Composition Formulas of Cu-Ni-Al Cupronickel Alloys Derived by the Cluster Plus Glue Atoms Model

Cu-Ni-Al cupronickel alloy is a precipitation-strengthening alloy with γ' (Ni₃Al) and γ″ (Ni₃Nb) phases embedded in the γ phase (Cu solid solution), enhancing strength and corrosion resistance. However, it is difficult to clarify the correlation between composition and properties due to the complex elements and microstructure. This study attempts to construct the composition formulas and microstructure constitution (e.g., phase volume fractions) of Cu-Ni-Al cupronickel alloys via the cluster plus glue atoms model. Based on the solubility behavior of alloying elements, a systematic classification was established as follows: γ phase elements (Cu-like elements, including Cu, Ni, Al, Mg, Mn); γ' phase elements: Al-like elements (including Al, Nb, Si, Cr, Mn, Fe); and Ni-like elements (including Ni, Fe, Cu). Using this classification and phase composition (γ and γ' phase under heat-treated conditions), the cluster formula of the structural units that carry the composition information were formulated as follows: γ-{(Cu,Ni,Al,Mn,Mg)₁₆}_1-x and (γ' + γ″)-{(Al,Nb,Si,Cr,Mn,Fe)₄(Ni,Fe)₈Cu₄}_x, where x represents the volume fraction of precipitates (γ' + γ″). The representative Cu-Ni-Al cupronickel alloys were also analyzed by the cluster formula, and theoretical phase volume fractions were obtained (13.21-26.32%). Furthermore, Young's modulus, predicted by the cluster formula, shows good agreement with the practical alloys, verifying its applicability for alloy design.