Investigating strain hardening by simulations of dislocation dynamics
- Materials deform, though different materials do not deform in the same way. They should not be expected to, as they can have very different underlying structures. In ductile metals, deformation occurs in three stages: elastic deformation, plastic deformation, and fracture. Elastic deformation is defined as recoverable deformation. Ductile metals are elastic for small deformations. This is in contrast to plastic deformation, which is unrecoverable deformation. Dislocations are thought to be the carriers of plasticity in metals, and as such play a key role in material deformation. This thesis presents simulation results that explore particular aspects of plasticity, specifically work hardening and cross-slip, at the mesoscale and microscale, respectively. In order to obtain these results, a number of mathematical and numerical advancements were made to improve the efficiency of the simulations and the resulting physics. With the code enhancements, a variety of novel results were obtained. The implementation of an expanded and more powerful 2D Dislocation Dynamics (DD) code, named DD2D, has resulted in efficiency gains of an order of magnitude or more. The advances included mathematical subtleties (implementing nonsingular stresses and a correction for conditional convergence), physical models (adding sources, obstacles, and sessile junctions), and numerical improvements (implementing different integrators and utilizing subcycling). Because of these, a large number of different simulations in DD2D were run and the stress-strain curve results are presented. These results are used to investigate the hardening behavior and propose a tensile yield surface for various configurations of glide planes and compare them to what Schmid's law predicts. Improving the algorithms of an atomistics code, MD++, resulted in the ability to explore stress regimes previously unable to produce results. A new "Reparametrization with Trimming" algorithm allowed the energy barrier for cross-slip to be calculated under high applied stress configurations at temperatures of 0 K. An equilibration algorithm allowed decreased equilibration times at finite temperature by an order of magnitude, greatly increasing the number of temperature and stress configurations that could be investigated. The 0 K simulations of cross-slip explored the effect of different stress components on the energy barrier for cross-slip. Two different theoretical mechanisms were explored: the Friedel-Escaig (FE) mechanism and the Fleischer mechanism. Over three hundred cross-slip simulations were run at different stress configurations at 0 K. The data were fit to 1D curves (one for each mechanism) using an "effective stress, " τ*, comprised of a combination of the applied stresses. Finite temperature simulations produced results used to find the cross-slip rate coefficient that, together with the energy barrier fit, was implemented into a 3D DD code.
|Type of resource
|electronic; electronic resource; remote
|1 online resource.
|Stanford University, Department of Mechanical Engineering.
|Nix, William D
|Nix, William D
|Statement of responsibility
|Submitted to the Department of Mechanical Engineering.
|Thesis (Ph.D.)--Stanford University, 2016.
- © 2015 by William Patrick Kuykendall
- This work is licensed under a Creative Commons Attribution Non Commercial 3.0 Unported license (CC BY-NC).
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