Development of a diffusion mobility database for Ni-base superalloys

Modelling the thermal oxidation of ternary alloys—compositional changes in the alloy and the development of oxide phases

Modelling of composition and phase changes in multiphase alloys due to growth of an oxide layer

The effect of platinum addition on the interdiffusion behavior of γ-Ni + γ′-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 °C for up to 100 hours. An Al-enriched γ′-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 γ + γ′ alloys. For a given diffusion couple end member, the thickening kinetics of the γ′ layer that formed increased with increasing Pt content in the Ni-Al-Pt γ + γ′ alloy. The γ′-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 γ + γ′ coating on CMSX-4 showed excellent oxidation resistance when exposed to air for 1000 hours at 1150 °C. Moreover, the Al content in the coating was maintained at a relatively high level due to Al replenishment from the CMSX-4 substrate.

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

Effect of reactive element oxide inclusions on the growth kinetics of protective oxide scales

Ternary interdiffusion coefficients were measured in the Ni solid solution γ (fcc) phase of the Ni-Cr-Al system at 1100 and 1200 °C. Extensive use was made of both γ/γ and γ/γ + β (β-NiAl structure) diffusion couples. Two analysis techniques were employed to calculate the interdiffusion coefficients. When the Matano planes for Al and Cr were not coincident, numerous integral calculations were made to determine an average diffusion coefficient and to assess the effect of the noncoincidence of the Matano planes. The results of the diffusivity measurements showed that
 
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$$\tilde D_{AlAl}^{N1} $$

 is approximately four times greater than
 
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$$\tilde D_{AlCr}^{N1} $$

 , while
 
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$$\tilde D_{CrAl}^{N1} $$

 and
 
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$$\tilde D_{CrCr}^{N1} $$

 are of the same magnitude. For all concentrations,
 
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$$\tilde D_{AlAl}^{N1} $$

 is two to three times greater than
 
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$$\tilde D_{CrCr}^{N1} $$

 . Both
 
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$$\tilde D_{AlAl}^{N1} $$

 and
 
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$$\tilde D_{AlCr}^{N1} $$

 increase with increasing Al concentration, whereas
 
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$$\tilde D_{CrAl}^{N1} $$

 and
 
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$$\tilde D_{CrCr}^{N1} $$

 show little concentration dependence on Cr alone. A ternary, finite-difference interdiffusion model was employed to predict concentration profiles for the γ/γ couples utilizing the concentration dependence of the measured diffusivities. Good agreement was observed between the predicted and measured concentration profiles for both 1100 and 1200 °C.

J. A. Nesbitts

Papers

Interdiffusion in Ni-rich, Ni-Cr-Al alloys at 1100 and 1200 °C: Part II. Diffusion coefficients and predicted concentration profiles