  High-temperature, high-dose, neutron irradiation of W results in the
formation of Re-rich clusters at concentrations one order of magnitude lower
than the thermodynamic solubility limit. These clusters may eventually
transform into brittle W-Re intermetallic phases, which can lead to high levels
of hardening and thermal conductivity losses. Standard theories of radiation
enhanced diffusion and precipitation cannot explain the formation of these
precipitates and so understanding the mechanism by which nonequilibrium
clusters form under irradiation is crucial to predict materials degradation and
devise mitigation strategies. Here we carry out a thermodynamic study of W-Re
alloys and conduct kinetic Monte Carlo simulations of Re cluster formation in
irradiated W-2Re alloys using a generalized Hamiltonian for crystals containing
point defects parameterized entirely with electronic structure calculations.
Our model incorporates recently-gained mechanistic information of
mixed-interstitial solute transport, which is seen to control cluster
nucleation and growth by forming quasi-spherical nuclei after an average
incubation time of 20 s at 1800 K. These nuclei are seen to grow by attracting
more mixed interstitials bringing solute atoms, which in turns attracts
vacancies leading to recombination and solute agglomeration. The clusters grow
to a maximum size of approximately 4-nm radius, and are not fully dense with
Re, containing 50% or less near the center. Our simulations are in reasonable
agreement with recent atom probe examinations of ion irradiated W-2Re systems
at 773 K.
