An analysis of small clusters formed in thermally aged and irradiated FeCu and FeCuNi model alloys

the pressure vessel of a pressurised water reactor, containing copper impurities, can be embrittled by the radiation-enhanced formation of copper-rich clusters during operation at 2900C. Precipitation has been studied in FeCu and FeCuNi alloys thermally aged for up to 10h at 5500C or irradiated to doses up to 6.95 x 1019n.cm-2 (E > 1 MeV) at 2900C. For the thermally aged alloys the most notable results include the clear observation of copper rich precipitates, evidence for the retention of a significant copper fraction in solid solution at the peak of hardening, evidence for a bi-modal distribution of particle sizes in the overaged condition, which was enhanced by the presence of Ni, and evidence for a Ni ''shell'' around the precipitates in the ternary alloy. For the irradiated alloys the FeCu matrix was virtually depleted of copper at the lowest dose but copper was retained to the highest dose in FeCuNi. The amount of Ni associated with the irradiation produced precipitates was significantly higher than for those produced thermally, however it has not been established whether this was within the particles or in the form of a ''shell''. The results are discussed in the light of the observed changes in hardness properties. (author)