New aspects of martensite stabilization in Ni–Mn–Ga high-temperature shape memory alloy

Abstract

We report on new aspects of martensite stabilization in high-temperature shape memory alloys. We show that, due to the difference in activation energies among various structural defects, an incomplete stabilization of martensite can be realized. In material aged at high temperatures, this gives rise to a variety of unusual features which are found to occur in the martensitic transformation. Specifically, it is shown that both forward and reverse martensitic transformations in a Ni–Mn–Ga high-temperature shape memory alloy can occur in two steps. The observed abnormal behaviour is evidence that, in certain circumstances, thermoelastic martensitic transformation can be induced by diffusion.

Keywords:

• martensitic transformation
• martensite stabilization
• Heusler alloys
• shape memory alloys
• ageing

Acknowledgements

This work was partially supported by Grant-in-Aids for Scientific Research from the Ministry of Education, Science and Culture, Japan, and by the Russian Found for Basic Research (project Nos. 06-02-16266, 06-02-39030 and 07-02-13629).