Groundbreakingly combined Mg-based dual-phase metallic glass and gradient nanocrystalline structure, successfully designed a multi-level nanostructured magnesium alloy, which increased the elongation of the gradient nanocrystalline magnesium alloy to 20%, and restored it to the non-SMAT (coarse-grained) magnesium alloy Malleability. At the same time, the alloy yield strength can be kept at 230MPa, which is equivalent to the strength of SMAT magnesium alloy. The excellent mechanical properties of multi-level nanostructured magnesium alloys are synergistically effected by three deformation mechanisms: dual-phase metallic glass undergoes multiple shear bands and nano-crystallization, metallic glass prevents crack extension, and SMAT nanocrystalline layer grains grow. By combining heterogeneous metallic glass and gradient nanocrystals, similar new nanostructures can obtain high-strength and high-plasticity copper. This alloy design concept is expected to achieve the combination of high strength and high elongation in other alloy systems.
Due to the characteristics of high strength and low density, magnesium alloys have broad application prospects in aerospace, automotive automation, biomedical alloys and other fields. In previous work, the team of Academician Lu Jian found that the super-nano dual-phase magnesium alloy with amorphous and nanocrystalline can achieve near-theoretical strength (Nature 545, 80-83 (2017)).
Based on this, the researchers designed a new multi-level structure of magnesium alloy: first use surface mechanical polishing (SMAT) magnesium alloy surface to obtain gradient nanocrystals, and then deposit Mg-based dual-phase metallic glass film on the alloy surface by magnetron sputtering. This design concept combines the advantages of dual-phase metallic glass and gradient nanostructures, successfully maintaining good tensile plasticity (20%) on the basis of a 31% increase in alloy strength. The excellent plastic deformation ability of the alloy is provided by the multiple shear band deformation and nano-crystallization of the dual-phase metallic glass, the dual-phase metallic glass blocking the crack extension of the SMAT nanocrystalline layer, and the grain growth of the SMAT nanocrystalline layer during deformation.
Relevant results were published in "Advanced Science" (IF=15.84) with the title "Nano-dual-phase metallic glass film enhances strength and ductility of a gradient nanograined Magnesium alloy". The first author of the thesis is Dr. Liu Chang (now a postdoctoral fellow at Max Planck Institute of Steel Research in Germany). The corresponding authors are Dr. Wu Ge (now a postdoctoral fellow at Max Planck Institute of Steel Research in Germany) and Professor Lu Jian. Other authors include Professor Liu Yong, Researcher Wang Qing, Associate Researcher Liu Xiaowei, and Doctoral Student Bao Yan.
Paper link: https://onlinelibrary.wiley.com/doi/full/10.1002/advs.202001480
Surface mechanical polishing (SMAT) can effectively improve the strength of the alloy by introducing gradient nanostructures into the alloy. However, previous studies have shown that the nanocrystalline layer on the surface of the SMAT magnesium alloy undergoes brittle fracture during plastic deformation, which severely reduces the alloy's deformability. In this study, a 13 μm thick Mg-Zn-Ca dual-phase metallic glass (NDP-MG) was deposited on the surface of SMAT Mg alloy by magnetron sputtering to prevent the crack extension of the nanocrystalline layer and initiate the grain growth mechanism of the nanocrystalline layer. High strength and high elongation.