Effect of heat treatments on thermally formed martensite phases in monocrystalline cu-ai-ni shape memory alloy

Abstract

The effect of heat treatments on martensitic transformations in monocrystalline Cu-14.1AI–4.2Ni (mass%) (alloy 1) and Cu-14.7AI–4.2Ni (mass%) (alloy 2) shape memory alloys has been investigated by means of differential scanning calorimetry measurements, optical microscopy and tensile tests of martensitic specimens. As a result, it was found in alloy 1 that not only transformation temperatures shifted but also thermally formed martensite changed from ß1, ’ to ϒ1, ’ with heat treatment. Meanwhile, in alloy 2, the ϒ1-, ’ martensite was thermally formed regardless of heat treatment. To clarify the reason for the change of thermally formed martensite, equilibrium temperatures, To, for the β1, ⇄ ϒ1’ and β1, ⇄ β1,’ transformations and their variation with heat treatment were estimated by utilizing previous data on stress-induced martensitic transformations and associated pseudoelasticity of the alloys (Trans. Jpn. Inst. Met, 28 (1987), 264). Consequently, by taking into consideration the variation of To temperatures and, in addition, supercooling neccessary for the nucleation of the ϒ1/ and ß1 martensites, the origin was successfully elucidated in terms of a simple scheme that the martensite phase with the highest Ms temperature changed from the β1, ’ to the ϒ1, ’ with heat treatment. Applicability of this scheme was discussed on similar phenomena occurred in other alloy systems in which martensite phases changed with chemical composition. © 1989, The Iron and Steel Institute of Japan. All rights reserved.