Effects of vacuum brazing on microstructure and pseudoelasticity of NiTi shape memory alloy brazements

Abstract

NiTi shape memory alloys exhibit pseudoelasticity, allowing them to fully recover from prior deformations under varying thermal or mechanical loads. Unlike conventional materials like steels, these alloys can endure elastic strain rates up to ten times higher, owing to a diffusion-free transformation between the austenite and martensite phases in their crystal lattice induced by temperature or stress. Given their material properties, NiTi shape memory alloys find applications as actuators, implants, and stents, demanding high reliability and biocompatibility standards. To maintain maximum pseudoelasticity when joining NiTi components, any microstructural changes have to be kept to a minimum. In this regard, vacuum brazing is a promising joining technique, as it is capable to produce joints at comparatively low joining temperatures without melting the base material. Hence, this paper is aimed at evaluating the influence of different holding times and brazing temperatures on the deformation behavior of NiTi alloys in vacuum brazing applying AgCuTi braze alloy. For this purpose, microstructural analyses by means of SEM and EDS as well as tensile tests were conducted. Furthermore, the fracture surfaces were analyzed by SEM. It could be observed that a stress plateau was present in the brazed samples leading to the assumption that brazing with AgCuTi is a suitable joining technique to preserve the properties of NiTi. Nevertheless, the brazed samples fractured in the stress plateau.