Tunable transformation temperatures in a shape memory alloy via free energy landscape engineering

Martensitic transformations in shape memory and superelastic materials are governed by the free energy landscape between austenite and martensite. This energy landscape can be modified by the incorporation of coherent second phases and leads to unique materials properties such as ultra-low stiffness, reduction in transformation temperature hysteresis, and enhanced functional fatigue. In this work, we will present a new shape memory alloy exhibiting tunable martensitic transformations temperatures between 80 and 150 ºC via careful annealing and precipitate growth. The design process will be presented, which initially relied on the theory of free energy landscape engineering combined with atomistic and thermodynamic calculations. Subsequent Ni-Ti-Hf-Al alloys were fabricated and characterized, and high-resolution transmission electron microscopy revealed the presence of Heusler precipitates. We will show how these precipitates alter the martensitic transformation behavior and enable tunability within a single composition. Design strategies for discovering new alloys with tunable transformation temperatures will be discussed.