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Improving the Plasticity of Metallic Glass through Heterogeneity Induced by Electropulsing-assisted Surface Severe Plastic Deformation

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2019, Doctor of Philosophy, University of Akron, Mechanical Engineering.
Chi Ma, University of Akron, May 2019. Improving the plasticity of metallic glass through heterogeneity induced by electropulsing-assisted surface severe plastic deformation. Advisor: Yalin Dong. Co-advisor: Chang Ye. Metallic glasses have raised tremendous interests to industrial and academic community due to its superior properties (e.g. ultimate strength, wear resistance, and soft-magnetic property) since their invention. The amorphous structure, on the one hand, gives rise to advanced material properties while, on the other hand, causes poor ductility hindering the wide application of metallic glasses. Aiming at designing ductile metallic glasses, this thesis research investigates surface severe plastic deformation (SSPD), electropulsing, and their combined treatment to achieve the transition from brittle to ductile, and unveil the relationship between microstructure and mechanical deformation behavior through molecular dynamics simulation and fracture theory. Ultrasonic nano-crystal surface modification (UNSM), as a member of the SSPD family, is a recent developed technology possessing high controllability and good surface finish. By applying ultrasonic mechanical peening, UNSM leads to the dilation of local atomic structure and thus induces extra free volume heterogeneously. Extra free volume is generated after UNSM treatment and consequently plasticity, i.e. yielding and more plastic strain, starts to appear. The observation of fracture surface suggests that before fracture occurs, higher density of shear bands exists in the UNSM-treated samples and severe interaction between shear bands impedes propagation of themselves, which contributes to material plasticity. In addition to extra free volume, the secondary crystalline phase induced by UNSM and its effect on increasing plasticity is also explored. In this thesis, for the first time, the composite structure of nanocrystalline phase co-existed with extra free volume is fabricated by electropulsing-assisted UNSM treatment. It is found that the composite structure is much more effective to cause the transition from brittle to ductile than UNSM or electropulsing alone. The underlying mechanisms revealed by molecular dynamic simulation and experimental characterization are twofold: as one kind of heterogeneity, free volumes nucleate more shear bands, and thus increase their density, leading to the delay of propagation. As another kind of heterogeneity, nanocrystals not only promote the atomic plastic deformation at interface but also prevent brittle fracture due to their inherent ductility resulting from crystalline structure. In summary, an innovative surface engineering technology, electropulsing-assisted severe plastic deformation, is invented to design a composite microstructure with both free volume and nanocrystal, which leads to improved plasticity of metallic glasses.
Yalin Dong, PhD (Committee Chair)
Chang Ye, PhD (Committee Co-Chair)
Guo-Xiang Wang, PhD (Committee Member)
Rajeev Gupta, PhD (Committee Member)
Jun Ye, PhD (Committee Member)
Kwek-Tze Tan, PhD (Committee Member)
130 p.

Recommended Citations

Citations

  • Chi, M. (2019). Improving the Plasticity of Metallic Glass through Heterogeneity Induced by Electropulsing-assisted Surface Severe Plastic Deformation [Doctoral dissertation, University of Akron]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=akron1555595868348676

    APA Style (7th edition)

  • Chi, Ma. Improving the Plasticity of Metallic Glass through Heterogeneity Induced by Electropulsing-assisted Surface Severe Plastic Deformation. 2019. University of Akron, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=akron1555595868348676.

    MLA Style (8th edition)

  • Chi, Ma. "Improving the Plasticity of Metallic Glass through Heterogeneity Induced by Electropulsing-assisted Surface Severe Plastic Deformation." Doctoral dissertation, University of Akron, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=akron1555595868348676

    Chicago Manual of Style (17th edition)