APFIM [atom probe field-ion microscope] investigations of solute clustering and precipitation in irradiated RPV [reactor pressure vessel] steels

After exposure to low and intermediate fluence neutron irradiation, a variety of reactor pressure vessel (RPV) steels, including A533B-type surveillance specimens, Gundremmigen KRB-A ex-service plate, and test reactor irradiated materials, have been analyzed in the atom probe field-ion microscope (APFIM). With this instrument, it is possible to characterize the ultrafine microstructural features associated with irradiation embrittlement. The statistical significance of the solute clustering has been evaluated with the mean separation technique of Hetherington and Miller. Atom probe analysis has provided direct evidence of very diffuse phosphorus-enriched and copper-enriched atmospheres. Phosphorus appears to play a more dominant role than copper in these low fluence irradiations. 14 refs., 5 figs., 5 tabs