Although NiTiNb alloys are well known as wide-hysteresis SMAs with important applications as coupling materials, the full significance of one aspect of the Ni-Ti-Nb ternary system has not been fully appreciated.  In their 1998 patent [US4770725] Duerig, Melton and Simpson identified quasi-binary eutectic microstructures in NiTiNb alloys that contained nearly pure NiTi and Nb phases.  Since then, additional work [Prima, et al., Pow. Met. and Met. Cer. 34, 155, 1995] has identified the probable existence of a quasi-binary isopleth in this system, with a eutectic at (NiTi)-26 at% Nb at 1170 oC.  This means that when nitinol and pure niobium are brought into contact at temperatures above 1170 oC, spontaneous melting will occur, forming a liquid that will subsequently solidify into microstructures containing only ordered NiTi and near-pure disordered bcc-niobium. 

We show that this reaction can form the basis for a new brazing system for nitinol.  The eutectic liquid, which contains >35 at% Ti, is extremely reactive and not only wets NiTi surfaces, but also apparently dissolves oxide scales, obviating the need for fluxes and providing for efficient capillary flow into joint crevices.  The solidified braze metal is shown to be both strong and ductile, and is known from the literature to be machinable, corrosion resistant, and biocompatible.  This paper will review the metallurgy of the NiTi-Nb quasi-binary system and present recent results on brazing nitinol using niobium.  The technique will enable a wide variety of new forms of low-density superelastic and shape-memory nitinol, such as honeycombs, spaceframes, and shape-memory/superelastic hybrid structures of nearly arbitrary complexity.