Geometry and energy barrier of martensite in the initial stage martensitic transformation in B19’ TiNi shape memory alloy

This study addresses the selectivity of self-accommodation microstructures in the initial stage of martensitic transformation in B19’ TiNi. It is known that specific habit plane variant (HPV) pairs, including {111¯}mtype I twin planes that are called initial stage microstructure (ISM) in this study, are preferentially formed in the initial stage of martensitic transformations. To elucidate on the selectivity of the ISM, the energy barrier at the formation of the twin plane, which is the candidate for the starting point of HPV pair formation, and the geometry of the HPV pair are theoretically analyzed in this paper. Using geometrical non-linear theory analysis, the compatible twin plane which has exact twin orientation relationship can be formed in the specific five groups of HPV pair. Using ab-initio simulations and Eshelby theory analysis results, {111¯}mtype I twin is determined to have a relatively small energy barrier among the twins in TiNi. Therefore, this paper shows that the HPV pair having a twin plane with a small energy barrier of formation and a structure that can effectively reduce the elastic strain is preferentially formed as the ISM.