Wetting and spreading of nickel–phosphorus brazes: detailed real til11eobservations of spreading on iron–chromium substrates

Abstract

Hot stage optical microscopy has been used in detailed studies of the flow of drops of Ni–11P braze over iron substrates containing 2–20 wt-%Cr. Wetting was good, but not complete at the experimental temperatures of about 1000°C. Spreading of the braze drops was initially rapid but progressively slowed and was impeded by chemically and physically tenacious substrate oxides, and by grain boundary grooves parallel to the drop periphery. Ultimately, flow ceased owing to isothermal resolidification of the braze, caused by alloying with the substrate material. Mathematical analysis of the spreading data shows that the time dependence is similar to that of theoretical models for inert systems, but these models do not predict the effect of substrate chemistry on spreading rates seen in the present study. The spreading data also demonstrate that increasing the chromium content of the substrates not only restricts the extent of spreading but also its rapidity and duration.

MST /2056