Quasi-honeycomb grain morphologies strengthen passivating layers in Inconel-718 superalloy: Lessons learned from Additive Manufacturing

Despite intensive studies on the effect of grain morphology, that results from Selective Laser Melting (SLM) fabrication, on the mechanical properties of Inconel-718 (IN718), there is a lack of insight into its effect on the alloy's oxidation behavior. This work compared the oxidation of IN718 alloy manufactured by SLM (SLM-IN718) with that of a cast-rolled commercially available sample (Comm-IN718) when heating to 900 °C. Our results showed that the passivating chromia layer that forms on Comm-IN718 was resistant to rupture because it is a thin oxide layer with a favorable distribution of internal stress due to the alloy's well-defined quasi-honeycomb primary grain morphology. In comparison, SLM-IN718 had a poorly defined grain morphology, with two distinct length-scales best described as dendritic structures. This morphology resulted in the formation of a thicker oxide scale without favorable distribution of the stress into the alloy grain boundaries. This led to the rupture of the passivation layer. These findings give us a new insight into how the primary microstructure of Inconel alloys can affect their oxidation behavior by affecting the stress distribution within surface oxide films. We are the first to show that morphology characteristic control plays a key role in improving Inconel oxidation.