The Correlation Between Electrical Conductivity And Strength Properties Of Copper Alloys Based On Short-range-order Model

The key problem in the development of Cu alloys is the contradictory coupling between electrical conductivity and mechanical strength:any strength increase would lower conductivity.Since both properties are closely related to chemical short-range ordering,the increment induced by alloying should be rooted in the short-range structure in solid solutions.The present thesis attempts to understand the alloying principles of electrical conductivity and mechanical strength and investigates the quantitative relationships between the two properties and chemical compositions of Cu alloys,taking Cu-Ni-Mo as the example.The common structural origin of both properties is targeted from the viewpoint of chemical short-range ordering.Our research group previously developed a so-called cluster-plus-glue-atom model for the description of chemical short-range ordering.By applying this model as the composition bridge linking resistivity and strength,an intrinsic parameter characteristic to the alloy nature is proposed and used as criterion to classify Cu alloys.The thesis work covers three aspects as follows.(1)First,the cluster model for chemical short-range-ordering is used in the analysis of the resistivity mechanism in Cu-Ni-Mo alloys and quantitative relationships between resistivity and composition are established.Since the cluster-plus-glue-atom model for solid solutions is only confined to the first and next near-neighbor interactions,the complex short-range orders are largely simplified.Specifically,when the cluster model based on[Mo1-Ni12]are used,the Matthiessen law can be quantified that expresses the relationship between solute content and resistivity.Upon varying Mo/Ni ratios,the alloys can show three structural rates,the ideal cluster-solution state(Mo/Ni=1/12),extra Ni in solution with Cu matrix(Mo/Ni<1/12),and extra Mo precipitated from the Cu matrix(Mo/Ni>1/12).By considering the individual contributions from the three structural states,the quantitative resistivity-composition relationships are obtained.The additional resistivities arising from these three states are respectively 1.08?1.21 and 0.09,× 10-8?·m.These relationships validate the cluster model based on[Mo1-Ni12]and verifies the effectiveness of using this model in the calculation of electrical resistivity.(2)Next,by theoretical analyzing the strengthening mechanism in Cu-Ni-Mo alloys due to alloying,it is pointed out that the common structural origin of both strength and resistivity is chemical short-range ordering so that the relevant cluster model suits for quantifying the strength in Cu-Ni-Mo.The cluster model for chemical short-range ordering is then introduced to analyze the relationship between HV hardness and composition.According to the previously mentioned cluster model,the strengthening due to alloying,termed as residual hardness following residual resistivty,comes from the contributions from solutioning and precipitation.When the alloy is in the ideal cluster solution state,satisfying short-range-order[Mo1-Ni12]cluster,the residual hardness HR(Kgf· mm-2)and residual resistivity pR(10-8?·m)are both linearly dependent on the total solute contents,with the increment of residual hardness being 1.5 HV upon 1 at.%solute increase.It is therefore possible to remove the solute content term and connect directly hardness HR with resistivity pR as pR=0.72HR.Here a new parameter is defined,strength-over-resistivity ratio HR/?R,which is a characteristic constant in a given alloy system,irrelevant to alloy compositions,here HR/?R = 1.50/1.08?1.39.This parameter classifies the function of Cu-Ni-Mo alloys in terms of both strength and resistivity.(3)Finally,based on the above relationship between electrical resistivity and mechanical strength obtained in Cu-Ni-Mo alloys,Cu alloys are classified using the new parameter HR/?R or the strength-over-resistivity ratio,which reflects the fundamental property of the alloy system.Cu alloys for electrical conduction purpose is characterized by a nearly pure Cu matrix plus fine precipitation.Such a complex structure actually originates from a parent solid-solution state.The introduction of the cluster model leads to the quantification of the contributions from both matrix and precipitates.In the ideal cluster solution state for Cu-Ni-Mo alloys,HR/pR=7×10~8 MPa/?·m,and in the fully precipitated state without any solute in solution,it is 31O×10~8 MPa/?·m.A composite model is then proposed that satisfactorily explain the last value and unveils the importance of 50%IACS that is used commonly in the industries for conductive Cu alloys.Using the basic strength-over-resistivity ratio of the fully precipitated state,all Cu alloys are classified in the electrical resistivity and tensile strength coordination map into two major groups.It is pointed out that the specific 310×10~8 MPa/?·m line in the map separates the Cu alloys that possess both high strength and low resistivity,such as Cu-(Cr,Zr,Mg,Ag,Cd)systems,and those that do not,such as Cu-(Be,Ni,Sn,Fe,Zn,Ti,Al)systems.The unveiling of such a universal strength-over-resistivity ratio of 310×10~8 MPa/?·m,which is obtained from a special Cu-Ni-Mo system,provides a quantitative parameter to evaluate comprehensively the overall performance of all Cu alloys for this parameter is composition independent and is relevant only to the alloy system.This parameter is useful in guiding the materials selection and alloy development.