Atomic diffusion and phase equilibria at the interfaces of the CoAl/Ir multilayer on Nb₅Si₃-base alloys

Abstract

Atomic diffusion and phase equilibria have been investigated at the interfaces of Ir/CoAl and Ir/Nb₅Si₃ to evaluate the suitability of a diffusion-barrier layer of Ir between an oxidation-resistant layer of B2-CoAl and a base material Nb₅Si₃. Diffusion couples were prepared by hot pressing and annealed at 1573 K for up to 178 hours. Diffusion layers of (Ir, Co) solid solution and B2-(Ir, Co)Al were formed at the Ir/CoAl interface. The concentration of Al dramatically dropped at the interface, which indicates that the Ir layer effectively works as the diffusion barrier against the inward diffusion of Al. To quantitatively evaluate the potential of Ir as a diffusion barrier, the Boltzmann-Matano analysis was employed to determine the diffusion coefficient of Al using Ir-8 at. pct Al/Ir diffusion couples annealed at temperatures of 1573, 1673, and 1773 K. For instance, an extremely low value of 7.0×10⁻¹⁹ m²/s is evaluated for Ir-4 at. pct Al at 1573 K. At the Ir/Nb₅Si₃ interface, the intermetallic phases Ir₃Si and Ir₃Nb are formed on the Ir side and the Nb₅Si₃ side, respectively. The formation of Ir₃Si is controlled by the diffusion of Si through Ir₃Nb in which the solubility of Si is limited quite small.