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

Abstract

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 × 10 18 n/cm 2 (the low fluence) to 1 × 10 20 n/cm 2 (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 〈110〉 crowdions and move long distances in response to small strain fields. Interstitial clusters move along a 〈110〉 direction and can switch to other 〈110〉 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. The structure consists of vacancies which are connected in a curved string shape. Along the vacancy strings, many relaxations of a tri-vacancy of Damask- Dienes-Weizer type (3v-sft) were observed. Such a relaxation to the 3v-sft type makes it difficult for a single vacancy evaporation. Small vacancy clusters move and coalesce into larger vacancy clusters. The linkage of the results of experiments and computer-simulations suggests that voids nucleate as a metastable defects at coalesced vacancy clusters at high temperature. The nucleation of voids is not affected by the influence of gas atoms dissolved in the material. Micro-voids migrate in the specimens after their nucleation. During their movement, gas atoms are trapped in the voids. The trapping of a larger number of gas atoms limits the movement of voids. This leads to a higher number density of voids in the as-received copper than in residual-gas-free specimens at the high fluence. Voids form uniformly in specimens at the low flunce and they migrate to dislocation lines. Dislocations are also trapped at voids during climbing by absorbing interstitial clusters. These finally lead to the preferential formation of voids along dislocation lines.