At temperatures of interest for reactor materials some point defects cluster together with their own kind, and if large enough, their geometry and the type of point defect involved can be determined in the electron microscope. Depending upon their structure and the temperature, these clusters can move through the solid under the influence of externally applied elastic stresses, or internal stresses.
The interaction of dislocation lines with point defect clusters dominates the irradiation-induced mechanical property changes. The hardening which irradiation induces is dependent on the nature, size, number, and distribution of these clusters, which frequently depend on impurities and the temperature. However, above a certain temperature the clusters either become too large to be effective, or they disappear.
The mechanical properties can be modified if irradiation introduces impurities by nuclear transmutation. In practically all but some of the most pure materials, transmutations occur, and if the transmutation has a high probability and the product is noxious, the effects can be important. Because of their low solubility, and because they are normally gaseous at the temperatures of concern, the inert gas elements, which are frequent transmutation products, can have marked effects on the mechanical properties. These elements cluster together and form gas bubbles, which at suitable temperatures migrate bodily through the solid, colliding and coalescing with others to form larger bubbles, and collecting on dislocation lines, precipitates, and boundaries within the solid.
The bubbles impede the motion of dislocations through the solid and harden it, the effects depending mainly on the size and distribution of the bubbles and the number of gas atoms produced. However, the bubbles on the grain boundaries enlarge under tensile stress and can weaken the solid and reduce its ductilitv.