Structure of the Al–Ni intermetallic layers produced on nickel alloy by duplex treatment

Hard and wear and corrosion resistant Al–Ni type intermetallic layers, with an external Al2O3 zone, were successfully produced on Inconel 600 using a duplex method. The duplex method combines glow discharge assisted oxidizing with pre-coating the Inconel substrate with aluminum by magnetron sputtering. The oxidizing process carried out at 560 °C for 4 h leads to a diffusion-induced transformation of the aluminum coating and adjacent Inconel into a 15 μm thick composite layer of Al2O3+AlNi+AlCr2+AlNi3+Cr(Fe,Ni)+Ni(Cr,Fe,Al). The structure of the layer was examined in cross-section by transmission electron microscopy (TEM). The surface zone of the layer is constituted by nanocrystalline Al2O3 covering the main zone of the AlNi layer. In this zone, near its border with the oxide zone, are small agglomerates of nanocrystalline Al2O3. The upper and thicker part of the AlNi zone also contains precipitates of the AlCr2 phase. The AlNi zone is separated from Inconel by a diffusion zone of Ni(Cr,Fe,Al). In this region grains of AlNi3 are found with groups of Cr(Fe,Ni) phase grains. As a consequence, the region is locally strongly enriched with chromium. This suggests that the formation of AlNi induces an uphill diffusion of chromium into the Inconel substrate.