SUBJECT: Ph.D. Dissertation Defense
BY: Laurent Capolungo
TIME: Friday, June 8, 2007, 10:00 a.m.
PLACE: MRDC Building, 4211
TITLE: Modeling of the size effect in the plastic behavior of polycrystalline materials
COMMITTEE: Dr. Jianmin Qu, Co-Chair (ME)
Dr. Mohammed Cherkaoui, Co-Chair (ME)
Dr. David L. McDowell (ME)
Dr. Hamid Garmestani (MSE)
Dr. Steven Johnson (MSE)


Size effect in polycrystalline materials are known to lead to “abnormal responses” such as the breakdown of the Hall-Petch law, when the grain size is decreased to the nanocrystalline range. This thesis focuses on the study of the size effect in the elastic-viscoplastic response of pure face centered cubic polycrystalline materials. First, the effect of vacancy diffusion is studied via the use of a two-phase self-consistent scheme in which the inclusion phase represents grain interiors and the matrix phase represents grain boundaries. The behavior of the inclusion phase is driven by the activity of dislocations, described with typical strain hardening laws, and by the activity of Coble creep. The behavior of the matrix phase is modeled as elastic-perfect plastic. This model is then extended to account for the possible activity of Lifschitz sliding. Second, grain boundary assisted deformation is studied. Detailed atomic motions within pure tilt bicrystal interfaces are predicted via molecular simulations. It is revealed that grain boundaries containing C structural units exhibit particular atomic motions in the elastic regime characterized by “bifurcation” events. The active role of grain boundaries to the viscoplastic deformation is studied with the introduction of a novel method allowing the scale transition from the atomistic scale to the macroscopic scale. A model describing the mechanism of grain boundary dislocation emission and penetration is informed with molecular simulations and finite element simulations. The macroscopic response of the material is then predicted with use of several self-consistent schemes, among which two novel schemes are introduced. The most refined micromechanical scheme proposed is based on a two-phase representation of the material and is valid in the elastic-viscoplastic regime and accounts for the effect of slightly weakened interfaces.