The inherent mechanical properties of beryllium

The extent to which the plastic and fracture properties of metals in general and beryllium in particular are known and can be understood from the nature of the interatomic bonding is reviewed in this paper. A previous study by the author revealed a correlation between the elastic constants of pure metals and their susceptibility to cleavage fracture. The elastic constants were so chosen as to reveal the relative ease of bond bending and bond breaking, a subject only recently attempted using the electron theory of metals. The extreme position of beryllium among the metals was revealed, and although by purification and grain refinement it is possible to produce specimens with elongation to fracture of over 10% at room temperature the final fracture is still by cleavage. An attempt is made in this paper to explain the correlation in mechanistic terms. Linear anisotropic elasticity theory does not predict correctly either the choice of slip mode or the fracture plane of metal crystals. Furthermore measured values of critical resolved shear stress for slip and specific energies of cleavage fracture also show no simple correlation with the relative ease of cleavage of metals. The present review reveals gaps in experimental data and understanding of critical mechanisms required to relate interatomic forces to fracture properties of metals. Behaviour in the non linear regime must dominate events at the crack tip and at the cores of dislocations where the interatomic strains are large. Consequently success of the correlation of elastic constants with incidence of cleavage must therefore rest on the empirical assumption, made in the previous paper, that the non linear stress-strain relationships for metals at high strain are simply related to the linear elastic behaviour. (author)