Interdiffusion behavior of Pt-modified γ-Ni+γ′-Ni3Al alloys coupled to Ni-Al-based alloys

The effect of platinum addition on the interdiffusion behavior of gamma-Ni + gamma'-Ni3Al alloys was studied by using diffusion couples comprised of a Ni-Al-Pt alloy mated to a Ni-Al, Ni-Al-Cr, or Ni-based commercial alloy. The commercial alloys studied were CMSX-4 and CMSX-10. Diffusion annealing was at 1150 degrees C for up to 100 hours. An Al-enriched gamma'-layer often formed in the interdiffusion zone of a given couple during diffusion annealing due to the uphill diffusion of Al. This uphill diffusion was ascribed to Pt addition decreasing the chemical activity of aluminum in the gamma + gamma', alloys. For a given diffusion couple end member, the thickening kinetics of the gamma' layer that formed increased with increasing Pt content in the Ni-Al-Pt gamma + gamma' alloy. The gamma'-layer thickening kinetics in diffusion couples with Cr showed less of a dependence on Pt concentration. Inference of a negative effect of Pt and positive effect of Cr on the Al diffusion in this system enabled explanation of the observed interdiffusion behaviors. There was no or minimal formation of detrimental topologically close-packed (TCP) phases in the interdiffusion zone of the couples with CMSX-4 or CMSX- 10. An overlay Pt-modified gamma + gamma' coating on CMSX-4 showed excellent oxidation resistance when exposed to air for 1000 hours at 1150 degrees C. Moreover, the Al content in the coating was maintained at a relatively high level due to Al replenishment from the CMSX-4 substrate.