Molecular Dynamics Simulations Of Metals

This dissertation describes the development and testing of modified embedded atom method (MEAM) interatomic potentials for Al, Si, Mg, Cu, Fe, and their alloys, with primary concentration on Mg-Al system. We performed the density functional theory (DFT) based ab initio calculations to determine the structural and elastic properties of element pairs that are impractical to obtain from experimental measurements. Specifically, we estimated the cohesive energy, equilibrium atomic volume, bulk modulus, and elastic moduli of every element pair in the NaCl reference structure. Based on the results of DFT calculations, MEAM parameters for each element pair were constructed. We extensively tested the new MEAM potential for Mg-Al alloy system. The new Mg-Al MEAM potential was compared with DFT calculations, previously published semi-empirical interatomic potentials, and experiments. Applicability of the new MEAM potential to atomistic modeling was demonstrated by calculating stress-strain responses from molecular dynamics (MD) simulations of Mg and Al systems in a variety of configurations. The effects of alloying, porosity, and strain rate conditions on the stress-strain response were quantified. The underlying mechanisms for tension-compression asymmetry observed in the macroscale experiments of Mg alloys were investigated at the nanoscale. This work presents a contribution to the task of bridging quantum-mechanical and classical atomistic scale simulations. Information from ab initio electronic structure calculations was used to construct parameters of semi-empirical MEAM potentials for large-scale atomistic simulations of alloys. The results of the new MEAM models compare extremely well to those from other published interatomic potentials. The applicability of the new MEAM potential to investigate nanoscale mechanisms of the deformation and fracture for Al, Mg and Mg-Al alloys was demonstrated. It has been shown that the MEAM provides a single universal formalism for classical atomistic simulations of a wide range of elements and their alloys

Large-scale parallel lattice Boltzmann-Cellular automaton model of two-dimensional dendritic growth Large-scale parallel lattice Boltzmann -cellular automaton model of two-dimensional dendritic growth Manuscript Title: Large-scale parallel lattice Boltzma

Abstract An extremely scalable lattice Boltzmann (LB) -cellular automaton (CA) model for simulations of two-dimensional (2D) dendritic solidification under forced convection is presented. The model incorporates effects of phase change, solute diffusion, melt convection, and heat transport. The LB model represents the diffusion, convection, and heat transfer phenomena. The dendrite growth is driven by a difference between actual and equilibrium liquid composition at the solid-liquid interface. The CA technique is deployed to track the new interface cells. The computer program was parallelized using Message Passing Interface (MPI) technique. Parallel scaling of the algorithm was studied and major scalability bottlenecks were identified. Efficiency loss attributable to the high memory bandwidth requirement of the algorithm was observed when using multiple cores per processor. Parallel writing of the output variables of interest was implemented in the binary Hierarchical Data Format 5 (HDF5) to improve the output performance, and to simplify visualization. Calculations were carried out in single precision arithmetic without significant loss in accuracy, resulting in 50% reduction of memory and computational time requirements. The presented solidification model shows a very good scalability up to centimeter size domains, including more than ten million of dendrites

Three Dimensional Simulation of Solutal Dendrite Growth Using Lattice Boltzmann and Cellular Automaton Methods

To simulate three dimensional (3D) solute-driven dendrite growth, a new numerical technique is introduced that combines the lattice Boltzmann (LB) and cellular automaton (CA) methods to calculate the transport phenomena and capture the solid/liquid interface, respectively. The effect of undercooling and degree of anisotropy on the kinetics of dendrite growth and the parallel performance of the model are studied. The present model can be used as a novel tool for simulating 3D dendrite growth with high computational efficiency and large-scale parallelization

Three-Dimensional Lattice Boltzmann Modeling of Dendritic Solidification under Forced and Natural Convection

A three-dimensional (3D) lattice Boltzmann (LB) model is developed to simulate the dendritic growth during solidification of Al-Cu alloys under forced and natural convection. The LB method is used to solve for solute diffusion and fluid flow. It is assumed that the dendritic growth is driven by the difference between the local actual and local equilibrium composition of the liquid in the interface. A cellular automaton (CA) scheme is adopted to capture new interface cells. The LB models for solute transport and fluid flow are first validated against two benchmark problems. The dendrite growth model is also validated with available analytical solutions. The evolution of a 3D dendrite affected by melt convection is investigated. Also, density inversion caused by solute concentration gradient is studied. It is shown that convection can change the kinetics of growth by affecting the solute distribution around the dendrite. In addition, the growth features of two-dimensional (2D) and 3D dendrites are briefly compared. The results show that decreasing undercooling and increasing solute concentration decelerates the growth in all branches of the dendrite. While increasing fluid velocity does not significantly influence upstream and transverse arms, it decreases the growth rate in the downstream direction considerably. The size ratio of the upstream arm to the downstream arm rises by increasing inlet velocity and solute content, and decreasing undercooling. Similarly, in the case of natural convection, redistribution of solute due to buoyancy-induced flow suppresses the growth of the upward arm and accelerates the growth of the downward arm. Considering the advantages offered by the LB method, the present model can be used as a new tool for simulating 3D dendritic solidification under convection