Dislocation Barriers and Plastic Flow of Metals:  Experimental Observation and Computation

Professor Sia Nemat-Nasser
Center for Excellence for Advanced Materials
Department of Applied Mechanics and Engineering Sciences
University of California - San Diego

Sponsored by: the University Research Council and the Center for Materials Research and Analysis

Date:  Friday, May 29, 1998
Time:  1:30 p.m.
Place:  W128 Nebraska Hall


The theoretical basis of the rate– and temperature–dependent plastic flow of metals is examined at the dislocation scale, modeling the barriers to the dislocation motion by the lattice structure, other intersecting dislocations, and impurities such as substitutional atoms. Based on this, specific constitutive models for bcc and fcc crystalline solids are developed. Several of the constitutive parameters in these relations are estimated, based on the physical properties of the material at the atomic scale, leaving very few free parameters. Using the results obtained through some novel experimental techniques, the remaining constitutive parameters are estimated for commercially pure tantalum (bcc) and copper (fcc). Excellent correlation is obtained between the model predictions and the experimental results, over a broad range of strains (from a few percent to greater than 100%), strain rates (from quasi-static to greater than 10 4/s), and temperatures (from 77K to 1,200K). These and related results will be presented. The localization of the inelastic flow and crack propagation in single crystals are studied experimentally and by numerical simulations, focusing on anisotropic inelastic response of the crystal, and mechanisms of possible crack initiation and growth, produced upon loading by the residual inhomogeneous plastic strains.