Effect of carbon on the annealing hardening of irradiated vanadium

Measurements of electrical resistivity and mechanical property, and microscopic observation were carried out in order to clarify the role of carbon in the annealing hardening of neutron-irradiated vanadium specimens containing various amounts of carbon. The results obtained are as follows: (1) Two peaks appeared in yield stress-annealing temperature curves of the specimens irradiated to 3.4 and 4.7x1020 neutrons/cm2 (>1 MeV) and annealed for 1 hr at various temperatures: The first one is at about 2000C and the second is at about 5000C. (2) The first peak increased in height and shifted slightly to lower temperature side with increase in carbon content. Correspondingly, the isochronal recovery curve of electrical resistivity also shifted to lower temperatures with increase in carbon content. However, there was no significant change in the microstructure with annealing temperature up to 3000C, and the structure was characterized by a high density of very fine and irregularly-shaped defects. (3) The second peak extremely increased in height with increase in carbon content, but did not shift. The defect clusters changed to dislocation loops above 3000C and decreased their density with increase in annealing temperature. (4) It was considered that the first peak was caused by segregation of interstitial impurities dissolved in the matrix to the defect clusters, and that the second was by the dislocation loops reinforced with segregated carbon atoms which came out at annealing temperatures by decomposition of cabide pre-existed at lower temperatures and probably precipitated again around the loops by cooling to the ambient temperature. (auth.)