PhD Student, Missouri University of Science and Technology
Computer simulation of realistic experiment of solidification in metals in presence of oxygen.
As we increase the temperature of any metallic materials such as Aluminum, Iron, Steel etc. it melts. In the same way if the melted materials are left out in a lower temperature environment it solidifies. The melting-solidification phenomena changes the physical, chemical, mechanical properties of a material. Melting-solidification have wide range of applications in manufacturing industries (such as steel making, automobiles). The properties of a metal or alloys can be controlled by controlled melting solidification. If each metal or alloy are studied individually by experiments, it will take a large amount of resources (in terms of budget and equipment) and manpower to create a database of these properties in different experimental conditions. Instead of doing each of the different experiment we can run computer simulation and accelerate the process of understanding the behavior of materials during and after the melting-solidification. Computer simulations can give a guideline to the experimentalists how the materials may behave in a certain experimental condition and it will behave after the experiments. This will accelerate the process of running different experiments and also, we can design new materials. We can predict materials with superior properties, which will be lengthy and time consuming process to find it by performing experiments. All materials are consisted of atoms/molecules. When there is any kind of change of properties of a materials, it must start at atomic or molecular level. For an example, when we see a piece of metal or a wood is broken, we see it macroscopically (at a very large scale). If we look closer and try to see it under microscope we will find, the larger crack was originated from much smaller identical cracks. If we keep magnifying it, we will find the crack started at an atomic scale. So, to study the properties of melting-solidification we need to look at atomic scale to understand how material properties evolve by melting or solidification. The method I use for my research is called molecular dynamics. Molecular dynamics based models can simulate a small portion (Few atoms to Billions of atoms) of the materials and it can predict how it will behave a larger scale. Computer simulations act as a bridge between microscopic length and time scales and the macroscopic world of the laboratory. This is the frontiers of new material development.
Abstract: Can completely homogeneous nucleation occur? Large scale molecular dynamics simulations performed on a graphics-processing-unit rich supercomputer can shed light on this long-standing issue. Here, a billion-atom molecular dynamics simulation of homogeneous nucleation from an undercooled iron melt reveals that some satellite-like small grains surrounding previously formed large grains exist in the middle of the nucleation process, which are not distributed uniformly. At the same time, grains with a twin boundary are formed by heterogeneous nucleation from the surface of the previously formed grains. The local heterogeneity in the distribution of grains is caused by the local accumulation of the icosahedral structure in the undercooled melt near the previously formed grains. This insight is mainly attributable to the multi-graphics processing unit parallel computation combined with the rapid progress in high-performance computational environments.Nucleation is a fundamental physical process, however it is a long-standing issue whether completely homogeneous nucleation can occur. Here the authors reveal, via a billion-atom molecular dynamics simulation, that local heterogeneity exists during homogeneous nucleation in an undercooled iron melt.
Pub.: 07 Apr '17, Pinned: 30 Jun '17
Abstract: We present a fluid dynamics video showing the results of a 9-billion atom molecular dynamics simulation of complex fluid flow in molten copper and aluminum. Starting with an atomically flat interface, a shear is imposed along the copper-aluminum interface and random atomic fluctuations seed the formation of vortices. These vortices grow due to the Kelvin-Helmholtz instability. The resulting vortical structures are beautifully intricate, decorated with secondary instabilities and complex mixing phenomena. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
Pub.: 16 Oct '08, Pinned: 30 Jun '17
Abstract: Molecular Dynamics is an important tool for computational biologists, chemists, and materials scientists, consuming a sizable amount of supercomputing resources. Many of the investigated systems contain charged particles, which can only be simulated accurately using a long-range solver, such as PPPM. We extend the popular LAMMPS molecular dynamics code with an implementation of PPPM particularly suitable for the second generation Intel Xeon Phi. Our main target is the optimization of computational kernels by means of vectorization, and we observe speedups in these kernels of up to 12x. These improvements carry over to LAMMPS users, with overall speedups ranging between 2-3x, without requiring users to retune input parameters. Furthermore, our optimizations make it easier for users to determine optimal input parameters for attaining top performance.
Pub.: 14 Feb '17, Pinned: 30 Jun '17
Abstract: Nano mechanical behavior of Mg<img border="0" alt="single bond" src="http://cdn.els-cdn.com/sd/entities/sbnd" class="glyphImg">Li nanowire is investigated under tension and compression to elicit property alteration due to Li alloying in Mg within hexagonal range. Embedded atom method (EAM) is employed to carry out present simulation work. Nanowire under consideration is supposed to be isotropic and mechanical behavior is uninfluenced by material texture. The elastic modulus, yield strength both in tension and compression is assessed with change in strain rate. Effects of temperature in tension and compression are studied. Results of present simulation work elicit serrated yielding under uniaxial tension, however, twin mediated deformation under compression is completely tuned with previously reported experimental works. This investigation bridges nanometer scale properties to microscale material response, which in turn can be applied for designing suitable robust processing routes of this material.
Pub.: 02 Mar '16, Pinned: 30 Jun '17
Abstract: We investigate the dependency of strain rate, temperature and size on yield strength of hexagonal close packed (HCP) nanowires based on large-scale molecular dynamics (MD) simulation. A variance-based analysis has been proposed to quantify relative sensitivity of the three controlling factors on the yield strength of the material. One of the major drawbacks of conventional MD simulation based studies is that the simulations are computationally very intensive and economically expensive. Large scale molecular dynamics simulation needs supercomputing access and the larger the number of atoms, the longer it takes time and computational resources. For this reason it becomes practically impossible to perform a robust and comprehensive analysis that requires multiple simulations such as sensitivity analysis, uncertainty quantification and optimization. We propose a novel surrogate based molecular dynamics (SBMD) simulation approach that enables us to carry out thousands of virtual simulations for different combinations of the controlling factors in a computationally efficient way by performing only few MD simulations. Following the SBMD simulation approach an efficient optimum design scheme has been developed to predict optimized size of the nanowire to maximize the yield strength. Subsequently the effect of inevitable uncertainty associated with the controlling factors has been quantified using Monte Carlo simulation. Though we have confined our analyses in this article for Magnesium nanowires only, the proposed approach can be extended to other materials for computationally intensive nano-scale investigation involving multiple factors of influence.
Pub.: 17 Nov '16, Pinned: 30 Jun '17
Abstract: Homogeneous nucleation from aluminum (Al) melt was investigated by million-atom molecular dynamics (MD) simulations utilizing the second nearest neighbor modified embedded atom method (MEAM) potentials. The natural spontaneous homogenous nucleation from the Al melt was produced without any influence of pressure, free surface effects and impurities. Initially isothermal crystal nucleation from undercooled melt was studied at different constant temperatures, and later superheated Al melt was quenched with different cooling rates. The crystal structure of nuclei, critical nucleus size, critical temperature for homogenous nucleation, induction time, and nucleation rate were determined. The quenching simulations clearly revealed three temperature regimes: sub-critical nucleation, super-critical nucleation, and solid-state grain growth regimes. The main crystalline phase was identified as face-centered cubic (fcc), but a hexagonal close-packed (hcp) and an amorphous solid phase were also detected. The hcp phase was created due to the formation of stacking faults during solidification of Al melt. By slowing down the cooling rate, the volume fraction of hcp and amorphous phases decreased. After the box was completely solid, grain growth was simulated and the grain growth exponent was determined for different annealing temperatures.
Pub.: 22 Jun '17, Pinned: 30 Jun '17
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