Functionally graded NiTi strips prepared by laser surface anneal

In this study, two-dimensional functionally graded NiTi thin plates were created by laser surface scanning anneal. Owing to the natural degradation of heat penetrating into the material, a temperature gradient within the thickness was created by laser surface interaction and heat conduction. A microstructural gradient was created due to partial annealing within the temperature field. The microstructural variation through the thickness was characterized by hardness measurement and layered differential scanning calorimetry analysis. The microstructural gradient led to a unique shape memory effect, involving shape change in two opposite directions upon one heating, analogous to stingray motion. Such behaviour also experiences enlarged temperature windows for both the forward and reverse martensitic transformations, rendering high controllability of actuation.