Influence of Elastic Energy on the Unloading Behaviour of NiTi Shape Memory Alloys

¹ Department of Mechanical and Materials Engineering, University of Western Australia, Nedlands, WA 6907, Australia
² Laboratoire Sols Solides Structures, Université Joseph Fourier, BP. 53X, 38041 Grenoble cedex, France

Abstract

A non-linear unloading path of deformed NiTi shape memory alloys was observed during mechanical testing at various temperatures, giving a strain recovery larger than what expected from elasticity. Whilst the nonlinear unloading path at above the A_s temperature may be rationalised by a simultaneous reverse transformation from the oriented martensite to austenite, the non-linear strain recovery during unloading at below the A_s temperature must have a different mechanism, because the thermodynamic driving force for a reverse transformation is negative. This second mechanism is ascribed to a 'self-deorientation' process of the fully oriented martensite and the driving force for this process is attributed to the elastic energy stored in the matrix during the forward deformation process.