperature of 450°C. However, at a lower HT temperature of 350°C, the TTs are found to decrease with increasing CW. For a given CW, the A→R and R→A transformations decrease with decreasing HT temperature and the effect is greatest at high CW (>50%) conditions. An effort is made to identify the factors responsible for the observed behavior in the R-phase transformation.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 459: Symposium Y – Materials for Smart Systems II , 1996 , 281
Copyright
Copyright © Materials Research Society 1997

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References

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