Nondestructive evaluation for microstructural changes of materials caused by neutron irradiation, which makes the material brittle, is necessary to operate nuclear power plants more safely and efficiently. One of the most important microstructural changes in irradiated materials is the formation of solute atom clusters, which pinned down the dislocations and make materials hard and brittle. In this study, we evaluated solute atom clusters in Fe-Cu alloy specimens by using the nonlinear ultrasonic method. Solute atom clusters were formed by annealing Fe-Cu alloy specimens at 500 ℃ for various lengths of time. These solute atom clusters were observed by the atom probe tomography (APT) and the transmission electron microscope (TEM), and their radius and number densities were evaluated. As a result of cluster analysis, solute atom clusters were not formed in the specimen without annealing, while they were formed in the specimen annealed for 25 hours. In addition to observing solute atom clusters, we also evaluated the acoustoelastic constant (AEC), which is one of the parameters depending on the nonlinearity of the stress-strain relation. The result of the experiment shows that the mean values of AEC were significantly different between the specimen without annealing and the one annealed for 25 hours. This suggests that AEC is sensitive to the formation of solute atom clusters.
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