The influence of dynamical structural relaxation of point defect clusters on void formation in irradiated copper

In the neutron-irradiation experiment with a temperature controlled capsule at JMTR, residual-gas-free copper was irradiated at 200 C and 300 C together with as-received copper. The fluences were 5 x 1018 n/cm2 (the low fluence) to 1 x 1020 n/cm2 (the high fluence). TEM observation of the irradiated specimens showed that interstitial clusters form a colony at the low fluence which develops into a dislocation structure at the high fluence. Between the colonies only vacancy clusters in the form of voids and stacking fault tetrahedra (sft) were observed. There are no effects of residual gas atoms on the formation of voids at the low fluence although the effects become appreciable at the high fluence. The number of vacancies which are accumulated in a void is 350 times larger than that in a sft at the low fluence. The number density of voids decreased with increasing neutron fluence while the number density of sft increased. The voids form uniformly in copper irradiated to the low fluence while they were observed along dislocations at the high fluence. Computer simulations by molecular dynamics show that small interstitial clusters relax to a bundle of left angle 110 right angle crowdions and move long distances in response to small strain fields. Interstitial clusters move along a left angle 110 right angle direction and can switch to other left angle 110 right angle directions, and form groups of clusters. At high temperature, a dense colony of the clusters forms and develops into a dislocation structure. It is shown that small vacancy clusters relax to movable structures at high temperature. (orig.)