Colloquium: Maryam Ghazisaeidi, Atomic scale study of defects in hcp metals

Abstract

Accurate and quantitative study of defects is a crucial step in understanding materials properties. The focus of this talk is to explain atomic scale methods of modeling defects in titanium and magnesium. Improving the usability of these light metals affects the energy efficiency in automotive and aerospace industries. In the first part, I introduce a first-principles approach to study the interaction between lattice defects in Ti. While recent first-principles methods predict dislocation core structures and boundary geometries and energies, modeling a dislocation near a boundary requires new techniques to treat the long range strain field of the dislocation near a boundary. Using flexible boundary conditions with a new method to compute the lattice Green’s function for crystals containing a planar interface, we present a general method to study line defects interacting with interfaces with a tractable number of atoms. We use the interfacial lattice Green’s function to model a screw dislocation interaction with Ti (1012) twin boundary for the first electronic structure prediction of a dislocation in a boundary. The interaction energy of an oxygen interstitial with the Ti (1012) is also computed. While we applied our method to a systematic study of defects interactions in titanium, the method is general and opens up the possibility of investigating line defects/interface interactions with chemistry changes in arbitrary systems.

The second part of the talk is focused on an atomic-scale study of deformation mechanisms in magnesium. Mg alloys have excellent strength to weight ratio but their use is limited by their poor formability due to the highly anisotropic Mg deformation; basal deformation is easily activated while high stresses are required to accommodate the deformation along ⟨c⟩ axis. We study the atomic-scale mechanisms of Mg non-basal deformation modes that involve ⟨c + a⟩ slip and twinning. Our study includes two aspects: accurate modeling of defect structures and investigation of possible twin nucleation mechanisms. We perform first-principles calculations of ⟨c + a⟩ screw and edge dislocations and twinning dislocations to obtain the stable core structures and explore the alloying effects. In addition, we examine the Mendelson pole mechanism for ⟨c⟩ dislocations and show that it is possible to initiate twins from existing lattice dislocations from a continuum mechanics analysis and check these mechanisms with atomistic calculations.

Bio

Maryam Ghazisaeidi received B.S and M.S degrees in Civil Engineering (2003) and Mechanics of Materials and Structures (2005) respectively from Sharif University of Technology in Tehran, Iran. She conducted her doctoral studies at University of Illinois at Urbana-Champaign under the supervision of Prof. Dallas Trinkle. Her thesis focused on first-principles modeling of defects in Ti. After receiving her PhD in Theoretical and Applied Mechanics in 2011, she joined the Brown/GM collaborative Lab for Computational Materials Research at Brown University as a post-doctoral research associate where she studies Mg deformation mechanisms with Prof. Bill Curtin. She received the H. Langhaar award from Department of Mechanical Science and Engineering at University of Illinois in 2010 and appeared on the UIUC list of teachers ranked as excellent by their students in 2008.