ATOMISTIC MODELING OF DISLOCATION MOTION AT EXPERIMENTAL TIME-SCALES
An accurate prediction of the rate of dislocation motion is key to the fidelity of multi-scale plasticity models of metals and alloys. In this dissertation, atomistic simulations and rate theories based on statistical mechanics are used to accurately predict the rate of three main dislocation motion mechanisms: 1) Dislocation motion across precipitates 2) Dislocation motion through a field of obstacles 3) Dislocation motion via kink-pair nucleation For these mechanisms, the accuracy of both conventional and modern rate theories is examined by comparing their predictions to benchmarks obtained from MD simulations. Different variants of the Harmonic Transition State Theory, as the most common rate theory in the literature, are found to provide grossly inaccurate predictions for all three problems. It is shown that the inaccuracy of these approaches stems from their assumptions about the entropy barrier. The original version of HTST estimates the entropy barrier by the harmonic vibrational entropy, which is found to be inaccurate for all three problems due to thermal softening. Other versions of HTST based on simple estimates of the attempt frequency consider smaller values for the vibrational entropy, and hence provide even more inaccurate predictions. Furthermore, all variants of HTST neglect the configurational entropy, which turns out to be significant for the kink-pair nucleation problem. The utility of the Finite Temperature string method for computing a reaction coordinate and a free energy profile was examined for the three problems. The method provides an accurate reaction channel for dislocation-obstacle interactions but fails to provide a reasonable free energy profile. The reasons are investigated and discussed in the first paper presented in this dissertation. The original version of the method fails to provide a reaction channel for the kink-pair nucleation problem because it has not been designed for problems with multiple reaction channels. To address this issue, a modification to the approach based on physical intuitions about the problem is proposed and is shown to be effective. Different variants of the Transition Interface Sampling approach, as a modern rate theory, are found to be capable of accurately predicting the rate for all three problems. TIS and its Path Swapping version are found to be effective for dislocation-precipitate interactions. The method is also accurate in the jerky motion regime of dislocation motion through a field of solutes. For the smooth motion regime, however, the Partial Path version of TIS --- designed for diffusive barriers --- had to be used. In order to provide accurate predictions for the kink-pair nucleation problem, TIS had to be modified based on physical intuitions about the problem because the method has not been designed for problems with multiple reaction channels such as the glide of screw dislocations in BCC transition metals. The performance of the Meyer-Neldel (MN) rule, as the most common entropy estimation approach in material mechanics, is examined for the three problems. It is shown that the method accurately predicts the entropy barrier for dislocation-precipitate interactions but fails to fully explain the entropy barrier for the other two problems. The assumptions and theoretical justification of the method for dislocation processes, which are often neglected in the literature, are revisited and simple alternative models are proposed.
Applied mathematics; Alloys; Atomistic Simulations; Crystal Plasticity; Dislocations; Material Mechanics; Molecular Dynamics; Mechanical engineering; Materials Science
Warner, Derek H.
Hennig, Richard G.; Bindel, David S.
Civil & Environmental Engr
PHD of Civil & Environmental Engr
Doctor of Philosophy
Attribution-NonCommercial-NoDerivatives 4.0 International
dissertation or thesis