Vacuum brazing and heat treatment of NiTi shape memory alloys

Abstract

The pseudoelasticity of NiTi shape memory alloys is a unique material property which can be characterized by a complete recovery of a previously impressed component shape by a change of the thermal or mechanical load conditions after deforming. In contrast to the elastic deformation of ordinary materials like steels, twentyfold higher elastic strain rates up to 10 % are possible due to a temperature or a stress induced diffusion-free transformation of the crystal lattice between the austenite and martensite phases. Therefore, these superelastic alloys are frequently used as actuators, implants or stents so that there is an extraordinary high requirement of reliability and biocompatibility. In terms of joining, vacuum brazing might be a particularly suitable method to produce joined components which preserve a maximum of pseudoelasticity. Within the present research, it was shown that the vacuum brazing process at 1180 °C using pure niobium is well integrable into a solution annealing and a shape annealing heat treatment in a single furnace run. This led to a distinct tension plateau at around 285 MPa with an almost R-phase-free conversion of NiTi. Furthermore, it was proven that the share of the superelastic and proeutectic NiTiNb-phase was significantly increased with the dwell time.