Molecular dynamics simulation of mechanical behavior and phase transformation of nanocrystalline NiTi shape memory alloy with gradient structure

Abstract

Severe plastic deformation can lead to surface amorphization of nickel-titanium shape memory alloys (NiTi SMAs), and amorphous layer can be transformed into nanocrystalline layer by means of appropriate heat treatment. Consequently, molecular dynamics simulation is used to investigate mechanical behavior and phase transformation of nanocrystalline NiTi shape memory alloy with gradient structure, where surface layers possess the average grain sizes of 6, 9, 12 and 15 nm, respectively, and the middle layer possesses the average grain sizes of 20 nm. The maximum tensile stress and the irrecoverable strain decrease with the increase of the average grain size in the surface layers. As for the maximum tensile stress, the maximum value is 2.46 GPa and the minimum value is 1.92 GPa. As for the irrecoverable strain, the maximum value is 0.5%, and the minimum value is 0.25%. Amorphous phase and grain boundaries (GBs) have an important influence on the superelasticity of NiTi SMA. The degradation of superelasticity is mainly attributed to plastic deformation of the amorphous phase and the GBs.