Evaluating Wagner Oxidation Criteria for Protective Al₂O₃ Scale Formation in Ni-Based Superalloys

Area of Science:

• Materials Science
• Metallurgy
• Surface Engineering

Background:

• Nickel-based superalloys are critical for high-temperature applications.
• Protective alumina (Al2O3) scale formation is essential for alloy oxidation resistance.
• Predicting the transition to continuous alumina scale formation is key for alloy design.

Purpose of the Study:

• To assess the validity of Wagner transition criteria for predicting Al2O3 scale formation in Ni-based superalloys.
• To compare predictions with experimental data from both custom and commercial alloys.
• To identify factors influencing discrepancies in transition temperature predictions.

Main Methods:

• Evaluation of Wagner transition criteria against experimental oxidation data.
• Comparison of predicted and experimentally determined transition temperatures.
• Analysis of oxide subscale morphology and its impact on oxygen diffusion.

Main Results:

• Wagner criteria successfully predicted transition temperatures for commercial superalloys.
• A significant underprediction (50-100°C) of transition temperature was observed for an experimental superalloy.
• A complex oxide subscale in the experimental alloy was identified as a key factor reducing oxygen ingress.

Conclusions:

• The Wagner transition criteria show general success but require refinement for alloys with complex subscale structures.
• The presence of internal oxide subscales can alter oxidation kinetics and transition behavior.
• Recommendations are provided to improve the predictive capability of the methodology for polycrystalline Ni-based superalloys.