Study On High Strength And High Conductivity Deformation Processed Cu-Based In Situ Composites

The higher strength, better plasticity and conductivity of high-strength and high-conductivity deformation processed Cu-based in situ composites are being required due to the rapid development of microelectronics, electric power, information, transportation and electromechanical industries. Deformation processed Cu-14Fe, Cu-7Cr, Cu-14Fe-0.1Ag and Cu-7Cr-0.1Ag in situ composites were designed and prepared by optimized alloying composition, cast, deformation and heat treatment. Effect of Ag micro-alloying, directional solidification and high magnetic field on the microstructure and properties of Cu-Fe/Cr during the solidification, cold drawing and heat treatment were investigated by using X-Ray diffraction (XRD), optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), LCD electronic tensile-testing machine, Vickers hardness tester and micro-ohmmeter. The main results of the research are as follows.The as-cast microstructure of Cu-Fe alloys included Cu matrix and Fe dendrite, and that of Cu-Cr alloys was made of Cu matrix, Cr dendrite and a small quantity of Cu-Cr eutectic. Ag micro-alloying refined the Fe and Cr dendrites in Cu-Fe and Cu-Cr alloys, which decreased the average size and spacing of the filaments in deformation processed in situ composites. During the cold deformation, the formation of the curved lamelliform filaments experienced the following four main steps in the longitudinal section, namely breaking of dendrites, flattening and rotating of particles, lapping and merging of filaments, refining and homogenizing of filaments.Sectionalized physical model of strengthening mechanism was developed. With small cold deformation strain, the strength of the composites can be calculated by the modified rule of mixing <σC=σMfM+σXfX, where σM=σ+k3m1/2X, and k3=52for deformation processed Cu-14Fe in situ composite at η≤5. As the cold deformation strain increasing, the strengthening mechanism deviated from the rule of mixing gradually, and the strength agreed with Hall-Petch relation. The mathematical model can be expressed as σC=108+1299λ-1/2for Cu-14Fe composite at5<η≤7.8. As the cold deformation strain further increasing, the dislocation density in the composites decreased, so that the strengthening mechanism disobeyed Hall-Petch relation gradually, and the strength can be forecasted by the interface obstacle strengthening model. Ag micro-alloying increased the strength of Cu-14Fe and Cu-7Cr composites. This is attributed to the microstructure refinement which makes Ag micro-alloying deformation processed Cu-b.c.c. in situ composites conform to Hall-Petch relation in smaller cold deformation strain. The strengthening mathematical model of Ag micro-alloying can be expressed as σC=139+1299λ-1/2for Cu-14Fe-0.1Ag composite at4<77<7.8.The fracture surface analysis of the samples showed that the macroscopic fracture morphology of the composites gradually changed into shear fracture from cup and cone shape with the cold deformation strain increasing, and the dimples size in the microstructure gradually decreased. The shear fracture was delayed to bigger cold deformation strain after the addition of trace Ag, which indicated that Ag micro-alloying could improve the plastic deformability of deformation processed Cu-b.c.c. in situ composite.The influence of cold deformation strain on the conductivity of the composites depends on the interface scattering resistivity generated by the interface between Cu matrix and filaments. The mathematical model of the interface scattering resistivity can be expressed as ρint=-0.09+kD(d0/d), where kD=0.02for Cu-14Fe composite. Ag micro-alloying improved the conductivity of Cu-14Fe and Cu-7Cr composites. The effect of trace Ag on the conductivity of the composites depends on the impurity scattering resistivity. Ag micro-alloying promoted the solid solution Fe/Cr atoms to precipitate due to the solid solution advantage of Ag in Cu matrix, and solid solution Ag has relatively little damage to the conductivity of Cu matrix contrast to Fe/Cr. Therefore Ag micro-alloying reduced the impurity scattering resistivity to improve the conductivity of the composites.Ag micro-alloying reduced the thermal stability of Fe/Cr filaments due to refined filaments, decreased the interface energy between Cu matrix and filaments, and increased the diffusion coefficient of Fe/Cr atoms in Cu matrix, which promoted the peak conductivity to migrate to lower temperature during heat treatment. Moreover, it also contributed to promote precipitation decomposition of Cu matrix at lower temperature, and accelerate aging precipitation of Fe/Cr atoms to form dispersed precipitates, which led to the peak strength to migrate to lower temperature during heat treatment.The combination of the strength, conductivity and plasticity of Cu-14Fe and Cu-7Cr composites was optimized by using Ag micro-alloying, deformation, intermediate heat treatment and aging treatment. A new micro-alloying thermo-mechanical treatment process was developed. The prior heat treatment under high magnetic field enhanced the diffusion coefficient of Fe atoms in Cu matrix, accelerated the precipitation of supersaturated solid solution Fe atoms, and promoted Fe dendrites spheroidizing, refining and homogenizing, which made both strength and conductivity of the Cu-Fe composites increase. Directional solidification technology made primary Cr phase arrange along the drawing direction, decreased the average size and spacing of rod-like Cr phase, and made Cr distribute more uniform, which improved the strength of Cu-Cr composites significantly and retained the relatively high conductivity. Combining optimized subsequent heat treatment processes made the composites obtain good combination property of strength, conductivity and elongation. The strength/conductivity/elongation of Cu-14Fe-0.1Ag composite reached1149MPa/60.3%IACS/3.3%,1093MPa/61.9%IACS/3.5%and1006MPa/63.7%IACS/3.7%at η=7.8after isochronic aging treatment1h under10T magnetic intensity; The strength/conductivity/elongation of Cu-7Cr-0.1Ag composite reached1067MPa/74.9%IACS/2.9%,1018MPa/76.0%IACS/3.0%and906MPa/77.6%IACS/3.3%at η=8after isochronic aging treatment1h.