Strain localization and failure in irradiated zircaloy with crystal plasticity
Abstract This paper presents a micromechanical and mechanistic study of irradiation-induced crystallographic softening known to accelerate failure in irradiated zircaloys typically used as cladding material in pressure water nuclear reactors. The irradiation is known to lead to an increase in yield strength, and reduced ductility is anticipated to result from the progressive reduction in slip system strength. Extensive studies using transmission electron microscopy (TEM) show the formation of type dislocation channels in irradiated zircaloys anticipated to affect basal and prismatic systems. A crystal plasticity approach is established to incorporate basal and prismatic crystallographic softening, both of which are shown to be required in order to capture independent experimental observations for irradiated zircaloy. Representative irradiated zircaloy textures subjected to cyclic loading regimes were modelled to provide an understanding of the failure processes during in-service conditions. Under both strain and stress-controlled cyclic loading, irradiation softening led to the development of strain localization, and the formation of slip banding and its coalescence. This was found to lead to localized ratcheting and macroscale softening, and in strain-controlled loading, ultimately to plastic shakedown. Stress-controlled cyclic loading, however, especially with non-zero mean applied stress, led to pronounced local and macroscale ratcheting, influenced profoundly by the irradiation softening, and hence finally to ductile failure. It was also observed that local strain hardening due to GND development was small compared to irradiation-induced softening processes, supporting the notion that slip system softening dominates shear band formation.