Effect of Short-Time Annealing on Austenite Finish Temperature and Phase Transformation Characteristics of NiTi Wire

Manufacturing safe and reliable NiTi implants for minimal invasive application requires careful adjustment of the materials properties, e.g. the austenite finish (A_f) temperature and the phase transformation characteristics. The materials properties are likely to be affected during the production of NiTi implants, especially by the shape setting procedure that is routinely carried out at ~500°C for <30min. Whereas the impact of annealing treatments of 1h to 100h has been investigated extensively, little is known about the impact of short-time annealing treatments <30min. In the present investigation, the annealing parameters of NiTi wire samples were varied from 450°C to 600°C and 2min to 30min, respectively. The A_f temperature and the phase transformation characteristics were altered significantly. The A_f temperature varied in an interval of 40K. For the actual medical-grade NiTi wire with an initial A_f temperature close to 5°C, annealing for a time span as short as 2min at ~450°C resulted in a shift of the A_f temperature to above 30°C, exceeding the threshold for safe and reliable medical application. Accordingly, the shape setting of NiTi implants needs to be precisely controlled and adjusted for each new batch of material.