Phase field study on the temperature dependence of the shape memory effect and superelasticity of NiTi alloys with different grain sizes

Abstract

The shape memory effect (SME) and superelasticity (SE) of NiTi shape memory alloys (SMAs) were reported to be severely affected by grain size (GS) and temperature. The effect of temperature on the GS-dependent SME and SE of polycrystalline NiTi SMAs and underlying mechanisms have not yet been comprehensively understood. In this work, a new diffuse grain boundary energy related to the misorientation among adjacent grains is introduced into the phase field model. The effect of temperature on the GS-dependent SME and SE of nanocrystalline and polycrystalline NiTi SMAs is studied by systematic phase field simulations. It turns out that both the critical GSs for twinned martensite nucleation and the termination of twinned martensite propagation increase with increasing the temperature since the internal stress field (decreases with decreasing the GS) gradually becomes unable to break through the hindrance of both the grain boundary energy (increases with decreasing the GS) and the energy barrier (increases with increasing the temperature) on the martensite transformation in nanocrystalline NiTi SMAs. After the SME and SE processes, the residual strain in the alloys increases with increasing the GS because the (deformation incompatibility dependent) stress field is insufficient to prompt a complete reverse martensite transformation when the GS is relatively large. This phenomenon can be relieved by increasing the temperature (due to the reduced energy barrier).