Microstructure–property relationships in a high-strength 51Ni–29Ti–20Hf shape memory alloy

NiTiHf alloys exhibit remarkable shape memory and pseudoelastic properties that are of fundamental interest to a growing number of industries. In this study, differential scanning calorimetry and isothermal compression tests have revealed that the 51Ni–29Ti–20Hf alloy has useful shape memory properties that include a wide range of transformation temperatures as well as highly stable pseudoelastic behavior. These properties are governed by short-term aging conditions, which may be tailored to control transformation temperatures while giving rise to exceptionally high austenite yield strengths which aid transformation stability. The yield strength of the austenite phase can reach 2.1 GPa by aging for 3 h at 500 °C, while aging for 3 h at 700 °C produced an alloy with an austenite finish temperature (A _f ) of 146 °C. High-resolution scanning transmission electron microscopy has revealed a new precipitate phase, H′-phase, under the homogenized and extruded conditions and under the 500 °C-3-h-aged condition, but only the previously identified H-phase precipitate was observed after aging at temperatures of 600 and 700 °C for 3 h. Finally, dislocation analysis indicated that plastic deformation of the austenite phase occurred by 〈100〉 type slip, similar to that observed in binary NiTi.