Solid-state reactive diffusion between Ni and W

Abstract: Various alloys are interconnected with W using a Ni intermediate layer by the diffusion bonding (DB) technique. During solid-state heating in the DB technique, however, reactive diffusion occurs at the interconnection between Ni and W. In order to examine the kinetics of the reactive diffusion, sandwich Ni/W/Ni diffusion couples were isothermally annealed at temperatures of T =1023–1173K for various times up to t =366h. Owing to annealing, Ni4W is produced as a layer at the Ni/W interface in the diffusion couple and grows predominantly towards Ni. The thickness of the Ni4W layer increases in proportion to a power function of the annealing time. The exponent of the power function is close to 0.5 at T =1173K and gradually decreases with decreasing annealing temperature. Furthermore, grain growth takes place in Ni4W due to annealing. Therefore, volume diffusion is the rate-controlling process for the growth of Ni4W at T =1173K. At T <1173K, however, boundary diffusion contributes to the rate-controlling process, and the contribution of boundary diffusion increases with decreasing annealing temperature. On the other hand, a region alloyed with W is formed in Ni from the Ni4W/Ni interface by diffusion-induced recrystallization (DIR). The growth rate of the DIR region is much greater than the penetration rate of W into Ni by volume diffusion, and the concentration of W in the DIR region at the Ni4W/Ni interface is equal to the solubility of W in Ni. Such growth behavior of the DIR region was numerically analyzed using a mathematical model. The analysis indicates that the growth of the DIR region is controlled by the interface reaction at the moving boundary of the DIR region as well as the boundary diffusion along the grain boundaries across the DIR region under the present annealing conditions. [Copyright &y& Elsevier]