Design of functionally graded NiTi by heat treatment

A novel design of the functionally graded near-equiatomic NiTi shape memory alloys that exhibit expanded ranges of transformation temperature and stress is developed. The approach utilises the sensitivity of the alloys' thermomechanical properties with respect to heat treatment conditions. The functionally graded properties are achieved by anneal within a temperature gradient after cold work, thus to create a continuous structural variation. The optimum annealing temperature range for the Ti-50.5 at.% Ni alloy is determined to be 630-800K. Gradient-anneal of the alloy wire in this temperature range resulted in varying thermal transformation behaviour along its length and unique Luders-type deformation behaviour with a positive stress gradient for both the forward and the reverse transformations. Such behaviour enhances the functionality of the alloy for actuation applications.