The metallic bond: Elastic properties

Establishing relationships between electronic structure and the properties of metals and alloys has relied heavily on assigning various attributes to the metallic bond. Principal among these is that of a directional bond. Bond directionality has been evoked as an explanation for the magnitudes of elastic shear constants, elastic anisotropy, the susceptibility to brittle failure, and preferred allotropic and alloy structure. Though bond directionality is frequently used as an explanation of metallic and alloy properties, it has never been rigorously defined. In this paper, a definition of bond directionality is provided which allows for its quantification. Using this definition, it is shown that the relative magnitudes of the single crystal shear elastic constants for f.c.c. metals can be correlated with bond directionality. The only information which is required to provide a measure of bond directionality is the total charge density corresponding to the given metal or alloy. This information can now be easily determined through the use of modern electronic structure techniques.