Influence of diffusion processes on structure of brazed joints and brazing filler metal after spreading over Ni3Al-based alloy

Abstract

This paper presents the results of investigations on wetting with Pd-Ni-Cr(Me)-Ge system brazing filler metal of pure nickel, high-temperature alloy based on nickel aluminide (Ni3Al). Micro-X-ray spectrum examinations showed that spread of Pd-Ni-Cr(Ме)-Ge system brazing filler metal over pure nickel substrate, which has no aluminum, promoted crystallization of brazing filler metal in the form of a solid solution of palladium-nickel and single disperse inclusions of (PdNiCr)-14,13Ge phase. This inclusion precipitates along the boundaries of solid solution grains and can be the reason for strengthening phase. In the spread of this alloy over Ni3Al (Ni-Al-Mo-В)-based high-temperature alloy, containing around 10 wt.% of aluminum, there is another composition and morphological structure of brazing filler metal after crystallization. Thus, the solid solution crystallizes on the base metal substrate. Phase precipitations based on palladium (60.59%) saturated with aluminum (12.3%) is formed in it. In brazing of Ni3Al (Ni-10Al-14Mo-В)-based alloy using this brazing filler metal, the high concentration of aluminum in base metal and the presence of concentration gradient at interface result in active diffusion of the latter from base metal in the brazed seam. Diffusion processes lead to the substitution of germanium on aluminum in (PdNiCr)xGey phase. It results in the formation of (PdNiCr)xAly intermetallic phase, the concentration of aluminum which corresponds to the concentration of aluminum in base metal and it precipitates in a central zone of the brazed seam and leads to brittle fracture of the brazed joints. The use of the brazing filler metal on the Ni-Cr-Al-Me for brazing of nickel aluminide leads to the formation of a structure without intermetallic phases and eutectics in the central zone of the brazed seam.