Metallic alloys show complex chemistries that are not yet understood so far. It has been widely accepted that behind the composition selection lies a short-range-order mechanism for solid solutions. The present paper addresses this fundamental question by examining the face-centered-cubic Cu-Zn α-brasses. A new structural approach, the cluster-plus-glue-atom model, is introduced, which suits specifically for the description of short-range-order structures in disordered systems. Two types of formulas are pointed out, [Zn-Cu12]Zn1~6 and [Zn-Cu12](Zn,Cu)6, which explain the α-brasses listed in the American Society for Testing and Materials (ASTM) specifications. In these formulas, the bracketed parts represent the 1st-neighbor cluster, and each cluster is matched with one to six 2nd-neighbor Zn atoms or with six mixed (Zn,Cu) atoms. Such a cluster-based formulism describes the 1st- and 2nd-neighbor local atomic units where the solute and solvent interactions are ideally satisfied. The Cu-Ni industrial alloys are also explained, thus proving the universality of the cluster-formula approach in understanding the alloy selections. The revelation of the composition formulas for the Cu-(Zn,Ni) industrial alloys points to the common existence of simple composition rules behind seemingly complex chemistries of industrial alloys, thus offering a fundamental and practical method towards composition interpretations of all kinds of alloys.
It was recently known that the compositions of industrial alloy specifications generally satisfy simple composition formulas issued from short-range-order structural units in their basic solid solutions. In present work, Cu-Ni face-centered-cubic alloys were further addressed by introducing the cluster-plusglue-atom model for the short-range-order structural descriptions. Composition formulas covering only the first twelve and the second six neighbor shells in the face-centered-cubic lattice are proposed, [Cu-Cu12](Cu,Ni)6 for the Cu-rich alloys and [Ni-Ni12]Cu6 ~ [Ni-Ni12](Cu5Ni) for Monel alloy, the only Ni-rich alloy specficiation, where the square-bracketed part represents a cuboctahedral cluster. The alloy specification selection is also discussed in terms of valence electron numbers per unit formula. The present work confirms that well-established industrial alloys have simple composition rules that reflect the intrinsic short-range-order local structures in the solid solutions.
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