Study of Defects in Aluminium using Large Scale Electronic Structure Calculations.

Abstract

Defects in materials play an important role in determining their behavior. Defects, such as vacancies, dislocations, surfaces, dopants, interstitials, though present in small concentrations of the order of few parts per million (ppm), can influence properties of the materials at macroscopic scales. Various computational techniques have been used to model the influence of defects on the properties of materials. Density functional theory based electronic structure calculations have been very accurate in predicting material properties. Electronic structure calculations have provided various insights into the properties of materials such as bulk properties, surface energetics and phase transformations. But, their applicability in studying defect properties is restricted due to limitations on simulation cell-size, to the order of a few hundred to thousand atoms. Recently developed coarse graining technique Quasi Continuum orbital free density functional theory (QCOFDFT) addresses these issues using a real-space local variational formulation of the orbital free density functional theory, finite element discretization of the formulation and an adaptive coarse graining technique. Using this technique, multi-million atom simulations with arbitrary boundary conditions are accessible, which are instrumental in modeling defects in crystalline materials. In this thesis, we extend the QCOFDFT technique to the more accurate non-local kinetic energy functionals that describe materials systems whose electronic structure is close to that of a free electron gas. First, we present a local variational reformulation of the non-local functionals. The accuracy of this local formulation is validated with calculations on bulk properties of Aluminium and compared against plane-wave basis implementations. The coarse graining technique, QCOFDFT, is then used with this local reformulation to achieve multi-million atom simulations of vacancies in Aluminium. Cell-size studies are performed on mono and di-vacancies to underscore the need of large scale electronic structure calculations for an accurate understanding of the eneregtics of defects. The real-space formulation is then used to study an isolated edge dislocation to characterize the defect core and investigate the influence of external deformations on defect-core energetics. Finally, we use QCOFDFT to study vacancy clustering and nucleation of dislocation loops which have important consequences in embrittlement of metals due to radiation damage and quenching processes.