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
