Bulletin of the American Physical Society
19th Biennial Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 60, Number 8
Sunday–Friday, June 14–19, 2015; Tampa, Florida
Session C5: First-Principles and MD II: Damage and Defects in Metals |
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Chair: Mark Elert, United States Naval Academy, Luis Zepeda-Ruiz, Lawrence Livermore National Laboratory Room: Grand I/J |
Monday, June 15, 2015 11:15AM - 11:30AM |
C5.00001: Nucleation and Evolution of Dynamic Damage at Cu/Pb Interfaces using Molecular Dynamics S.J. Fensin, S.M. Valone, E.K. Cerreta, P.A. Rigg, G.T. Gray III For ductile metals, the process of dynamic fracture occurs through nucleation, growth and coalescence of voids. For high purity single phase metals, it has been observed by numerous investigators that voids tend to heterogeneously nucleate at grain boundaries and all grain boundaries are \textit{not} equally susceptible to void nucleation. However, for materials of engineering significance, especially those with second phase particles, it is less clear if the type of bi-metal interface between the two phases will affect void nucleation and growth. To approach this problem in a systematic manner two bi-metal interfaces between Cu and Pb have been investigated: \{111\} and \{100\}. Qualitative and quantitative analysis of the collected data from the spall simulations suggests that Pb becomes disordered during shock compression and is the preferred location for void nucleation under tension. Despite the interfaces being aligned with the spall plane (by design), they are not the preferred location for void nucleation \textit{irrespective} of interface type. [Preview Abstract] |
Monday, June 15, 2015 11:30AM - 11:45AM |
C5.00002: Quasi-Coarse-Grained Dynamics (QCGD): Modeling of Defect/Damage Evolution at Mesoscales using Atomic Scale Interatomic Potentials Avinash Dongare, Karoon Mackenchery, Garvit Agarwal, Ramakrishna Valisetty, Arunachalam Rajendran, Raju Namburu A computationally efficient modeling method called ``quasi-coarse-grained dynamics'' (QCGD) is developed to expand the capabilities of molecular dynamics (MD) simulations to model behavior of metallic materials at the mesoscales. This mesoscale method is based on solving the equations of motion for a chosen set of representative atoms from an atomistic microstructure and retaining the energetics of these atoms as would be predicted in MD simulations through scaling relationships for the interatomic potentials. The success of the mesoscale method is demonstrated by the prediction of the high temperature thermodynamics, deformation behavior of interfaces, phase-transformation behavior, heat generation during plastic deformation as well as the wave-propagation behavior in metallic systems under various conditions, as would be predicted using MD simulations. The reduced number of atoms and the improved time-steps allow the modeling of metallic materials at the mesoscale in extreme environments. The applicability of the QCGD simulations to predict the evolution of defect structures and the microstructure during deformation and failure in FCC metals at the mesoscales will be discussed. [Preview Abstract] |
Monday, June 15, 2015 11:45AM - 12:15PM |
C5.00003: Simulation of shock-recovered samples: dislocations, twins, porosity and phase transformations Invited Speaker: Eduardo Bringa Atomistic molecular dynamics (MD) simulations of shock compression of metals often display tremendous plastic activity in the material, including immense dislocation densities during loading, of the order of 1e16-1e17m$^{\mathrm{-2}}$. These high densities are in stark contrast with dislocation densities in recovered samples, measured by electron microscopy to be the order of 1e13-1e14m$^{\mathrm{-2}}$. This large disparity sometimes generated lack of confidence in simulation results as representative of experimental results, but the outlook has changed in the last few years thanks to novel experiments estimating dislocation densities during loading, and thanks to new simulations trying to mimic recovery of shocked samples. Experiments measuring instability growth, or using dynamic X-Ray Diffraction seem to agree with the immense dislocation densities predicted by MD. Simulated recovery has been shown to provide significant reduction of dislocation densities in fcc samples. Here it will be shown that this is also true in bcc Ta samples, where low dislocation mobility might have been expected to reduce dislocation disappearance. In addition to dislocations, recovered samples might display twins and porosity. Twinning has been observed in many loading simulations, but it might be reversed upon unloading. On the other hand, unloading itself might cause twinning, as it will be discussed for the case of polycrystalline Fe. Regarding porosity, it might appear due to different scenarios. For instance, dislocation recovery in fcc Au leads to vacancy clusters which would then decay into the stacking fault tetrahedra (SFTs) observed experimentally in recovered samples. Finally, phase transformations might occur during loading, including solid-solid transformations and melting. Recovery might revert those phase transformations, as it has been shown for Fe, or it might lead to further transformations and changes in the microstructure, as it will be shown for additional examples. [Preview Abstract] |
Monday, June 15, 2015 12:15PM - 12:30PM |
C5.00004: Physics of Shock Compression and Release: NEMD Simulations of Tantalum and Silicon Eric Hahn, Marc Meyers, Shiteng Zhao, Bruce Remington, Eduardo Bringa, Tim Germann, Ramon Ravelo, James Hammerberg Shock compression and release allow us to evaluate physical deformation and damage mechanisms occurring in extreme environments. SPaSM and LAMMPS molecular dynamics codes were employed to simulate single and polycrystalline tantalum and silicon at strain rates above 10$^{8}$ s$^{-1}$. Visualization and analysis was accomplished using OVITO, Crystal Analysis Tool, and a redesigned orientation imaging function implemented into SPaSM. A comparison between interatomic potentials for both Si and Ta (as pertaining to shock conditions) is conducted and the influence on phase transformation and plastic relaxation is discussed. Partial dislocations, shear induced disordering, and metastable phase changes are observed in compressed silicon. For tantalum, the role of grain boundary and twin intersections are evaluated for their role in ductile spallation. Finally, the temperature dependent response of both Ta and Si is investigated. [Preview Abstract] |
Monday, June 15, 2015 12:30PM - 12:45PM |
C5.00005: Shock waves and recovery in polycrystalline iron Nina Gunkelmann, Diego R. Tramontina, Eduardo M. Bringa, Herbert M. Urbassek It is well known that shocks create not only plasticity in Fe, but also phase transform the material from its bcc phase to the high-pressure hcp phase. These two mechanisms were recently examined in several simulation studies. However, there are still important questions that are not answered in our current understanding of shocks in Fe. In particular, the morphological properties of shock recovered samples have not been extensively explored in experiments, and are still unexplored in atomistic simulations. In this work, we study shocks and recovery of large polycrystalline iron samples by molecular dynamics simulations. With increasing shock strength, we find a transition from a 2-wave structure (elastic and plastic wave) to a 3-wave structure (an additional phase-transformation wave), in agreement with experiments. The phase transformation is preceded by dislocation generation at grain boundaries. Our analysis shows that recovery leads to twinning inside the recovered bcc grains. The structure of the twins is in good agreement with experimental results and a semi-analytical model which assumes a critical shear stress for twinning. [Preview Abstract] |
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