Session R21: Materials at Extremes: Dynamic Compression

Ultrafast X-ray Studies on the Dynamics of Structural Transitions

Understanding the processes which dictate physical properties such as strength, elasticity, plasticity, and the kinetics of phase transformation/crystallization requires studies at the relevant length-scales (e.g., interatomic spacing and grain size) and time-scales (e.g., phonon period). Experiments performed at the Matter in Extreme Conditions end-station at the Linac Coherent Light Source, SLAC combine a laser-driven dynamic compression pump and X-ray free electron laser probe. To showcase some of the capabilities of this end-station, we present time-resolved structural and/or electronic transformations in a suite of materials over a pressure range of a few to tens of GPa, including: 1) quartz/fused silica, 2) water, 3) Fe-bearing pyroxene, 4) iron and 5) titanium .   

Nanosecond homogeneous nucleation and crystal growth in shock-compressed SiO$_{\mathrm{2}}$

Understanding the kinetics of shock-compressed SiO2~is of great importance for mitigating optical damage for high-intensity lasers and for understanding meteoroid impacts. Experimental work has placed some thermodynamic bounds on the formation of high-pressure phases of this material, but the formation kinetics and underlying microscopic mechanisms are yet to be elucidated. Here, by employing multiscale molecular dynamics studies of shock-compressed fused silica and quartz, we find that silica transforms into a poor glass former that subsequently exhibits ultrafast crystallization within a few nanoseconds. We also find that, as a result of the formation of such an intermediate disordered phase, the transition between silica polymorphs obeys a homogeneous reconstructive nucleation and grain growth model. Moreover, we construct a quantitative model of nucleation and grain growth, and compare its predictions with stishovite grain sizes observed in laser-induced damage and meteoroid impact~events.   

Order parameter aided phase space exploration under extreme conditions

Efficient exploration of configuration space and identification of metastable structures in condensed phase systems are challenging from both computational as well as algorithmic perspectives. In this talk I will illustrate how we can extend the recently proposed order-parameter aided temperature accelerated sampling schemes to efficiently and systematically explore free energy surfaces, and search for metastable states and reaction pathways within the framework of density functional theory based molecular dynamics. I will illustrate how this sampling scheme can be used to explore the relevant parts of configuration space in prototypical materials, like SiO2 and identify the different metastable structures, transition pathways and phase boundaries.   

Multi-frame X-ray Phase Contrast Imaging of Impact Experiments at the Advanced Photon Source

Recent advances in coupling synchrotron X-ray diagnostics to dynamic compression experiments are providing new information about the response of materials at extremes conditions.~ For example, propagation based X-ray Phase Contrast Imaging (PCI) which is sensitive to differences in density (or index of refraction) has been successfully used to study a wide range of phenomena including jet-formation in metals, crack nucleation and propagation, and detonator dynamics.~ These experimental results have relied, in part, on the development of a robust, optically multiplexed detector system that captures single X-ray bunch images with micrometer spatial resolution on the nanosecond time scale.~~ In this work, the multi-frame PCI (MPCI) system is described along with experiment highlights that include the compression of an idealized system of spheres subjected to impact loading. Additional advances to the detector system will be presented that are designed to increase the efficiency of the detector system and to retrieve phase information from the X-ray images which is required for determining the density during dynamic loading. Experimental results, implications, and future work will be discussed.~   

\textbf{The Challenge of Time-Dependent Control of Both Processing and Performance of Materials at the Mesoscale, and the MaRIE Project}

DOE and NNSA are recognizing a mission need for flexible and reduced-cost product-based solutions to materials through accelerated qualification, certification, and assessment. The science challenge lies between the nanoscale of materials and the integral device scale, at the middle or "mesoscale" where interfaces, defects, and microstructure determine the performance of the materials over the lifecycle of the intended use. Time-dependent control of the processing, structure and properties of materials at this scale lies at the heart of qualifying and certifying additive manufactured parts; experimental data of high fidelity and high resolution are necessary to discover the right physical mechanisms to model and to validate and calibrate those reduced-order models in codes on Exascale computers. The scientific requirements to do this are aided by a revolution in coherent imaging of non-periodic features that can be combined with scattering off periodic structures. This drives the need to require a coherent x-ray source, brilliant and high repetition rate, of sufficiently high energy to see into and through the mesoscale. The Matter-Radiation Interactions in Extremes (MaRIE) Project is a proposal to build such a very-high-energy X-ray Free Electron Laser.   

