Short Duration Heat Treatments before Aging Increase Mechanical Hysteresis of Pseudoelastic NiTi Alloy

High temperature (550 °C) and short duration (10, 15, 20 s) heat treatments were performed on pseudoelastic NiTi alloy in addition to the conventional aging process in order to increase the mechanical hysteresis of the material. Pre-treated and aged samples were compared with plain aged ones through mechanical testing, powder X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The introduction of the pre-treatment, in combination with the aging process, was shown to increase the mechanical hysteresis of the aged samples up to ~30% after stabilization. XRD analysis showed how the introduction of the pre-treatment introduces a recovery of the residual stresses in the material microstructure, as well as the nucleation of different sets of metastable precipitates (such as TiNi3). We thus observe that the combination of pre-treatment and aging affects the alloys microstructure, causing a series of effects that synergise in increasing the material hysteresis.