Effects of Sc addition and aging on microstructure and martensitic transformation of Ni-rich NiTiHfSc high temperature shape memory alloys

Abstract

In this study, the effects of Sc addition and aging on the microstructure and martensitic transformation of Ni50.3Ti29.7-xHf20Scx(x = 0, 0.5, 1, and 2 at.%) alloys were investigated. The addition of Sc resulted in the formation of Sc2O3 phase along with the martensite and (Ti,Hf)4Ni2Ox at room temperature. The Ni content of the matrix increased slightly with an increase in the Sc content up to 1 at.% and then decreased with a further increase in the Sc content to 2 at.%. The transformation temperature of the solution-treated NiTiHfSc alloys first increased at a rate of 52 °C/1 at.% Sc and then decreased at a rate of 42 °C/1 at.% Sc. The effect of aging at 550 °C for 3 h on the precipitation of the H-phase in the alloys depended on their Sc content. With an increase in the Sc content, the volume fraction of the H-phase first decreased and then increased attributing to the Ni content in the matrix and the evolution of the secondary phase. After aging, the characteristic temperatures of martensitic transformation were elevated as compared to those of solution-treated alloys because of the precipitation of the H-phase. With an increase in the Sc content up to 1 at.%, the increment in transformation temperature was first reduced and then increased with further increasing Sc content to 2 at.%. This can be attributed to the change in the H-phase volume fraction with the Sc content.