Phase Stability and Transformations in Niti from Density Functional Theory Calculations

Abstract

We use density functional theory to characterize various crystalline phases that govern the response of NiTi alloys including shape memory and super elasticity. We also investigated possible transition pathways between the various phases and the energetics involved. We predict a new phase of NiT i, denoted B 1 9 ’’, which is involved in the transition between B 1 9 ’ (the experimentally obser ved mar tensite phase) and B CO (the theoretically predicted ground s tate). B 1 9 ’’ is monoclinic and can exhibit shape memor y. Fur thermore, its presence reduces the internal stress required to s tabilize the experimentally obser ved B 1 9 ’ phase. We also characterize the surface energy of the various phases and show how reducing the specimen size to the nanoscale affects the relative stability of the phases and, consequently, their mechanical response.