Effect of Hafnium Addition in 60NiTi Alloy Hardened Under Open Atmosphere Conditions

Vacuum furnaces or furnaces backfilled with inert argon gas are commonly employed to conduct hardening treatment on 60NiTi alloy. However, vacuum furnaces with the possibility of drop-quenching the solutionized parts are not easily available, and backfilling the furnaces with inert argon gas is costly. In this research, the hardening treatments were carried out in an air-exposed environment. As expected, reaction of Ti with oxygen gas existing in air during heat treatment resulted in formation of an oxide layer on the surface. This led to the depletion of Ti from the NiTi matrix and consequent formation of Ni-rich zones causing precipitation of soft Ni-rich phases. It was found that the addition of 1 at.% hafnium (Hf) prevented the soft Ni-rich phases formation in the matrix of the outer surface regions. This result suggests that Hf addition can be considered as a solution to harden a 60NiTi alloy under cost-effective open atmosphere heat treatment conditions.