A comparative study of the microstructure and shape memory behaviour of NiTi, NiTiHf, and Pt-added NiTiHf alloys

Abstract The microstructure, substructure, transformation temperature, and recovery ratio of a novel Ni 45 Pt 5 Ti 30 Hf 20 alloy are reported in the current work. The properties of binary TiNi and ternary TiNiHf alloys are also characterized to establish a comparative basis for shape memory behaviour. The microstructure of the cast structure of Ni 45 Pt 5 Ti 30 Hf 20 alloy showed a dendritic matrix and interdendritic eutectic-like mixture, and that of the homogenized structure consisted of martensitic matrix and secondary phase of dark contrast having the composition (Ti+Hf) 2 (Ni+Pt). Pt substitution of Ni in the ternary alloy with 20 at.% Hf by 5 at.% leads to a decrease in the A f , A s , M s , and M f . M s and M f represent the temperatures at which austenite to martensite transformation starts and completes, respectively, while A s and A f represent the temperatures at which martensite to austenite transformation starts and completes, respectively. The XRD studies showed that the structure of martensite is B19′ and TEM studies showed that the substructure of martensite in Pt modified NiTiHf alloy is similar to that of ternary NiTiHf alloy. The shape memory recovery of the Pt-modified alloy is similar to the ternary alloy at higher strength levels. The recovery ratio determined using Vickers indentation is compared with that determined using compression tests to provide an efficacy of small volume tests for screening of these alloys.