Molecular dynamics simulations on nanocrystalline super-elastic NiTi shape memory alloy by addressing transformation ratchetting and its atomic mechanism

The transformation ratchetting of super-elastic nanocrystalline (NC) NiTi shape memory alloys (SMAs) under cyclic tension-unloading conditions and its deformation mechanism in atomic scale were studied by molecular dynamics (MD) simulations. The effects of ambient temperature, applied peak stress and stress rate on the transformation ratchetting of NC NiTi SMAs were discussed under an isothermal condition; in addition, the effect of temperature rise derived from transformation latent heat and inelastic dissipation on the transformation ratchetting of NC NiTi SMA was further investigated in an adiabatic case by setting different stress rates. The MD simulations show that the transformation ratchetting in NC NiTi SMA occurs in atomic level and depends on different thermal boundary conditions. Under an isothermal condition, the transformation ratchetting is caused by both the plastic deformation occurred at grain boundaries and in the disordered structures within the grains and the accumulation of residual B19' martensite phase; but, it is determined only by the plastic deformation at grain boundaries and in the disordered structures within the grains under an adiabatic condition. The MD simulations are verified further by qualitatively comparing them with corresponding experimental observations of NC NiTi SMA.