Reinitiation of substrate oxide undermining by Ni-P filler metals during wetting of nickel-base alloys

The process of wetting of nickel and nickel-chromium substrates by nickel-phosphorus high-temperature brazing filler metals has been examined. The filler was found to wet the substrate by undermining the oxide layer on the substrate. Evidence was found that the undermining process is halted temporarily at discontinuities in the substrate oxide which typically occur above substrate grain boundaries. It is suggested that it is initially more favourable energetically for the filler to spread along the oxide defects than to continue undermining. Mechanisms for the reinitiation of undermining have been considered. It is proposed that undermining recommences after the onset of isothermal solidification of the filler spreading along oxide defects. 11 refs.