Ageing effects on phase transformations in NiTi alloys

Abstract
Phase transformation behaviour of Ti-rich and Ni-rich NiTi SMAs subjected to two rounds of heat treatments are compared. The first group of samples, received as straight-annealed, was subjected to a series of heat treatments in the temperature range from 200 to 900°C (designated in the text as 1st round of heat treatments). A second group of samples was subjected to a previous annealing at 900°C, followed by heat treatments in the temperature range from 200 to 900°C (designated in the text as 2nd round of heat treatments). Phase transformation temperatures are determined by electrical resistivity (ER) measurement in the temperature range from -140 to +130°C. It was observed that Ti-rich NiTi alloy, previously annealed at 900°C during the 1st round of heat treatments, does not show any change in transformation characteristics after the 2nd round of ageing treatments, whereas, for Nirich NiTi alloy, it was found that there is a difference in the transformation behaviour after the 2nd round of ageing treatments.