Shape Memory Response of Polycrystalline NiTi12.5Hf Alloy: Transformation at Small Scales

Abstract The transformation behavior of NiTiHf alloys is intriguing. In NiTiHf alloys, the experimental transfor-mation strains have been reported to be considerably lower than theoretical transformation strains. In this study, the transformation strain is established with very careful strain measurements at small scales in isobaric and isothermal experiments. Because of the heterogeneity of strain distri-butions, the results depend on the sub-region considered. The measured local transformation strain can be as high as 6.0 % in compression which is in very good agreement with theoretical calculations for NiTi12.5Hf. The compre-hension of NiTi12.5Hf alloy was furthered upon extensive microstructural characterization including high-resolution electron microscopy, establishing the volume fractions of precipitates and twin type. The volume fraction of pre-cipitates is similar to that of Ni-rich binary NiTi alloys. Meanwhile, the twinning modes in the martensite are compound and Type I twins which were used in the the-oretical calculations of transformation strains. This mate-rial also generates a high work output and represents a foundation for understanding higher Hf compositions