Molecular dynamics simulations of plastic deformation in iron and aluminum

Plasticity, which is driven by the movement of line defects known as dislocations, is one of the most important metal properties. Many factors, including the number of impurities, grain size, twin boundaries, crystal structure, and local phases, influence the movement of these defects at the nanoscale. For investigating material properties at the nanoscale, molecular dynamics (MD) simulations is one of the most powerful and widely used tools in Computational Material Science. In this work, the plastic deformation of two metal materials, iron and aluminum, is investigated using MD simulations. Pure iron undergoes pressure-induced phase transformation at a pressure around 13 GPa. Large scale MD simulations of shock waves in micrometer sized nanocrystalline iron with two different ramp times depict for the first time the relation of ramp times and stress relaxation during plasticity in agreement with theory and experiments. The grain boundaries influence the nucleation of dislocations, and the plasticity region shows evidence of dislocation nucleation, dislocation loops, and also vacancies. A detailed analysis of the samples has been done by X-Ray diffraction patterns, showing that the shear stress facilitates the transformation to the hcp phase. In agreement with experiments, this work shows that carbon atoms interfere the phase transformation process from body-centered cubic (bcc) to the hcp phase in iron. The simulations show that carbon atoms hinder the propagation of the new phase. Dislocations surrounded by carbon clouds known as Cottrell atmospheres are pinned during the hydrostatic compression, and the hcp nuclei are not able to grow due to the presence of the carbon cloud. Apart from shock wave modeling, nanoindentation and tension tests are often used in experiments and simulations to examine material properties while taking surface deformation into consideration. During the uniaxial loading, the oxide surface strongly influences the mechanical response of the core aluminum because of the continuous reorganization of Al-O bonds during the loading. Nanowire materials with an oxide layer begin to plastically respond sooner than those without an oxide layer. The Al–O layer rearrangement stabilizes the structure and aids in the improvement of the mechanical characteristics. Since a prototypical mechanisms for plasticity generation is indentation, the last part of this thesis focuses on a thorough examination of the plastic deformation of Al and Fe surfaces during nanoindentation with different parameters including sample sizes, indenter size, indentation velocity. For both evaluated materials, the domain size strongly influences the mechanical response. To summarize, the present work leads to an increased understanding of materials under high pressure. The results may be very beneficial for developing simulation models and also for developing novel materials with properties that withstand high-velocity impact loading, such as nanomaterials. This study provides an assessment of the influence of surface and interfaces on plastic deformations in both bcc and fcc materials using molecular dynamics simulations.