Shock−wave compression of single−crystal beryllium

Abstract

Beryllium single crystals were shock loaded by planar impact along the a and c axes as well as along off−axis directions. The orientation of primary (basal), secondary (first−order prism), and tertiary (second−order pyramidal) slip systems, in addition to the relative magnitudes of their yield stresses, permits the study of dynamic yield behavior on each system separately. Slip on the primary system is activated by shock loading in directions which make angles of ϑ=23°−61° with respect to the c axis. The dynamic shear yield strength on the basal plane varies from 0.6 to 1.9 kbar depending on the shock−propagation direction: this effect is thought to be due to differences in compressive strain perpendicular to the basal plane. In all single crystals the rise times of the elastic waves are on the order of a few nanoseconds in contrast to the relatively long rise times reported for shock compression of polycrystalline beryllium. Dynamic yielding on secondary and tertiary planes results in significant stress relaxation behind the elastic wave front. This suggests that dislocations on these planes are undergoing rapid multiplication on a submicosecond time scale. The method of characteristics for rate−dependent material response is used to study dislocation multiplication and plastic wave propagation in crystals shock loaded along the a axis. The phenomenon of multiple plastic wave propagation is also observed and compared with finite−difference solutions for plane shock−wave propagation in anisotropic media. The shock−wave behavior of single−crystal beryllium is more complicated than one would expect on the assumed basis of elastic−perfectly plastic response of the three known slip systems.