Extended Defects and Precipitations

We have already mentioned clustering occurring during the recovery of irradiated material. Experimentally one finds interstitial and vacancy clusters, the size of which increases with temperature. While it is not possible to detect any visible clusters at V 100°K by electron microscopy, this is no problem at room temperature. Neutron-irradiated Cu, for example, contains clusters from v 25 Aup to 175 A in diameter, the smaller ones being predominantly vacancy clusters, the bigger ones interstitial types  

The large vacancy clusters are called voids. At higher temperatures these voids may collapse and form loops. These loops may be regarded as a special type of dislocation. Dislocations are present in every non-ideal material and determine its mechanical properties. The two main types are the edge and the screw dislocations. Defects are called edge dislocations when one plane of atoms in the lattice is missing or supernumerary screw dislocations are formed when a part of the crystal is displaced by an atomic layer. Fig. 14 illustrates the two types of dislocation. 

In an ideal situation dislocation lines would penetrate the whole crystal. In reality they mostly extend from one grain boundary to another one or they are pinned by impurities. If the lines form a closed circle inside the crystal, they are called loops. Summarizing, one may say that dislocations can arise from vacancy clusters as well as from interstitial clusters due to their pressure on the lattice. Very often they are the final products of an annealing procedure. Dislocations already existing interact with point defects and impurities acting as traps or sinks. 

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