Microstructure and Oxidation Mechanism Evolution of Co–17Re–25Cr–2Si in the Temperature Range 800–1,100 °C

Abstract

The oxidation behavior of the newly developed Co–Re–Cr-based alloy Co–17Re–25Cr–2Si (at.%) has been studied in laboratory air at 800–1,100 °C. A transition of oxidation mechanism was observed in the alloy within this temperature range. At 800 and 900 °C, the oxide scale mainly consists of an outermost Co-oxide layer and an inner spinel CoCr2O4 layer. Few SiO2 particles were found in the inner oxide layer. Re oxidizes and then evaporates during the exposure to air since both of these two layers are not protective. Above 1,000 °C, the main oxide product is a continuous compact Cr2O3 layer. SiO2 was found as inner oxides particles underneath the Cr2O3 layer. Thus, the evaporation of Re-oxide is strongly retarded. Due to the outward diffusion of Cr during the formation of the Cr2O3 scale, the original Cr-rich sigma (σ) phase at the subsurface dissolves. Furthermore, the hcp Co solid-solution matrix supersaturated in Cr an Re brings about the precipitation of the secondary σ phase in the substrate during oxidation test.