Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session P2: Materials in Extremes VIIFocus

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Sponsoring Units: DCOMP DMP SHOCK Chair: Jon Belof, Lawrence Livermore National Laboratory Room: 261 
Wednesday, March 15, 2017 2:30PM  3:06PM 
P2.00001: Ultrafast studies of shockinduced melting and phase transitions at LCLS Invited Speaker: Malcolm McMahon The study of shockinduced phase transitions, which is vital to the understanding of material response to rapid pressure changes, dates back to the 1950s, when Bankcroft {\it et al} reported a transition in iron [1]. Since then, many transitions have been reported in a wide range of materials, but, due to the lack of sufficiently bright xray sources, the structural details of these new phases has been notably lacking [2]. While the development of nanosecond {\it in situ} xray diffraction has meant that latticelevel studies of such phenomena have become possible [35], including studies of the phase transition reported 60 years ago in iron [6], the quality of the diffraction data from such studies is noticeably poorer than that obtained from staticallycompressed samples on synchrotrons. The advent of xray free electron lasers (XFELs), such as the LCLS, has resulted in an unprecedented improvement in the quality of diffraction data that can be obtained from shockcompressed matter. Here I describe the results from three recent experiment at the LCLS that looked at the solidsolid and solidliquid phase transitions in Sb, Bi and Sc using single 50 fs xray exposures [7,8]. The results provide new insight into the structural changes and melting induced by shock compression. [1] D. Bancroft {\it et al}, J. Appl. Phys. {\bf 27}, 291 (1956). [2] G.E. Duvall and R.A. Graham Rev. Mod. Phys. {\bf 49}, 523 (1977). [3] Q. Johnson and A. Mitchell, Phys. Rev. Lett. {\bf 29}, 1369 (1972). [4] T. dâ€™Almeida and Y.M. Gupta, Phys. Rev. Lett. {\bf 85}, 330 (2000). [5] J. R. Rygg {\it et al}, Rev. Sci. Instrum. {\bf 83}, 113904 (2012). [6] D.H. Kalantar {\it et al}, Phys. Rev. Lett. {\bf 95}, 075502 (2005). [7] M.G. Gorman {\it et al}, Phys. Rev. Lett. {\bf 115}, 095701 (2015). [8] R. Briggs{\it et al}, Phys. Rev. Lett. In Press (2016). [Preview Abstract] 
Wednesday, March 15, 2017 3:06PM  3:18PM 
P2.00002: The Matter in Extreme Conditions at LCLS: present and future capabilities Eric Galtier, Hae Ja Lee, Bob Nagler, Andy MacKinnon, Shaughnessy Brown, Inhyuk Nam, Eduardo Granados, Alan Fry, Brice Arnold, Oliver Hickman Less than five years ago, the Matter in Extreme Conditions at the Linac Coherent Light Source executed its first experiments. This unique endstation, bringing together high intensity optical lasers with the high peak brightness Xray source of the LCLS, offers new capabilities to study a broad range of scientific topics of the exteme, from high pressure systems to high energy density physics. In this talk, we present recent high pressure studies using nanosecond optical laser systems for dynamic compression. Then, we describe an upgrades plan to extend and strengthen its capabilities in order to explore even further the pressure/temperature phase space. [Preview Abstract] 
Wednesday, March 15, 2017 3:18PM  3:30PM 
P2.00003: Investigating deformation mechanisms in shock compressed tantalum via femtosecond diffraction David McGonegle, Marcin Sliwa, Justin Wark, Cynthia Bolme, Andrew Higginbotham, Amy Jenei, HyeSook HyeSook, Bruce Remington, Rob Rudd, Damian Swift, Chris Wehrenberg, Luis ZepedaRuiz, Hae Ja Lee, Bob Nagler When materials are compressed beyond their Hugoniot elastic limit, they act to relieve built up shear stress by deforming plastically. Tantalum provides an interesting case to study owing to its multitude of competing plasticity mechanisms, a combination of dislocation flow (slip) and deformation twinning. We present the first direct, insitu observation of twinning in shockcompressed metals using femtosecond xray diffraction performed at the MEC beamline at LCLS. Tantalum with an initial (110) fiber texture was subjected to shock compression in the 10300 GPa pressure range at strainrates above $10^9 \mathrm{s}^{1}$ and the ultrafast texture evolution was recorded via insitu, timeresolved xray diffraction. The onset of twinning was observed at 25 GPa, reaching a twindominated response for shock strengths of $\sim$5075 GPa. At high shock pressure ($>$150 GPa) the twin fraction is lower and the response is slip dominated. These results compare favourably with molecular dynamics simulations performed using the Ravelo EAM potential, which also show a twinslip transition. [Preview Abstract] 
