Deformation Behavior of Heat-Treated Ni-Rich NiTi Shape Memory Alloy

While much is known about the shape memory and supereleasticity of equiatomic alloy of Ni and Ti (NiTiNoL), the deformation behavior of Ni-rich NiTi has attracted relatively some attention. The increase in nickel content, however, also hardens the alloy, which can make the alloy difficult to process. A slightly Ni-rich composition of NiTi can be, on the other hand, advantageous in applications where a higher stiffness of NiTi coupled with its shape memory and superelasticity is required e.g. in orthodontic wires, cardiovascular stent or pressure valves. In this study, we investigate the influence of heat treatment on the deformation behavior of superelastic nickel–titanium for biomedical applications. For this, NiTi alloy composed of 56 wt.% (51 at.%) nickel has been investigated after heat treatment within the thermal window of between 400 and 800°C. Heat treatment significantly influenced both the plasticity and the transformation behavior of Ni-rich NiTi. A detailed examination of the microstructural evolution, calorimetric response and tensile test response with respect to the superelasticity allowed us to establish protocols for obtaining nearly ideal superelastic properties in Ni-rich NiTi shape memory alloys. Our findings can enable use of these alloys in e.g. medical devices that require higher stiffness and a larger surface area.