Solidification Structure And Properties Of Cu-Fe In-situ Composites Prepared Under Alternating Magnetic Field

The applications of electromagnetic solidification technique not only enrich and develop theories of metal solidification,and promote the emergence of new material and advanced technology.Cu-Fe deformation processed in-situ composites is considered as a high strength and high conductivity Cu based material for industrial applications,because of its excellent mechanical properties and low production cost.But due to the coarse and non-uniform Fe dendrites and residual Fe solute atoms in Cu matrix,the practical application of Cu-Fe composites is restricted greatly.Alternating magnetic field(AMF)has the advantages of no pollution,no contact,etc.,it can change the heat and solute transfer process of melt in solidification process,and affects the microstructure,solute distribution,and material properties significantly.In this work,the general thoughts are improving the solidification structure and promoting the precipitation of Fe from Cu matrix under AMF.The key issues to research are how to regulate the growth and distribution of primary Fe phase and promote the precipitation of Fe solute atoms in Cu matrix.By means of Cu-14Fe and Cu-8Fe in-situ composites,the influence and mechanism on solidification behavior under AMF was studied.Based on the optimization of solidification process parameters,the AMF solidification technique for high strength and high conductivity Cu material has been established.According to phase equilibrium thermodynamic calculation,the compound technique of Ag micro-alloying and AMF solidification are proposed,influences of microstructure and properties on Cu-Fe in-situ composites from solidification to deformation process were analyzed,The main results of research are as follows:(1)The solidification behavior of Cu-Fe in-situ composite show that AMF influences the primary Fe phase nucleation and growth process on three modes,the dissociation of falling crystal nucleus,the fusing of dendrite arm,and solid-liquid interface stability.The intense convection of melt produced by AMF,in terms of macrosegregation,the floating of Fe dendrites due to density difference has been inhibited,and Fe phase distributes more uniformly in ingots.With the increasing of the magnetic flux density,the secondary dendritic arm of Fe dendrites become shorten and transform to rosette or spherality grains,meanwhile,the grains distribute more dispersion and uniform.The application of AMF improve the impurity float up so as to reduce the total oxygen content of ingot,and can affect the solute distribution in Cu matrix and Fe phase significantly.Correspondingly,AMF influence the strengthening mechanism of Cu-Fe in-situ composites by solid solution strengthening,precipitation strengthening and fine grain strengthening.A physical strengthening model of Cu-Fe composites can be described as mixing rule and Hall-Petch relation with these strengthening mechanisms.Influence on conductivity of Cu-Fe composites depends on the impurity scattering resistivity in Cu matrix.With the application of AMF,the decrease of oxygen impurities content and Fe content in the Cu matrix are beneficial to the improving of conductivity.We tested AMF technique in the solidification process of Cu-Fe-P-RE alloy which used for electric railway,the grain size of ingots is refined significantly,and mechanical properties and corrosion resistance are improved,the material comprehensive performance is better than the level of it with conventional casting technique.(2)The application of AMF in solidification process of Cu-Fe composites reduces Fe solute atoms content in Cu matrix effectively.After treat with AMF of various voltages and frequency,Fe content in different areas of Cu matrix are reduced,and decreased with the increasing of magnetic flux density.According to the test and calculation on microhardness and conductivity,thermodynamic and kinetic mechanisms are analyzed.The increase of melt undercooling caused by intense convection could reduce the melt free-energy.The potential barrier of the atomicgroup jumped from liquid phase to solid phase has reduced,which is conducive to the nucleation of Fe atoms during the solidification process,and reduce the Fe content in Cu matrix.(3)The phase diagram calculation shows that Ag can reduce the Fe solid solubility in Cu,experimental result approves addition of trace amount of Ag can refine the primary Fe phase size and decrease Fe content in Cu matrix effectively.As a result of influences on promoting the Fe atoms precipitation by AMF and inhibiting Fe atoms solution by Ag element,the optimization on the solidification structure and properties of Cu-Fe in-situ composites is more significant under the compound technique.Compare with the conventional solidification process,the average size of primary Fe phase decreases by about 25%,and the conductivity increases by about 20%.The morphology and distribution of primary Fe phase and Fe content in Cu matrix in the solidification structure have decisive influences on the strength and conductivity of Cu-Fe deformation processed in-situ composite.The strengthening mechanism of Cu-Fe deformation processed in-situ composite considered as interaction of dislocations strengthening,Fe filament strengthening and Fe precipitation strengthening.The dispersed fine primary Fe grain are beneficial to decrease the filaments dimension and spacing in deformation processed in-situ composites,and improve the tensile strength and plastic deformability of the composite on equal strain.The conducting mechanism of Cu-Fe deformation processed in-situ composites depends on the impurity scattering resistivity and the interface scattering resistivity.Impurities and defects in Cu matrix are the main factor which determine the electrical resistivity in lower strain stage,and the phase interface density is the main factor that determine the electrical resistivity when ?>3.7.Through the regulation on primary Fe phase growth behavior and promotion on nucleation of solid solution Fe atoms during solidification process,AMF/Ag microalloying compound technique improve the comprehensive performance of Cu-Fe deformation processed in-situ composite significantly.The tensile strength of Cu-14Fe deformation processed in-situ composites increases by about 16%and the conductivity increases by 20%.