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This study investigates radiation-induced solute segregation at grain boundaries in dilute Ni-X alloys (X=Fe, Cr, Mn) using ion irradiation and phase-field modeling to elucidate underlying mechanisms. By performing irradiations across a temperature range and employing STEM-EDS for characterization, the research distinguishes between kinetic and thermodynamic segregation pathways for different solutes. The key result is the identification of distinct segregation behaviors: Fe depletes via inverse Kirkendall effect, Cr exhibits temperature-dependent kinetic competition, and Mn segregates thermodynamically, with microstructural evolution further complicating these processes.
Fe, Cr, and Mn exhibit fundamentally different grain boundary segregation mechanisms under irradiation, revealing a complex interplay of kinetic and thermodynamic forces that dictates alloy stability.
Radiation-induced composition redistribution at grain boundaries (GBs) in Ni-based structural alloys poses a critical challenge for the long-term deployment of advanced nuclear reactors. The mechanisms governing solute-specific segregation remain poorly understood because defect-mediated transport, thermodynamic driving forces, and competing microstructural sinks can all contribute under irradiation. We perform systematic Ni2+ ion irradiations on dilute Ni-Fe, Ni-Cr, and Ni-Mn binary alloys at 500, 800, and 1000 K to 2 dpa and characterize GB chemistry using STEM-EDS. The segregation behavior is interpreted using rate-theory phase-field modeling informed by atomistic transport data. We find that Fe depletes at GBs across all temperatures via preferential vacancy-solute exchange (inverse Kirkendall effect). Cr segregation reflects a temperature-dependent competition between self-interstitial atom (SIA)-mediated transport toward GBs and vacancy-mediated transport away from GBs, yielding near-zero net segregation at 500 K and depletion at higher temperatures. In contrast, Mn exhibits distinct behavior dominated by thermodynamic segregation that drives local enrichment at the GB. We leverage depth-dependent characterization along the same GB, spanning irradiated and unirradiated regions, to separate radiation-induced and thermodynamic contributions and validate the unirradiated region as an internal thermodynamic reference. Void formation at 800 K and irradiation-induced GB migration at 800 and 1000 K further add mechanistic complexity by suppressing and redistributing the solute. Our combined experimental and computational assessments show that interpreting GB chemistry evolution under irradiation requires concurrent treatment of non-equilibrium kinetic transport, thermodynamic driving forces, bulk sink effects, and GB migration.