Effect of Co vs. Fe content on early stages of oxidation of Co-Cr-Fe-Mn-Ni-Si complex concentrated alloys at 800 °C

The effect of Co vs. Fe content on early stages of oxidation in air was studied in Co-Cr-Fe-Mn-Ni-Si complex concentrated alloys (CCAs). A higher Co content decreased the phase fraction of protective corundum-type oxide as determined quantitatively. This is consistent with presented thermodynamic calculations, predicting a decrease in Cr activity with increasing Co content. The formation of Cr-rich corundum-type oxide is hindered in favor of Fe/Co/Ni-rich spinel-type oxide. Accordingly, beneficial effects of Co on mechanical properties of 3d transition metal CCAs for high temperature applications need to be balanced with detrimental effects on oxidation resistance.