Effect of Crystallographic Texture on Sound Velocity Propagation in Polycrystalline Beryllium

Beryllium is being considered as a possible capsule material for ignition targets for the National Ignition Facility. The material and machining specifications may ned to be highly restrictive, especially with regard to isotropic sound propagation. Beryllium, a hexagonal metal, displays directionally dependent sound speeds due to its anisotropic Young’s modulus. Crystallographic texture transfers this anisotropic sound speed to the polycrystal to varying degrees depending on the texture strength. From published values for the elastic compliances for Be, the value of E for single crystals was seen to vary with azimuthal angle from the c axis, from about 350 GPa parallel to c to about 290 GPa parallel to a. The longitudinal sound velocity varies with E, and experimentally measured velocities on single crystal Be are in good agreement with the derived values. The value of E for polycrystalline Be was calculated from simulated textures ranging from 1 MRD (multiples of random distribution), i.e., random, to 2, 4, 8, 20, and 40 MRD. The difference in sound speed from the fastest to the slowest direction for those textured materials were 0, 0.5, 1.0, 1.9, 3.8, and 5.4 percent respectively. Experimentally measured textures, processed by hot-pressing, swaging, and HIPping, were used to illustrate the effect of process variables on the resulting texture. These types of differences in sound speed have tremendous implications for the manner in which the beryllium used for ignition capsules for the National Ignition Facility is fabricated.