Microstrure Design,Fabrication And Properties Of Ultrafine Grained Cu

The microstructural units like intragranular nanoparticles,nanotwins and heterogeneous microstructures are always introduced in ultrafine grained Cu for strengthening and toughening,while multiple strengthening strategies,higher density and lower impurity content can keep a better electrical conductivity of the ultrafine grained Cu.The traditional fabrication processes,including several plastic deformation（SPD）and high energy mechanical milling（HEMM）,are not suitable in fabricating ultrafine grained Cu with good comprehensive properties,because the SPD cannot introduce multiple strengthening,while impurities are easily introduced by HEMM.Dealloying can change an alloy powder into the nanostructured powder with controllable nanostructures and lower impurities.Based on the microstructural characters of the strengthening and toughening units in the ultrafine grained Cu,dealloying was used for designing and fabricating ultrafine grained or nano-grained Cu powders with unique nanostructures,then the as-fabricated Cu powders were consolidated into ultrafined grained Cu bulks with good overall performance of strength,ductility and electrical conductivity,which overcome the disadvantages of the ultrafine grained Cu fabricated by traditional processes.The first design strategy is an ultrafine grained Cu with the nacre-inspired lamellar structure.In this part,the nanograins attached and ultrafine grained Cu flake（NGA）powder was designed and fabricated by HEMM and dealloying.The NGA powder has a lot of surface-attached nanograins which are smaller than 100 nm.The fabrication processes and formation mechanism of the special nanostructures are well investigated.The NGA powder was consolidated into an ultrafine grained Cu bulk by thermomechanical processes.The interlamellar nano-bridges formed by sintering of surface-attached nanograins,while spherical and aligned nanopores were remained between nano-bridges.It is called the ultrafine grained Cu with nacre-inspired lamellar structure.It has a higher uniform elongation（2.7%）and better ability to resist work softening than the ultrafine grained Cu fabricated by SPD（～1.5%）.The second design strategy is an ultrafine grained Cu with Cu-Al₂O₃-Cu sandwich structure.Numerous nanopores and Al atoms deteriorate the electrical conductivity（75%IACS）of the Cu bulk.Selective oxidation between Al and O atoms in the NGA powder can happen to form surface-attached Al₂O₃ nanoparticles when the NGA powder is suitably heat-treated,and an ultrafine grained Cu bulk with Cu-Al₂O₃-Cu sandwich microstructure was fabricated by sintering of the heat-treated powder.Comparing with the ultrafine grained Cu bulk with nacre-inspired lamellar structure,it has a 30%higher elongation to fracture（9.3%）and a 33%higher electrical conductivity（94%IACS）.The above-mentioned lamellar structures only introduce grain boundaries strengthening and dislocation strengthening.In order to introduce Orowan strengthening for further improvement of strength and electrical conductivity of the ultrafine grained Cu,an ultrafine grained Cu strengthened with dispersed Al₂O₃ nanoparticles should be designed and fabricated.Cu-50wt.%Al powder was dealloyed in hydrochloric acid to form a nanoporous Cu-0.8%Al powder with Cu₂O on its surface.By changing the sealing methods of the powder during heat treatment,external oxidation and internal oxidation of Al₂O₃ can be controlled to introduce Al₂O₃ particles inside or outside of the Cu powder.In order to investigating the influence of the Al₂O₃ distribution on strength,ductility and electrical conductivity of the Al₂O₃/Cu composites,two kinds of Al₂O₃/Cu powders were consolidated into rods by powder compact extrusion（PCE）.The Al₂O₃ nanoparticles in the former sample are mainly distributed at grain boundaries,while that in the latter sample are mainly distributed inside the Cu grains.The latter sample has a higher strength and ductility and a somewhat lower electrical conductivity,but the combination of mechanical and electrical properties is better.Further research shows that high dislocation density in the as-extruded sample tend to interact with the intragranular Al₂O₃ nanoparticles and inhibit their Orowan strengthening effect.The strength,ductility and electrical conductivity can be improved by short-time and low temperature heat treatment of the Al₂O₃/Cu nanocomposite with uniform Al₂O₃ distribution.The combination of its tensile strength and electrical conductivity（522MPa;90%IACS）is better than most of the reported Al₂O₃/Cu composites.The property improvement is mainly because of synergistic strengthening by grain boundaries,dislocations and Al₂O₃ nanoparticles,while dealloying eliminates the impurity content and increases the electrical conductivity.