Oxygen-mediated phase evolution and elemental partitioning in a Ni-based superalloy during thermal-mechanical-chemical erosion

Phase evolution and elemental partitioning behavior of the GH4098 Ni-based superalloy were investigated in detail using multi-scale electron-microscopy-based analytical techniques after repeated testing cycles of thermal–chemical–mechanical erosion in a vented combustor. Localized melting and redeposition of molten materials have been confirmed, with the maximum temperature at the bore surface exceeding the melting point during transient thermal cycles upon exposure to high-temperature propellant gas. The major phase constituents in the redeposited materials evolve from Cr-containing oxides on a γ-matrix to partitioned clusters of Cr/Al-rich oxides, Ni–Co–Fe solid-solution, and Ti–Mo intermetallic phases during repeated oxidation processes, and finally to a continuous Cr–Al–Ti-containing oxide region enclosing isolated metallic domains. The results of first-principles calculations further reveal that Cr-rich oxides are preferentially formed due to a low energy barrier and low adsorption energy for O atoms on localized regions enriched with Cr. Segregation of W at the interface of Cr-rich oxides also takes place through energetically favored paths for migration. In addition, the formation of Ti–Mo intermetallic phases is preferred upon intermediate contents of interstitial O atoms, while the partitioning of major alloying elements including Ni, Co and Fe leads to the presence of solid-solution phases.