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Metals and other advances in the study of damage tolerance of gradient materials
[ Instrument Network Instrument Development ] The method of seeking to improve the mechanical properties of engineering structural materials at the same time is the direction of material scientists' long-term efforts. By taking inspiration from natural materials, material scientists create similar materials, which forms the concept of “learning to natureâ€. The unique structure of certain organisms in nature gives them good mechanical properties, enabling them to withstand the harsh environments of nature. One of the structures is a gradient structure, and the bamboo structure in nature is a typical gradient structure. The vascular bundle structure of bamboo has a density that gradually decreases from the outside to the center, and its core has better flexibility while maintaining overall strength and rigidity. Materials scientists have successfully replicated gradient structures in a variety of metallic materials and systematically studied their mechanical properties. Related studies have shown that gradient structure metal materials can break the inversion relationship of material "strength and plasticity" to a certain extent, and obtain excellent strong plasticity.
However, as an engineering structural material, acceptable fracture toughness (or damage resistance) must be further demonstrated to avoid catastrophic failure in practical applications. Fracture toughness is critical to the safe application of most materials and is often a comprehensive indicator of strength and plasticity. But in general: strength and plasticity are the global mechanical response controlled by the overall deformation of the material, and fracture toughness is the mechanical response of the "local" microstructure deformation at the crack tip. The deformation zone involved in the fracture tip of the fracture toughness is more than the tensile deformation. The volume is small by several orders of magnitude. Therefore, although an excellent combination of material strength and plasticity can be achieved by constructing a gradient structure, whether it can ensure the fracture toughness is also superior is still a key scientific problem to be solved. At the same time, local variations in the microstructure in the gradient structure material may cause changes in crack propagation resistance during crack propagation. Therefore, it is very important to reveal the influence of the non-uniformity of the microstructure in the gradient structure on the crack initiation and propagation resistance of the crack.
On the basis of the successful preparation of bulk gradient materials, Li Yi, associate researcher of the Materials Dynamics Research Department of the National Research Center of Materials Science, Institute of Metal Research, Chinese Academy of Sciences, and a professor at the University of California, Berkeley, Robert O. Ritchie Collaboration was carried out to evaluate the deformation and fracture behavior of gradient structure (GS) Ni with grain size spans from ~30 nm to ~4 μm. The study found that, compared with the brittle pure nanocrystals and the ductile coarse-grained Ni, the gradient structure Ni has high fracture toughness, showing the best combination of strength and toughness. Gradient materials consume significantly more plastic work during stretching to fracture than pure coarse and pure nanocrystalline materials, which are derived from the interaction between the internal gradual microstructures during crack propagation. 2. The fracture toughness and deformation behavior of the gradient material are related to the gradient direction. The initial fracture toughness (KJIc) in the direction from the coarse crystal to the nanocrystal gradient is higher than the initial fracture toughness in the direction from the nanocrystal to the coarse crystal gradient. When the crack propagates along the coarse crystal to the nanocrystal gradient, its R curve (crack resistance resistance curve) is similar to pure coarse grain Ni, showing the best combination of strength and toughness. At the initial stage of crack propagation, passivation occurs in the coarse-grained region, which is characterized by ductile fracture. However, in the late stage of crack propagation, brittle cracks are induced in the nanocrystalline region, and the failure is rapidly expanded, and unstable brittle fracture occurs. Since the crack propagation process is a transition process from ductile fracture to brittle fracture, careful consideration is required in safety applications. On the other hand, when the crack propagates in the direction from the nanocrystal to the coarse crystal gradient, it also exhibits an initial fracture toughness (KJIc) and an R curve superior to that of pure nanocrystalline Ni, and the toughness of the crack tip is continuously increased. In addition, when the crack propagates to the coarse crystal region, the crack tip is passivated. The crack tip toughness and the passivation process show that the gradient direction from nanocrystal to coarse crystal is a transition from brittle fracture to ductile fracture, which has excellent safety application prospects. The research results provide a theoretical basis for the practical application of gradient materials.
The above research results were recently published in Materials Today. The first author of the article is Cao Yuqing, a doctoral student at the Institute of Metals. The co-first author is Dr. Yu Qin from the Lawrence Berkeley National Laboratory. Relevant work was funded by the National Key R&D Program (Grant No. 2017YFB0702003) and the National Natural Science Foundation Project (Grant No.).