Role Played by Grain Boundaries in Plastic Deformation of Polycrystalline Metals: A Discrete Dislocation Dynamics Study

Plastic deformation in metals and their alloys results due to the motion of large number of dislocations. Grain Boundaries (GB), by affecting various aspects of dislocation motion, add to the complexity. Classical two-dimensional (2D) Discrete Dislocation Dynamics (DDD) framework was extended to include Grain Boundaries (GB) to study the deformation in a polycrystalline metallic material and the role played by the grain boundaries therein. During plastic deformation, GBs interact with dislocations, altering their natural course, thus affecting the macroscopic mechanical response of the material. Grain boundaries are qualified as hard and soft based on whether they hinder the dislocation motion by acting as obstacles or allow the dislocations to pass through them. A polycrystalline DDD framework was developed to account for these effects in order to model the plastic deformation in the material. The study considered an annealed material with a random distribution of Frank-Read sources and no initial dislocations distributed in the sample. Dislocations were nucleated from the Frank-Read sources when the stress at the location of a source reached a critical value. Obstacles that act as pinning points were distributed randomly in the specimen. Three-dimensional effects like junction formation that account for formation of junctions, which in turn give rise to stage-II hardening were accounted for. Square and rectangular grain morphology was considered to generate mechanical response from the specimen under plane strain deformation.