Beryllium-rich intermediate phases in beryllium alloys

The results of a survey of the factors affecting the formation of phases of stoichiometry MBe5, M2Be17, MBe12 and MBe13 are presented. Using published information it is shown that the structures adopted at the higher Be:M ratios involve different characteristics from those adopted at lower Be:M ratios. In the ThMn12 and NaZn13 structures adopted in the former case dsub(M-Be) > (rsub(M) + rsub(Be)), (where the atomic radii refer to coordination number 12) and the dsub(Be-Be) distances are contracted. In the CaCu5 structure adopted at composition MBe5, dsub(M-Be) < (rsub(M) + rsub(Be)) and interactions between unlike atoms are significant. The structural characteristics, occurrence, and the stabilities of these phases, and of the others mentioned above, are discussed in terms of atomic radius ratios, the position of the M component in the periodic table, and the value of the univalent ionic radius of the M component, taken as a measure of the extension in space of the hard incompressible ionic core. Though the compound-forming characteristics of beryllium are largely dictated by its small atomic diameter, other factors such as the nature of the bonding which can be exerted by the partner atoms are also significant. In particular, the proportions of the volumes of the atoms which are occupied by the hard incompressible ionic cores assume importance. (author)