Simulation of crystal plasticity in irradiated metals: A case study on Zircaloy-4

A classical crystal plasticity formulation based on dislocation slip was extended to include the mechanisms of dislocation channelling, with associated strain softening which is observed in many alloys post irradiation. The performance of the model was evaluated against experimental data on Zircaloy-4, which included engineering stress-strain response and high-resolution digital image correlation strain mapping. Variants of the model were developed to evaluate the influence on the strain hardening law used, comparing hardening based on a linear relationship for effective plastic strain with that based on the evolution of geometrically necessary dislocations. In addition, governing equations for simulating the interaction between gliding dislocations with various types of irradiation defect were investigated; this included the comparison of isotropic and anisotropic interactions based on the resultant reaction segments for each interaction. It was shown that the engineering stress-strain response measured by experiment could be captured by many of the model variants, but the simulation of characteristic strain heterogeneity was more sensitive to the model used. Direct modelling of the HRDIC experiments indicated that the model successfully predicted the activation of slip systems in many cases and exhibited localised strain distribution as observed in the experiment. In all models localised kink band formation was predicted, which is not observed experimentally, which highlights limitations in modelling of softening materials with a local crystal plasticity approach and a required area of development going forward.</p

Exploring the hydride-slip interaction in zirconium alloys

Hydrogen pick-up and hydride precipitation can lead to embrittlement and fracture strength reduction of nuclear fuel cladding tubes made of Zircaloy. Plastic deformation of hydride packets and its interaction with local plasticity in the zirconium matrix is a key linkage of microstructure feature to structural integrity of hydrided polycrystalline bulk Zircaloy. This work focuses on explicit representation of hydride packets from high spatial resolution electron backscatter diffraction onto a crystal plasticity finite element model for capturing and understanding slip localisation near hydride-matrix phase boundaries, based on the extracted material property of hydrides. The mechanisms behind slip evolution including slip nucleation, slip transfer, and slip inhibition are studied by combined high-resolution digital image correlation and crystal plasticity results. Through assessing various slip transfer parameters, new slip transfer criterion is proposed for α/δ phase boundaries. Prior to slip transfer criterion, local micromechanical quantities, specifically shear stress and stored energy density, are necessary to drive and provide pathway for subsequent slip transfer at α/δ phase boundaries.</p