ATOM-PROBE CHARACTERIZATION OF THE MICROSTRUCTURE OF NUCLEAR PRESSURE-VESSEL SURVEILLANCE MATERIALS AFTER NEUTRON-IRRADIATION AND AFTER ANNEALING TREATMENTS

Microstructural changes due to neutron irradiation of weld and forging materials were characterized using the atom probe field ion microscop e (APFIM). Neutron-induced clusters containing Cu, P, Ni, Mn and Si we re detected in the high copper weld (0.24 at.% Cu) after irradiation t o fluences of 6.6 X 10(22) and 3.47 X 10(23) nm(-2); only phosphorus a tmospheres were observed in the low copper forging material (0.02 at.% Cu) irradiated to an intermediate fluence of 1.5 X 10(23) nm(-2). The se results are in agreement with previous studies and with their respe ctive measured transition temperature shifts. In addition, APFIM exper iments were carried out on the high fluence weld material after two po st-irradiation annealing treatments. The first annealing treatment of 168 h at 454 degrees C is similar to the proposed condition for in sit u pressure vessel annealing and the second, 29 h at 610 degrees C, is similar to the final stress relief heat treatment employed in vessel f abrication. Annealing at 454 degrees C led to coarsening of the copper -enriched precipitates and a 92% recovery of the radiation-induced tra nsition temperature shift. Essentially complete rehomogenization of th e solutes was obtained in the simulated stress relief treatment at 610 degrees C. (C) 1997 Elsevier Science B.V.