The influence of alloy microstructure on the oxide peg morphologies in a Co-10% Cr-11%Al alloy with and without reactive element additions

Abstract

The addition of reactive elements such as yttrium or hafnium to heat-resisting alloys has marked beneficial effects on their high-temperature oxidation behavior, and in particular greatly improves the adhesion of the oxide scale to the substrate. In the case of alloys forming Al2O3 protective oxide scales, the improvement in adhesion appears to be related to the formation of intrusions of oxide into the metal—“pegs”—which hold the scale on. The morphology of these pegs is a function of alloy structure and of the reactive element. In particular, it appears that while the pegs of Hf-containing alloys grow as a result of a diffusion process, perhaps along the oxide-metal phase boundary, the pegs on Y-containing alloys seem to form too fast for this, and it is suggested that stresses established by the oxidation of the Co3Y inclusions lead to the formation of porous short-circuit paths.