Understanding the brittleness of metallic glasses through dynamic clusters
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Exploiting molecular dynamics simulation, this article investigates the dynamic process of atomic rearrangement in two metallic glasses (MGs), Cu50Zr50 and Fe80P20, which are well known as ductile and brittle MGs under compression, respectively. It was found that the local rearrangements can be identified clearly by the distribution of kinetic energy and atomic strain rate, and that they are always driven by several high-velocity atoms in the core and induce a large shear and tensile strain over a very short duration. The size, kinetic energy, strain rate, and cavitation rate of the clusters in Fe80P20 are markedly larger than those in Cu50Zr50, which explains the distinct strength and brittleness of these two MGs. This study further confirmed that localized rearrangement of atomic structure is the underlying mechanism of plastic deformation in MGs, which governs their macro-scale mechanical performance.
I. INTRODUCTION
Metallic glasses (MGs) are newcomers of the timehonored alloy family.1 They are generally fabricated by fast cooling from melt to maintain the structural disorder of the liquid state. Unlike traditional crystalline metals, the microstructure of a MG is completely amorphous, without any crystalline grain and long-range atomic periodicity.2–4 As the metallic bond is much weaker than a covalent or an ionic bond, MGs possess the advantage of both glass and metal, demonstrated by their excellent electrical and thermal conductivities, much higher strength than their crystalline counterparts, and superplasticity under an elevated temperature.5,6 However, the complicated mechanical behavior and the lack of ductility bottleneck the wide application of MGs. At room temperature, MGs are very brittle under tension but can become ductile under compression.7 The strengths are insensitive to a large range of strain rates but reduce if the strain rate is beyond 103 s1.8 In the supercooled liquid regime, however, MGs demonstrate superplasticity and rheological behavior with strong temperature and strain rate dependence.7,9 It is also noted that the mechanical properties vary pronouncedly in different MG systems.10,11 For example, Fe-based MGs possess much higher strength and much lower ductility than Zr-based MGs.5 To understand these phenomena, a deep understanding of their atomistic deformation mechanism is necessary. The plastic deformation of a MG is neither continuous nor homogenous,12 which is manifested by the a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.11 J. Mater. Res., Vol. 29, No. 4, Feb 28, 2014
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sudden bursts of plastic strain and shear banding.9 Their microscopic picture is the localized atomic structural rearrangements, which occur stochastically and swiftly, causing the discrete plasticity events both temporarily and spaciously. From the energetic point of view, the atomic rearrangement can be considered as a process that the atomic system changes from one relatively low-energy configur
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