The microstructure of fast neutron irradiated Nimonic PE16

Abstract

The microstructures of a number of conditions of Nimonic PE16, irradiated in EBR-II at temperatures from 394 to 636°C to displacement doses up to 74 dpa, have been examined. Volume changes, determined from density measurements, were less than 1% in the solution treated and aged (STA) and overaged (OA) conditions. Specimens which were cold worked by 20% or more prior to aging exhibited no significant void swelling. Redistribution of γ′ to point defect sinks occurred, particularly at higher temperatures, resulting in a skeletal γ′ form in both the STA and OA conditions. In cold worked conditions the γ′ precipitates tended to retain a more equi-axed shape. Continuous grain boundary layers of γ′ were produced in STA PE16 at intermediate irradiation temperatures. Grain boundary precipitation of M 6C and Laves phase occurred at higher temperatures and was more prevalent in a high-boron alloy. Mass spectrometry measurements confirmed that the helium 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 bubble surface energy of 3 J m −2. The mechanism of irradiation embrittlement in PE16 is discussed in the light of these observations.