LAUSR

Oxidation of a Ni–16Mo–7Cr–4Fe superalloy containing various yttrium concentrations (0.00, 0.05, 0.12, 0.21 and 0.43 wt%) was undertaken in air at 1273 K for times up to 250 h. The nature and the structure of the oxide scales were investigated by synchrotron radiation techniques, TEM, SEM, XPS, etc. The oxidation kinetics of the alloys containing a low Cr content of 7 wt% sectionally obeyed the parabolic law. The oxidation rate of the alloy in the steady-state stage was reduced by about a factor of 30 by the micro-addition of 0.05 wt% yttrium. Yttrium microalloying greatly enhanced the selective oxidation of chromium and promoted the formation of a compact inner Cr2O3-enriched layer, which can inhibit the outward diffusion of oxidizable elements, especially the volatile-oxide-forming-element Mo, and remarkably improve the adhesion of the oxide scale to the matrix. The oxide scale of the alloy containing 0.05 wt% Y had a thin duplex structure: an outer NiO/NiFe2O4 layer and an inner Cr2O3/YCrO3/spinel oxide layer. In comparison, the oxide scale of the Y-free alloy and the alloys containing excess Y roughly had a thick triple-layer structure: an outmost NiO/NiFe2O4 layer, an intermediate Mo0.84Ni0.16/Cr2O3/spinel oxide layer and an inner Cr2O3/spinel oxide layer. Increasing the concentration of Y in solid solution and reducing the amount of Y-bearing compound are helpful to optimize the effect of Y on improving the oxidation resistance of the alloy.