The role of H-phase in thermal hysteresis and shape memory properties in Ni50Ti30Hf20alloy

Ni50Ti30Hf20 high temperature shape memory alloy was aged at 550 °C for 10 h and 450 °C for 50 h which corresponds to the peak hardening state with equal gain in strength caused by different size/volume fraction ratio of nano-precipitation of H-phase. In order to determine the effect of such structures on the martensitic transformation, thermal cycling and shape-memory properties were measured. It is shown that finer precipitates with higher volume fraction provide superior dimensional stability, good thermal cycling stability and small transformation hysteresis.