Microstructure And Properties Of Heavy Deformed Multi-phase Filamentary Composites

Cu-6wt.%Ag in situ filamentary composite was prepared by cold drawing. The filamentary microstructure, electrical resistivity and strain degree were investigated. With increasing the draw ratio, the equiaxed Cu-rich dendrites, eutectic colonies and Ag precipitates in the as-cast and homogenized structure developed into the fine filamentary structure and the electrical resistivity increased. There is a change of the electronic scattering effect of the dislocation, the phase interface between Ag precipitates and Cu grains and the interface between eutectic filamentary bundles and Cu matrix in the alloy under different strain condition. The change of the electrical resistivity with strain when the filamentary structure evolves into nanoscale at heavy draw ratios is generally in accord with the interfacial scattering model proposed in the presented investigation on the composites with higher Ag contents.The Cu-12wt.%Ag were prepared to investigate the role of Ag precipitates on the properties of the alloy. Two processes of heat treatment were performed to produce different amount of Ag precipitates in the alloy. The microstructure of Ag precipitates was systematically observed by optical microscopy, scanning electron microscope and transmission electron microscope. The interface between Cu matrix and Ag precipitates could significantly block dislocation movement and produce electron scattering. Therefore, the alloys containing more Ag precipitates would exhibit higher strength and electrical resistivity.Cu-12wt.%Fe in situ filamentary composite was prepared by casting and cold drawing. The microstructure was observed and the hardness was determined. The results show that the equiaxed Cu and Fe grains can develop into the crowded filamentary structure. There is a preferred orientation on the longitudinal direction different from that on the radial direction of the wire specimens. The reduction of fiber scale shows an exponential relationship with the drawing ratio. With the drwing ratio increasing up to6.0, the strain degree of Fe grains increases linearly. However, the increase in strain degree of Fe grains with the the drwing ratio deviates from the linear relationship once the drawing ratio is over6.0. With the increase in the ratio of the longness to the diameter of the filaments, the density of Cu/Fe interface increases exponentially. There is a Hall-Patch relation between hardness and filamentary space. The microstructure refinement from drawing strain can decrease hardness obviously. In special, the hardness reduction from the microstructure refinement is more significant.For the investigation on the microstructure evolution of Fe and Ag phases, the Cu-6wt.%Ag-6wt.%Fe alloys were prepared by casting and heavy cold drawing. The microstructure of the alloys at different drawing strains was studied by scanning electron microscopy and transmission electron microscopy. The microstructure of as-cast Cu-6wt.%Ag-6wt.%Fe is made up of Cu matrix, Fe dendrites and eutectic colonies. The microstructure components develop into filamentary bundles during cold drawing. The Ag-rich colonies show uniform line-like distribution while Fe dendrites show ribbon-like distribution at the heavy deformation state. Fe phase with b.c.c. structure in the composite can keep the similar strain behavior as Cu and Ag phases with f.c.c. structure during drawing deformation.Dislocations form the dislocation cell structure in Cu-6wt.%Ag-6wt.%Fe filamentary composites during cold deformation. After drawing ratio up to6.4dislocation cells lose stability and transform into subgrain boundaries. Meantime deformation twins in deformed Cu grains increase in number. Ag fibers result from eutectic colonies and precipitates. There is a cube-on-cube orientation relationship,{111}cu//{111} Ag and<110>cu//<111>Ag between Cu matrix and Ag precipitates, which ensures the co-defoemation and similar interface behavior. As drawn ratio increasing to8.6, Cu/Ag interface evolves into coherent interface partly. There still are some Fe particles among Cu filaments, which results in dislocation multiplication and crack generation. More Fe solute in Cu matrix can be found as strain increasing.Both Ag and Fe constituents can impair the conductivity in the composite. Fe filaments suppress more the electrical conduction in the microcomposite than Ag filaments. A formulation deduced from a parallel-circuit model for the resistivity of a microcomposite has been given for the evaluation of the resistivity.