High temperature oxidation of superalloys

Since 2006, the Institute for Surface Science and Corrosion successfully participates in the DFG-Graduate School (Graduiertenkolleg 1229 - Stabile und metastabile Mehrphasensysteme bei hohen Anwendungstemperaturen). Within the project OB 1 (Oxidation Resistance of Superalloys) the oxidation behavior of Ni-base superalloys was investigated [1]. In cooperation with the University of Bourgogne (Dijon) the influence of phase dependent surface deformation and mechanical stresses on oxide layer growth was explored for duplex stainless steels. First interesting correlations have been obtained between local (sub-µm) oxide layer thickness and composition on the one hand and microstructural caused surface deformation states on the other hand [2]. An interesting result is that differences in oxidation behavior can also be observed within a phase (e.g. ferrite) depending on local residual stresses which are distributed inhomogeneous within the grain. Therefore, the determining factor was the local stress state since other rate-determining factors such as chemical composition or grain orientation had been eliminated. In a follow-up project (since fall 2009) of the DFG-Graduate School the oxidation resistance of novel Co-base superalloys is investigated. The influence of the chemical composition and microstructure on oxidation mechanisms is determined by using custom model alloys. Previous isothermal oxidation results of polycrystalline γ/γ’-strengthened Co-Al-W alloys show an insufficient oxidation behavior [3] which is due to the formation of an inner heterogeneous Al2O3-layer (at 900 °C). Al-depletion in the base material (due to the formation of the Al2O3-layer) leads to microstructural changes at the alloy/oxide layer interface, especially at 800 °C (see Figure 1). Time-dependent studies of the oxidation behavior show that Al2O3-formation starts at preferred surface locations (Figure 2). However, lateral resolution does not allow for clear correlation of these locations to specific local microstructures. In addition, optimization of alloy composition concerning oxidation resistance is also necessary. First promising results show improvements in oxidation resistance by adding small amounts of boron (which also enhances oxide layer adhesion) and chromium [4]. The grain boundary strengthening effect of boron is well known but an improved oxidation behavior or oxide layer adhesion is not yet report for other alloy systems. Klein et al. [3, 4] now reported that addition of sufficient amounts of boron to polycrystalline Co-base superalloys benefits the formation of a continuous inner Al2O3-layer which protects the base material from further oxidation. ToF-SIMS mappings (Figure 3) reveal that tungsten borides within the inner oxide layer are very likely responsible for the improved oxidation resistance and oxide layer adhesion [4].

Fig. 1: SEM cross-sectional micrograph of a Co-W-Al-B alloy after 500 h of oxidation at 800 °C in air [3].

Fig. 2: SEM cross-sectional micrographs and EDX mappings of oxide layers grown on a Co-Al-W-B-alloy after different oxidation times at 800 °C in air [3].

Fig. 3: ToF-SIMS mappings of the tungsten (a) and boron (b) distribution in a Co-Al-W-B alloy after 500 h of oxidation at 800 °C in air [4].

References: [1] M. Bensch, J. Preussner, R. Hüttner, G. Obigodi, S. Virtanen, U. Glatzel, Modelling and Analysis of the Oxidation Influence on Creep Behavior of Thin-Walled Structures Based on the Single Crystal Nickel-Base Superalloy Rene N5 at 980°C Acta Materialia 58 (2010) 1607-1617. [2] D. Kempf, V. Vignal, N. Martin, S. Virtanen, Relationships between strain, microstructure and oxide growth at the nano- and microscale, Surface and Interface Analysis 40 (2008) 43-50. [3] L. Klein, A. Bauer, S. Neumeier, M. Göken, S. Virtanen, High temperature oxidation of γ/γ'-strengthened Co-Base superalloys, Corrosion Science 33 (2011) 2017-2034. [4] L. Klein, Y. Shen, M.S. Killian, Effect of B and Cr on the high temperature oxidation behaviour of novel γ/γ'-strengthened Co-base superalloys, Corrosion Science 53 (2011) 2713-2720.