Effect of Heat Treatments on the Thermomechanical Behaviour of Ni-Ti Superelastic Mini Coil Springs

Abstract

Shape Memory Alloys are susceptible to annealing heat treatments, which are capable of partially or fully recover atomic mobility and, therefore, affect the overall thermomechanical response of the material. In this work, NiTi SMA orthodontic minicoil springs in superelastic state, widely commercialized, were submitted to annealing treatments as a way to modify their thermomechanical response and adapt it to the use in mechanical systems in other fields besides orthodontics. The main objective is to study the influence of temperature and time of annealing on the thermomechanical behaviour of the coil springs, originally superelastic at room temperature. Using a factorial design, three mechanical properties of interest were studied: spring constant, shear modulus and energy dissipation capacity. It was demonstrated that annealing in the range of 500°C-600°C is capable of converting superelastic springs to an apparently actuator state, as residual strain after loading/unloading at room temperature was observed, when a maximum 7% shear strain was attained in the cross section of the spring’s wire.