Coarsening Behavior of Ni(Fe) Particles in the Inner Oxide Layer Formed on Fe-5 mass%Ni Alloy at 1200°C in N₂ Atmosphere

In our previous study, we investigated the high-temperature oxidation behavior of Fe-5 mass%Ni alloy and discussed the changes in the microstructure of the inner layer, particularly the size and distribution of Ni(Fe) particles, with oxidation time. The number density and area fraction of Ni(Fe) particles in the inner layer decreased, whereas the size of the particles increased with oxidation time. This coarsening was proposed to arise from Ostwald ripening. However, this evaluation of the coarsening of Ni(Fe) particles includes the effect of oxygen potential in the oxide scale. In this study, for minimizing the effect of the oxygen potential gradient in the oxide scale on the coarsening of Ni(Fe) particles, the samples oxidized at 1200°C were subsequently held in pure N₂ with low oxygen potential to decrease the oxygen potential gradient across the oxide scale, and the microstructural changes in the inner layer during heat treatment in N₂ were investigated to confirm the coarsening of Ni(Fe) particles by Ostwald ripening, with a minimal effect of the change in the oxygen potential on the microstructural evolution. After the equilibrium condition was established in N₂ with lower oxygen potential in the inner layer, the coarsening behavior of Ni(Fe) particles followed the relationship explained by Ostwald ripening. Ni(Fe) particles precipitated in the outer and inner layers because of a decrease in the oxygen potential. The precipitation of Ni(Fe) particles in the inner layer occurred only in the initial transient stage of heat treatment, until the equilibrium condition in N₂ was established.