Revelation of solid solubility limit Fe/Ni = 1/12 in corrosion resistant Cu-Ni alloys and relevant cluster model

Minor Fe additions are necessary to enhance the corrosion resistance of commercial Cu-Ni alloys. The present paper aims at optimizing the Fe content in three alloy series Cu₉₀(Ni,Fe)₁₀, Cu₈₀(Ni,Fe)₂₀, and Cu₇₀(Ni,Fe)₃₀ (at.%) from the viewpoint of their corrosion performance in a 3.5% NaCl solution. An Fe/Ni = 1/12 solid solubility limit line was revealed in the Cu-Ni-Fe phase diagram. Three Fe/Ni = 1/12 alloys, Cu₉₀Ni_9.23Fe_0.77 (at.%) = Cu-8.6Ni-0.7Fe (wt.%), Cu₈₀Ni_18.46Fe_1.54 = Cu-17.3Ni-1.4Fe, and Cu₇₀Ni_27.7Fe_2.3 = Cu-26.2Ni-2.1Fe, show the best corrosion performances in their respective alloy series. The Fe/Ni = 1/12 solubility limit is explained by assuming isolated Fe-centered FeNi₁₂ cuboctahedral clusters embedded in a Cu matrix. The three Fe/Ni = 1/12 alloys can be respectively described by cluster formulas [Fe₁Ni₁₂]Cu₁₁₇, [Fe₁Ni₁₂]Cu₅₂, and [Fe₁Ni₁₂]Cu_30.3. The Fe/Ni = 1/12 rule may serve an important guideline in the industrial Cu-Ni alloy selection because above this limit, easy precipitation would negate the corrosion properties of the Cu-Ni-based alloys.