Effects of varying temperature on microstructure of materials under irradiation can be very complicated. In this paper, evolution of microstructure in vanadium alloys during temperature changes is closely examined with emphasis on the effect of frequency of the temperature changes and the final temperature.

Pure vanadium, V-5Cr and V-5Nb were prepared. Neutron irradiation was conducted in JMTR at constant temperatures of 200°C, 400°C and under cyclic temperature changes of 200/400°C. The numbers of temperature cycles were 0.5, 5.5 and 6 cycles. TEM observations, Positron Annihilation Spectroscopy and micro-Vickers hardness test were performed.

By comparison between 0.5 and 5.5 temperature cycle, coarser microstructures are observed in 5.5 cycles. Frequent temperature change is considered to have restricted the defect growth. By comparison between 5.5 and 6 temperature cycle irradiation, which is focused on the effect of the final temperature, coarser microstructures are observed in 6 cycle irradiation.