Role of Dislocations, Dislocation Walls and Grain Boundaries in Void Formation during Early Stages of Fast Neutron Irradiation

High purity aluminium was used to study the details of microstructural evolution during early stages of neutron irradiation. Aluminium specimens were irradiated at 120°C to fluences between 2.10²¹ and 1.10²⁴ n/m² (E > 0.1 MeV).  Transmission electron microscopy (TEM) investigation demonstrated that, even in fully annealed material, irradiation-induced dislocations and voids evolve heterogeneously. In addition, voids and dislocations were found to segregate such that the groups of voids and dislocations are spatially separated from each other. This kind of heterogeneity and segregation is further enhanced by the introduction of microstructural heterogeneity (in the form of dislocation walls) prior to irradiation. Another form of heterogeneity was found to occur in a relatively wide band in the vicinity of the void denuded zone along grain boundaries; in this region, both formation and growth of voids were enhanced compared to that observed in the grain interior. It is argued that these results cannot be rationalized in terms of a conventional biasdriven mechanism operating in a continuous sink medium. Both cell size and grain boundary effects would indicate an unusually high rate of transport of self-interstitial atoms from cell- and grain-interiors to cell walls and grain boundaries.