Wednesday, March 15, 2017 3:30PM  3:42PM 
P2.00004: Tensile strength and failure mechanisms of tantalum at extreme strain rates Eric Hahn, Saryu Fensin, Timothy Germann, Marc Meyers Nonequilibrium molecular dynamics simulations are used to probe the tensile response of monocrystalline, bicrystalline, and nanocrystalline tantalum over six orders of magnitude of strain rate. Our analysis of the strain rate dependence of strength is extended to over nine orders of magnitude by bridging the present simulations to recent laserdriven shock experiments. Tensile strength shows a powerlaw dependence with strain rate over this wide range, with different relationships depending on the initial microstructure and active deformation mechanism. At high strain rates, multiple spall events occur independently and continue to occur until communication occurs by means of relaxation waves. Temperature plays a significant role in the reduction of spall strength as the initial shock required to achieve such large strain rates also contributes to temperature rise, through pressurevolume work as well as viscoplastic heating, which leads to softening and sometimes melting upon release. At ultrahigh strain rates, those approaching or exceeding the atomic vibrational frequency, spall strength saturates at the ultimate cohesive strength of the material. [Preview Abstract] 
Wednesday, March 15, 2017 3:42PM  3:54PM 
P2.00005: An atomistic study of shock front evolution in a nanocrystalline Al R. Valisetty, A. Rajendran, G. Agarwal, A. Dongare, J. Ianni, R. Namburu The precursor decay phenomenon in solids was investigated using large scale atomistic molecular dynamics simulations.? The main objective was to study the strain rate effects on the evolution of stress wave profiles at the elasticplastic transition point (Hugoniot Elastic Limit  HEL) . The simulations employed a multibillion nanocrystalline aluminum atom system with an average grain size of 100nm for five impact velocities ranging 0.7 km/s to 1.5 km/s.? The results comprised of the histories of stress, strain rate, and precursor decay with respect to time and distance from 100s of terabytes of data were analyzed towards evaluating the transient shock fronts. The stress amplitude (HEL) at all strain rates exhibited high strain rate dependency and as one would expect the steady state value was never reached. The HEL data from computational results at nanoscales when extrapolated reasonably matched with the experimental data across maso to macro scales.? Finally using a crystal analysis algorithm and a twin dislocation identification method, dislocation densities were reduced and reported in terms of type of dislocation vs. axial stress.? The analyses also showed that certain types of dislocations seemed to strongly influence the elasticplastic transition response of the aluminum atom system. [Preview Abstract] 
Wednesday, March 15, 2017 3:54PM  4:06PM 
P2.00006: A dislocation dynamics model of the plastic flow of fcc polycrystals Abigail Hunter, Dean Preston Plastic constitutive models applicable at strain rates of roughly 10$^{5}$s$^{1}$ and higher are essential for simulations of material deformation and failure under shock wave loading. Accurately describing deformation physics in this strain rate regime remains a challenge due to the break down of fundamental assumptions that apply to material strength at low strain rates. Furthermore, continuumscale models traditionally have difficulty accounting for specific mesoscale deformation behavior due to the larger length scales (tens to hundreds of microns) at which these models are applicable. Nevertheless, it is possible to construct a reliable, analytic dislocation dynamics based model of the flow stress as a function of strain, strain rate, temperature, and material density. We present a dislocation dynamics model of the plastic flow of fcc polycrystals from quasistatic to very high strain rates, pressures from ambient to 1000 GPa, and temperatures from zero to melt. The model is comprised of three coupled ordinary differential equations: a kinetic equation, which relates the strain rate to the stress, mobile and immobile dislocation densities, mass density, and temperature, and two equations describing the evolution of the mobile and immobile dislocation densities. Preliminary results will be presented. The relative importance of various deformation mechanisms vs. strain, strain rate, and temperature will be discussed. [Preview Abstract] 
