Cluster Structure Model Of Stable Solid Solutions And Composition Design Of Corrosion-resistant Cu-Ni-based Multi-element Alloys

Cu-Ni alloys are widely employed as tube and vessel materials due to their excellent resistance to seawater corrosion. Minor Fe and Mn additions are necessary to enhance the corrosion resistance of commercial Cu-Ni alloys. The corrosion resistance decreases with further increasing Fe and Mn contents due to precipitation of a Ni-(Fe, Mn) phase. So the key issue is the optimum modification element contents in different Cu-Ni alloys with high temperature stability. Generally, single-phase homogeneous solid solution alloys are desirable for good corrosion resistance performance. Such a single phase state is easily retained by quenching when the solid solution alloy has the maximum thermal stability. Therefore, the amounts of modification element additions are determined by the stability issue of the parent Cu-Ni alloys. The aim of this investigation was two-fold, first to experimentally verify the optimum M (M=Fe, Ni and Cr) levels in different Cu-Ni based alloys from the view point of homogeneous solid solution and corrosion performances, and second to theoretically establish a cluster-based structural model for stable solid solutions that quantitatively explains the solid solubility. A series of studies were carried out focusing on the modification element contents of Cu-Ni alloys and the relationships among solid solution alloy composition-atomic cluster structure model-alloy heat treatment process-microstructure-macroeconomic performances have been established, which is important to theoretical research and practical applications.In this work, an atomic cluster-based structure model was presented to explain the stable solid solution solubility limit of M elements in Cu-Ni-based alloys, the M elements having negative enthalpies of mixing with Ni and positive enthalpies of mixing with Cu. By this model, it assumed that one M (M=Fe, Ni and Cr) atom and twelve Ni atoms formed a M-centered and Ni-surrounded cube-octahedron and these isolated M1Ni12 clusters are embedded in the Cu matrix conforming to a cluster formula [M1Ni12]Cux. The ratio of M and Ni is equal to 1/12, and the stable solid solution compositions of the M-modified Cu-Ni alloys is [M1/13Ni12/13]1/(1+x)Cux/(1+x), (M=Fe, Ni and Cr). The high stability of the resultant Cu-Ni-Fe alloys have been confirmed by first-principles calculations.XRD, SEM, TEM and electrochemical corrosion measurements were used to characterize the microstructure and corrosion-resistance performance of Cu-(Ni, M) alloys. Vickers hardness measurement revealed a two-stage hardening process as a function of Fe/Ni ratios. These two stages correspond to the Fe solution stage (Fe/Ni(?)1/12) presumably dominated by solute hardening, and to the precipitation stage (Fe/Ni>1/12) dominated by dispersion strengthening but also counter-balanced by softened Fe-Ni deprived Cu matrix. The alloy [(Fe0.6Mn0.25Cr0.15)1Ni12]Cu30.3 of Cu-Ni modified by Fe, Ni and Cr additions has the best corrosion resistance in 3.5% NaCl aqueous solution, the immersion corrosion rate being 0.0008μm/h after 240 hours.