Change in the Transformation Sequence due to Stress-Free Thermal Cycling of Ni-Ti Shape Memory Alloy

The stress-free thermal cycling with help of a differential scanning calorimeter (DSC) is executed through the transformation range of a fully annealed Ni-Ti alloy sample, continuously observing the variations in the calorimetric profiles and related thermal parameters. The influence of two different cooling rates right after annealing on the transformation properties is extra and separately examined. It is found that as the thermal cycle number increases, the transformation sequence changes into a two-stage behaviour on cooling, and the first visible symptoms related to the appearance of intermediate R-phase occur after 10^th full cycle. Apart from a progressive decrease of the critical temperatures, there is no DSC evidence for the two-stage reaction on heating within the whole thermal cycling test. As a result of the higher cooling rate after annealing, somewhat higher transformation temperatures are measured for the Ni-Ti alloy.