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

Atom probe field ton microscopy (APFIM) investigations of the microstructure of as-fabricated, long-term thermally aged ({approximately}100,000h. at 540{degrees}F (282{degrees}C)) and neutron-irradiated (6.6{times}10{sup 18} and 3.47{times}10{sup 19} n cm{sup {minus}2} (E>1 MeV) at {approximately}550{degrees}F (288{degrees}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)