Under the influence of irradiation materials creep at low temperatures. The precise mechanisms of irradiation creep are not certain but in general terms it is clear that, as atoms displaced from their sites move about, they rearrange the crystal structure in such a way as to reduce the elastic energy, and if the material is under stress this gives rise to strain in the direction of the stress. For example one mechanism by which this might happen is that interstitial atoms produced by neutron scattering may tend to migrate to defects such as edge dislocations, causing them to climb so that the material strains. Another possible mechanism is that vacancies may coalesce on a plane in the crystal and if there is a compressive stress normal to the plane the resulting disc-shaped void may collapse causing the material to strain. At high temperature thermal agitation gives rise to such creep mechanisms: irradiation allows creep to take place at much lower temperatures. For example, substantial irradiation creep has been observed in 316 stainless steel at 280 °C whereas thermal creep is not significant below about 600 °C.

The creep strain is in most cases proportional to the stress, almost proportional to D, and nearly independent of temperature. Figure 3.14

shows the dependence of the ratio of creep shear strain to stress on D for various materials.

Irradiation both hardens materials (i. e. it raises the yield stress) and makes them more brittle (i. e. it reduces the elongation before failure). The ultimate stress usually changes relatively little, but there is a loss of work hardening. Typical stress-strain curves are shown in Figure 3.15.

There are two mechanisms that cause these effects. At low test temperatures the defects caused by irradiation damage reduce the mobility of dislocations and inhibit plastic strain so that the uniform elongation is very low. At higher test temperatures (500-600 °C in 316 stainless steel), the material anneals, the defects are removed, and the properties of the unirradiated material tend to be recovered.

Above 700 °C in 316 stainless steel the second mechanism comes into play. As shown in Figure 3.15 there is a loss of ductility which can outweigh the increase in ductility in the unirradiated material due to thermal effects. This is the result of helium generated by (n, a) reactions. At high temperatures it diffuses to the grain boundaries

where it collects in the form of small bubbles. These cause a loss of cohesion between the grains, so that they can be torn apart.