In situ EBSD studies of the selection of stress-induced martensitic variants in NiTi alloy

In situ EBSD investigations were conducted to polycrystalline NiTi alloy during tensile deformation to study the variation selection of martensitic transformation and its effect on pseudoelasticity. Experiments show that more than one martensite variants are formed in most grains, and the directions of traces for some variants are close to the direction of macroscopic Lüders band. Then the Schmid factors of activated variants in each grain were calculated, the results show that Schmid factors of activated shear systems can be divided into three categories, I: maximum Schmid factor; II: sub-maximum Schmid; III: small Schmid factor. The appearance of the latter two categories is mainly influenced by constraints of neighboring grains and variety of local stress state caused by Lüders band. And the Schmid factors of residual martensites belong to the third category. It is mainly because dislocations are easily introduced at the front of the Lüders band during the transformation process, which decrease the order degree of B2 phase and increase the difficulty of reverse phase transformation, and the sample shows poor pseudoelasticity.