THE MICROSTRUCTURE OF FAST-NEUTRON IRRADIATED NIMONIC PE16

The microstructures of a number of conditions of Nimonic PE16, irradia ted in EBR-II at temperatures from 394 to 636 degrees C to displacemen t doses up to 74 dpa, have been examined. Volume changes, determined f rom density measurements, were less than 1% in the solution treated an d aged (STA) and overaged (OA) conditions. Specimens which were cold w orked by 20% or more prior to aging exhibited no significant void swel ling. Redistribution of gamma' to point defect sinks occurred, particu larly at higher temperatures, resulting in a skeletal gamma' form in b oth the STA and OA conditions. In cold worked conditions the gamma' pr ecipitates tended to retain a more equi-axed shape, Continuous grain b oundary layers of gamma' were produced in STA PE16 at intermediate irr adiation temperatures. Grain boundary precipitation of M(6)C and Laves phase occurred at higher temperatures and was more prevalent in a hig h-buron alloy, Mass spectrometry measurements confirmed that the heliu m generation rate in PE16 was about 1.2 appm per dpa, and assessments of the total helium contents of gas bubbles yielded a value for the bu bble surface energy of 3 J m(-2). The mechanism of irradiation embritt lement in PE16 is discussed in the light of these observations.