Wednesday, March 15, 2017 4:06PM  4:18PM 
P2.00007: Proton radiography measurements of ejecta structure in shocked Sn J.E. Hammerberg, W.T. Buttler, A. Llobet, C. Morris We have performed ejecta measurements at the Los Alamos proton radiography facility on 7 mm thick 50 mm diameter Sn samples driven with a PBX9501 high explosive. The surface of the Sn, in contact with He gas at an initial pressure of 7 atmospheres, was machined to have 3 concentric sinusoidal features with a wavelength of $\lambda =$2mm in the radial direction and amplitude h$_{0}=$0.159mm (kh$_{0}=$2$\pi $h$_{0}$/$\lambda =$0.5). The shock pressure was 27 GPa. 28 images were obtained between 0 and 14 $\mu $s from the time of shock breakout at 500 ns intervals. The Abel inverted density profiles evolve to a selfsimilar density distribution that depends on a scaling variable z/v$_{s}$t where v$_{s}$ is the spike tip velocity, z is the distance from the free surface and t is the time after shock breakout. Both the density profiles and the time dependence of the mass per unit area in the evolving spikes are in good agreement with a RichtmyerMeshkov instability based model for ejecta production and evolution. [Preview Abstract] 
Wednesday, March 15, 2017 4:18PM  4:30PM 
P2.00008: Formation mechanism of the shockinduced particle jetting kun xue Granular shells or rings dispersed by the impulsive shock loadings disintegrate into macroscopic particle agglomerates which soon protrude into particle jets. Predicting the number of shockinduced particle jets requires the knowledge of the formation mechanism of the particle jetting. We carried out the numerical simulations of the shock dispersal of the semitwo dimensional particle rings using the discrete element method. The simulations reveal a twostaged jetting formation process. The first phase features the transition of the homogeneous particle flows to the localized shear flows which are the precursors of the incipient jets. The incipient jets undergo the substantial annihilation. The number of jets equals to the number of incipient jets subtracted by that of eliminated ones. The former depends on the heterogeneous structure of the network of force chains which is a function of the perimeter of the inner surface, the packing density and the material properties. The latter is determined by the shock loading, the packing density and the thickness of the ring. A physics based model has been proposed to account for the number of jet, which formularizes the initiation and the elimination processes of the incipient jets. [Preview Abstract] 
Wednesday, March 15, 2017 4:30PM  4:42PM 
P2.00009: Modeling jet formation around copper notch using dual domain material point method combined with molecular dynamics Tilak Dhakal, Duan Zhang Under a strong impact in metals, the material around a notch forms a jet shooting outward. We use this phenomenon as an example to test our new multiscale computation method intended for modeling extreme material deformation and thermodynamic nonequilibrium. Starting from the Liouville equation, we derive macroscopic momentum equation with a stress tensor directly related to the molecular interactions. Such derived momentum equation can be used for thermodynamic nonequilibrium problems as long as we can calculate the stress. In cases of thermodynamic nonequilibrium, it is often difficult to obtain a constitutive relation for the stress. In this work we perform molecular dynamics (MD) simulations to calculate the stress, while the continuum level calculation is performed using dual domain material point (DDMP) method to consider extreme deformation of the material. In this multiscale computation, each material point is a MD system. The MD systems communicate with the continuum level calculation through the strain rate and stress. An algorithm to address the extreme shear deformation in MD is developed. Since the material points do not need to communicate among each other, the MD simulations are performed in parallel in GPU using CUDA to accelerate the computation. [Preview Abstract] 
