Molecule-like chemical units in metallic alloys

Each conventional alloy has its own specific compositions but the compositional origin is largely unknown due to our insufficient understanding about chemical short-range ordering in the alloy, in particular, in the solid-solution state. In the present paper, the compositions of metallic alloys are discussed and formulated, by unveiling the basic moleculelike structural units in solid solutions. Friedel oscillation theory, which describes the partial charge screening behavior in solid solutions, and henceforth the origin of short-range ordering, is applied to pin down the ideal chemical compositions of conventional metallic alloys. We propose that, at a specific composition, atoms self-assemble into an ideally ordered structure consisting of atoms residing in the nearest-neighbor shell (denoted as cluster) plus those in the next outer shell (denoted as glue atoms), which can be formulated as [cluster](glue atoms). This simplified version of short-range-order structure represents the smallest charge-neutral and mean-density zone (termed as “chemical units”) and can be regarded as the ‘molecules’ of solid solutions. Accordingly, the chemical units and the corresponding molecule-like formulas for face-centered-cubic (FCC), hexagonal close-packed (HCP), and body-centered cubic (BCC) structures are analyzed and equations are obtained to identify the chemical formulas for FCC solid solutions. For instance, well-known α-brass Cu-30Zn alloy is formulated as [Zn-Cu₁₂]Zn₄. Examples of aluminum alloys, superalloys and stainless steels are also illustrated, demonstrating the versatility of the present model to interpret chemically complex alloys.