The local atomic environment of Cu and Ni in Fe-Cu-Ni alloys following thermal ageing and neutron irradiation: A study using fluorescence mode X-ray absorption fine-structure spectroscopy

X-ray absorption fine structure (XAFS) spectroscopy using fluorescence mode detection has been successfully extended to quantitative examination of the local atomic environment of dilute analytes in a strongly scattering host matrices. This is applied to the characterization of thermally aged and neutron-irradiated Fe-Cu and Fe-Cu-Ni alloys which, despite their metallurgical simplicity and high Cu content, are believed to model some aspects of the behaviour of steels used in the fabrication of pressurized-water reactor reactor pressure vessels. Specifically, neutron-irradiation- and thermal-ageing-induced changes in the local atomic environment of Cu and Ni in binary Fe-1.3 wt% Cu and ternary Fe-1.28 wt% Cu-1.43 wt% Ni alloys aged at 550°C for up to 760h and irradiated to 6.95 × 10¹⁹ neutrons cm^-2 (E > 1 MeV) at 288°C have been examined. The close similarity of the atomic numbers of Fe, Cu and Ni make this a particularly demanding test of the experimental technique. Examination of the atomic structure around Cu using XAFS demonstrate similar and continuous evolution of the mean Cu environments of binary and ternary alloys during thermal ageing associated with the formation of bcc Cu microprecipitates. The data are consistent with the subsequent emergence of a 'fee-like' phase proposed previously but indicate that even after extended thermal ageing (760 h) the proportion of Cu in such structures is small (about 10%). Ni appears to remain predominantly in solid solution, inhibiting Cu precipitation. The mean Cu environments of Fe-Cu and Fe-Cu-Ni irradiated to 6.95 × 10¹⁹ neutrons cm^-2 (E > 1 MeV) are essentially identical and each can be approximated by alloy specimens thermally aged beyond peak hardness. The results are discussed in relation to previous findings and a possible limitation of thermally aged alloys as models of irradiated specimens is identified.