Modeling the Influence of Phase Boundaries and Oxygen Interstitials on the Nucleation and Growth of Deformation Twins in the Alpha-phase of Titanium Alloys

Title of dissertation: MODELING THE INFLUENCE OF PHASE BOUNDARIES AND OXYGEN INTERSTITIALS ON THE NUCLEATION AND GROWTH OF DEFORMATION TWINS IN THE ALPHA-PHASE OF TITANIUM ALLOYS William Joseph Joost, Doctor of Philosophy, 2015 Dissertation directed by: Professor Sreeramamurthy Ankem Professor Maija M. Kuklja Department of Materials Science and Engineering Twinning is an important deformation mechanism in many hexagonal close packed metals, including α-titanium alloys. However, the processes of twin nucleation, growth, and interaction with other defects are not well understood. Further, many aspects of deformation twinning are difficult to interrogate experimentally owing to the small time and length scales of the governing mechanisms. In this study we apply a combination of theoretical and computational materials science techniques, leveraged with experimental data, to quantify the effects of α-β phase boundaries and oxygen interstitials on twin nucleation, twin growth, and ultimately mechanical behavior in titanium alloys. Combined results from finite element method and analytical dislocation modeling demonstrate that elastic and plastic interaction stresses across the interface between the αand β-phases are responsible for the experimentally observed anisotropy in the deformation behavior of dual-phase alloys. Interaction stresses also promote slip and twinning at up to 30% lower applied stress than predicted from Schmid’s Law, significantly affecting performance in many applications. The complex interactions of phase boundaries, dislocations, and deformation twins modify the preferred deformation mechanism and promote twinning for some loading orientations. In order to quantify the interaction between oxygen interstitials and (101̄2) twin boundaries, we employ atomistic simulations using a newly developed modified embedded atom method potential and density functional theory. Our investigation reveals that a twin boundary alters interstitial formation energy by as much as 0.5 eV while also stabilizing a tetrahedral interstitial, which is unstable in the bulk. Further, the activation barriers for diffusion in the region near a twin are uniformly lower than in the bulk; an atom diffusing across the twin boundary moves through several paths with peak activation barriers more than 0.3 eV lower than for comparable diffusion far from the twin. Despite accelerated kinetics, oxygen diffusion still occurs much more slowly than twin growth, suggesting that oxygen interstitials contribute to experimentally observed time-dependent twinning. Together, these results provide new insight while enabling predictive modeling and purposeful development of improved titanium alloys across a wide range of applications. MODELING THE INFLUENCE OF PHASE BOUNDARIES AND OXYGEN INTERSTITIALS ON THE NUCLEATION AND GROWTH OF DEFORMATION TWINS IN THE ALPHA-PHASE OF TITANIUM ALLOYS