Chemical units of binary solid-solution alloys directly derived from short-range-order parameters

Short-range ordering is a typical structural characteristic of solid solutions. According to our cluster-plus-glue-atom model, the chemical unit of a solid solution covers a nearest-neighbor cluster and a few next-nearest-neighbor glue atoms, [cluster](glue atoms). However, the determination of the number of atoms located in these two shells requires complex calculations. The present work provides a simple method to obtain the chemical unit in a direct manner using Cowley's short-range-order parameters α₁ and α₂, by graphically illustrating the relationship between the total number of glue atoms and the ratio of atomic radii. For example, according to this graphical illustration, the Cu_68.9Zn_31.1 alloy should have four glue atoms. Using its measured α₁ = −0.137 and α₂ = 0.149, a new form of chemical unit is obtained, [Zn–Cu_9.40Zn_2.60]Zn_1.65Cu_2.35, which is further simplified into integer forms of [Zn–Cu₁₀Zn₂]Zn₃Cu₁ = Cu₁₁Zn₆ or [Zn–Cu₉Zn₃]Zn₁Cu₃ = Cu₁₂Zn₅, explaining the cartridge brasses C27000 (65Cu–35Zn) and C26000 (70Cu–30Zn), respectively. This provides a new way of understanding the composition of industrial alloys from their atomic origins and offers a new alloy design strategy.