The role of yttrium in high-temperature oxidation behavior of Ni-Cr-Al alloys

Abstract

Oxidation of five nickel-chromium-aluminum alloys with yttrium additions of between 0.005 and 0.7wt.% has been studied in the temperature range 800–1200°C in oxygen at pressures of 1, 10, and 720 Torr. The yttrium additions improve the oxidation behavior of the nickel-chromium-aluminum alloys, and generally an addition of 0.1 or 0.2 wt.% yttrium gives the best improvement in the oxidation resistance. In this case, the oxidation kinetics indicate asymptotic approach toward zero scale growth with time. This is suggested to be caused by the formation of subgrains in the alloy which (a) provide enhanced diffusion of aluminum to the surface and (b) increase the number of oxide nucleation sites. Preferred oxidation of aluminum occurs, resulting in the formation of an α-Al ₂O₃ layer. Additions of more than 0.3 wt.% yttrium result in preferential grain boundary oxidation and a convoluted alloy-oxide interface. This effect, “key-on effect” along with the formation of an aluminum and yttrium double oxide, produces increased oxide adherence for these alloys.