Reactive eutectic brazing of high temperature shape memory alloy Nickel-Titanium-Hafnium.

摘要

The existence of a quasi-binary isopleth between NiTi and niobium allows a robust metallurgical joint to be formed from contact melting between pure phases, once the system exceeds the eutectic temperature. NiTiHf, an isomorph of NiTi, has been found to behave in a similar manner. Contact melting occurs between pure NiTiHf and pure niobium when they are brought into contact at temperatures exceeding 1170°C. Liquid formation continues as a concentration gradient forms between the two solid surfaces, NiTiHf and niobium. This liquid region, upon solidification, forms a eutectic microstructure. The eutectic composition of the NiTiHf-Nb quasi-binary system was measured using X-ray florescence spectroscopy. The NiTiHf-Nb eutectic point (14at%-Nb) has a lower niobium content than the established NiTi-Nb eutectic point (26at%-Nb). Mechanical testing revealed a ductile braze that has greater strength than the NiTiHf base metal; hardness studies indicate that the braze procedure has no effect on the base metal. When NiTi is replaced with NiTiHf, one new phase is observed in the braze joint; this phase remains unidentified.