Atomic diffusion and phase equilibria were 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 (1573K, up to 178h). 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-8at%Al/Ir diffusion couples annealed at 1573, 1673 and 1773K. An extremely low value of 7.0 x 10–19m2/s was evaluated for Ir-4at%Al at 1573K. At the Ir/Nb5Si3 interface, the intermetallic phases Ir3Si and Ir3Nb were formed on the Ir side and the Nb5Si3 side, respectively. The formation of Ir3Si was controlled by the diffusion of Si through Ir3Nb in which the solubility of Si was limited quite small.

Y.Kimura, T.Shimizu, S.Shiina, Y.Mishima: Metallurgical and Materials Transactions A, 2005, 36[3], 591-600