A mechanical criterion for Lüders-type deformation of polycrystalline NiTi

NiTi shape memory alloys are known to exhibit a Lüders-type deformation behavior associated with its stress-induced martensitic transformation. Such phase-transformation-caused Lüders deformation presents a different metallurgical mechanism to the dislocation-related Lüders deformation of mild steels. To explain this phenomenon, many hypotheses have been proposed in the literature. However, all these explanations seem to contradict some experimental evidence in one way or another. In this work, a new explanation for the occurrence of the Lüders-type deformation in NiTi is proposed and a model is derived based on the criterion of conservation of sample length changes produced by the stress-induced transformation, the crosshead movement of the testing machine and the elastic contraction. The model is verified by tensile experiment and digital image correlation measurement. The new model is able to describe the upper-lower yielding phenomenon and the plateau stress of the Lüders-type deformation behavior.