Radiation-induced precipitation in fast-neutron irradiated tungsten-rhenium alloys: An atom-probe field-ion microscope study

The phenomenon of radiation-induced precipitation has been studied in W-10 at.% Re and W-25 at.% Re alloys, utilizing the atom-probe field-ion microscope technique. Specimens of these alloys were irradiated in the Experimental Breeder Reactor II to a fast-neutron fluence of ∼ 4 × 10²⁶ neutrons m^-2 (E > 0.1 MeV) at 575, 625 and 675°C. This corresponds to 8.6 dpa and an average displacement rate, for the two year irradiation time, of 1.4 × 10^-7dpa s^-1. Extrapolation of the solvus lines on the W-Re phase diagram indicates that the 10 at.% Re alloy is subsaturated with respect to the solvus line of the primary solid solution, while the 25 at.% Re alloy is supersaturated, with respect to the same solvus line. In the case of the 10 at % Re alloy coherent, semicoherent and possibly incoherent precipitates with the composition ∼ WRe and a disc-shaped morphology - one or two atomic planes thick - were detected at a number density of ∼10²² m^-3, and a mean diameter of ∼ 5.7 nm. For the 25 at.% Re alloy coherent, semicoherent and incoherent precipitates with the composition ∼ WRe₃ were detected; the precipitate's number density is ∼ 10²³ m^-3 with a mean diameter of 4.0 nm. None of the ∼ WRe or the ∼ WRe₃ coherent precipitates were associated with either line or planar defects or with any impurity atoms. Therefore, a true homogeneous radiation-induced precipitation occurs in these alloys. A physical argument is presented for the nucleation of WRe or WRe₃ precipitates in the vicinity of displacement cascades produced by primary knock-on atoms.