The behavior of single-crystal silicon to dynamic loading using in-situ X-ray diffraction and phase contrast imaging

Hydrostatic and uniaxial compression studies have revealed that crystalline silicon undergoes phase transitions from a cubic diamond structure to a variety of phases including orthorhombic Imma phase, body-centered tetragonal phase, and a hexagonal primitive phase [1, 2]. The dynamic response of silicon at high pressure, however, is not well understood. Phase contrast imaging has proven to be a powerful tool for probing density changes caused by the shock propagation into a material [3]. In order to characterize the elastic and phase transitions, we image shock waves in Si with high spatial resolution using the LCLS X-ray free electron laser and Matter in Extreme Conditions instrument. In this study, the long pulse optical laser with pseudo-flat top shape creates high pressures up to 60 GPa. We measure the crystal structure by observing X-ray diffraction orthogonal to the shock propagation direction over a range of pressures. We describe the capability of simultaneously performing phase contrast imaging and in situ X-ray diffraction during shock loading and discuss the dynamic response of Si in high-pressure phases. [1] Jamieson, Science, 139, 762 (1963); Hu et al. Phys. Rev.B 34, 4679 (1986) [2] McMahon and Nelmes, Phys. Rev. B 47, 8337 (1993); Mogni et al. Phys. Rev. B 89, 064104 (2014) [3] Nagler et al. J. Synchrotron Rad. 22 (2015); Schropp et al. Scientific Reports 5, 11089 (2015)   

Calculation of Si L2,3-edge Non-Resonant Inelastic X-ray Scattering Spectra of Compressed Silica Glass

Despite the abundance of silica in Earth\textquoteright s crust and its corresponding importance to geological processes, debate continues as to the local coordination of Si in silica at geological pressures. Recent {\em ab initio} molecular dynamics simulations [M. Wu et al., Sci. Rep. 2, 398 (2012)] predicted a change from 4-fold coordinated Si at ambient pressure to 6-fold coordination by 22 GPa. This was consistent with experimental non-resonant inelastic x-ray scattering (NRIXS) measurements at the O K-edge that also suggested a conversion to 6-fold coordination by 22 GPa [J. Lin et al., Phys. Rev. B 75, 012201 (2007)]. However, NRIXS measured at the Si L-edge found the spectra to be largely independent of pressure up to 74 GPa [H. Fukui et al., Phys. Rev. B 78, 12203 (2008)] indicating that 4-fold coordination is maintained. The discrepancy may potentially be due to low instrument resolution of the Si L-edge measurements. We present calculated Si L-edge NRIXS spectra at multiple pressures between ambient and 150 GPa and for several momentum transfer values. This allows us to identify spectral features that may be used to better distinguish 4-fold and 6-fold coordinated environments as experimental resolutions improve.   

Probing off-Hugoniot states in Ta, Cu, and Al to 10 Mbar compression with magnetically driven liner implosions

We report on a technique for obtaining off-Hugoniot equation of state data on solid metals by a magnetically driven cylindrical liner implosion on Sandia’s Z-machine (Z). The sample material is in an inner tube with an outer tube composed of Al that serves as the current carrying cathode. A shaped current pulse quasi-isentropically compresses the sample as it implodes. Photonic Doppler velocimetry measures the implosion velocity of the free inner surface of the sample material, and the explosion velocity of the return current anode free outer surface. The velocimetry measurements are used in conjunction with magnetohydrodynamic simulations and optimization to infer pressure and density in the sample. Results are presented for experiments on the Z-machine in which Ta, Cu, and Al samples were compressed to peak pressure ~10 Mbar. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Spall Response of Tantalum at Extreme Strain-Rates

Strain-rate and microstructure play a significant role in the ultimate mechanical response of materials. Using non-equilibrium molecular dynamics simulations, we characterize the ductile tensile failure of single and nanocrystalline tantalum over multiple orders of magnitude of strain-rate. This comparison is extended to over nine orders of magnitude including experimental results from resent laser shock campaigns. Spall strength primarily follows a power law dependence with strain-rate over this extensive range. In all cases, voids nucleate heterogeneously at pre-existing defects. Predictions based on traditional theory suggest that, as strain-rate increases, tensile strength should increase. Alternatively, as grain size decreases, tensile strength may decrease due to an increased propensity to fail at a growing volume fraction of grain boundaries. Strain-rate and grain size dictate void nucleation sites by changing the type and density of available defects: vacancies, dislocations, twins, and grain boundaries.   

