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

The transformation behavior of NiTiHf alloys is intriguing. In NiTiHf alloys, the experimental transformation 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 distributions, 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 comprehension 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 precipitates 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 theoretical calculations of transformation strains. This material also generates a high work output and represents a foundation for understanding higher Hf compositions.