Modelling of High Temperature Oxidation of Alumina-Forming Single-Crystal Nickel-Base Superalloys

Microstructure evolution in single-crystal superalloys is modelled as the interplay between oxide growth and substrate response. This is of particular importance for specimens with reduced wall thicknesses, where the affected substrate constitutes a significant fraction of the material. A model based on thermodynamic and kinetic data only is presented in order to predict the growth kinetics of oxides and the resulting influence on microstructure evolution of the substrate. The present work focuses on alumina (Al2O3) growth as it is the most important oxide regarding long-term behaviour. Al2O3 growth is described using a dynamic growth parameter which is derived using thermodynamic and kinetic principles. The substrate response model calculates the distribution of the alloying elements as well as the evolution of the phase fractions as a function of depth and oxidation time. The model has been applied on the strong alumina-forming alloy René N5 and the weak alumina-forming alloy SCA425+. Since γ′ fraction is one of the most relevant factors for high temperature creep properties, the present work concentrates on the calculation of the time- and space-dependent γ′ precipitate fraction profile. The model predictions were verified with very good agreement with respect to Al2O3 growth, element distribution and γ′ fraction distribution.