Effect of Chromium Content on the Oxidation Behavior of a Ta Stabilized γ'-Strengthened Polycrystalline Co-30Ni-10Al-4W-4Ti-2Ta Alloy.

The high-temperature oxidation behaviors of polycrystalline Co-30Ni-10Al-4W-4Ti-2Ta superalloys with Cr contents ranging from 1 to 5 at.% are characterized at 900 °C to provide benchmark data for the alloy design of the CoNi-based superalloys. The mass gain curves for all three alloys exhibit parabolic growth, and the addition of 5Cr at.% is sufficient to decrease the oxidation rate by two orders of magnitude compared to the Cr-free alloy. Furthermore, cross-sectional findings reveal that these three alloys form qualitatively similar oxide scales composed of an outer oxide layer of CoO and CoAlO phase on top of an AlO scale, following the inner oxide layers of CrO, TiO, and TiTaO, and internally oxidized AlO precipitate. The alloy forms a chromium-rich oxide scale as the Cr addition increased, and the concentration of Cr in the scale/alloy interface increases, promoting the growth of CrO, while CoAlO and CoO nucleation is inhibited. The results further indicate that Cr has a superior effect on improving the oxidation resistance of CoNi-based alloys and that a higher content of Cr can assist the formation of a continuous AlO, CrO, and TiTaO layers, which in turn hampers outer Co and Ni, and inward oxygen flux.