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Pushing The Limit to Achieve NiTi SMA Actuating Members that are Dimensionally Stable and have High Transformation Temperatures

Published online by Cambridge University Press:  25 February 2011

Paul E. Thoma ,
Alexander M. Blok  and
Ming-Yuan Kao
Paul E. Thoma
Affiliation:
Johnson Controls, Inc., Controls Research Dept., 507 East Michigan Street, Milwaukee, WI 53202, USA
Alexander M. Blok
Affiliation:
Johnson Controls, Inc., Controls Research Dept., 507 East Michigan Street, Milwaukee, WI 53202, USA
Ming-Yuan Kao
Affiliation:
Johnson Controls, Inc., Materials Research Dept., 5757 North Green Bay Avenue, Milwaukee WI 53209, USA
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Abstract

In binary NiTi shape memory alloys (SMA), the highest martensite (M) and austenite (A) transformation temperatures (TT) occur in the annealed condition. The highest TT also occur in near equiatomic NiTi alloys that have an excess of Ti. However, the NiTi alloy composition and condition that have the highest TT produce actuating elements that are generally short lived, have poor mechanical properties, and plastically deform (creep) under low stress levels. Cold working the SMA followed by a memory imparting stress relieving heat treatment (HT) produces actuating elements that are long lived, have good mechanical properties, and are resistant to creep under moderate stress levels. However, in obtaining these desirable properties through thermal-mechanical processing, the M and A TT are significantly decreased, which limits the upper ambient temperature in which the actuating element can operate.

A dimensionally stable actuating member with high TT can be achieved by thermal cycling (under stress) a NiTi SMA wire that has received prior thermal-mechanical processing. Cycling under an applied axial stress can increase the M TT of a SMA wire. Data showing the influence of thermal cycling on the TT of axially stressed SMA wires, that were cold drawn followed by HT at different memory imparting temperatures, are presented and discussed. For a NiTi SMA wire (A finish TT = 111 °C in the annealed condition) having approximately 40% cold reduction in area, 400°C for 1 hour memory imparting HT, and 10 Ksi axial stress, the M start TT (Ms) and A finish TT (Af) increase from 26°C and 79°C respectively after 10 thermal transformation cycles to 62 °C and 83.5 °C respectively after 10,000 thermal transformation cycles.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 246: Symposium M – Shape-Memory Materials and Phenomena--Fundamental Aspects and Applications , 1991 , 321
Copyright
Copyright © Materials Research Society 1992

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References

1. AbuJudom, D.N., Thoma, P.E., and Fariabi, S. (The order of names was changed to Fariabi, S., AbuJudom, D.N., and Thoma, P.E. by letter to the editor after the proceedings were published.) in Proceedings of the 6th International Conference on Martensitic Transformations'89), held in Sydney, Australia, July 3-7, 1989, edited by Muddle, B.C. (Materials Science Forum, Volumes 56–58, 1990, Part II-Trans Tech Publications, Ltd) pp. 565–570.Google Scholar
2. Thoma, P.E., AbuJudom, D.N., and Fariabi, S., U.S. Patent No. 4,881,981 (November 21, 1989).Google Scholar