Bonding nature and the origin of ductility of metallic glasses

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Understanding the structure-property relationship of metallic glasses (MGs) at an atomic- or electronic level is a challenging topic in condensed matter physics. MGs usually exhibit low macroscopic plasticity, owing to the localized plastic flow in nano- and micro-meter scale shear bands upon deformation, which impedes their wide application as new structural materials. Thus, a detailed description of internal structure and establishing the structure-property relationship would underpin our knowledge of the mechanisms for the ductility/brittleness of MGs and further improve their plasticity. Due to the lack of structural defects such as dislocations and grain boundaries, the short- or middle-ranged ordered clusters are the typical deformation units in MGs, where the bonding strength and direction between atoms are the key factors that affect the cooperative displacements inside deformation unit. However, the bonding nature of MGs and their structure-property relationship are little studied systematically, which hinders our comprehensive understanding the basic problems about mechanical behaviors of MGs, such as fracture and plasticity deformation mechanism.In this paper, the potential correlation between the flexibility of bonding and ductility of MGs is discussed in detail. The first section gives a simple introduction of this topic. In the second section, the latest research progress of the electronic structural study of MGs is presented. Here, the corresponding studies of electronic structures of crystal alloys and their relationship with the mechanical properties are also presented for comparison. In the third section, the traditional and new experimental techniques employed for electronic structure measurements are presented, such as X-ray photoelectron spectroscopy, ultraviolet photoemission spectroscopy and auger electron spectroscopy and the parameters such as nuclear magnetic resonance knight shift, susceptibility (χ) and specific heat (C) are also given in order to introduce electronic structure analysis methods of MGs and further reveal the bonding character of MGs and recent experimental findings of the relationship between the electronic structure and the mechanical properties of MGs.Numerous studies show that in the typical transition metal (TM)—metalloid metallic glass systems, the bond flexibility or mobility of atoms at the tip of crack that depends on the degree of bonding hybridization, determines the intrinsic plasticity versus brittleness. For instance, in these transition metal (TM)-based MGs, when metalloid element M with sp-element shells is alloyed in the TM matrix, the s-density of states (DOS) at M sites is scattered far below the Fermi level due to the pd hybridization between the p orbitals of M element and the d orbitals of TM. This causes the reduction of s-DOS at the Fermi energy (gs(EF)) at the solute M sites and exhibits a strong character. Thus, it is proposed that the gs(EF) can be employed as an effective order parameter to characterize the nature of bonding, especially in the aspect of evaluating bond flexibilities in amorphous alloys. This shows that the plastic flow and fracture process of MGs on an atomic scale can be well described using a simple bonding model where the deformation process is accompanied with the broken-down and reforming of atomic bonding inside short- or middleranged ordered clusters, since the defects are absent in MGs. We hope that this introduction can provide a much clearer picture of the bonding character of MGs, and further guide us in understanding the mechanism for ductile-to-brittle transition in MGs and exploring the novel MGs with intrinsic plasticity.directional boning