The phenomena of radiation-induced precipitation and solute segregation effects in a W-25 at.% Re alloy have been investigated using the atom-probe field-ion microscope. This alloy is supersaturated with respect to the solvus line of the primary β-solid solution. The specimens had been irradiated in the Experimental Breeder Reactor (EBR-II) to a fast neutron fluence of ~4 × 1022 neutrons cm-2 (E > 0.1 MeV) at 575, 625 and 675°C. This fluence corresponds to 8.6 dpa and an average displacement rate, for the 2 year irradiation time, of 1.4 × 10-7 dpa s-1. The results of the present work show significant alteration of the microstructure of this alloy as a result of neutron irradiation. Coherent, semicoherent and incoherent precipitates with the composition ~Wre3 were detected; the precipitate's number density is ~ 1017 cm-3 with a mean diameter of ~40Å. The coherent WRe3, precipitates were not associated with either line or planar defects or with any impurity atoms. Therefore, a true homogeneous radiation-induced precipitation occurs in this alloy. The semicoherent and incoherent WRe3 precipitates were associated with 4He atoms; i.e. these precipitates may have been heterogeneously nucleated. Voids at a number density of ~1017cm-3 and a mean diameter of ~90Å were detected. Neither a significant enrichment or enhancement of Re was found at these voids. A two-dimensional WRe3, phase has been observed at a grain boundary. A physical argument is presented for the nucleation of WRe3 precipitates in the vicinity of displacement cascades. It is suggested that the first step in the nucleation of WRe3 precipitates is the formation of tightly-bound mobile mixed dumbbells which react to form an immobile di-rhenium cluster. A possible sequence of point-defect reactions is detailed which can lead to a WRe3 cluster. The growth of this cluster into a precipitate is most likely driven by the irreversible vacancy: self-interstitial atom (SIA) annihilation reaction, as suggested recently by Cauvin and Martin. Point defect mechanisms for all the other observations are also discussed.
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