Effects Of Cold Drawing And Aging On Microstructure And Properties Of Cu-Cr Alloy

In this work, Cu-Cr wires were prepared by melting, casting, aging and cold drawing. The micro structure evolution of the alloys during deformation, especially the Cu/Cr phase interface structure and element diffusion on the Cu/Cr phase interface, were investigated by TEM. The effects of the Cu/Cr phase structure on the mechanical and electrical properties were analyzed. The mechanism of the electrical conductivity change during aging was investigated. Moreover, the strengthening mechanism of the alloys during aging and cold drawing was also discussed.With the increase of the drawing strain, most of the original Cr particles in Cu-3%Cr gradually refine into fibers with a diameter of only20nm. The Cu grains also transform into filaments with an average diameter of100nm. The dislocation density increases rapidly and tends to decrease in certain degree at η>4. Local dynamic recover and recrystallization are observed in Cu grains.There is no special crystal orientation relationship between Cu and Cr phases before deformation in Cu-3%Cr. Besides, there is no element diffusion at the Cu/Cr interface. With the increase of deformation, Cu and Cr phases gradually produce K-S relationship and the original non-coherent Cu/Cr interface gradually evolves into semi-coherent and even full coherent interface. Elements diffusion region at the Cu/Cr interface appears gradually and the region widens to about4nm at η>4.The ultimate tensile strength of Cu-3%Cr increases with the increase of drawing strain and tends to keep constant at η>4.5. The amount, diameter and space of Cr fibers play important roles in the strength. At high strain, non-coherent interface evolves into coherent interface, resulting in the reduction of dislocation slip resistance. Therefore, the strength can hardly increase.The electrical resistivity of Cu-3%Cr rises with the increase of drawing strain. The increase of interface area is responsible for the resistivity increasing. There is a plateau in the electrical resistivity curve at η=4.5resulting from the formation of Cu/Cr coherent interface. This means the coherence of Cu/Cr interface reduces electron scattering at interface. The element diffusion at interface also has an influence on the electrical resistivity increase.The variation of electrical conductivity of Cu-3%Cr during aging can be characterized by Avrami empirical formula. The change of electrical properties is controlled by phase transformation rate. The experiment results are consistent with Matthiessen rule. The electron scattering from solute atoms is the dominant source of electrical resistance.Dispersed precipitates are formed in Cu-0.5%Cr after aging treatment. The alloy is strengthened by coherent strengthening mechanism. As the increase of drawing strain, Cu grains evolve into fibers and dislocation density increases rapidly. At high strain (η>3), dislocation cell walls evolve into low angle grain boundaries. The strength of Cu-0.5%Cr rises with the increase of drawing strain, and tends to keep constant at η>3. The fraction of precipitation strengthening keeps47%～49%at η>3.