Atomic Scale Study on the Tension-Compression Asymmetry of Single Crystalline and Nanocrystalline NiTi Shape Memory Alloy

Abstract

The tension-compression asymmetry of single crystalline and nanocrystalline NiTi shape memory alloys (SMAs) is investigated at atomic scale utilizing molecular dynamics (MD) simulation. This study is based on the second nearest neighbor modified embedded-atom method interatomic potential for Ni-Ti binary systems. First of all, multiple variants of martensite are simulated via temperature-induced martensitic transformation in single crystal and nanocrystalline. The characteristics of forward and reverse martensitic transformation is derived by means of the atomic structural evolution. Secondly, it is observed that the tension-compression asymmetry for single crystalline NiTi is attributed to types of stress induced martensitic variants and martensitic reorientation, which respectively leads to stress asymmetry and strain asymmetry. However, the phenomenological tension-compression asymmetry in nanocrystalline is mainly credited to stress-induced martensitic variants, no obvious martensitic reorientation occurs under tension and compression loads. The corresponding atomic-scale structural evolution also explains why the length of tensile stress plateau in nanocrystalline is shorter than that of the single crystal. Finally, the nanocrystalline tension-compression asymmetric behavior is simulated under a broad range of temperature to obtain the critical stress-temperature relation. The simulation results are compared with the corresponding results obtained by experiment reported in the literature. The tension-compression asymmetry mechanism of single crystalline and nanocrystalline NiTi shape memory alloy is derived based on numerical results at atomic scale.