Development of a solute and defect concentration dependant Ising model for the study of transmutation induced segregation in neutron irradiated W-(Re, Os) systems.

In this study, radiation-induced precipitation of transmutation products is addressed via the development of a new solute and vacancy concentration dependant Ising model for the W-Re-Os system. This new model includes interactions between both Os and Re atoms with vacancies, thus facilitating more representative simulations of transmutation in fusion reactor components. Local solute concentration dependencies are introduced for the W-Re, W-Os and Re-Os pair interactions. The model correctly accounts for the repulsion between small clusters of vacancies and the attraction between larger clusters/voids, via the introduction of local vacancy concentration dependant interaction coefficients between pairs of atoms and vacancies. To parameterise the pair interactions between atoms and/or vacancies, the enthalpy of mixing, Δ, for various configurations and solute/defect concentrations, was calculated using density functional theory, within 6 binary systems: W-Re, W-Os, Re-Os, W-vacancy, Re-vacancy and Os-vacancy. The new energy model was implemented into the SPPARKS Monte Carlo code, and successfully used to predict the formation of voids decorated with Re and Os solute atoms. Analysis suggests that there is a strong thermodynamic tendency for Os to bind to these voids with a comparatively weaker binding from Re atoms. The binding energies of various solute/vacancy clusters were calculated and showed that Re and Os solute atoms tend to stabilise small clusters of vacancies, increasing the attractive binding energy between the constituents.