Title: Towards a Predictive Criterion for Homogeneous Dislocation Nucleation In Crystalline Solids

Speaker: Ronald Miller, Mechanical and Aerospace Engineering, Carleton University

Date/Time: Monday, Sept. 21, 2008, 10:00 – 11:00 am MT

Location: CSRI/90 (NM), Videoconference to 940 Auditorium (CA)

Brief Abstract: Plasticity is governed by the motion, interaction and multiplication of dislocations during deformation.  A fundamental mechanism of plastic flow is the nucleation of new dislocations, but a complete understanding of the mechanical conditions which lead to nucleation has remained elusive.  A number of dislocation nucleation criteria have been proposed, but none appear to accurately describe the phenomenon.  The goal of the current work is to develop a criterion that is accurate and at the same time practical for use in larger-scale models like Discrete Dislocation Dynamics.

The presentation will focus on the test problem of a spherical, frictionless indenter pressed into an initially defect-free single crystal, which is studied using molecular dynamics and statics with empirical interatomic potentials.  The simulations provide a full understanding of the details of the nucleation mechanism and the mechanical conditions within a crystal just prior to nucleation.

Next, each of the existing nucleation criteria will be examined in turn, demonstrating that they neither correctly describe the phenomenon nor accurately predict the nucleation of a dislocation. Finally, a new criterion will be presented that is a successfully predictive model.

The new criterion is based on a stability analysis of sub-matrices from the dynamical matrix of the atomistic crystal.  It accurately predicts the load level at which a defect nucleates and correctly determines both the location and the type of defect that forms.  The presentation will conclude with a discussion of the remaining challenges to be confronted before the criterion can be practically applied in larger-scale models.

CSRI POC: Mike Parks, (505) 845-0512