Dislocation structure evolution induced by irradiation and plastic deformation in the Zr-2.5Nb nuclear structural material determined by neutron diffraction line profile analysis

Zr-2.5Nb samples removed after 7 years of service from a nuclear power reactor were investigated by traditional mechanical testing and whole pattern neutron diffraction line profile analysis of the irradiated and deformed materials. A significant increase in yield strength and subsequent strain softening are observed in the as-irradiated material. The line profile analysis allows the change in mechanical properties to be directly related to evolution of the microstructure. A fourfold increase in overall dislocation density accomplished entirely by an increase in the 〈a〉 Burgers vectors dislocations, and profound change in the dislocation network arrangement, are found to be created by the fast neutron irradiation. Comparison to the microstructural evolution during plastic deformation of the unirradiated sample shows a similar increase in dislocation density, but the increase is equally distributed amongst 〈a〉 and 〈c + a〉-type dislocations. Finally, plastic deformation of the previously irradiated material again increases the dislocation density significantly but, in contrast, does so through a 10-fold increase in the 〈c + a〉 dislocation density relative to the as-irradiated material, while the 〈a〉 Burgers vector density does not change. The different evolution of the 〈a〉 and 〈c + a〉 Burgers vector ratios in the unirradiated and irradiated Zr-2.5Nb during plastic deformation can perhaps be explained by the strain localization effect previously reported in irradiated Zircaloy subjected to deformation.