Wednesday, March 15, 2017 4:42PM  4:54PM 
P2.00010: Molecular dynamics simulations of shock induced deformation twining of FCC single crystal copper Anupam Neogi, Sunil Rawat, Nilanjan Mitra Multimillion atom nonequilibrium molecular dynamics simulations has been carried out to demonstrate shock induced deformation pathway of FCC single crystal Cu loaded along $<$100$>$, $<$110$>$ and $<$111$>$ directions for a varying range of shock intensities. For the shock traveling along $<$100$>$ crystal direction, slippage based plasticity has been observed to occur up to the shock pressure of $\sim$58 GPa i.e piston velocity of 1.1 km/s. For higher intensity of shock i.e. shock pressure above $\sim$60 GPa (piston velocity of 1.2 km/s), predominating nature of twining mechanism has been identified during postshock relaxation. The underlying atomistic mechanism of this conversion of slipbased to twinbased plasticity has been studied along with distribution of shock induced strain in the deformed matrix and its associated postshock relaxation. Postshock relaxation upon unloading has been identified to play a crucial role over the resultant texturing in the target sample. The activation and temporal evolution of different twin systems has been studied. Moreover, a complete atomistic mechanism of this sliptotwin transition in FCC shocked metal will be discussed. [Preview Abstract] 
Wednesday, March 15, 2017 4:54PM  5:06PM 
P2.00011: Anisotropic shock response of single crystal titanium: Molecular dynamics investigation Anupam Neogi, Nilanjan Mitra A comprehensive atomistic simulation study has been done to investigate anisotropic shock response of single crystal $\alpha$ Ti upto 350 GPa of shock pressure. Details characterization of the shocked microstructure, including hexagonal closed packed $\alpha$to$\omega$ martensitic phase transformation, has been done from mechanistic perspective by analyzing radial distribution function, neighbor based structural identification, xray diffraction etc. Four different interatomic potentials, FinnisSinclair (FS) manybody potential, embedded atom method (EAM) potential, 2NNMEAM potential and splinebased modified embedded atom method (MEAM) potential, has been chosen to explore the capability, accuracy and applicability of these interatomic potentials for typical dynamic shock study i.e. in high pressure applications. [Preview Abstract] 
Wednesday, March 15, 2017 5:06PM  5:18PM 
P2.00012: Silicene growth on Ag(111) thin film covered Si(111) substrates. TsuYi Fu, HungChang Hsu, YiHung Lu, TaiLung Su Silicene, the two dimensional monolayer films and the same group IV element with graphene, is expected for its abundant properties. However, the difficulty of structure growth and the demand for special substrates limit the possibility of further applications. Using scanning tunneling microscopy (STM) and scanning tunneling spectrum (STS), the formation of silicene on the 6\textasciitilde 12ML Ag (111) thin films was studied. The Ag films were pregrown on the Si (111) substrates. The classical silicene superstructures, such as 4$\times$ 4, $\surd $13$\times$ $\surd $13, 2$\surd $3$\times$ 2$\surd $3, can be observed on this Ag (111) thin film covered Si(111) surfaces. The more continuous silicene sheet formed on 6\textasciitilde 12 ML Ag (111)/Si(111) substrate than on the single crystal Ag (111) surface. Various silicene superstructures are usually discontinuous on the single crystal Ag (111) surfaces, but continuous on the thin film Ag (111) surfaces by Ag domain rotation on Si (111) surfaces. A flexible thin film Ag (111) surface seems a better substrate than solid crystal Ag (111) for silicene growth. The STS of each superstructure of silicene was measured and found the similar electron properties. It indicates the possible application for multisuperstructure silicene. [Preview Abstract] 
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