Effect of Hf solute addition on the phase transformation behavior and hardness of a Ni-rich NiTi alloy

Hf substituted NiTi alloys are crucial for high temperature shape memory applications owing to their martensitic phase transformation temperatures around 100 degrees C. For further high temperature applications above this limit, a balanced combination of high phase transformation temperatures, high strength and low thermal hysteresis is the foremost criterion. Here, we show that substitution of an array of Hf concentrations (5-25 at.%) to Ni50.3Ti29.7 alloy leads to a wide range of phase transformation temperatures from sub-zero to 280 degrees C. Our studies establish complete solid solution solubility of Hf in the Ni50.3Ti29.7 alloy up to 25 at.% Hf. As a result, the hardness Ni50.3Ti29.7 alloy increases linearly with Hf concentration on account of solid solution strengthening effect. We further show that the Ni50.3Ti29.7Hf20 alloy exhibits least thermal hysteresis combined with high phase transformation temperatures and hardness amongst the compositions investigated. Our study paves the way for designing Ni-rich NiTiHf high temperature shape memory alloys, with operating temperatures up to 250 degrees C and thermal hysteresis as low as 30 degrees C.