Unraveling the effect of grain size distribution on tension-compression asymmetry of polycrystalline NiTi shape memory alloys

The tension-compression asymmetry of nanograined NiTi shape memory alloy (SMA) under different loading modes is well recognized, yet how grain size distribution governs this asymmetry remains to be elucidated. Here, the influence of grain size distribution on the tension-compression asymmetry of nanograined NiTi SMA is systematically investigated using molecular dynamics simulations. In homogeneous nanograined NiTi SMA, the ratio of critical transformation stress under compression to that under tension is found to increase linearly with grain size, with larger grains leading to markedly greater differences in energy dissipation and residual strain between the two loading modes. For gradient nanograined NiTi SMA, an increase in the gradient rate is observed to effectively mitigate tension-compression asymmetry in terms of residual strain, energy dissipation, and martensite content, while concurrently endowing the material with lower residual strain, reduced energy dissipation, and enhanced martensitic transformation capability. These findings highlight the unique advantage of gradient structures in achieving synergistic optimization of mechanical properties. By elucidating the atomic-scale mechanisms governing the evolution of deformation modes such as grain boundary sliding and martensitic transformation with grain size and gradient rate, this study provides a theoretical foundation for tailoring the mechanical performance of NiTi-based smart materials through gradient microstructure design.