Structural origin of reversible martensitic transformation and reversible twinning in NiTi shape memory alloy

Abstract

Much progress has been made over the past few decades in resolving the mechanism for the transformation from ordered body-centered-cubic (B2) austenite to monoclinic martensite (B19′) in NiTi shape memory alloy, as well as the twinning mechanisms in martensite. However, so far no lattice correspondence analyses on the atomic scale have been conducted, as a result, these mechanisms have not been completely understood in terms of the pathways for reversible phase transition and reversible twinning in martensite. In this work, atomistic simulations were performed to investigate how B2 austenite transforms to B19′ martensite and how twins are formed during phase transformation. In particular, lattice correspondence in the structural evolutions was carefully analyzed to reveal the martensitic transformation mechanism and twinning mechanism with much better clarity. The simulation results show that, when martensite grains were nucleated in austenite, they already formed a twin relationship as a natural result of the crystal structure. Thus, self-accommodation is achieved naturally. Coalescence occurred subsequently and some martensite grains grew at the expense of their neighbors, indicating that the interfaces are elastically mobile and their migration is reversible. Eventually twinned martensite variants were created. It is shown that, in the B2 → B19′ transformation, the B19′ monoclinic structure can be treated as a distorted hexagonal close-packed (HCP) structure. With this treatment, it is demonstrated that the martensitic transformation is essentially similar to BCC ↔ HCP transformation which only involves atomic shuffles and is reversible, and twinning in martensite is essentially similar to {101¯2} twinning in HCP metals which only involves atomic shuffles and is reversible as well. Another significant discovery is that the B19′ martensite exhibits a very special dual-lattice structure: the relatively weak Ti-Ti bonding of the monoclinic unit cell permits easy reorientation of lattice units. As a result, two differently oriented lattice units co-exist in the same martensite grain which loses long range periodicity, providing an extra degree of freedom for accommodating external and internal strains. These results explain well why B19′ martensite is extremely adaptive to straining and why deformation is reversible.