Effect of grain size on the microstructure and mechanical anisotropy of stress-induced martensitic NiTi alloys

Stress-induced martensite always exhibits an oriented microstructure, which leads to mechanical anisotropy in NiTi alloys. In this study, the effects of grain size on the microstructure and mechanical anisotropy of stress-induced-martensitic NiTi were investigated. Nanocrystalline NiTi exhibited single-variant and twin-free nanograins; ultrafine-grained NiTi exhibited (111)m type І, (2‾01) deformation twins, (100) deformation twins and (001)m compound twins. Microstructural differences suggest that the nanocrystalline and ultrafine-grained alloys had different textures, grains were reoriented, and alloy deformed plastically upon indentation loading. Stress-induced nanocrystalline martensitic NiTi exhibited considerably stronger elastic-modulus anisotropy than ultrafine-grained NiTi when a lower indentation load was applied, whereas stress-induced ultrafine-grained martensitic NiTi exhibited stronger hardness anisotropy.