The effects of stress on the martensitic transformation in Ni–Al

Previous research on a Ni–37.5at%Al crystal has shown that there are significant differences between the thermal expansion behaviour measured along the three 〈100〉 directions. It was believed that an internal, defect strain field, as a result of the crystal production, served to determine its morphology on cooling it below the martensite start temperature (Ms). The current research has focused on the effects of an applied biaxial stress on the thermal strain measured along one of these directions. Measurements taken using capacitance dilatometry show that the applied stress has a dramatic effect on the thermal expansion of the crystal, significantly increasing both the thermal strain detected in the martensite phase and the pretransformation strain. The application of biaxial stress was found to modify the martensite morphology significantly. Extensive cracking was observed on the crystal surface as it was cooled through the martensitic transformation.