Thermogravimetric and Metallographic Study of the Oxidation Behaviors at Elevated Temperature of Cr– Free Al–Containing Cobalt and Nickel Alloys Reinforced by TaC Carbides

Three alloys, two based on cobalt and one on nickel, containing 5 or 10 wt.%Al for their resistance to hot oxidation, and Ta and C for forming TaC carbides for they creep–resistance at high temperature, were synthesized by casting. They were subjected to the control of their as–cast microstructures and to oxidation tests at 1200°C for 50 hours in a thermobalance. The initial microstructures of the two low Al alloys, both containing 5 wt.%Al, are not significantly modified by the Al introduction by comparison to the more usual {25 to 30 wt.%Cr}–containing original alloys. In contrast, their oxidation behaviors are either catastrophic (Co alloy) or acceptable but not really alumina–forming. To improve the oxidation resistance of the Co alloy a version with 10 wt.%Al was additionally elaborated. Increasing the Al content improved significantly the oxidation behavior but also induced obvious modifications in the microstructure, with the appearance of the Co3Al intermetallic replacing almost a half of the volume fraction of the Co solid solution matrix. Except the 5wt.%Al cobalt alloy over which a thick double–structured scale made of CoO and of a mix of CoO and spinel formed, the 5wt.%Al nickel alloy and the 10wt.%Al cobalt alloy were covered a duplex external oxide scale with an outermost spinel oxide and an innermost alumina oxide, rather protective considering the parabolic constants but threatened by spallation even for the rather slow cooling. The responsibility of tantalum, the oxide of which seems deleterious for adherence, was pointed out.