On the temperature and stress dependence of transformation strain in single crystalline Cu-Al-Ni shape memory alloys

Abstract
In the austenite to martensite phase transformation of shape memory alloys the transformation strain, ε^tr, usually has definite dependence on the external fields while the change in entropy or volume can be considered constant. Under uniaxial loading this dependence of ε^tr is related to the change of the martensite variant distribution with increasing field parameters. The stress and temperature dependence of the elastic and dissipative energy contributions to the transformation can be correlated with the η dependence of ε^tr, where η is the volume fraction of the stress induced (single) variant martensite structure. Obviously, this dependence leads to deviation from the linear Clausius-Clapeyron relation too. In this paper we discuss and compare experimental results in single crystalline Cu17.9wt.%Al-2.6wt.%Ni and Cu-11.5wt%-Al5.0wt%Ni alloys obtained from stress-strain and strain-temperature hysteretic loops at different constant temperatures and stresses, respectively. Furthermore, it is shown that the field dependence of ε^tr is reflected in the field dependence of the dissipative and elastic energy terms too.