An atomic resolution study of radiation-induced precipitation and solute segregation effects in a neutron-irradiated W-25 at.% Re alloy

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.