Revealing effect of Al content on oxidation of novel Co-Cr-Nb-W carbide-strengthened superalloy

The high-temperature oxidation behavior of novel Co-Cr-Nb-W carbide-strengthened wear-resistance alloys with different Al contents (1wt%, 2wt% and 3wt%) at 950, 1000 and 1050 °C was thoroughly investigated by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and field emission electron probe analyzer. The porous property of NbC in-situ oxidation products (Nb₂O₅, CoNb₂O₆ and Co₄Nb₂O₉) induces a multi-layered oxide scale with micropores and cracks. Co-Cr-Nb-W alloy with 1 wt% Al undergoes catastrophic oxidation and spalling above 1000 °C. The outward transportation of Cr and Co is effectively restrained by a continuous Al₂O₃ scale formed around NbC in-situ oxidation region when Al content reaches 3 wt%. The β-CoAl in Co-Cr-Nb-W alloy with 3 wt% Al has an oxidation priority over eutectic carbides and the alloy matrix which are both enriched with Cr, thereby preventing the formation of Cr-depletion area and improving the self-healing ability of the oxide film. A slight change in Al content has a remarkable influence on the cooperative effect of Al and Cr and multiplies the antioxidant capacity of Co-Cr-Nb-W alloy above 1000 °C.