EVOLUTION OF NICKEL BORIDES IN NI-SI-B BRAZED JOINTS OF NICKEL DURINGPOST-BRAZE HEAT-TREATMENT

In brazes of pure nickel with Ni-Si-B filler metals, nickel boride for ms in the central region of the brazed gap by solidification, and in t he pure nickel base metal along both joint faying surfaces by solid st ate diffusion and precipitation. During subsequent heat treatment, the gap boride gradually decomposes and dissolves, while the lining borid e along the joint faying surfaces increases both in size and quantity. This phenomenon is the result of an uphill diffusion of boron from th e brazed gap, which remains very much a ternary Ni-Si-B system, into t he base metal, which is largely a binary Ni-B system owing to the rapi d diffusion of boron ahead of silicon ahead of silicon into the base m etal. The driving force for such diffusion is the high chemical potent ial of boron in the gap owing to the presence of silicon, which substa ntially lowers the solubility of boron in gamma-Ni. On the other hand, the separation between the opposite lining boride layers increases wi th brazing time and is controlled by the diffusion of silicon from the gap into the base metal. The formation of lining boride in the base m etal can be suppressed by suitable alloying elements, such as chromium , which can effectively retard the diffusion of boron. (C) 1997 The In stitute of Materials.