The Metal Material Microdamage Molecular Dynamics Study

Understanding evolution mechanisms of microscopic damage is the foundation for constructing fracture model of metal materials. Molecular Dynamics (MD) simulation is an effective method of studying microcosmic mechanical rules of metal materials, such as microcosmic plastic deformation, phase change, damage fracture and so on.In this thesis we use MD simulations to investigate (ⅰ) the nucleation and initial growth of voids in face-centered cubic copper crystal, (ⅱ) the microscopic mechanisms of dislocation nucleation in single crystal copper with nanovoid inclusion, (ⅲ) the size effects on void growth and coalescence under high strain-rates. We analyzed the evolution behaviors of defect structures and void interfaces via molecular dynamics post-processing methods of bond-pair analysis, cluster identification, surface construction. Detailed results are as follows.For nucleation and initial growth of voids under high strain-rates, Simulation results show that voids nucleate though clusters of vacancies. The latter are generated via dislocations climbing in dislocation concentration regions of perfect single crystal copper. Stresses do not fluctuate with large amplitude in the damage areas. It is confirmed that local stress concentration is not the main reason for void nucleation at a given point. With plastic deformation of surrounding material, the shape of void evolves from being elongated to being cylindrical, then ellipsoidal and spherical-like. Finally they coalesce with neighboring voids to form macroscopic fracture. In single crystal copper with two nanoscale void inclusions, the void nucleates at a fixed point where dislocations intersect. Influenced by the moving directions of dislocation lines, their shapes are first flat-triangle-like, and then gradually evolve to spherical-like with slipping and climbing of dislocations.For microscopic mechanism study of dislocation nucleation in single crystal copper with nanovoid inclusion, simulation results show that, four sliding systems are activated simultaneously in the void surface under high uniaxial tension rate in single crystal copper with one nanvoid inclusion. The dislocations prefer to nucleate on the four activated sliding systems. When applying constant stress on infinite boundaries, the stress component along slip direction of slip plane perfectly account for the phenomena of dislocation nucleation. Influenced by the activating sliding systems, the void shape evolves to octahedral-like. For the single crystal with two nanovoid inclusions that locate in different positions, surrounding areas of voids present different morphologies, velocities of dislocation and growth rates of void. It is because that the interaction of the two voids leads to different stress concentration regions in surroundings, and that the dislocation nucleation requires a critical width of stress concentration region.For the size effects on void growth and coalescence in face-centered cubic copper crystal, simulation results show that, voids with various sizes grow and coalesce via dislocation nucleating on the void surface. With decreasing the void size, the critical yield stress increases. If the void radius is large, dislocations symmetrically nucleate and migrate on the void surface, and elongated in elongated in the loading direction. The evolution processes are similar. If the void radius is small, dislocations asymmetrically nucleate on the void surface and elongated along the vertical direction. The process of void growth may be characterized by four stages, the stage for elastic deformation, the stage for independent growth, the coalescence and steady growth stages. The second stage, independent growth stage, diminishes gradually when the void size becomes smaller and smaller.