Controlled Initiation and Propagation of Stress-Induced Martensitic Transformation in Functionally Graded NiTi

Abstract Near-equiatomic NiTi alloys are known to exhibit a thermoelastic B2–B19′ martensitic transformation, which gives rise to their unique properties of the shape memory effect and pseudoelastic effect. This transformation can be induced by the application of a stress. The stress-induced transformation nucleates locally and randomly within the body of the material, making the initiation and propagation of the transformation unpredictable. In addition, the propagation of the transformation occurs often in a Luders-like manner, which defines a typical condition of mechanical instability, thus making the deformation behaviour uncontrollable. To overcome these challenges, this study explored a design strategy to create geometrically graded NiTi components in which the stress-induced martensitic transformation can be controlled to initiate and propagate in a pre-designed direction within a wide stress or temperature window for improved controllability. This paper reports the design, experimental evaluation, and finite element simulation of several geometrically graded NiTi structures. The samples in series design configuration exhibited full controllability with complete stress gradient in deformation over the stress-induced transformation, whereas the samples in parallel design configuration exhibited a partially controllable stress window.