Sound velocity in shock compressed molybdenum obtained by ab initio molecular dynamics

The sound velocity of Mo along the Hugoniot adiabat is calculated from first principles using density-functional theory based molecular dynamics. These data are compared to the sound velocity as measured in recent experiments. The theoretical and experimental Hugoniot and sound velocities are in very good agreement up to pressures of 210 GPa and temperatures of 3700 K on the Hugoniot. However, above that point the experiment and theory diverge. This implies that Mo undergoes a phase transition at about the same point. Considering that the melting point of Mo is likely much higher at that pressure, the related change in the sound velocity in experiment can be ascribed to a solid-solid transition.   

Atomistic Simulation of shock induced dislocation dynamics and evolution of different plasticity mechanisms in Single Crystal Copper

Deformation and observation of different types of plasticity mechanisms of FCC metals (e.g. Copper) under shock loading of various intensities has been investigated by several groups of researchers around the globe through different types of experiments and/or atomistic simulations. However, there still exists lacuna in this well researched area. In this study the temporal details of dislocation dynamics are provided. Simulations also demonstrate different types of temporal evolution of different loops observed for single crystal Cu under different intensities of shock loading. Observance of formation of twins and their temporal evolution at higher intensities of shock loading are also demonstrated as part of this study. Comparisons of these NEMD simulations using EAM potential are discussed with regards to different experimental and simulation studies in literature.   

Effects of porosity on shock-induced melting of honeycomb-shaped Cu nanofoams.

Metallic foams are of fundamental and applied interests in various areas, including structure engineering (e.g., lightweight structural members and energy absorbers), and shock physics (e.g., as laser ablators involving shock-induced melting and vaporization).Honeycomb-shaped metallic foams consist of regular array of hexagonal cells in two dimensions and have extensive applications and represent a unique, simple yet useful model structure for exploring mechanisms and making quantitative assessment. We investigate shock-induced melting in honeycomb-shaped Cu nanofoams with extensive molecular dynamics simulations. A total of ten porosities (phi) are explored, ranging from 0 to 0.9 at an increment of 0.1. Upon shock compression, void collapse induces local melting followed by supercooling for sufficiently high porosity at low shock strengths. While superheating of solid remnants occurs for sufficiently strong shocks at phi\textless 0.1. Both supercooling of melts and superheating of solid remnants are transient, and the equilibrated shock states eventually fall on the equilibrium melting curve for partial melting. However, phase equilibrium has not been achieved on the time scale of simulations in supercooled Cu liquid (from completely melted nanofoams). The temperatures for incipient and complete melting are related to porosity via a power law and approach the melting temperature at zero pressure as phi tends to 1.   

Dynamic Behaviors of Materials under Ramp Wave Loading on Compact Pulsed Power Generators

The technique using intense current to produce magnetic pressure provides a unique way to compress matter near isentrope to high density without obvious temperature increment, which is characterized as ramp wave loading, and firstly developed by Sandia in 1998. Firstly recent advances on compact pulsed power generators developed in our laboratory, such as CQ-4, CQ-3-MMAF and CQ-7 devices, are simply introduced here, which devoted to ramp wave loading from 50GPa to 200 GPa, and to ultrahigh-velocity flyer launching up to 30 km/s. And then, we show our progress in data processing methods and experiments of isentropic compression conducted on these devices mentioned above. The suitability of Gruneisen EOS and Vinet EOS are validated by isentropic experiments of tantalum, and the parameters of SCG constitutive equation of aluminum and copper are modified to give better prediction under isentropic compression. Phase transition of bismuth and tin are investigated under different initial temperatures, parameters of Helmholtz free energy and characteristic relaxation time in kinetic phase transition equation are calibrated. Supported by NNSF of China under contract No.11327803 and 11176002