MICROSTRUCTURAL CHARACTERIZATION OF OXIDE SCALES FORMED ON ⌈-⌈' Co-AI-W- BASED SUPERALLOYS

Cobalt superalloys based on the Co-AI-W system are a new group of heat-resistant materials, which have attracted attracted the interest of researchers in 21^thcentury. The advantage of the new materials over conventional cobalt alloys is the strengthening mechanism based on a microstructure of the γ-γ' type. This study concerned microstructural study of the oxidation products formed on the Co-9AI-9W (at.%) alloy during isothermal oxidation at different temperatures. Therefore, the alloy was prepared via vacuum induction melting (VIM) and casting to a graphite mold. The alloy was subjected to a two-step heat treatment. In the first step, the alloy was annealed at 1250 °C for 16 h in a tubular furnace and are then water quenched. Afterwards, the ingot was annealed at 900 °C for 100 h. The oxidation tests were carried out on cylindrical samples, which were ground, polished and degreased. The oxidation oxide scale formation of the alloy was examined under isothermal conditions in the range 500-1000 °C. The morphology of oxides was examined by scanning electron microscopy (SEM), and their composition was analyzed energy dispersive spectroscopy (EDS). The structures of scales formed at different temperatures were compared. Oxidation at 500 °C results in growth of monolayer scale of Co-oxides, whereas at higher temperatures, multilayer scales are formed. The scales usually consists of outer scale of cobalt oxides, inner oxide layers composed of a mixture of CoO and Co-spinels, and internal oxidation zone.