Abstract
Formation of diffusion-barrier coating system with two-layered structure has been developed. The system comprises an inner χ-phase of the Re-Cr-Nb system as a diffusion-barrier layer and an outer Al-reservoir layer. In this study, Re film was formed on Nb substrate via electroplating and the film changed into χ-Re3Nb phase by preliminary heat treatment at 1500℃ for 1 h. Cr-pack cementation was conducted under vacuum of 10−5 Pa at 1300℃ and 1400℃ for 6 min and 1 h, respectively, using an Al2O3 crucible in which the specimens were buried in a powder mixture of pure Cr and Al2O3. The packed specimens were examined using an electron probe microanalyzer to determine the concentration profile of each element. The layer structure of the film on the Nb substrate after the Cr-pack cementation was discussed on the basis of composition paths plotted in the Re-Cr-Nb phase diagram.
In the case of the specimen induced by Cr-pack cementation at 1300℃ for 6 min, the alloy layers in the Re-Cr-Nb system consists of the γ, Re-rich, χ and α phases. In the case of the specimen induced by Cr-pack cementation at 1300℃ for 1 h and 1400℃ for 6 min, the alloy layers in the Re-Cr-Nb system consisted of the γ, χ and α phases. By contrast, in the specimen induced by Cr-pack cementation at 1400℃ for 1 h, the alloy layers in the Re-Cr-Nb system consists of the γ, Nb(Cr, Re)2 and α phases.
