| The variations in thermal transformation properties due to annealing heat-treatment within the temperature range from 400œC to 600œC at regular temperature intervals of 50œC were studied for the near equiatomic Ni-Ti shape memory alloy by the Differential Scanning Calorimetry (DSC) measurements. It was found that thermal characteristics were sensitive to the processing temperatures, whereat there was a critical temperature, where the specimens demonstrated remarkably different transformation behaviour. In the as-received specimen and for annealing at 400 and 450œC, the two-stage transformations on both cooling and heating (A↔R↔M) were observed, although the reverse reactions appeared as not-well separated. The same sequence during cooling, but a one-stage transformation during heating (M→A) was detected when the annealing temperature reached 500 and 550œC. As the heat-treatment temperature was about 600œC, the phase transformation upon cooling and heating occurred in one-stage process, i.e. without revealing an intermediate R-phase. In addition to the changes of phase transformation sequences, there were noticeable variations in transition temperatures and heats depending on the annealing temperature. It is thought that the alterations of the microstructure in the Ni-Ti alloy during annealing are responsible for such behaviour. |
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