Atomic dynamics and energetics of martensitic transformation in nickel-titanium shape memory alloy

Microscopic mechanism of martensitic transformation in nickel (Ni)-titanium (Ti) alloy is investigated by molecular dynamics (MD) simulation using embedded atom method (EAM) potentials. The computational parallelepiped specimen with nano-size dimension is surrounded by Ti-terminated free surfaces and constrained regions for loading. The detection method of martensite phase is newly exploited. The crystalographic B19' monoclinic crystal form can be identified as martensite phase by checking up atomic lengths and angles of neighborhood and by comparing them with possible values of lattice parameters already proposed. In tensile loading, the specimen shows a kind of stress-induced transformation from parent phase (B2 structure) to martensite phase (B19' structure). Outbreak of martensitic transformation occurs immediately after stress reaches maximum value. In outbreak of martensite, distortion of unit structure is observed as an actual change in atomic coordinates. It is found that there are two major transformation paths both resulting in martensite structure, each of which has contrary sequence of changes in atomic length or angle. There is also the other route of atomic movement for completing martensitic transformation with relatively long-range atomic migration. The EAM potential used in the present study is discussed as to crystalline energies of periodic B2 or B19' unit structures. Dynamic energies in transformation are also obtained from MD results and they show that there are energy barriers in martensitic transforming. Static evaluation of energy, on the assumption of uniform transformation, is carried out and is compared with energy change obtained by MD method.