Lüders-like deformation associated with martensite reorientation in NiTi

Abstract

It is known that near-equiatomic NiTi shape memory alloys may exhibit a Lueders-like deformation behavior under a variety of testing conditions. These conditions include the tensile deformation associated with the stress-induced martensitic transformation from the austenite, the reverse transformation of the stress-induced martensite to austenite in pseudoelasticity, and the deformation in martensitic state via a martensite variant reorientation process. The Lueders-like deformation behavior is characterized by a stress plateau and a stress-drop at the beginning of the process for the forward transformation upon loading and a stress minimum for the reverse transformation on unloading. Based largely on this observation in pseudoelasticity, it has been suggested that the stress peaks at the beginning of the stress plateau are associated with the nucleation of the product phase for the corresponding transformations and that the stress plateau corresponds to the process of the transformations. This common belief, however, is challenged by a number of experimental observations. This seems to suggest that the occurrence of the stress plateau on a stress-strain curve is mechanical in nature instead of being determined by the transformation. This uncertainty, however, needs to be clarified.