Unveiling the Mechanism of Improved Oxidation Resistance for CaO Crucible Melting Using Ni-Al Alloy

Melting the ingot or alloy using a CaO crucible is a method known to substantially improve the oxidation resistance of superalloys. But the mechanism is unclear due to the lack of investigation on the inclusions that supposedly prevent S segregation at the oxide/substrate interface, which suppress oxide spallation. To clarify the reaction during the melting process, simplified binary Ni-9.8 wt pct Al single-crystal alloys were cast via melting in both an Al 2 O 3 crucible and a CaO crucible. Cyclic oxidation tests were done on both samples at 1100 °C to compare the oxidation resistance between these samples. Samples were also oxidized at 1100 °C only for 1 hour to investigate the inclusions found within the substrate melted in a CaO crucible using aberration-corrected scanning transmission electron microscope with energy-dispersive X-ray spectroscopy (STEM-EDS). Crystalline CaS and MgAl 2 O 4 inclusions were found near the sub-grain boundary of the sample. This finding proved that the Ca from the CaO crucible reacts with S in the melt to form CaS, preventing S from segregating at the oxide/substrate interface.