PHASE FIELD SIMULATION ON THE SUPER-ELASTICITY AND SHAPE MEMORY EFFECT OF GRADIENT NANOCRYSTALLINE NiTi SHAPE MEMORY ALLOY

A two-dimensional phase field model was established to simulate and predict the super-elasticity, one-way and stress-assisted two-way shape memory effects of gradient nanocrystalline NiTi shape memory alloy system. The simulated results show that in the super-elastic process of gradient nanocrystalline NiTi alloy, the characteristics of martensite transformation and its reverse in the traditional coarse-grained NiTi alloy, i.e., nucleation-expansion and reduction-disappearance of local martensite band, are retained in the relatively coarse-grained region, but with the decrease of grain size, a homogeneous transformation mode, i.e., without the formation of local martensite band, is observed in the fine-grained region; moreover, in the super-elastic and shape memory processes, both the martensite transformation and reorientation originate from the relatively coarse-grained region and then propagate progressively to the fine-grained one, while the reverse transformation is opposite. The gradual propagations of martensite transformation and reorientation make the stress-strain and strain-temperature curves of gradient nanocrystalline NiTi alloy show a remarkable “hardening”, which can be attributed to the grain size-dependence of martensite transformation and re-orientation in nanocrystalline NiTi alloy, i.e., with decreasing the grain size, the transformation or re-orientation barrier increases gradually, and the nucleation and expansion of martensite transformation or re-orientation becomes more and more difficult. It is concluded that the gradient nanocrystalline structure has wider transformation stress, reorientation stress and transformation temperature windows than the traditional uniform-grained NiTi alloys, which means that the controllability of the inelastic deformation of such alloy is significantly improved.