Low electrical resistivity and high strength are a basic requirements for copper alloys.However,it has been widely known that these two properties are contradictory to each other:high electrical resistivity means extensive electron scattering by obstacles in the alloy,which in turn blocks dislocation movement to enhance mechanical strength.That is to say,any increase in strength necessarily brings about an increase in electrical resistivity.Essentially,strength and electrical resistivity are coupled in metal alloy as both are issued from a similar microstructural mechanism. That is why it is generally difficult to evaluate these alloys comprehensively and to select the materials appropriately.
The present work addresses this fundamental problem by analyzing the dependence of hardness (in relation to strength) and electrical resistivity on solute content for deliberately designed ternary[Mo_{y/(y+ 12)}Ni_12/(y+12)]_xCu_100-x alloys (at.%),where x=0.3-15.0 is the total solute content,y=0.5-6.0 is the ratio between Mo and Ni.The Mo-centered and Ni-nearest-neighbored[Mo₁-Ni₁₂]cluster structure are used to construct a short-range-order structure model of solid solution.The cluster[Mo₁-Ni₁₂]in solution enhances the strength,without increasing the electrical resistivity much,for the solutes are organized into cluster-type local atomic aggregates that reduce the dislocation mobility more strongly than electron scattering.The short-range-order structure has an essentially identical function for strength and electrical resistivity. In this solution state,both hardness and resistivity increase linearly with solute content increasing.When the solute constituents do not meet the requirement for ideal solution,i.e.,Mo-Ni ratio exceeds 1/12,the maximum value that the cluster[Mo₁-Ni₁₂]can accommodate,the solid solution should be destabilized and precipitation should occur,such as Mo precipitation in this case.The deviation from the linear change of resistivity and strength with solute content are caused by different alloy states,that is,solid solution and precipitation,which contribute to the resistivity and strength differently.Here we define a new term,the ratio of residual tensile strength to residual electrical resistivity,i.e.the “strength/resistivity ratio” in short,which represents an essential property of the alloy system.This ratio is 7×10⁸ MPa/Ω· m) for the Cu-Ni-Mo alloy in complete solid solution state,and it is in a range of (310-490) 10⁸ MPa/Ω·m) for the Cu-Ni-Mo alloys in a fully precipitation state (i.e.,most of Mo solute atoms precipitate out of the Cu matrix).
Finally this new parameter is applied to the classification of common copper industrial alloys for the purpose of laying the basis for material selection.It is found that the strength/resistivity ratio of 310 effectively marks the boundary between the fully precipitated state and precipitation plus solution state.Using this criterion,it is concluded that alloys based on Cu-(Cr,Zr,Mg,Ag,Cd) are suitable for high-strength and high-conductivity applications.However,alloys based on binary systems Cu-(Be,Ni,Sn,Fe,Zn,Ti,Al) cannot realize the same purpose.The finding of the line dividing the characteristic properties of alloy having a strength-resistivity-ratio of 310 provides a key quantitative basis for comprehensively evaluating the alloy performance,which can effectively guide material selection and development of high strength and high conductivity copper alloys.