Molecular dynamics simulations on one-way shape memory effect of nanocrystalline NiTi shape memory alloy and its cyclic degeneration

The one-way shape memory effect (OWSME) of nanocrystalline NiTi shape memory alloy (SMA) and its cyclic degeneration under thermo-mechanically coupled cyclic loading conditions were investigated by molecular dynamics (MD) simulations. Firstly, the critical temperatures of martensite transformation were obtained by carrying out the MD simulations of temperature-induced martensite transformation; then, the microscopic mechanisms of martensite reorientation-induced plasticity, the deformation during the subsequent heating and cooling, and the cyclic degeneration of OWSME were investigated based on the correspondent MD simulations. The results show that the martensite reorientation of nanocrystalline NiTi SMA occurs via the migration of the interfaces between martensite variants, the growth of martensite variants with favorable orientations and their swallowing to the variants with unfavorable orientations under specific mechanical loading conditions; the cyclic degeneration of OWSME in nanocrystalline NiTi SMA is caused by the interaction among the martensite reorientation, martensite transformation, martensite reorientation-induced plasticity and martensite transformationinduced plasticity occurred in the martensite region and at the interfaces between martensite variants. The martensite reorientation in each OWSME cycle is reflected by the migration of martensite variant interfaces and the growth and swallowing of martensite variants; the martensite transformation in each OWSME cycle is the nucleation and growth of martensite phase; and the accumulation of plastic deformation is caused by the migration of martensite variant interfaces and the dislocation slipping at the martensite variant interfaces which leads to the cyclic degeneration of OWSME. The MD simulations are verified further by comparing them with the available experimental results of NiTi SMA.