Evaluation of solute atom cluster in Fe-Cu alloy by acoustoelastic constant

Abstract

Non-destructive evaluation of microstructural change in reactor pressure vessel (RPV) steel is necessary to operate aged nuclear power plants more safely. According to previous studies, microstructural change not only results in the embrittlement of the materials but also influences the material nonlinearity. However, these studies are not sufficient to identify what is the main cause of change in the material nonlinearity. In this study, to evaluate the effect of solute atom clusters, which is an important microstructural change due to neutron irradiation, on the material nonlinearity, we measured the material nonlinearity in Fe-Cu alloy specimens containing Cu-rich solute atom clusters. In particular, the material nonlinearity was evaluated by acoustoelastic constant (AEC). To form different number densities and diameters of solute atom clusters, the specimens were annealed for a different time. The number density and diameter of the solute atom clusters were evaluated by atom probe tomography (APT). As a result, solute atom clusters observed by APT were a few nanometers in size and had a number density of about 1×10²³ m^-3, which was comparable to the diameter and number density of that in the irradiated RPV steel. There was a significant difference between the mean values of AEC measured for the specimen without annealing and the specimen with annealing for 25 hours. This result suggests that the AEC is depending on the formation of fine and dense solute atom clusters.