Precipitation of metallic Ni clusters in alumina scale during high-temperature oxidation of NiAl in water vapor

Water vapor is ubiquitously present for energy conversion and combustion systems, yet its effect on the oxidation of crucial structure components of heat-resistant alloys remains elusive. Departure from the well-established phenomenon of exclusive alumina growth from the NiAl oxidation in dry O ₂, we demonstrate that the NiAl oxidation in H ₂O results in the precipitation of a significant number of metallic Ni clusters within the Al ₂O ₃ scale by adsorbing Ni atoms from the alloy through a vacancy-assisted solute capture solute capture process of the enhanced oxide/alloy interface migration. The Ni clusters initially have a hcp structure at small sized, transitioning into an fcc structure when their size exceeds ∼12.5 nm. Using atomistic modeling, we show that the dissolution of a high concentration of H derived from H ₂ O into the oxide scale is the key to stabilizing the hcp Ni structure. Subsequent coarsening of the hcp Ni clusters lowers the H concentration, leading to the hcp → fcc transformation. These results provide mechanistic insight into the effect of water vapor on the long-term stability of the structure alloys by influencing the oxidation-induced mass transport across the scale/alloy interface and microstructure evolution within the oxide scale.