Doctor of Philosophy, The Ohio State University, 2019, Materials Science and Engineering

Titanium (Ti) and its alloys have a wide range of applications in biomedical, automotive and aerospace industries due to their excellent strength to weight ratio and corrosion resistance. Alpha phase Ti has hexagonal closed packed (hcp) structure that shows anisotropic plastic deformation; 〈 a 〉 type slip on prism planes is the easiest to activate but cannot accommodate deformation along the 〈 c 〉 axis. The low temperature ductility of Ti is linked to twinning. Therefore, understanding the mechanisms behind the twin nucleation and growth in Ti alloys is important from both theoretical and industrial application points of view. To that end, the present study seeks a better understanding of the atomic scale processes involved in twin nucleation mechanisms and the effect of alpha-stabilizing solutes such as interstitial oxygen, substitutional aluminum and rare earth elements on twinning. Systematic molecular dynamics (MD) simulations are used to identify the underlying mechanism of twin nucleation from dislocation/grain boundary interactions. Density functional theory (DFT) simulations are employed to examine the effect of oxygen interstitials on the twinning behavior of Ti. A systematic framework has been developed to predict the diffusion of interstitial elements near the twin boundaries in hcp alloys. Next, uncertainty that arises from first-principles calculations in predicting diffusion coefficients are quantified. Finally, solute segregation to the twin boundaries as a new mechanism for dynamic strain aging (DSA) is investigated in Ti and other hcp alloys.

Committee: Maryam Ghazisaeidi (Advisor); Michael Mills (Committee Member); Wolfgang Windl (Committee Member) Subjects: Computer Science; Materials Science

Master of Science, University of Akron, 2014, Applied Mathematics

We study the problem of detachment of a graphene sheet from a rigid substrate. Our study is partially motivated by an interest in the mechanism of unrolling of carbon nanoscrolls. We consider a simplified two-dimensional model that consists of two chains of carbon atoms. One chain representing a substrate is assumed to be fixed while the second chain corresponding to a detachable graphene sheet is unconstrained, except for one endpoint that is assumed to be at a prescribed distance from the substrate. We derive a continuum model of this system by upscaling the corresponding atomistic model. Specifically, we suppose that the distance between the adjacent atoms of the flexible chain is fixed to reflect the strength of covalent bonds. Then the discrete energy of the system consists of two contributions - due, respectively, to deviations of angles between the adjacent bonds from their equilibrium values and to van der Waals interactions. We formally derive an expression for macroscopic continuum energy by expanding the discrete energy in the small parameter equal to the ratio of the interatomic distance to the length of the sheet. The scalar function that describes the equilibrium shape of the graphene sheet can then be determined by solving the variational problem for the macroscopic energy. We conclude by analyzing the solutions of the Euler-Lagrange ODE corresponding to this energy.

Committee: Dmitry Golovaty Dr. (Advisor); Patrick Wilber Dr. (Advisor); Malena Espanol Dr. (Advisor) Subjects: Applied Mathematics