Part XII – December 1968 – Papers - Deformation and Fracture of Beryllium Bicrystals

Beryllium bicrystals were Prepared by a seeding and floating zone-melting technique from high-purity beryllium single-crystal material. The starting material was given six floating zone purification passes prior to producing the bicrystals. Symmetric, asymmetric, and antisymmetric bicrystals were tested in tension. Included were bicrystals with boundaries both parallel and perpendicular to the tension axis. Modified symmetric bicrystals were also examined in detail. In these cases, the angles made by the slip planes with the boundary were not equal. Failure by (0001) cleavage, (1180) cleavage, and boundary separation were all observed depending on the orientation of these cleavage planes and the boundary with respect to the tension axis. Failure occurred on whichever system experienced the greatest normal stress. The origin of the fractures observed and the operative dislocation mechanisms were investigated by frac-tography and transmission electron microscopy. The results indicate that low-angle boundaries form in close proximity to the bicrystal boundary and act as sites for crack nucleation. It has been concluded that, although macroscopic compatibility was satisfied in many of the bicrystals examined, the bicrysta1 boundary was effective in Producing "early" failure by local bend plane formation as plastic deformation proceeded, followed by cleavage initiation and crack propagation. SINGLE crystals of high-purity beryllium oriented with the basal plane inclined 45 deg to the tension axis exhibit a critical resolved shear stress of about 300 psi and 200 pct extension by slipping only on the basal plane before failure by cleavage on that plane. Single crystals, oriented with the basal plane parallel to the tension axis, deform by prism slip at a critical resolved shear stress of 9000 psi and exhibit an extension of over 100 pct before failure in a ductile manner.' In contrast to the behavior of single crystals, poly-crystalline beryllium deformed in tension or bending exhibits little or no ductility at room temperature. Beryllium's lack of ductility at room temperature has been attributed to its lack of a sufficient number of independent slip systems. Beryllium has only four independent slip modes at room temperature and according to the Taylor2 and von Mises3 criteria at least five independent shear modes are necessary to completely satisfy the strain compatibility conditions at all the boundaries in a random polycrystalline sample. Studies of beryllium bicrystals thus appear to provide a convenient way in which to determine whether the condition of strain compatibility is a necessary and sufficient condition to prevent failure and whether obviously incompatible boundary conditions result in boundary separation. Bicrystal studies, although not providing all of the constraints encountered in polycrystalline samples, do allow one to select very specific orientation relation-