James Marrow

© 2019 Calibrating and verifying crystal plasticity material models is a significant challenge, particularly for materials with a number of potential slip and twin systems. Here we use digital image correlation on coarse-grained α-uranium during tensile testing in conjunction with crystal plasticity finite element simulations. This approach allows us to determine the critical resolved shear stress, interaction mechanisms and hardening rate of the different slip and twin systems. The constitutive model is based on dislocation densities and twin volume fractions as state variables, and the simulated geometry is constructed from electron backscatter diffraction images that provide shape, size and orientation of the grains, allowing a direct comparison between virtual and real experiments. An optimisation algorithm is used to discriminate between different models for the slip-twin interactions and to find the parameters that reproduce the evolution of the average strain in each grain as the load is increased. A tensile bar, containing four grains aligned with the load direction, is used to calibrate the model with eight unknown parameters. The approach is then independently validated by simulating the strain distribution in a second tensile bar. Different mechanisms for the hardening of the twin systems are evaluated, based on the interaction between coplanar and non-coplanar twins. The latent hardening of the most active twin system turns out to be determined by coplanar twins and slip. The hardening rate of the most active slip system is lower than in fine-grained α-uranium. The method outlined here can be applied to identify the critical resolved shear stress and slip-twin interaction mechanisms of other coarse-grained materials.

Three-electrode studies coupled with tomographic imaging of the Na/Na-β″-alumina interface reveal that voids form in the Na metal at the interface on stripping and they accumulate on cycling, leading to increasing interfacial current density, dendrite formation on plating, short circuit, and cell failure. The process occurs above a critical current for stripping (CCS) for a given stack pressure, which sets the upper limit on current density that avoids cell failure, in line with results for the Li/solid-electrolyte interface. The pressure required to avoid cell failure varies linearly with current density, indicating that Na creep rather than diffusion per se dominates Na transport to the interface and that significant pressures are required to prevent cell death, >9 MPa at 2.5 mA·cm-2.