Molecular dynamic simulation on nano-indentation of NiTi SMA

Micro-indentation has been widely used for testing the mechanical properties of NiTi SMAs. In the present study, large-scale molecular dynamics (MD) simulations of nano-indentation on equiatomic NiTi SMA were performed with a 2NN MEAM potential. The temperature- and stress-induced martensitic transformation was simulated, where the phase transformation temperatures were determined and the critical stress for transformation was found to rise with the increase of temperature. The nano-indentation results show that a typical superelasitic recovery occurs during unloading, and the slope of P-L curves increases with the increase of temperature, which can be attributed to the increase of critical stress. The MD simulation results also show that the affect region of phase transformation beneath the indenter tip can be divided into elastic zone, twinned martensitic zone and detwinned martensite zone. Differently orientated martensitic variants appear during loading and most of them recover during unloading. In addition, the influencing region of phase transformation decreases with the increase of temperature and a theoretical explanation was suggested. The morphologies and profiles of indented surfaces were presented, which exhibit sink in during indent and turn into pile-up after unloading once severe plastic deformation occurs during indentation. The MD simulation results are consistent satisfactorily with the experimental results, implying that the present work could lead to a better understanding of the mechanical properties of equiatomic NiTi SMAs at nano-scale.