On the microstructure and mechanical properties of a two-way shape memory NiTi/NiTiCu bi-layer diaphragm

Abstract

In this research, a NiTi/NiTiCu bi-layer composite consist of austenitic and martensitic layers was made by the diffusion bonding method, to make a diaphragm to show two-way shape memory actuation. The initial phase of the single sheets and the quality of the bonding after diffusion annealing were investigated using X-ray diffraction (XRD) and microstructural analyses, respectively. The effects of applied force on both sides of the diaphragm were studied using loading-unloading steps in the bending test. The observations implied the dependence of mechanical response on the loading mode; pseudo-elastic displacement and thermal recovery showed various figures when the loading mode changes. For both the loading modes, interestingly, the bi-layer showed two-way shape memory behavior in a diaphragmatic mode at a certain amount of force, without any complicated thermomechanical process and training and just by one cycle of loading-unloading. However, the results illustrated a more extensive range of total displacement in which the bi-layer showed two-way shape memory behavior when the load applied on the austenitic side. The stress and strain profile in the cross-section of the diaphragm was studied using finite element simulation. The results could be useful in the design of diaphragms with two-way shape memory behavior. Keywords: Shape memory alloys, Bi-layer diaphragm, FEM simulation, Diaphragmatic actuation, Two-way shape memory effect