Effects of Point Defects on the Monoclinic Angle of the B19" Phase in NiTi-Based Shape Memory Alloys

This study investigated the effects of a variety of point defects on the monoclinic angle of the B19 '' martensite in NiTi, including Ni and Ti vacancies, Ni-Ti anti-sites in the lattice, third element substitution, and Ni and Ti enrichments in the B2 stoichiometry. Their effects were studied by density functional theory calculations on the monoclinic B19 '' phase as a proxy for the experimentally observed B19 ' phase. It was found that vacancies and Ni-Ti anti-sites reduce the monoclinic angle of B19 '', whereas third element substitution of Ni and Ti may affect the monoclinic angle in both directions depending on the third element and the host element it replaces. Enriching Ni or Ti content to deviate from the B2 stoichiometry decreases the monoclinic angle. A change in the monoclinic angle of the B19 '' phase implies variation in the characteristic strains associated with the transformation. Thus, these findings may provide a new angle on possible interpretations of the variations of recoverable transformation strains experimentally observed of NiTi shape memory alloys.