WETTING AND SPREADING OF NICKEL-PHOSPHORUS BRAZES - DETAILED REAL-TIME OBSERVATIONS OF SPREADING ON IRON-CHROMIUM SUBSTRATES

Hot stage optical microscopy has been used in detailed studies of the flow of drops of Ni-11 P braze over iron substrates containing 2-20 wt -%Cr. Wetting was good, but not complete at the experimental temperatu res of about 1000 degrees C. Spreading of the braze drops was initiall y rapid but progressively slowed and was impeded by chemically and phy sically tenacious substrate oxides, and by grain boundary groves paral lel 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 t ime dependence is similar to that of theoretical models for inert syst ems, 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 no t only restricts the extent of spreading but also its rapidity and dur ation. (C) 1996 The Institute of Materials.