Effects of cooling rate on the shape memory effect thermodynamics of NiTi

The cooling rate of near-stoichiometric NiTi after annealing at 500 degree C is shown to have dramatic effects on the transformation thermodynamics. The slower the rate of cooling, the higher the transformation temperatures with less rhombohedral phase (R-phase) observed in room-temperature X-ray diffraction spectra. Given that this effect is due to the amount of time spent at intermediate temperatures, i.e an ageing effect, calorimetric analysis of specimens aged at 400 or 450 degree C revealed progressively higher transformation temperatures and latent heats with ageing. These observations are associated with the evolution of a secondary first-order transformation to the R-phase.