An atomic resolution study of homogeneous radiation-induced precipitation in a neutron irradiated W-10at.% Re alloy

The phenomenon of radiation-induced precipitation has been investigated in a W-10at.% Re alloy using the atom-probe field-ion microscope. This alloy is subsaturated with respect to the solvus line of the primary solid solution (β phase). The specimens had been irradiated in the Experimental Breeder Reactor II (EBR-II) to a fast-neutron fluencc of ~4 × 10²² neutrons cm^-2 (E > 0.1 MeV) at 575, 625 and 675°C. This 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 a significant alteration of the microstructure of this alloy as a result of the fast-neutron irradiation. Precipitates with the composition ~WRe were detected at a density of 10¹⁶cm^-3. Coherent, semicoherent and possibly incoherent precipitates of the σ phase have been observed. They were not associated with either linear or planar defects, or with any impurity atoms; i.e. a true homogeneous radiation-induced precipitation occurs in this alloy. A physical argument is presented for the nucleation of the WRe precipitates in the vicinity of displacement cascades produced by primary knock-on atoms. It is suggested that the nucleation of WRe is due to the formation of tightly-bound mobile mixed dumbbells which react to form an immobile rhenium cluster. A possible sequence of point-defect reactions is detailed which can lead to a WRe 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. A mechanism for the suppression of voids, in this alloy, is presented which is self-consistent with the homogeneous radiation-induced precipitation mechanism.