Solidification phenomena in nickel base brazes containing boron and silicon

Abstract

1. The intermetallic compounds formed in brazes of nickel base filler metals containing chromium, boron and silicon are nickel boride, nickel silicide and chromium boride. 2. The intermetallic compounds are found to form as constituents of binary or ternary eutectics with γ-nickel. The binary eutectics are γ-nickel/nickel boride, γ-nickel/nickel suicide and γ-nickel/chromium boride. The ternary eutectics are γ-nickel/nickel boride/chromium boride and γ-nickel/nickel boride/nickel silicide. 3. Upon cooling from the brazing temperature, the first formed solid is primary γ-nickel, which either nucleates within the melt as nodules or nodular dendrites or simply grows onto the partially melted filler metal particles. On further cooling, boron and silicon in the melt tend to exclude one another, solidifying into different types of eutectics the constituents of which depend on the composition of the filler metal. 4. When only silicon is present, the remaining melt simply solidifies into an eutectic of γ-nickel and nickel silicide. 5. For boron-containing filler metals, the first formed eutectic is usually the binary eutectic of γ-nickel and nickel boride; this in turn enriches the melt with chromium. In the absence of silicon, this chromium-enriched melt will eventually solidify into a ternary eutectic of γ-nickel, nickel boride and chromium boride. When silicon is present, the formation of the above ternary eutectic is suppressed and a binary eutectic of γ-nickel and chromium boride is formed instead. When this happens, the remaining melt will be enriched in silicon and the last portion of the melt solidifies into a ternary eutectic of γ-nickel, nickel boride and nickel silicide. 6. For the filler metal containing nickel, boron and silicon but no chromium, after the formation of binary eutectic of γ-nickel and nickel boride, the melt will become gradually enriched with silicon and eventually solidify into the ternary eutectic of γ-nickel, nickel boride and nickel silicide.