Modeling and simulation of deformation pattern evolution during stress-induced martensite phase transition in NiTi microtubing

A number of thermoelastic theories have been established to interpret the phase transformation phenomena of shape memory alloys (SMAs) at the lattice level. The SMAs used in real applications are polycrystals such as NiTi microtubing in medical surgery and human implants. Recent experiments on the mechanical response of NiTi SMA in a thin-walled tube configuration revealed many interesting phenomena such as the dynamic formation, self-merging, topology transition and interface instability of a macroscopic domain. So far there is no theoretical model that can satisfactorily describe the above macroscopic level domain formation and evolution in polycrystalline materials. In this paper, we construct a continuum Ginzburg-Landau type free energy function based on the experimental data and the material symmetry (anisotropy) considerations of the NiTi polycrystal. The established theoretical model is implemented into finite element method (FEM) codes to simulate the martensitic domain formation and evolution in the tube under applied stress. The computational results captured many important features of the experimental observations.