Study On Ultra High Strength Cu-Cr In Situ Composites

With the development of the high pulse and high field magnets technology and the high speed electric railway, there is a new urgent need by the magnetic field conductor and the electric contact wire, which needs the best combination of high strength, high electrical conductivity and high thermal conduction. Thus, deformed Cu-Cr in-situ composites were developed.Deformation processed Cu-15Cr-0.1Zr, Cu-15Cr-0.2Zr and Cu-15Cr-0.1Zr-RE in situ composites were manufactured by inductive melting, casting, swaging, appropriate wire drawing and intermediate annealing. Effect on the microstructure and properties of Cu-Cr in situ composite caused by the content of Zr, the addition of RE, intermediate annealing of alloys were studied through scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution electron microscopy(HREM), tensile test, and resistivity measurement, on the basis of the combination of Yu's empirical electron theory in solid and molecules (EET) and Cheng's improved Thomas-Fermi-Dirac (TFD) theory. The relationship between the valence electron structure of a biphase interface and its properties is explained further through the research on the effect of the fiber phase and Cu/Cr interface in in-situ composites. The main results of the research are listed below:A new type of high strength and high conductivity Cu-15Cr-0.1Zr and Cu-lOCr-0.1Zr in situ composites were successfully prepared using the cold drawing and intermediate heat treatment, When alloy was drawn to 6.43 the strength/conductivity can reach 1212MPa/71.5%IACS and 1096MPa/76.3%IACS respectively, and all the softening temperatures are up to 550℃.The addition of 0.1wt%Zr accelerate the precipitation of Cr in the matrix and keep the conductivity of the micro-alloying Cu-15Cr composite; the ultimate tensile strength of Cu-15Cr-0.1Zr can increases about 15% and the soften temperature of Cu-15Cr-0.1Zr can improves 50℃with addition the trace Zr. The addition of Zr about 0.2% in Cu-15Cr alloy could promote alloy work hardening in the process of alloy deformation, and against to the deformation of alloy, and reduced the conductivity of alloy. The addition of RE in Cu-15Cr-0.1Zr alloy improved electrical conductivity. But, at the same time it also reduced the strength.As cast, the alloy include of Cu matrix, gross Cr dendrite and a small quantity of Cu-Cr eutectic. In the process of alloy deformation, Cr phases with heat forged and solution treatment were elongated along the direction of drawing. At lower drawing strains, some of Cr fibers have the same bcc single crystal structure as that of dendrites in the as cast state. At higher drawing strains, a single fiber was divided into sub-grains. These sub-grains were separated by sub-grain boundaries. Relative angle differences across the sub-grain boundaries are about 5°～30°.With the increasing of strain, Cr phases formed fibers and Cr ribbons were curl and fold in cross section, observation at the interface showed that the interface of Cu/Cr were coherent or shows moire pattern contrast atη=6.43.The study on Cu-15Cr-0.1Zr alloy prepared by two times intermediate annealing showed appropriately intermediate annealing and obviously improved the electrical conductivity, and the strength did not terribly reduced. The first intermediate annealing was very important for the properties of alloy. When the alloy was drawn to 6.43, the alloy prepared by the first intermediate annealing at 450℃can obtain best strength, and the alloy prepared by the first intermediate annealing at 500℃can obtain best electrical conductivity. In this paper, the relationship of three times intermediate annealing and the properties of Cu-15Cr-0.1Zr alloy were also studied. The results showed that when alloy prepared by three times intermediate annealing was drawn to 6.43, the strength of alloy was smaller than the alloy prepared by two times intermediate annealing, but the electric conductivity had a little increasing.The strength of deformation-processed Cu-Cr in-situ composite was measured. The results showed to be anomalously higher than those predicted by rule of mixture equations. The strength increased with increasing of the deformation. The strengthening mechanism was discussed. The analysis indicate that the sub-structural especially phase boundary strengthening plays the crucial role. The experimental data are in good agreetment with the predictions of the geometrically-necessary dislocation model and interface as dislocation source model.The results of thermal stability test for Cu-15Cr-0.1Zr alloy showed that the softening temperature of the alloy exceed about 550℃, and achieved the performance figure. When alloy was annealed during 400℃～600℃, electric conductivity gradually increased with the increasing of annealing temperature. The electric conductivity of alloy reached maximum value (about 78%IACS) when alloy was annealed at 600℃. At last, with the continuous increasing of annealing temperature, the electric conductivity rapidly reduced. Investigate the fracture behavior of Cr fibers in heavily drawn Cu-15Cr-0.1Zr in-situ composite wires as a function of temperature. Results of SEM and TEM show the fibers undergo interfacial diffusion shape change from cavitation, longitudinal spliting plus, cylinderization plus, breakup to complete spheroidization failure. Predicted result showed the fracture behavior of the Cr fibers were consistent with interfacial diffusion, Experimental results were compared with existing models and the appropriate physical model for breakup of filaments were provided.The interface conjunction factors of the Cu/Cr interface of Cu-Cr in-situ composites show that:the electron density of Cu(lll)/Cr(110) interface Ap is the largest in other interface, and the number of atom state groupsσ' which keeps continuous interface electron density is larger, according to the theory of Yang Zhilin, The larger theΔρof Cu/Cr interface, the larger the stress on the interface is, if the larger theσ' or a, that is the steady combine at the higher stress station, so that accord with strength and toughness at higher strain.