Influence of strain rate on mechanical properties and deformation texture of hot-pressed and rolled beryllium

Plastic deformation of hexagonal metals such as beryllium occurs by a mix of dislocation slip and deformation twinning mechanisms. Slip and twinning are controlled by different mechanisms at the atomic scale, and thus respond differently to variations in strain rate. In general, deformation twinning is expected to be favored by high strain rate conditions. Both textured and randomly textured polycrystalline beryllium samples were deformed at strain rates from 0.0001/s to 5000/s. The yield point was found to be strain rate insensitive over the 7+ orders of magnitude of strain rate. The hardening, however, is strongly rate dependent for some of the initial textures, depending on loading direction relative to the basal poles. Optical microscopy and neutron diffraction measurements of the crystallographic texture were carried out to monitor the evolution of the microstructure and, specifically, the activity of deformation twinning as a function of strain rate. The relative roles of the active slip and twin deformation mechanisms are linked to the observed rate dependence of the flow stress.