Interdiffusion between the platinum-modified aluminide coating RT 22 and nickel-based single-crystal superalloys at 1000 and 1200°C

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Abstract
Th e interdiffusion between the Pt-modified aluminide coating RT 22 and the single-crystal supera lloys CMSX-6 and SRR 99 has been investigated at 1000 and 1200°C up to 1600 h of exposure time in air and vacuum. Both coating/substrate systems possess remarkable stability at 1000°C, in air as well as in vacuum. It appears that the plate-like precipitates of intermetallics in the interd iffusion zone between coating and substrate act as a barrier layer for interdiffusion. Some inward diffusion of Pt and Al and outward diffusion of Ti occur at 1000°C. Rapid degradation of the coating takes place at 1200°C mainly by interdiffusion. The AI di ffusion into the substrates leads to conversion of β-NiAI into γ'-Ni3AI and finally into γ-phase. Tirapidly diffuses outwards and becomes incorporated into the scale rich in AI. Thus, the Ti content of the substrate affects the scaling behaviour of the coa ting. The intermetallic precipitates are dissolved and, therefore, so is the interdiffusion barrier. The somewhat better behaviour of the RT 22/ SRR 99 system is probably caused by the greater stability of the W-rich intermetallics in this system.

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