A New Method for Brazing Nitinol Based on the Quasibinary TiNi-Nb System

Abstract

(Abstract) We lay out the metallurgical basis for a new and robust method for joining NiTi shape-memory and superelastic alloys. It is based on the use of pure niobium as a melting-point depressant that can induce contact melting with NiTi at tractable temperatures. A strong, ductile, corrosion-resistant, biocompatible braze joint is achieved without the use of fluxes and allows processing in normal industrial vacuum conditions. The discovery may have far-reaching implications for the use of NiTi in complex aerospace structures, and will allow expanded use of NiTi with with dissimilar materials, including ceramics. 3 in the form of globular bcc precipitates in a NiTi matrix. The purely mechanical behavior of these particles depresses the martensite start temperature (M s ), relative to the austenite start (A s ), and thus expands the service temperature range. Alloys with higher Nb content have been shown to solidify into eutectic microstructures 4 . More recent data 5-7 suggest that a quasibinary eutectic isopleth exists between the intermetallic compound TiNi and pure Nb. The eutectic isotherm is 140 K below the congruent melting temperature of NiTi, and the eutectic composition lies somewhere near Ni 38 Ti 36 Nb 26 . For this reason, spontaneous contact melting occurs when pure Nb comes in contact with NiTi at high temperature, and on solidification only two phases appear: austenitic NiTi and bcc-niobium. The large titanium fraction in the eutectic liquid makes it highly reactive. It readily wets NiTi surfaces, as well as many dissimilar materials including Al 2 O 3 . It flows readily in to capillary spaces and aggressively dissolves surface oxides, eliminating the need for fluxes. The absence of embrittling ternary intermetallic phases in the solidified braze is conducive to good mechanical properties We show that the solidified eutectic microstructure is in fact strong and ductile. Furthermore, with appropriate post-braze heat treatment, full shape-memory and superelastic functionality can be readily obtained in the joined NiTi sections. The technique should thus enable a wide variety of new designs that seek to exploit the special constitutive properties of nitinol in adaptive structures. In particular, new very-low-density forms of superelastic and shape-memory nitinol, such as honeycombs and space-frames, will be possible, as well as shape-