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
