Chromium in steel protects against corrosion. But under neutron radiation — inside a nuclear reactor — chromium migrates to or away from grain boundaries, and the direction depends on temperature. Below a critical temperature, chromium enriches at boundaries. Above it, chromium depletes. The transition is sharp, and its location determines whether the steel corrodes from the inside.
Oplinger et al. (arXiv:2603.14574) build a rate-theory model that reproduces this flip. The mechanism is a race between two defect-mediated transport processes. Radiation creates vacancies and interstitials in roughly equal numbers. Vacancies drag chromium toward boundaries (enrichment) because chromium-vacancy binding is favorable. Interstitials push chromium away (depletion) because self-interstitial atoms preferentially replace chromium at boundary sites.
Temperature controls which process wins. At low temperatures, vacancy mobility is limited — vacancies accumulate near where they form and interact strongly with nearby chromium. The drag is local and effective. At high temperatures, vacancies are mobile enough to reach sinks before binding with chromium, weakening the drag mechanism. Meanwhile, interstitial transport becomes dominant, and the flow reverses.
The elegant detail: dose rate and dislocation density change only the magnitude of segregation, not its direction. Temperature alone controls the sign. But the transition temperature itself shifts when you account for realistic conditions — biased defect production at cascades, selective absorption at dislocations. The simple transport model predicts the flip at one temperature; the full model shifts it lower. The physics is qualitatively right but quantitatively wrong without the secondary effects.
One variable controls the direction. Every other variable controls only the speed. The material's corrosion fate is set by a single parameter crossing a threshold that itself depends on the details you thought were negligible.