Development of Al₂O₃ scale during oxidation of a wrought nickel-base alloy

Oxidation exposures consisting of 1 min to 1000 h at 950-1150 °C in still air were used to study the development of surface scale on an Ni-16Cr-4.5Al-4Fe-0.04C-0.01Y alloy. Growth kinetics of the oxide scale were studied thermogravimetrically. Scanning electron microscopy and thin-foil analytical electron microscopy combined with energy dispersive X-ray spectroscopy as well as X-ray diffraction were used to characterize the morphology, structure and composition of oxide scale. After a primary stage of rapid kinetics, oxide growth followed a parabolic rate law with an activation energy of about 400 kJ mol^-1. During primary oxidation, the alloy developed an inner layer of Ni(Al, Cr)₂O₄ spinel and the outer scale contained discrete particles of NiO and Cr₂O₃. When steady-state conditions were established, the inner spinel was replaced by a yttrium-modified layer of α-Al₂O₃. Under steady-state conditions, oxide growth appeared to be controlled at least partially by outward diffusion of aluminium as indicated by the effect of exposure time on its concentration in the outer scale. Near the oxide-metal interface, yttrium was found to segregate to grain boundaries of α-Al₂O₃. Experimental results suggested that yttrium maintained a fine-grained structure of α-Al₂O₃ of improved elevated temperature mechanical strength.