Irradiation of metals and alloys at temperatures below recovery Stage V typically produces pronounced radiation hardening, as discussed in Chapter 1.04, Effect of Radiation on Strength and Ductility of Metals and Alloys. The matrix hardening is typically accompanied by reduction in tensile elongation and in many cases lower fracture toughness.21 -222 The uniform elongation measured in tensile tests for

metals and alloys irradiated in this temperature regime usually decreases to <1% for damage levels above 0.1-1 dpa, which may require use of more con­servative engineering design rules for the allowable stress of structural materials.223 The hardening is largely due to the creation of high densities of sessile defect clusters, which act as obstacles to dislocation motion in the matrix. The defect cluster densities decrease rapidly with increasing temperature above recovery Stage V. Figure 25 compares the

temperature-dependent defect cluster densities224,225
observed in neutron-irradiated Cu, austenitic stainless steel, and V-4Cr-4Ti. Stage V annealing of defect clus­ters is evident for temperatures above 150, ^200, and ^275 °C for Cu, stainless steel, and V-4Cr-4Ti, respectively. The mechanical properties in irradiated nonmetals at temperatures below recovery Stage V exhibit variable behavior, with observations of increased hardness,226,227 unchanged strength,228 and decreased hardness or flexural strength.229-232