Microstructural investigation of the initial oxidation at 1450°C and 1500°C of a Mo(Si,Al)2-based composite

The initial oxidation behaviour of a Mo(Si,Al)2-based composite (Kanthal Super ER) was investigated. The microstructures of unexposed reference specimens and samples oxidized isothermally in air at 1450°C for 5 and 50 minutes and also at 1500°C for 5 minutes were evaluated.Thin foil specimens were prepared using a focused ion beam/scanning electron microscope and analysed in an SEM with scanning transmission electron microscopy detectors and an energy dispersive X-ray system for high-resolution imaging and chemical analysis.The samples formed relatively pure α-Al2O3 scales during the oxidation. The Al supply to the growing alumina scale is mainly provided by the Mo(Si,Al)2 phase in the bulk material. The outward Al transport is counterbalanced by inward diffusion of Si into Mo(Si,Al)2. Therefore an Al poor Mo5(Si,Al)3 phase forms directly below the scale. The Al concentration in the subjacent Mo(Si,Al)2 decreased to 20 at.%, corresponding to Mo(Si1.4,Al0.6).