Mechanisms of plastic deformation in amorphous silicon by atomistic simulation using the Stillinger-Weber potential

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2005.Includes bibliographical references (p. 205-212).Molecular dynamics simulation of amorphous silicon (a-Si) using the Stillinger- Weber potential reveals the existence of two distinct atomic environments: one solidlike and the other liquidlike. The mechanical behavior of a-Si when plastically deformed to large strain can be completely described by the mass fraction [phi] of liquidlike material in it. Specifically, samples with higher [phi] are more amenable to plastic flow, indicating that liquidlike atomic environments act as plasticity "carriers" in a-Si. When deformed under constant pressure, all a-Si samples converge to a unique value of [phi] characteristic of steady state flow. Discrete stress relaxations were found to be the source of low-temperature plastic flow in a-Si in deformation simulations by potential energy minimization. These relaxations are triggered when a local yielding criterion is satisfied in a small cluster of atoms. The atomic rearrangements accompanying discrete stress relaxations are describable as autocatalytic avalanches of unit shearing events. Every such unit event centers on a clearly identifiable change in bond length between the two split peaks of the second nearest neighbor shell in the radial distribution function (RDF) of bulk a-Si in steady-state low.by Michael J. Demkowicz.Ph.D

Phenomenology and kinematics of discrete plastic deformation events in amorphous silicon : atomistic simulation using the Stillinger-Weber potential

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2004.Includes bibliographical references (p. 62-64).The need to understand plastic deformation in amorphous covalently bonded materials arose from the unique mechanical properties of disordered intergranular layers in nc-TiN/a-Si₃N₄ ceramic composites. Silicon was chosen as a model disordered network solid for the purpose of conducting feasible atomistic computer simulations of plastic deformation. Amorphous silicon structures were created by melting and quenching using a molecular dynamics algorithm. These structured were plastically deformed by conjugate gradient static energy minimization. Atomic level analysis was carried out using appropriately generalized notions of stress and strain. Plastic deformation was found to occur in a series of discrete stress relaxations, each one of which was accompanied by a well localized atomic level rearrangement. The transforming regions were roughly ellipsoidal in shape and involved the cooperative motion 100-500 atoms spanning a length scale of 0.7-2.5nm. This length scale is large in comparison to the typical thickness of disordered intergranular layers in nanocrystalline ceramic composites, indicating that the plastic relaxation process in such intergranular layers cannot be the same as the one found in bulk amorphous covalent solids.by Michael J. Demkowicz.S.M