Mechanism of ductility-loss in neutron-induced metals

Irradiation of metals with high-energy particles such as neutron, ion or electron produces points defects (atomic voids, interstitial atoms) and depleted zones where the atomic density is low. The formation of point defect clusters (dislocation loops) causes a decrease in hardening coefficient, leading to a decreased ductility. The life of concentrated gliding (dislocation channelling) in dislocation-loop-free large gliding zones is short, contrary to past predictions, indicating that channelling is not the dominant factor in the decreased hardening coefficient, or decreased ductility. Bud-like secondary gliding occurs to eliminate the elastic strain accompanying the bending of crystals in a large gliding zone while secondary propagation dislocation takes place to complete the bend gliding required to increase plastic deformation, resulting in the cancellation of the dislocation loops in the non-gliding zones. As a result, the resistance to the primary gliding deformation in such zones is greatly reduced to cause a decrease in hardening coefficient, hence a decrease in extension. The decrease in ductility becomes smaller as the strength after neutron irradiation is maintained more strongly by stable obstacles which do not disappear through reaction with dislocation loops. (Nogami, K.)