Characterizing dislocation loops in irradiated polycrystalline Zr alloys by X-ray line profile analysis of powder diffraction patterns with satellites

This work extends the convolutional multiple whole profile (CMWP) line profile analysis (LPA) procedure to determine the total dislocation density and character of irradiation-induced dislocation loops in commercial polycrystalline Zr specimens. Zr alloys are widely used in the nuclear industry as fuel cladding materials in which irradiation-induced point defects evolve into dislocation loops. LPA has long been established as a powerful tool to determine the density and nature of lattice defects in plastically deformed materials. The CMWP LPA procedure is based on the Krivoglaz–Wilkens theory in which the dislocation structure is characterized by the total dislocation density ρ and the dislocation arrangement parameter M. In commercial Zr alloys irradiation-induced dislocation loops broaden the peak profiles, mainly in the tail regions, and occasionally generate small satellites next to the Bragg peaks. In this work, two challenges in powder diffraction patterns of irradiated Zr alloys are solved: (i) determination of the M values from the long tail regions of peaks has been made unequivocal and (ii) satellites have been fitted separately, using physically well established principles, in order to exclude them from the dislocation determination process. Referring to the theory of heterogeneous dislocation distributions, determination of the total dislocation density from the main peaks free of satellites has been justified. The dislocation loop structure has been characterized by the total dislocation density of loops and the M parameter correlated to the dipole character of dislocation loops. The extended CMWP procedure is applied to determine the total dislocation density, the dipole character of dislocation loops, and the fractions of 〈a〉- and 〈c〉-type loops in proton- or neutron-irradiated polycrystalline Zr alloys used in the nuclear energy industry.

Prediction of Grain-Boundary Interfacial Mechanisms in Polycrystalline Materials

A multiple slip dislocation-density based crystalline formulation has been coupled to a kinematically based scheme that accounts for grain-boundary (GB) interfacial interactions with dislocation densities. Specialized finite-element formulations have been used to gain detailed understanding of the initiation and evolution of large inelastic deformation modes due to mechanisms that can result from dislocation-density pile-ups at GB interfaces, partial and total dislocation-density transmission from one grain to neighboring grains, and dislocation density absorption within GBs. These formulations provide a methodology that can be used to understand how interactions at the GB interface scale affect overall macroscopic behavior at different inelastic stages of deformation for polycrystalline aggregates due to the interrelated effects of GB orientations, the evolution of mobile and immobile dislocation-densities, slip system orientation, strain hardening, geometrical softening, geometric slip compatibility, and localized plastic strains. Criteria have been developed to identify and monitor the initiation and evolution of multiple regions where dislocation pile-ups at GBs, or partial and total dislocation density transmission through the GB, or absorption within the GB can occur. It is shown that the accurate prediction of these mechanisms is essential to understanding how interactions at GB interfaces affect and control overall material behavior.