Atomistic simulations of lattice defects

University dissertation from Stockholm : KTH

Abstract: Mechanical properties of solids are governed by crystal imperfections. Computational materials science is largely concerned with the modelling of such defects, e.g. their formation, migration, and interaction energies. Atomistic simulations of systems containing lattice defects are inherently difficult because of the generally complicated geometrical structure of the defects, the need for large simulation cells, etc.In this thesis, the role of lattice defects in the mechanism behind homogeneous melting is demonstrated. Also, a generic calculational scheme for studying atomic vibrations close to extended defects (applied to a dislocation) has been considered. Furthermore, heat capacities in the solid and liquid phases of aluminium have been calculated, as well as various thermophysical defect properties.The work was carried out using classical atomistic simulations, mainly molecular dynamics, of aluminium and copper. The interatomic forces were modelled with effective interactions of the embedded-atom type.The main results of this thesis are the following:• The thermal fluctuation initiating melting is an aggregate typically with 6-7 interstitials and 3-4 vacancies.• In the initial stage of melting, no signs of a shear modulus melting mechanism, or the presence of line-like defects (dislocations), can be seen.• The typical time interval from when melting initiates to the time at which the liquid phase is fully developed is of the order of 1000?, where the period ? corresponds to the maximum vibrational frequency in the solid.• The solid-liquid boundary advances at a pace comparable to that of thermal transport by vibrating atoms in the crystal at high temperatures.• The seemingly small anharmonic effect in the heat capacity of aluminium is caused by a partial cancellation of the low-order term linear in the temperature and anharmonic terms of higher order in the temperature.• The core region of an edge dislocation in face-centred cubic aluminium has compressed and expanded regions. The excess volume associated with the dislocation core is small, about 6 percent of the atomic volume, as a result of a partial cancellation between the volume changes of the compressed and expanded regions.• The compressed and expanded regions of the edge dislocation core give negative and positive contributions, respectively, to the excess vibrational entropy. The overall effect is a positive vibrational excess entropy of the dislocation core which is about 2kB per atomic repeat length along the dislocation core.• The atomic vibrations near the dislocation core are modelled by considering an atomic cluster with about 500-1000 atoms containing the core of dislocation, embedded in a large discrete, but relaxed, lattice of about 23 000 atoms. An atomic region that is four atomic layers thick and about 18 atomic diameters long in the direction parallel to the Burgers vector, accounts for most of the excess entropy.• The constant-pressure heat capacity of aluminium shows a minimum as a function of temperature in the liquid phase.