High-density liquid-like component facilitates plastic flow in a model amorphous silicon system
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High-density liquid-like component facilitates plastic flow in a model amorphous silicon system
Michael J. Demkowicz and Ali S. Argon Department of Mechanical Engineering, Massachusetts Institute of Technology 77 Massachusetts Ave., Cambridge, MA 02139
ABSTRACT Molecular dynamics simulations show that plastic deformation behavior of model Stillinger-Weber amorphous Si is very sensitive to the density of the initial unstressed state. Low-density systems exhibit a pronounced yield phenomenon, strain softening, and a dramatic drop in pressure during deformation at constant volume. This behavior is explained by the interplay in every system of the prevailing solid-like and liquid-like components, with the latter being denser and more amenable to plastic flow.
INTRODUCTION Plastic deformation in amorphous covalently bonded materials attracted our interest because of their potential to explain the ultrahardness of covalently bonded nanocrystalline ceramic coatings [1-3]. High levels of confinement by neighboring non-deformable crystallites force the intergranular disordered layers present in such flaw-free materials to undergo large strain shear flow (strains much larger than the shear strain). Due to lack of crystalline order in amorphous covalently bonded solids, atomic-scale investigations of their mechanisms of plastic flow have remained—with few exceptions [4, 5]—outside the capabilities of experimental techniques. Computer simulations have therefore played a deciding role in the study of these materials [6-8]. To date, however, most simulations of plastic deformation in disordered solids have focused on metallic glasses [9, 10] and amorphous polymers [11]. The former do not involve strongly directional covalent bonding. The latter include strong bond length and bond angle constraints along polymer backbones, but also weak intermolecular interactions that are out of place in the context of bulk amorphous covalent network materials. Therefore, covalent network materials represent a state of local atomic constraint more extreme than that in both metallic glasses and glassy polymers. As such, they have been studied primarily from the standpoint of rigidity percolation and its implications for amorphous structure [12]. Little was known, however, about their behavior when deformed plastically to large strain.
SIMULATION DETAILS We chose the Stillinger-Weber empirical potential for silicon [6] as a model covalently bonded system because its design fits the level of local atomic constraint required for studying the characteristics of covalently bonded disordered materials and how they differ from those of metallic and polymeric glasses. This potential consists of a combination of sums of 2-body and
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3-body terms, the latter accounting for bond-bending contributions to system energetics. More complex potentials that reproduce the specific low temperature behavior of silicon were also considered [13], but were not chosen because the Stillinger-Weber (SW) potential offers simplicity in a study of amorpho
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