Furnace brazing of Ti-15Mo-5Zr-3Al (Ti-15-5-3, β-Ti) alloy using clad Ti-15Cu-15Ni foil as the filler in a high vacuum has been carried out. In the brazed joints, the chemical compositions of distinct phases were quantified by electron probe micro-analyzer (EPMA), and the phase structures were identified by electron backscatter diffraction (EBSD). The as-brazed joint composed of α-Ti, retained β-Ti, Ti2Ni, and Ti2Cu. The embrittlement of the brazed joint was correlated mainly with the formation of intermetallics, especially cellular Ti2Ni dendrites in the brazed zone. It was noticed that the molten filler liquated the β-Ti grain boundaries and assisted the eutectic reaction therein, resulting in forming grain boundary Ti2Ni surrounded by the retained β-Ti. The results indicated that proper brazing conditions were able to eliminate all the harmful phases effectively, and increased the shear strength of the Ti-15Mo-5Zr-3Al brazed joint.

Dissimilar brazing of Ti–15Mo–5Zr–3Al (Ti-1553) to commercially pure titanium (CP-Ti) using Ti–15Cu–15Ni foil was performed in this work. The microstructures in different sites of the brazed joint showed distinct morphologies, which resulted from the distributions of Mo, Cu, and Ni. In the brazed zone adhered to the Ti-1553 substrate, the partitioning of Mo from the Ti-1553 into the molten braze caused the formation of stabilized β-Ti without Ti2Cu/Ti2Ni precipitates. In the CP-Ti side, the brazed joint displayed a predominantly lamellar structure, composed of the elongated primary α-Ti and β-transformed eutectoid. The decrease in the Mo concentration in the brazed zone caused the eutectoid transformation of β-Ti to Ti2Cu + α-Ti in that zone. The diffusion of Cu and Ni from the molten braze into the CP-Ti accounted for the precipitation of Ti2Cu/Ti2Ni in the transformed zone therein. The variation in the shear strength of the joints was related to the amount and distribution of brittle Ti2Ni compounds. Prolonging the brazing time, the wider transformed zone, consisting of coarse elongated CP-Ti interspersed with sparse Ti2Ni precipitates, was responsible for the improved shear strength of the joint.

Failure analysis was carried out on a ruptured C-276 pipe heated externally at 1050 °C, which had been used for a few months in a controlled decomposition reactor (CDR) system. To catch the decomposed perfluorinated compounds (PFCs, e.g., CF4, SF6, NF3, C3F8 and C4F8) present in the exhaust gas, the C-276 reactor was periodically purged with water mist, which caused a temperature gradient from the external to the inner surface of the pipe. The precipitation of large amounts of intermetallic compounds along the grain boundaries were found to be corroded preferentially. The internal surface of the used pipe was covered with many fine cracks. The corrosion and cracking of grain boundary precipitates accounted for the short service life of the C-276 pipe. Compositional measurements by electron probe micro-analyzer (EPMA) and phase identification by electron backscatter diffraction (EBSD) confirmed the presence of δ and μ phases in the ruptured pipe. The coarse intergranular precipitates were the δ phase (Mo7Ni7), which were enriched in Mo and Cr. Moreover, the fine precipitates dispersed intergranularly and intragranularly were the μ phase (Mo6Ni7), which were abundant in Mo and W. The numerous precipitates present in the matrix and along the grain boundaries were responsible for an obvious loss in the strength and ductility of the used C-276 pipe.