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

Abstract

The cooling rate of near-stoichiometric NiTi after annealing at 500° 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° C revealed progressively higher transformation temperatures and latent heats with ageing. These observations are associated with the evolution of a secondary firstorder transformation to the R-phase. Analysis of stress-strain data of near-stoichiometric NiTi helices, water quenched from its secondary anneal at 500° C, indicated that a Carnot efficiency of 16% can be expected, compared with 13.5% for the same material when furnace cooled.