Grain size dependence of the elastic modulus in nanostructured NiTi

Nanostructured NiTi with a graded surface nanostructure was produced by surface mechanical attrition treatment (SMAT). Nanoindentation measurements revealed that the elastic modulus of SMAT samples increased significantly with decreasing distance from the SMAT surface and grain size, reaching ∼85 GPa for a grain size of ∼6 nm at the surface. This grain size dependence of the elastic modulus was found to be due to suppression of the stress-induced martensitic transformation in nanostructured NiTi.