Modeling of the Influence of Oxidation on Thin‐Walled Specimens of Single Crystal Superalloys

The microstructural evolution of uncoated single crystal superalloys is modeled taking into account the interplay between oxide growth and substrate response. Experimental investigations demonstrate that γ' fraction of specimens with thicknesses less than 1 mm are strongly affected by oxidation. A model based on thermodynamic and kinetic data only, is presented calculating the growth kinetics of oxide scales and the resulting influence on microstructure evolution of the substrate. The model combines models for oxide growth and substrate response. Currently the main focus is on alumina (Al2O3 ) scale growth as it is the most important oxide for long term behavior. A dynamic growth parameter is used to describe the growth rate of the alumina scale. The model predicts the distribution of the alloying elements as well as the evolution of the generated phases as functions of depth and oxidation time. The model has been applied to three different alloys: the strong alumina forming alloy René N5, the moderate alumina forming alloy M247LC SX and the weak alumina forming alloy SCA425+. Since the 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 profile, which is most important for thin wall specimens. The predictions have been verified with very good agreement at an oxidation temperature of 980°C with respect to alumina scale growth and γ' fraction distribution. Predicted and measured alumina scale growth and γ' fraction distribution for oxidation at 980°C are in very good agreement. Introduction Single crystal nickel-based superalloys are widely used for most heavily loaded parts in the hottest sections of modern stationary gas turbines and jet engines [1]. Naturally, significant interest exists in weight saving of rotating parts, due to the leverage effect on weight saving of disc, shaft, bearing, casing and thereby overall engine weight. Therefore efforts were made in recent years to reduce turbine blade wall thickness [2-4]. Emphasis on thin-walled parts has to be placed on degradation by oxidation. Many publications [5-8] point out that oxidation plays a major role for the inferior creep performance of thin-walled (< 1 mm) specimens in comparison to regular size specimen (> 5 mm). Oxidation mechanisms of multi-component single crystal nickel-based superalloys are rather complex. In the case of the superalloy René N5 and at temperatures around 1000°C, a sequence of three different oxide scales forms: a NiO and Co3O4 containing outer scale, a multi-phase intermediate layer and next to the substrate a dense and compact alumina (Al2O3) layer (see Fig. 1 and 5a). Each of these oxide species influences the overall oxidation kinetics in a subtle and as yet non-quantified way. At the beginning of the oxidation process, oxide growth is observed to be rather rapid, only to slow down later in the process, due to the formation of the continuous Al2O3 scale. The extraction of specific alloying elements (especially Al) due to oxide scale formation results in the generation of the γ'-free layer and, further away from the surface, a reduction of γ' fraction [9,10]. The model reported in this paper has been developed with these mechanisms in mind. The rate of alumina formation is of major importance due to its role in changing the oxidation rate and oxidation resistance. The proposed model has to reflect the underlying force-flux equations which drive the diffusion (especially of Al) necessary for oxidation (especially of Al2O3) to occur. Of course, the rate of scale formation and depletion of alloying elements underneath the oxide scales are coupled [11-13]. The goal is to predict the dependence of the overall oxidation rate, the extent of the γ'-free layer and the distribution of γ' reduced fraction on alloy composition. Theory The microstructure evolution of single crystal nickel-based superalloys in the vicinity of the oxide layer is determined by the interplay between oxide growth and substrate response. The system after oxidation consists of the oxide layer, a specific oxidation affected substrate thickness and the non-affected substrate. In case of thin specimen and sufficient long oxidation exposure (thickness of 0.3 mm, temperature of 980°C, after 6 h), the latter can even disappear completely. The described layered structure forms in all cases. It is obvious that the kinetics of this process depends not only on the driving force of oxide formation, but also on the transport properties of the oxide layer and the substrate. The growth rates of different oxides can differ significantly but the fundamental mechanism of interplay with the substrate is the same. The model illustrated in Fig. 1 couples an oxide growth model (upper part) with a substrate response model (lower part). The lower boundary at d = 0 μm is the symmetry axis of an infinite extended (in z-direction) thin sheet. Fig. 1: Diagram of the coupling between oxide growth model with the substrate response model. Oxide Growth Model The formation of alumina is the rate-controlling oxidation mechanism. The prediction of the oxidation kinetics is based on two factors: (i) Gibbs free energy ΔGf (thermodynamic driving force) and (ii) a kinetic factor. Gibbs free energy of alumina