Martensitic transformations in nanostructured nitinol: Finite element modeling of grain size and distribution effects

A computational model of martensitic phase transformation in nanostructured nitinol is developed which takes into account the grain size effect. On the basis of the theoretical analysis of the thermodynamic transformation criterion and the energy barrier for phase transformation, it was demonstrated that the energy barrier for martensitic phase transformation in nanocrystalline nitinol increase drastically with decreasing the grain size. Finite element simulations of phase transformations and structure evolution in nanocrystalline nitinol under mechanical (tensile) loading are carried out for different structures of the materials. It was observed that the volume content of martensitic phase decreases drastically with reducing the grain size. When the grain size is smaller than some critical value (around 50–80nm, both in our simulations and in experimental data), the martensitic phase transformation are totally suppressed. Graded and localized distributions of grain sizes of nitinol were compared with nitinol samples with homogeneous grain size distribution. In the materials with localized region of small grains, it was observed that the martensite rich regions form first on the border between the coarse and fine grained regions, and expand inside the region with small grains along the shear band direction.