Microstructural investigations of as-fabricated, long-term thermally aged and neutron irradiated RPV materials: an atom probe study

Atom probe field ion microscopy (APFIM) investigations of the microstructure of as-fabricated, long-term thermally aged (= 100,000 h. at 540 F (282 C)) and neutron-irradiated (6.6x1018 and 3.47xl019 n cm-2 (E>1 MeV) at 550 F (288 C)) surveillance materials from commercial reactor pressure vessel steels were performed. This combination of materials and conditions permitted the investigation of potential thermal aging effects as well as the neutron-induced effects. The microstructural study focused on the quantification of the matrix chemistry and the detection and characterization of radiation-induced precipitates. The APFIM results indicate that there was no microstructural evolution after a long term-thermal exposure in weld (0.28 wt % Cu), plate (0.13 wt. % Cu) or forging (0.02 wt. % Cu) materials. Conversely, matrix depletion of copper and phosphorus solutes and the presence of Cu, P, Ni, Mn and Si rich clusters were observed in the neutron-irradiated weld material. These APFIM comparisons of materials in all three conditions are consistent with the measured change in mechanical properties (transition temperature shift)