Vacancy Clusters and Loops in Ion Bombarded Copper

Copper foils of 1000 Å thickness have been bombarded with 150 keV zinc ions, in the [011] direction, at 300 °K, and examined by quantitative electron microscopy. The damage has an attenuation length of 3000 Å. Each stopped ion creates, on average, three vacancy clusters or loops, the interstitials either reaching the foil surfaces or remaining invisible. Of the created vacancies 80 to 100 % survive. Ions create, on average, three secondary knockons in each cascade, and those of energy greater than 20 keV escape to form further cascades. The largest loops (ca. 1200 vacancies) correspond to a single cascade from a 65 keV ion. The smallest clusters (250 to 300 vacancies) correspond to the hard sphere radius at which further knockons are too large to escape from the cascade. These clusters are well above the visibility limit. Clusters and loops are formed at their final size, and there is no general growth during subsequent irradiation. The cluster configuration is relevant to irradiation creep and irradiation hardening; the lattice strain at the edge of a cluster is ca. 5 %, within a cluster only one lattice site in four is vacant, and each vacancy is bound to the cluster with an energy of ca. 0·15 eV. This is the minimum energy configuration for up to 600 vacancies. A cluster is not a void (it does not possess a free surface) nor is it a tetrahedron of stacking faults. Experiment and theory agree that these require higher energies. Clusters of between 600 and 900 vacancies collapse to sessile loops, thereby lowering the strain energy in the surrounding lattice. Larger clusters collapse to prismatic loops. This collapse does not require the nucleation of a Shockley partial dislocation.