Tension-Compression asymmetry of single-crystalline and nanocrystalline NiTi shape memory alloy: An atomic scale study

In this study, the tension-compression asymmetry of single-crystalline and nanocrystalline NiTi shape memory alloys (SMAs) was investigated by molecular dynamics (MD) simulations. Compound twinning martensite variants were simulated via thermally-induced martensitic transformation. The characteristics of the forward and reverse martensitic transformations were derived using atomic structural evolution. The tension-compression asymmetry of single-crystalline NiTi was attributed to different stress-induced martensitic variants and different deformation modes, which led to stress asymmetry and strain asymmetry, respectively. However, the phenomenological tension-compression asymmetry in nanocrystalline NiTi was mainly attributed to stress-induced martensitic variants, and no clear martensitic reorientation occurred under tension and compression loads. The corresponding atomic-scale structural evolution also explained the shorter tensile stress plateau in the nanocrystalline NiTi when compared to that in the single crystal samples. Moreover, the nanocrystalline tension-compression asymmetric behaviour was simulated under a broad temperature range to obtain the critical stress-temperature relation. Finally, the tension-compression asymmetry mechanisms of single-crystalline and nanocrystalline NiTi SMAs were numerically derived at the atomic scale.