Evolution of nickel borides in Ni-Si-B brazed joints of nickel during post-braze heat treatment

Abstract

In brazes of pure nickel with Ni-Si-B filler metals, nickel boride forms in the central region of the brazed gap by solidification, and in the pure nickel base metal along bothjointfaying surfaces by solid state diffusion and precipitation. During subsequent heat treatment, the gap boride gradually decomposes and dissolves, while the lining boride along the joint faying surfaces increases both in size and quantity. This phenomenon is the result of an uphill diffusion of boron from the brazed gap, which remains very much a ternary Ni-Si-B system, into the base metal, which is largely a binary Ni-B system owing to the rapid diffusion of boron ahead of silicon into the base metal. The driving force for such diffusion is the high chemical potential of boron in the gap owing to the presence of silicon, which substantially lowers the solubility of boron in y-Ni. On the other hand, the separation between the opposite lining boride layers increases with 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 metal can be suppressed by suitable alloying elements, such as chromium, which can effectively retard the diffusion of boron.