Mechanisms of Microstructure Evolution in an Austenitic Stainless Steel Bond Generated Using a Quaternary Braze Alloy

In typical braze joints, melting point depressants degrade the structural robustness by concentrating as brittle phases into continuous seams along the centerline. The objective of the present article is to sufficiently understand the mechanisms governing the microstructure of a typical braze so that approaches for modifying the fabrication to eliminate brittleness can be identified. A characterization conducted for a quaternary braze used for stainless steel bonds, containing P and Si melting point depressants, reveals that the thermochemical interactions governing the microstructure include solution/reprecipitation, solid-state diffusion, and solidification. It is shown that the Si can be incorporated into a solid solution γ-Ni(Fe, Si) phase that forms by reprecipitation. Moreover, this interaction can suppress the formation of silicides within a permissible braze cycle. However, the P is only diluted through solid-state diffusion into the parent alloy. This happens slowly because of its low solubility rendering it impractical to eliminate the phosphide intermetallic. Approaches that obviate this impediment to joint robustness are described. They involve the spatial disruption of the continuous phosphide-containing eutectic with a ductile γ-Ni(Fe, Si) phase.