Unit Shearing Events in Plasticity of Amorphous Silicon
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0904-BB03-07.1
UNIT SHEARING EVENTS IN PLASTICITY OF AMORPHOUS SILICON Michael J. Demkowicz1 and Ali S. Argon2 1 MST-8: Structure-Property Relations, Los Alamos National Laboratory, Los Alamos, NM 87545 2 Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139
ABSTRACT Plasticity in amorphous silicon (a-Si) as modeled by the Stillinger-Weber (SW) potential was investigated using structure relaxation by potential energy minimization. Irreversible stress drops in the observed mechanical response are the source of plastic deformation in this model directionally bonded material. Every such stress drop was found to be accompanied by atomic rearrangements that give evidence of the existence of unit inelastic shearing events that are characteristic of a-Si and account for plasticity in this material.
INTRODUCTION Apart from some speculative discussions on covalently bonded (oxide and compound) glasses [1, 2], until recently not much consideration had been given to mechanisms of plastic flow in directionally bonded network glasses. The excuse for this omission has often been the claim that such glasses are brittle (in tension). Since brittleness is a separate phenomenon from plastic flow and since much evidence exists that such solids can indeed undergo plastic flow under compression [3, 4], the subject has been ripe for serious consideration. The above rationale as well as the remarkable indentation resistances of nanostructured composites of crystalline and amorphous nitride coatings [5, 6] led us to explore the mechanisms of plastic flow in amorphous silicon (a-Si) [7, 8, 9] as a generic example of directionally bonded amorphous network glasses. Here we summarize the simulation results that led to the conclusion that the atomic rearrangements responsible for plasticity in a-Si consist of unit inelastic shearing events.
SIMULATION APPROACH The Stillinger-Weber empirical potential for silicon [10] was chosen as a model covalently bonded system because its design fits the level of local atomic constraint required for studying the mechanical behavior of covalently bonded disordered materials and how it differs from that of metallic and polymeric glasses. Furthermore, its design is simpler than that of other commonly used empirical models [11-13], its behavior has been explored extensively in previous studies, and it reproduces the characteristics of real Si in broad brushstrokes [14-18]. An immediate question that arises about the simple construction of the SW potential is whether it results in a
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completely accurate and precise description of the behavior of real silicon. The answer to this question cannot be unequivocal, for neither the SW potential nor any alternatives to it—being, after all, only empirical potentials—can do so perfectly. This shortcoming does not in any way invalidate their use, however, since the deviations from the behavior of real Si exhibited by many of them—including SW—are acceptable and the results acquired from their use can be viewed
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