The Development of Nickel-Titanium-Hafnium Superelastic Alloys for Biomedical Implants

NiTiHf-based shape memory alloys have been receiving considerable attention in aerospace, defense and biomedical applications due to their ability to recover large deformations at high temperatures and/or stress levels, high damping capacity and wear resistance. Precipitation strengthening (formation of coherent H phase precipitate structures) provides exceptional strength during transformation under isothermal (superelastic) and isobaric (actuation) uses, even without cold work. This behavior of NiTiHf alloys makes them a promising candidate for biomedical implants produced by additive manufacturing. However, they still need to be engineered in terms of strength, ductility and superelastic cyclic-behavior for secure employment. This study investigates superelastic behavior of low Hf content (<10 at.%) NiTiHf-based alloys by changing composition/aging treatments. It is suggested that variations in precipitate size, chemical composition of the matrix and morphology of precipitates through aging may affect critical stresses for martensitic transformation, hysteresis, ductility and dislocation slip.