Modern industrial materials are often required to have excellent comprehensive properties. Contact wire used in high-speed train needs possess high strength and toughness, high conductivity and wear resistance, which are often trade-off with each other. The fundamental mechanism that determines the strength and conductivity paradox is that all four strengthening mechanisms inevitably introduce lattice defects, which further lead to electron scattering, reducing the conductivity. In fact, strength and conductivity are not exactly the same thing because strength is related to dislocation behavior, and conductivity is related to electron motion. In fact, we need to find a structure that only blocks dislocations without scattering electrons. In order to achieve the best comprehensive performance, after hundreds of millions of years, organisms have evolved fractal structure, which was first proposed by mathematician Lewis Fry Richardson It is a geometric concept referring to the infinite repetition of similar morphology from macro to micro. The fractal structure is widely existing in nature, such as trunk branches, leaf veins, romaine cauliflower, biological bones and muscles, and so on. Compared with biological fractal structure, man-made materials are simple and rarely have complex multi-level fractal structure. In this work, we constructed a fractal structure with high-density nano-precipitates in CuCrZr alloy via rotary swage plus aging, and break the strength, conductivity and ductility limits of existing Cu alloys. The CuCrZr alloy exhibits unprecedented comprehensive properties of a high ultimate tensile strength of 626 MPa, a ductility of 19% and an electrical conductivity of 82% international annealed copper standard (IACS). Microstructural analysis indicates that the fractal structure and high-density nano-Cr precipitates block and accumulate dislocations, but allow electrons to flow unimpededly along the axis of CuCrZr wire, resulting in the high toughness and conductivity. Our finding verifies fractal structure has the potential to obtain materials with super excellent comprehensive properties.
 

Fractal structure,Strength and ductility,Conductivity,Cu alloys,Nano-precipitates