Multiscale study of helium diffusion in Ni-Cr alloys: Short-range trapping versus long-range channeling

Ni-Cr alloys are promising structural materials for nuclear reactor applications, but are susceptible to high-temperature helium (He) embrittlement (HTHE) under neutron irradiation due to He generation by transmutation reactions and subsequent He bubble formation. The diffusion of He governs the kinetics of He bubble formation and HTHE, requesting a full understanding of He diffusion under the influence of solute atoms. This study investigated He diffusion in Ni-Cr alloys by integrating density functional theory (DFT) with atomic kinetic Monte Carlo (AKMC) simulations. Our findings reveal that trapping basins around Cr atoms, which hinder He diffusion while being isolated in the dilute concentration regime, can form interconnected channels at high Cr concentrations for accelerated He diffusion under 600 K. The size of the channels and thereby their impact on He diffusion increase with Cr concentration. The competition between short-range trapping and long-range channeling leads to a non-monotonic dependence of He diffusivity on Cr concentration, first decreasing from 0-5 at% Cr and then increasing from 6-12 at% Cr, overturning the conventional wisdom that adding Cr monotonically slows down He diffusion. These atomic-scale insights are critical for designing radiation-tolerant Ni-based alloys. Furthermore, the combined DFT-AKMC approach and the concept of random walker diffusion through interconnected energy basins provide a broadly applicable framework for studying transport phenomena in disordered systems.