Effect of yttrium on oxidation behaviour of γ-γ′-Cr3C2eutectic during thermal cycling to 1373 and 1493 K

Abstract

Small additions of yttrium metal lead to large changes in both the cyclic oxidation resistance and the nature of the oxidation products of the γ-γ′-Cr3C2 eutectic composite. Doped alloys containing 0.1, 0.2 and 0.5 wt-% of yttrium all form a stable adherent scale of Al203 which spalls from the undoped material leading to subsequent formation of chromium and nickel oxides. Auger electron spectroscopy shows that at the early stages of oxidation the Al203 in the doped alloys grows at about a third the rate of that in the undoped material at 1273 K and that the yttrium concentration is enhanced in the region of the oxide/metal interface. The yttrium is thought to segregate to the oxide/metal interface leading to improved adhesion and to the oxide grain boundaries inhibiting grain growth and leading to a smaller oxide grain size. The combination of these effects favours the accommodation of oxide growth stresses by oxide plasticity in the doped material rather than by scale spallation as in the undoped alloy.