Insights into the complexities of diffusion-induced grain boundary migration in Fe-Cr-Ni ternary alloys

The paper investigates the diffusion-induced grain boundary migration (DIGM) in six Fe-Cr-Ni ternary alloys exposed to steam at 480 °C. DIGM occurs due to the outward diffusion of Fe and Cr, essential for surface oxide formation, resulting in DIGM regions depleted in Fe and Cr and enriched in Ni. A significant finding is the tendency for preferential intergranular oxidation (PIO) to occur along these migrated grain boundaries, attributed to their enhanced element diffusivity. The extents of DIGM, PIO, and surface oxide thickness exhibited notable variations across the alloys with varying Ni contents, ranging from 11 wt.% to 75 wt.%. Further theoretical analysis and diffusion-barrier modeling were introduced to clarify the mechanistic role of Fe/Ni. The results reveal that Ni content modulates the competing diffusion behaviors of Fe, Cr, and Ni along grain boundaries, leading to a compositional transition from Fe-dominated to Ni-dominated diffusion regimes. The increasing Ni concentration reduces surface oxide thickness, thereby diminishing local diffusion and influencing the extent of both PIO and DIGM. However, due to the strong coupling among DIGM, PIO, and surface oxidation, their evolution with Ni content exhibits a complex, non-monotonic trend that is difficult to quantify precisely. This complexity arises from the interplay between oxidation-driven chemical potential gradients and composition-dependent grain boundary diffusivity. Generally, DIGM promotes PIO unless the outward and inward diffusivity of elements is significantly affected by surface oxides. However, in high corrosion-resistant alloys, DIGM inhibits the occurrence of PIO by facilitating the formation of an external protective chromia layer through enhanced Cr diffusion outwards.