Bulletin of the American Physical Society
20th Biennial Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 62, Number 9
Sunday–Friday, July 9–14, 2017; St. Louis, Missouri
Session B5: First Principles and Molecular Dynamics I |
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Chair: Nina Gunkelmann, Friedrich-Alexander-Universitat Erlangen-Nurnberg Room: Regency Ballroom B |
Monday, July 10, 2017 9:15AM - 9:30AM |
B5.00001: Characterizing Atomistic Geometries and Potential Functions Using Strain Functionals Edward Kober, Nithin Mathew, Sven Rudin We demonstrate the use of strain tensor functionals for characterizing arbitrarily ordered atomistic structures. This approach defines a Gaussian-weighted neighborhood around each atom and characterizes that local geometry in terms of n-th order strain tensors, which are equivalent to the n-th order moments/derivatives of the neighborhood. Fourth order expansions can distinguish the cubic structures (and deformations thereof), but sixth order expansions are required to fully characterize hexagonal structures. These functions are continuous and smooth and much less sensitive to thermal fluctuations than other descriptors based on discrete neighborhoods. Reducing these metrics to rotational invariant descriptors allows a large number of defect structures to be readily identified and forms the basis of a classification scheme that allows molecular dynamics simulations to be readily analyzed. Applications to the analysis of shock waves impinging on samples of Cu, Ta and Ti will be presented. The method has been extended to vector fields as well, enabling the local stress to be cast in terms of rotationally invariant functions as well. The stress-strain correlations can then be used as the basis for developing and analyzing potential functions. [Preview Abstract] |
Monday, July 10, 2017 9:30AM - 9:45AM |
B5.00002: ABSTRACT WITHDRAWN |
Monday, July 10, 2017 9:45AM - 10:15AM |
B5.00003: Limits of metastability in shock-induced phase transitions Invited Speaker: Ramon Ravelo In dynamic compression, kinetic effects may delay a phase transition and under extreme strain rates of deformation, the parent phase can be loaded into metastable states from which the transition takes place, beyond the equilibrium phase boundary. Large-scale atomistic simulations coupled with first-principles calculations can provide valuable insights in the study of phase changes under extreme conditions, such as those produced by strong shock waves. The size scale of systems that can be studied via atomistic simulations are now sufficient to study large defective or multiphase structures, and the time-scales sampled in non-equilibrium molecular dynamics (NEMD) are currently in the nanoseconds, a time scale accessible in high-energy laser driven shock experiments. Atomistic simulations of shocked single crystals, show that sub-nanoseconds rise times and strain rates $> 10^9 s^{-1}$ can lead to deformation paths and transformation mechanisms different from homogeneous nucleation or thermally activated processes. We present a formulation of the limits of metastability of elastic-plastic and structural transitions in single crystals, including amorphization, under high strain-rates of deformation, based on large-scale atomistic simulations and ab-initio calculations. [Preview Abstract] |
Monday, July 10, 2017 10:15AM - 10:30AM |
B5.00004: Atomic Scale Modeling of Laser Shock induced Spallation of FCC Metals Sergey Galitskiy, Dmitry Ivanov, Avinash Dongare An atomistic-continuum approach combining the molecular dynamics (MD) simulations with a two temperature model (TTM) was used to simulate the laser induced shock loading and spall failure in FCC metals. The combined TTM-MD approach incorporates the laser energy absorption, fast electron heat conduction, and the electron-phonon non-equilibrium interaction, as well as the shock wave propagation, plastic deformation, and failure processes (spallation) in metals at atomic scales. The simulations are carried out for systems corresponding to dimensions of up to 500 nm in the loading direction for various Cu and Al microstructures and laser loading conditions (intensity and pulse durations). The front end of the metal that absorbs the laser energy is observed to undergo melting and a shock wave is generated that travels towards the rear surface. The shock wave reaches the rear surface, reflects, and interacts with the its tail to create a high triaxial tensile stress region and initiates spall failure (void nucleation). The predicted values of spall strength and wave velocities of shock waves compare very well with experimentally reported values at these dimensions and laser loading conditions. The effect of microstructure and the defect evolution in the system on the predicted spall failure behavior will be presented. [Preview Abstract] |
Monday, July 10, 2017 10:30AM - 10:45AM |
B5.00005: Strain-Rate Dependence of Deformation-Twinning in Tantalum Jayalath Abeywardhana, Tim Germann, Ramon Ravelo Large-Scale molecular dynamics (MD) simulations are used to model quasi-isentropic compression and expansion (QIC) in tantalum crystals varying the rate of deformation between the range $10^{8}-10^{12} s^{-1}$ and compressive pressures up to 100 GPa. The atomic interactions were modeled employing an embedded-atom method (EAM) potential of Ta. Isentropic expansion was done employing samples initially compressed to pressures of 60 and 100 GPa followed by uniaxial and quasi-isentropically expansion to zero pressure. The effect of initial dislocation density on twinning was also examined by varying the initial defect density of the Ta samples ($10^{10} – 10^{12} cm^{-2}$). At these high-strain rates, a threshold in strain-rate on deformation twining is observed. Under expansion or compression, deformation twinning increases with strain rate for strain-rates $> 10^{9} s^{-1}$. Below this value, small fraction of twins nucleates but anneal out with time. Samples with lower fraction of twins equilibrate to defect states containing higher screw dislocation densities from those with initially higher twinning fractions. [Preview Abstract] |
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