Evidence for neutron irradiation-induced metallic precipitates in model alloys and pressure-vessel weld steel

We have used positron Doppler-broadening spectroscopy to examine a series of neutron-irradiated model alloys (1 × 1023 n/m2, E>0.5 MeV) and 73W-weld steel (to 1.8 × 1023 n/m2, E>1 MeV. The copper, nickel and phosphorus content of the model alloys was systematically varied. The samples were examined in the as-irradiated state and after post-irradiation isochronal anneals to temperature up to 600 °C. By following the S and W parameters, and especially by plotting the results in (S,W) space, we can infer that the damage is a combination of irradiation-induced metallic precipitates and vacancy-type defect clusters. Samples with either high Cu or with a combination of high Ni and medium Cu (and the pressure-vessel weld steel) showed evidence for both irradiation-induced metallic precipitation, and vacancy-type clusters, while samples without either high Ni or high Cu showed predominantly evidence of annihilations in vacancy-type clusters. These results are discussed in terms of embrittlement models.