Phase field simulation on the grain size dependent super-elasticity and shape memory effect of nanocrystalline NiTi shape memory alloys

Abstract

Based on the Ginzburg-Landau's theory, the free energy function of polycrystalline system was modified by introducing an extra grain boundary energy, and a new two-dimensional phase field model considering the continuous variation of temperature was proposed to investigate the grain size dependent super-elasticity (SE) and shape memory effect (SME) of nanocrystalline NiTi shape memory alloys (SMAs) and to reveal the microscopic mechanism of such a grain size dependence. The simulated results show that: in the SE process, the nucleation-expansion and reduction-disappearance mode of local martensite band observed in the nanocrystalline NiTi SMAs with relatively large grain size can be gradually converted to a uniform martensite transformation and its reverse in the ones with smaller grain size; in the process reflecting one-way shape memory effect (OWSME), with the reduction of grain size, the content of remained austenite phase in the martensitic polycrystalline system obtained by quenching an original austenite one increases, and the involved inelastic deformation mechanism during tension-unloading progressively changes from martensite reorientation to reversible martensite transformation; for the stress-assisted temperature-induced martensite transformation (SATIMT), the content of austenite phase in the polycrystalline system at the lowest applied temperature increases with decreasing the grain size, and the martensite transformation cannot occur when the grain size is below a certain critical one. Further analysis indicates that the dependence of the SE and SME of nanocrystalline NiTi alloys on the grain size can be attributed to the increased proportion of un-transformable grain boundary with decreasing the grain size, whose inhibition to the martensite transformation within grains becomes stronger and stronger, i.e., the energy barrier increases progressively.