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 0 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 stressinduced transformation from parent phase (B2 structure) to martensite phase (B19 0 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 0 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.

Molecular dynamics simulation (MDS), using the embedded atom method for interatomic interactions, is performed to reveal the microscopic mechanism of stress-induced martensitic transformation of Ni–Ti alloys. Stress-induced martensitic transformation was observed for tensile simulation using four different strain rates. The relationship between stress and martensite ratio does not depend on the strain rate. Investigation of the results of MDS for the transformation pathway makes it clear that there are multiple pathways between the parent phase and the martensite phase regardless of the strain rate. Multiple pathways appear owing to differences in the relationship (angle) between the tensile direction and the specific lattice lengths of the parent or martensite unit cell. Increase and decrease in the martensite ratio during tensile simulation varies according to the pathways, depending on the strain rate.

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