The effect of ageing and thermal cycling on the shape memory properties of nickel-titanium

The objective of the work described in this thesis is to use the technique of Differential Scanning Calorimetry to gather information about the irreversible processes causing changes in transformation temperatures for an industrial equiatomic NiTi alloy due to the effects of processing variables. The variables examined are heat treatment, ageing, thermal cycling, and cold deformation. In general this work has shown that first, increasing the number of thermal cycles reduces the martensitic peak temperature and height and results in the appearance of a two step premartensitic transformation. Secondly, heating to at least 450°C for 1 minute was required to regain cycle 1 peak temperatures after thermal cycling. Thirdly, for samples aged at 800°C and above the shorter the time at temperature the lower were the transformation peaks. Fourthly, cold work lowers the transformation peak temperatures, height and value of AH, and the greater the amount of cold deformation the lower the values obtained. Fifthly, after cold work, a temperature of at least 600°C was found to be necessary to restore samples to an unstressed state.