The correlation between the critical shear stress of neutron irradiated copper single crystals and the density of defect clusters

An attempt has been made to determine experimentally the relationship between the density of defect clusters, as observed by transmission electron microscopy, and the critical shear stress of single crystals during the annealing of neutron irradiated copper. Single crystals and foil specimens of the same purity (99·999%) were irradiated together for the same dose (2·5 × 1018 f.n.cm−2) and examined after annealing for various times at 275°, 306° and 336°c. It was found that a single linear relationship existed between the critical shear stress at 4·2°k and the square root of the density of clusters less than 50 Å in diameter after anneals at all of the three temperatures. For the larger clusters, which are thought to be interstitial loops, linear relationships were not obtained and the results for the three annealing temperatures did not coincide. This work suggests that irradiation hardening is almost entirely due to the small vacancy clusters and that the average value of the resistance F max of each cluster is not less than 1·25 × 10−4 dynes. It is also concluded that although clusters of different size are present they all have approximately the same F max.