Atom probe characterization of the microstructure of nuclear pressure vessel surveillance materials after neutron irradiation and after annealing treatments

Abstract

Microstructural changes due to neutron irradiation of weld and forging materials were characterized using the atom probe field ion microscope (APFIM). Neutron-induced clusters containing Cu, P, Ni, Mn and Si were detected in the high copper weld (0.24 at.% Cu) after irradiation to fluences of 6.6 × 10 22 and 3.47 × 10 23 n m −2; only phosphorus atmospheres were observed in the low copper forging material (0.02 at.% Cu) irradiated to an intermediate fluence of 1.5 × 10 23 n m −2. These results are in agreement with previous studies and with their respective measured transition temperature shifts. In addition, APFIM experiments were carried out on the high fluence weld material after two post-irradiation annealing treatments. The first annealing treatment of 168 h at 454°C is similar to the proposed condition for in situ pressure vessel annealing and the second, 29 h at 610°C, is similar to the final stress relief heat treatment employed in vessel fabrication. Annealing at 454°C led to coarsening of the copper-enriched precipitates and a 92% recovery of the radiation-induced transition temperature shift. Essentially complete rehomogenization of the solutes was obtained in the simulated stress relief treatment at 610°C.