Microstructural evolution and hardening of neutron irradiated vanadium alloys at low temperatures in Japan Material Testing Reactor

In order to clarify the mechanism of the large increment of DBTT due to low temperature neutron irradiation, the mechanical properties and the microstructural evolution were investigated in vanadium binary alloys irradiated in JMTR. From a series of microhardness tests conducted on these alloys after irradiation at 90-200 degrees C, the hardness increased with increasing irradiation temperature, except for V-5Ti and V-5Nb. However, the hardening decreased with increasing irradiation temperature from 350 degrees C to 400 degrees C. The dislocation loops were observed by TEM after irradiation from 90 degrees C to 200 degrees C, while no void was observed at these irradiation temperatures. Positron annihilation spectroscopy (PAS) exhibited the nucleation of vacancy clusters in undersized binary alloys. The dislocation loop density increased with increasing irradiation temperature. It is considered that the irradiation hardening is mainly caused by dislocation loops. It has also been suggested that interstitial oxygen promote dislocation loop nucleation since interstitial oxygen and solute atoms interacted strongly and bound at around 200 degrees C leading to enhanced nucleation of interstitial loops. (C) 1999 Published by Elsevier Science B.V. All rights reserved.