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Title: Plasticity improvement of metallic glasses by introducing structural heterogeneities : an atomistic study
Authors: Zhao, Lei
Degree: Ph.D.
Issue Date: 2019
Abstract: Metallic glasses (MGs) have aroused great interest due to their exceptionally high strength and elasticity, remarkable fracture toughness and attractive processing potential, all of which are originated from the unique liquid-like atomic structure. However, a major obstacle in utilizing MGs as load-bearing materials is their intrinsic brittleness at room temperature, especially under tensile loading. Over the past thirty years, great efforts have been dedicated to improving the tensile ductility of MGs. One effective approach is to enhance or introduce structural heterogeneities in MGs that can trigger nano- or micro-scale spatial fluctuations, which may act as effective sources for the formation of shear bands as well as barriers for their propagation to delay shear instability. Heterogeneous microstructures in MGs may be raised through various treatments without changing the full glass nature or by directly incorporating heterogeneous crystalline phases during the fabrication process. Motivated by these concepts of structural heterogeneities, many experimental works have been successfully conducted to improve the ductility of MGs through various approaches, such as mechanical pre-treatment, ion irradiation and the formation of MG/crystalline metal multilayers. Although much effort has been made into understanding the mechanisms of plasticity improvement in these experiments, the origin of structural heterogeneities and the atomic mechanisms of shear band generation and propagation are still far from satisfactory understanding. Atomistic modelling, which allows one to directly investigate the detailed deformation behaviour as a function of the tailored architectures and to effectively establish links between the atomistic deformation mechanisms and corresponding mechanical properties, is a useful tool for probing such issues. In the present work, the primary objective is to investigate the atomistic mechanisms of plasticity improvement of MGs by introducing structural inhomogeneities using atomistic simulations and establish a more explicit relationship between microstructural structures and mechanical properties. Firstly, wire drawing, as a mechanical pre-treatment for processing small-sized MG wires in experiments, was reproduced using atomistic simulations. The atomistic deformation mechanisms of MGs in wire drawing, the origin of deformation-induced heterogeneities as well as their influences on the tensile ductility were systematically investigated. The results revealed that the plastic deformation behaviour of MGs in wire drawing is closely ccorrelated with the area reduction ratio (R): at small R, the area reduction is realized via shear transformations of atoms in the surface shell, leaving the core intact; while at large R, it primarily relies on the formation of multiple spatially distributed shear bands that redistributes the plasticity throughout the sample. The steady plastic flow of MGs in wire drawing was found to originate from the high triaxial compressive stresses in the surface shell. The deformation-induced heterogeneities were further understood through the detailed analysis of the resultant residual strain and stresses, the gradient amorphous structures, the unique free volume distributions and spatially distributed shear bands. Moreover, the tensile simulations demonstrated a brittle-to-ductile transition synchronized with a softening trend of the drawn MGs. The transition can be ascribed to the synergistic effects of three underlying mechanisms: 1) The compressive residual axial stresses in the surface shell lead to a shift of the initial yield sites from the surface to the core, suppressing the rapid formation of shear bands on the surface; 2) The rejuvenated amorphous structures in the surface shell serve as softening phases, constraining the plastic deformation in the core; 3) The spatially distributed shear bands, generated at large R, act as heterogeneous nucleation sites for highly dispersed plastic shearing, which deliver more homogenous plastic deformation.
To further investigate the structural heterogeneities on plastic deformation in pure MGs without the influence of residual stress, a series of MGs were partially rejuvenated with gradient rejuvenated amorphous structures (GRASs) through ion irradiation simulations. Results revealed that the introduced ductile GRASs facilitate the formation and propagation of new shear bands in the interior unrejuvenated region by suppressing the catastrophic propagation of one dominating shear band across the GRASs, thus resulting in more dispersed plastic shearing throughout the sample. It is also demonstrated that increasing both the volume fraction and degree of structural disordering of GRASs can improve the tensile ductility of MGs and lead to a brittle-to-ductile transition of the deformation mode. Moreover, the critical volume fraction of GRASs required for switching the transition is found to depend on the specific degree of structural disordering. The observed structural state-dependent transition is further understood from a mechanical perspective by considering the competition between the macroscopic yield stress and the critical stress of the material required for shear delocalization, based on which a criterion is developed to predict the critical transition boundary in MGs with GRASs across a wide range of structural states. Attempts were also made by introducing crystalline Cu in MGs as heterogeneous structures, which were also found to play a crucial role in promoting shear delocalization. The effect of the Cu layers on the tensile deformation behaviour of Cu/MG/Cu sandwich structures was examined. It was demonstrated that yielding first occur in the Cu phases, which further trigger plastic deformation in the MG core through the transfer of crystal plasticity across the crystalline-amorphous interfaces. The Cu layers serve as an effective medium for nucleating multiple shear bands, promoting more distributed plastic deformation in the sandwich nanostructures. Meanwhile, through straining a series of tailored MG/Cu nanolaminates, the influence of the Cu layers on the atomistic mechanisms of yielding and plastic deformation behaviour in the nanostructures was also examined. The results revealed the Cu layers serve as sites for heterogeneous nucleation of embryonic shear bands, as well as barriers to their propagation into mature ones. Meanwhile, the coupled interplay between the crystal plasticity and the glassy plasticity in the nanolaminates promotes a more homogeneous redistribution of plastic deformation, providing a kind of hardening mechanism. A transition of deformation mode from localized to homogeneous-like deformation was observed by tailoring the relative volume fraction of the Cu layers. This dissertation provides a diverse discussion of structural heterogeneities introduced in MGs by different approaches and their influences on the atomistic mechanisms of plasticity improvement. A bridge between knowledge obtained from these atomistic simulations and the mechanical properties observed in experiments is presented. The findings could not only provide a deep understanding of the structure-property relationships in MGs but also serve as a useful guide for designing and processing MGs with excellent mechanical properties.
Subjects: Hong Kong Polytechnic University -- Dissertations
Metallic glasses
Pages: xxiv, 228 pages : color illustrations
Appears in Collections:Thesis

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