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 α 1 and α 2 , 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 68.9 Zn 31.1 alloy should have four glue atoms. Using its measured α 1 = −0.137 and α 2 = 0.149, a new form of chemical unit is obtained, [Zn–Cu 9.40 Zn 2.60 ]Zn 1.65 Cu 2.35 , which is further simplified into integer forms of [Zn–Cu 10 Zn 2 ]Zn 3 Cu 1 = Cu 11 Zn 6 or [Zn–Cu 9 Zn 3 ]Zn 1 Cu 3 = Cu 12 Zn 5 , 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.