Phase transformation and microstructure of quaternary TiNiHfCu high temperature shape memory alloys

The effect of Cu addition on the phase transformation and microstructure of TiNiHf high temperature shape memory alloy has been studied. The experimental results show that the TiNiHfCu alloy undergoes a B2↔B19′ transformation with a concentration of 3 at.% Cu. And a two-step phase transformation occurs upon heating when the Cu content is 5 at.%. The constitutional phases of TiNiHfCu quaternary alloys are the matrix and (Ti,Hf,Cu) 2Ni particles. The substructure of martensite is mainly (001) compound twin in TiNiHfCu alloys. The martensite variants are (011) type I twin related. The phase transformation temperatures decrease rapidly during the initial several thermal cycles and then keep constant with further increasing of the thermal cycles. It should be noticed that the R-phase transition is separated from the martensitic transformation during the cooling process in the TiNiHfCu alloys. The underlying reasons have been discussed.