Microstructure and Growth of Al₂O₃ on Ni-Al Alloys

Abstract

<p>The oxidation properties of Ni-2, Ni-6 and Ni-32wt.%Al alloys were investigated in one atm oxygen at the temperature range 1273 to 1573 K. The reaction kinetics were determined thermogravimetrically and by layer thickness measurements. The reacted specimens were analyzed using light microscopy, X-ray diffraction, X-ray topography and electron metallographic techniques (TEM, SEM, EPMA and AES). Particular interest was given to the growth of Al₂O₃ on these alloys.</p> <p>Ni-2wt.%Al alloy oxidizes parabolically, at a rate one order of magnitude larger than pure Ni, giving rise to a scale consisting of an outer AI-doped NiO layer, an inner NiO-NiAl₂O₄ layer and an Al₂O₃-alloy internal precipitation zone. The growth of the Al₂O₃ rodlike precipitates, which is interpreted by a NiO/Al(alloy) solid state displacement reaction, is controlled by oxygen diffusion through the Al-depleted alloy in the precipitation zone. The increase in the reaction rate is attributed to the doping effect of dissolved Al on NiO and the high affinity of Al for oxygen resulting in internal precipitation.</p> <p>The oxidation kinetics of Ni-6wt.%Al alloy are irreproducible due to the formation of an imperfect Al₂O₃ scale containing NiO nodules, localized at alloy grain boundaries, beneath which Al₂O₃ is precipitated internally. The transition from internal precipitation to continuous Al₂O₃ formation is interpreted by a model involving the impingement of favorably oriented rodlike precipitates and lateral diffusion of Al from the impingement sites to neighboring regions of the precipitation front. The steady oxidation stage is controlled by thickening of the Al₂O₃ film.</p> <p>The initial sub-microcrystalline film, which is formed on Ni-32wt.%Al alloy, "recrytallizes" subsequently to α-Al₂O₃ giving rise to a well oriented film containing regions of disarrayed polycrystalline oxide. Inert marker measurements indicated that the initial film grows by inward oxygen diffusion. The metallographic observations on the growth of the recrystallized α-Al₂O₃ scale are consistent with counter-current Al and oxygen boundary diffusion in the disarrayed oxide and outward Al lattice diffusion in the oriented film. Accordingly, the growth of this scale was interpreted by a short-circuit diffusion model involving simultaneous reactants lattice and boundary diffusion.</p>