Effects of temperature change on the microstructural evolution of vanadium alloys under ion irradiation

The evolution of microstructure in vanadium alloys after upward and downward temperature changes has been closely examined. Vanadium alloys have been irradiated in the high fluence irradiation facility at the University of Tokyo (HIT). Irradiations have been performed either at constant temperature of 500°C or in a stepwise temperature sequence of either 350/500°C, 400/500°C, 450/500°C, 500/350°C or 350/500/350°C up to 0.5 or 0.75 dpa. After 350/500°C and 400/500°C temperature change irradiations, small dislocation loops have been observed. The density of these dislocation loops decreased with the pre-irradiation temperature. After 450/500°C irradiation, the microstructure was coarse, indicating that the initial temperature (450°C) was high enough to be in the regime, where the growth of defects mainly occurs. In the case of downward temperature change, microstructures coarser than those of higher temperature irradiations were observed. This apparent anomaly may be understood in terms of the rate theory.