Superelasticity in a New BioImplant Material: Ni-rich 55NiTi Alloy

With the drive towards minimally invasive procedures, the medical industry is looking towards ‘avant-garde’ materials, with 50NiTi currently being the prime choice for many critical components/applications. This paper examines a new Ni-rich NiTi alloy that exhibits superelasticity (SE) and shape memory (SM) properties. Superelastic (SE) properties of 55NiTi* [all compositions are quoted in atomic% throughout the paper. The reader should note the following conversions: 50NiTi (at.%)≈55NiTi (wt.%) and 55NiTi (at.%)≈60NiTi (wt.%)] are studied here as a function of heat-treatment between 400–800°C, and compared with the corresponding response of 50NiTi*, with an aim to develop and optimize thermal treatment procedures to maximize recoverable elastic strains. While optimal tuning of the SE properties in 50NiTi necessitates cold working in conjunction with specific heat treatment/aging, 55NiTi does not require cold work to achieve its optimal SE behavior. Moreover, it can be heat treated to produce strong, stable SE and SM response from the same ingot, with transformation temperatures being a strong function of heat treatment. The main difference between the two alloys is that Ni–Ti alloys with Ni content greater than 50.6 at.% are sensitive to heat treatment; aging in these materials leads to precipitation of several metastable phases. The initial work focuses on SE properties relevant to biomedical use, such as: plateau stress, recoverable strains and strength, as a function of heat treatment and microstructure.