Strain-induced topological transition from planar to filamentous grain boundary oxidation in austenitic stainless steel

In pressurized water (PWR) nuclear reactors, intergranular oxidation of structural materials in the primary‑coolant circuit is accelerated by plastic deformation along random high‑angle grain boundaries (RHAGBs), providing a precursor state for intergranular stress corrosion cracking (IGSCC). However, because IGSCC initiation is inherently stochastic, conventional macroscopic strain measures are ineffective for identifying which oxidized RHAGBs will eventually crack. This limitation motivates a focused search for mechanistic signatures that distinguish vulnerable boundaries from benign ones. Here, we evaluate RHAGB oxidation morphology as a potential mechanistic signature by comparing solution‑annealed (SA) and cold‑tensile‑strained (CTS) Fe–18Cr–14Ni exposed to simulated PWR primary‑water environments. Electron microscopy reveals a deformation-driven transition in RHAGB oxidation morphology, from continuous oxides in SA to complex 3D morphologies in CTS comprising Cr-enriched filaments advancing ahead of the oxidation front. We propose a “Leading Filament” mechanism to explain this transition, where short-circuit transport enables high-aspect-ratio, stress-concentrating filaments. While macroscopic strain controls oxidation depth, boundary-specific strain heterogeneity likely governs filament morphology, offering a mechanistic signature relevant to the boundary-to-boundary variability in IGSCC initiation.