High temperature oxidation behaviour of single crystal superalloys RR3000 and CMSX-4

Abstract

This paper describes the oxidation behaviour of two technologically important nickel based superalloys, RR3000 and CMSX-4, used extensively in gas turbine blade applications. Oxidation tests were conducted in air at 1100°C for equal and extended periods of time, up to 100 h to compare the superalloys characteristic properties. The morphology of oxide growth in the test samples was investigated using metallography, SEM, EDX, X-ray diffraction (XRD) and Cr³⁺ photoluminescence spectroscopy. The results indicated that irrespective of the duration of exposure, alloy RR3000 develops a much thicker oxide scale than alloy CMSX-4. Examination of cross-sections revealed that the scale grown on the alloy RR3000 is porous and has a three layered structure consisting of an outer layer of (Ni, Co)O; a sublayer of CrTaO₄ rich oxide with different spinels; and an inner layer of α-Al₂O₃. By contrast, the scale that forms on alloy CMSX-4 is compact and consists of two layers. The upper layer is primarily composed of spinels (Ni,Co)Al₂O₄ and (Ni,Cr)₂O₄, and the inner layer of α-Al₂O₃. Residual stress measurements using luminescence spectroscopy point to a weak adhesion of the oxide scale to the substrate for alloy RR3000 and a strong adhesion for alloy CMSX-4.