Thickness and grain size dependence of B2–R martensitic transformation behaviors in nanoscale TiNi films

• Size reduction induced phase transformation segregation between B2–R and R–B19′.
• R-phase starting temperature (Rs) exhibited an abnormal change with film thickness and grain size decreasing.
• Substrate-induced internal stress contibuted to the final increase of Rs.

The size dependence of phase transformation behaviors were investigated for nanoscale TiNi thin films. It was found for the first time that size reduction favored B2–R phase transformation and that R-phase starting temperature (Rs) exhibited abnormal change with film thickness and grain size decreasing. The influence of grain refinement and Ni-enrichment zone was dominant for thickness between 300 and 90 nm, and substrate-induced internal stress played crucial role as thickness were below 90 nm, resulting in the initial decrease and final increase of Rs.