Oplinger, Bachhav, Ahmed, and Kadambi built a rate-theory model for chromium behavior at grain boundaries in irradiated iron-chromium alloys and reproduced a striking experimental observation: chromium enriches the grain boundary at low temperatures but depletes it at higher ones. The crossover arises from temperature-dependent transport coefficients that shift the balance between vacancy-mediated and interstitial-mediated chromium flux. Dose rate and microstructural factors affect the magnitude of segregation but not its direction. Critically, the model revealed that predictions based solely on transport coefficients are valid only when point defect fluxes are symmetric — an assumption that real irradiation conditions routinely violate.
The through-claim is that the same mechanism can produce opposite outcomes depending on a single continuous parameter. There is no phase transition, no threshold event — just a smooth crossover where the dominant transport mode switches sign. This means that extrapolating from low-temperature data to high-temperature conditions does not merely produce quantitative error; it produces qualitative reversal. The chromium that was concentrating at the boundary is now fleeing it, and no amount of precision at one temperature prepares you for the other.
This is the hazard of monotonic thinking in any system with competing transport mechanisms. Drug efficacy that reverses at high doses (hormesis). Fertilizer that promotes growth below a threshold and suppresses it above. Interest rate policy that stimulates borrowing in one regime and triggers deleveraging in another. When a system has two competing pathways with different temperature (or dose, or intensity) dependences, there exists a crossover point where the system's behavior inverts — and that point is invisible from either side.
(arXiv:2603.14574)