Understanding how properties of materials change due to nuclear transmutations is a major challenge for the design of structural components for a fusion power plant. In this study, by combining a first-principles matrix Hamiltonian approach with thermodynamic integration we investigate quasisteady state configurations of multi-component alloys, containing defects, over a broad range of temperature and composition. The model enables simulating transmutation-induced segregation effects in materials, including tungsten where the phenomenon is strongly pronounced. Finite-temperature analysis shows that voids are decorated by Re and Os, but there is no decoration by tantalum (Ta).
The difference between the elements is correlated with the sign of the short range order (SRO) parameter between impurity and vacancy species, in agreement with Atom Probe Tomography (APT) observations of irradiated W-Re, W-Os, W-Ta alloys in the solid solution limit. Statistical analyses of Re and Os impurities in vacancy-rich tungsten show that the SRO effects involving the two solutes are highly sensitive to the background concentration the species. In quaternary W-Re-Os-Vac alloys containing 1.5% Re and 0.1% Os, the SRO Re-Os parameter is negative at 1200K, driving the formation of concentrated Re and Os precipitates. Comparison with experimental Transmission Electron Microscopy (TEM) and APT data on W samples irradiated at the High Flux Reactor (HFR) shows that the model explains the origin of anomalous segregation of transmutation products (Re,Os) to vacancy clusters and voids in the high temperature limit pertinent to the operating conditions of a fusion power plant.