Functionally graded NiTi shape memory alloys

Abstract

In this study, in order to obtain a functionally graded material, NiTi strips were annealed at 350°C, 450°C and 550°C in a furnace using an assembly that allowed a temperature gradient along them, and their transformation temperatures were studied by Differential Scanning Calorimetry (DSC). Furthermore, the strips were bent at both ends and dipped into a water bath at room temperature which was then heated to 61°C in order to observe the influence of the gradient annealing on their strain recovery. It was found that the strips’ coolest regions presented the greatest strain recovery, particularly the strips annealed at 350°C and 450°C, although any strip exhibited a full strain recovery, due to plastic deformation during bending. These results, together with the DSC analysis at both regions (coolest and hottest), allow us to conclude that the graded annealing was successful for the intended functional gradient, as a gradient of transformation temperatures along the strips has been obtained, despite the primitive assembly, thus presenting an interesting result for a first approach. Further tests will be performed with a new experimental procedure especially designed for this purpose.