INFLUENCE OF LONG-TERM THERMAL AGING ON THE MICROSTRUCTURAL EVOLUTIONOF NUCLEAR-REACTOR PRESSURE-VESSEL MATERIALS - AN ATOM-PROBE STUDY

Atom probe field ion microscopy (APFIM) investigations of the microstr ucture of unaged (as-fabricated) and long-term thermally-aged (similar to 100 000 h at 280 degrees C) surveillance materials from commercial reactor pressure vessel steels were performed. This combination of ma terials and conditions permitted the investigation of potential therma l aging effects. This microstructural study focused on the quantificat ion of the compositions of the matrix and carbides. The APFIM results indicate that there was no significant microstructural evolution after a long-term thermal exposure in weld, plate and forging materials. Th e matrix depletion of copper that was observed in weld materials was c onsistent with the copper concentration in the matrix after the stress relief heat treatment. The composition of cementite carbides aged for 100 000 h were compared to the Thermocalc (TM) prediction. The APFIM comparisons of materials under these conditions are consistent with th e measured change in mechanical properties such as the Charpy transiti on temperature. (C) 1997 Elsevier Science B.V.