分子動力学法を用いたNi-Ti合金に対する相変態の観察と構造解析

抄録

An SMA (shape memory alloy) is a functional material with a unique property known as the shape memory effect, or superelasticity. Microscopically, the mechanical actuation of this effect is based on reversible transformation between different crystal phases. To clarify the atomistic mechanism in SMA, we perform molecular dynamics simulation (MDS) of phase transformations for a Ni-Ti alloy, a typical alloy used for engineering SMAs. EAM potential, which can produce both B2 and B19' structures (stable crystal phases of Ni-Ti), is adopted to model the interatomic interaction of the nickel and titanium atomic systems. Two different-sized MDS models are constructed with unit cell structures of B2 phase. These structures are surrounded by free surfaces and constrained regions so as to be subjected to uniaxial tensile loading. A series of reversible processes likely to occur in SMA, including loading, unloading, heating, and cooling, is reproduced by corresponding stages of the MDS. Phase transformation is thought to be observable by detecting the local interatomic distances and bonding angles peculiar to B19' structure (martensite). During tensile loading simulations, the martensite crystal structure is always observed, having transformed from the B2 structure. At a large strain, models exhibit a rapid propagation of local lattice distortions accompanied by the formation of martensite. In unloading simulations, the larger model contains relatively less martensite than the smaller model. The discrepancy with regards to model size may depend on a different degree of constraint strength. Reverse transformation (from B19' to B2) occurs mainly in the subsequent heating stage, in which the stress is observed to increase under constant residual strain.