High temperature shape memory behavior of Ni_47.3Ti_29.7Hf₂₀Pd₃ alloys

The shape memory properties of Ni_47.3Ti_29.7Hf₂₀Pd₃ alloys were investigated through stress-free thermal cycling, constant-stress thermal cycling and constant-temperature stress cycling experiments. The alloys showed reversible phase transformations above 180 °C, which makes them suitable for high temperature shape memory applications. Constant-stress thermal cycling experiments revealed that recoverable strains of about 2% were possible under compressive stress levels higher than 700 MPa and both the recoverable strain and thermal hysteresis values were strong functions of applied stress. Additionally, two-way shape memory strain of 0.7% was observed. Unlike most of the shape memory alloys that have Class I transformation, Class II transformation was observed in these alloys and its relationship to the elastic energy storage was discussed through thermo-elastic martensitic transformation thermodynamics approach.