Session K24: Matter at Extreme Conditions: Dynamic Compression I

Plasticity-induced temperature and texture evolution under dynamic uniaxial compression

A crystal uniaxially loaded to extreme pressures often relieves the extraordinary shear stresses built up during compression via rapid plastic deformation. Plasticity under extreme loading conditions is a complex process whose character varies dramatically with the details of the applied compression path. Innovations on both experimental and computational fronts have given us glimpses into the lattice-level nature of plasticity under dynamic compression: ultrafast x-ray diffraction platforms allow us to measure directly the changes in crystal structure and orientation (i.e. texture) precipitated by plasticity at pressures approaching the terapascal scale; such measurements can be complemented by both large-scale molecular dynamics simulations and by sophisticated continuum-level plasticity models built from lower-length-scale simulations. In spite of this progress, fundamental questions such as ‘Which plasticity mechanisms will become active under compression?’ and ‘How much heating will they cause?’ remain difficult to answer for all but the most intensively studied materials. In this talk, I review recent progress and outstanding questions regarding the temperature and texture evolution brought about by plasticity under dynamic compression conditions. I discuss two experiments investigating plastic-work heating, the first measuring heating during rapid dynamic release of tantalum by measurement of its thermally-induced strains [1], the second constraining the heating caused by plastic work in diamond ramp-compressed to two terapascals by analysis of its path through phase space [2]. I then present a series of investigations into plasticity-induced texture evolution under shock compression, focusing on a novel multiscale strength model of tantalum that predicts with unprecedented accuracy its texture evolution in nanocrystals loaded to around 100 gigapascals [3].   

Material strength in high energy density conditions

We derive a continuum-level plasticity model for polycrystalline materials in the high energy density regime, based on a single dislocation density and single mobility mechanism, with an evolution model for the dislocation density.  The model is formulated in terms of quantities connected closely with equation of state (EOS) theory, in particular the shear modulus and Einstein temperature.  We used the atom-in-jellium model to estimate the Einstein frequency, EOS, shear modulus, and Peierls barrier from ambient to white dwarf conditions.  The Peierls barrier was adjusted to match a single flow stress datum.  The configurational energy of the dislocations is accounted for explicitly, giving a self-consistent calculation of the conversion of plastic work to heat.  The dislocation and elastic strain energies are predicted to contribute to the mean pressure, and may be significant when inferring scalar EOS data from dynamic loading experiments.  The deduced flow stress reproduces systematic trends observed in elastic waves and instability growth experiments.   

Investigating the Performance of Strength Models for High Energy Density Applications

Laser-driven shock compression experiments on a nanosecond time scale were performed on Ag, Al, Be, and Cu samples using the Omega EP Laser System at the University of Rochester. The sample materials in this experiment are potential pushers for shock melt experiments, and thus one of the motivations for this experiment was to study the speed and amplitude of the elastic waves and hence investigate the conditions for these materials to transmit a clean shock. Radiation hydrodynamics simulations of these experiments using the Steinberg-Guinan yield stress model show significant disagreement from the experimental VISAR measurements, because of strain rate dependence and texture. An improved match was found using significantly modified values of the Steinberg-Guinan model parameters. Simulations were also performed using dislocation-based strength models, including a recently developed model for high energy density conditions (arXiv:2110.06345), which gave significantly closer agreement using parameters fitted to gas gun experiments.   

Thermomechanical conversion in metals: dislocation plasticity model evaluation of the Taylor-Quinney coefficient

Using a partitioned-energy thermodynamic framework which assigns energy to that of atomic configurational stored energy of cold work and kinetic-vibrational, we derive an important constraint on the Taylor-Quinney coefficient, which quantifies the fraction of plastic work that is converted into heat during plastic deformation. Associated with the two energy contributions are two separate temperatures -- the ordinary temperature for the thermal energy and the effective temperature for the configurational energy. We show that the Taylor-Quinney coefficient is a function of the thermodynamically defined effective temperature that measures the atomic configurational disorder in the material. Finite-element analysis of recently published experiments on the aluminum alloy 6016-T4, using the thermodynamic dislocation theory (TDT), shows good agreement between theory and experiment for both stress-strain behavior and temporal evolution of the temperature. The simulations include both conductive and convective thermal energy loss during the experiments, and significant thermal gradients exist within the simulation results. Computed values of the differential Taylor-Quinney coefficient are also presented and suggest a value which differs between materials and increases with increasing strain.   

Dynamic fracture of expanding cylinder wall: Experiment and Simulation using dynamic ductile framework

 

In this work, we present a dynamic fracture study of aluminum 6061 using gas gun driven expanding cylinder experiments. In this experiment, a polycarbonate cylinder flyer is accelerated by a gas gun to impact another polycarbonate cylinder plug, which is located inside a metal cylinder of interest. The impact between the flyer plug and the inserted plug causes rapid expansion and fracture of the cylinder wall. Conventionally, the cylinders tested are free of defects. In this work, a surface defect was added to the cylinder wall to evaluate its effect on strain to failure. The existence of the surface defect on cylinder wall helps localize hoop stress and hoop strain at the defect location and weakens the dynamic strength of the cylinder. The radial expansion of the cylinder wall without surface defect is used as a baseline for comparison purposes. Using a dynamic ductile framework, we modelled this fracture process and study the role of the defect on lowering the hoop strain at failure or weakening of the dynamic strength of the cylinder wall.   

Using PAGOSA with FLIP+MPM to simulate and analyze the recompression of spall with 3D effects induced by double-shock waves

Many grid-based methods for fracture/fragmentation induced by shock wave in material simulate the shock waves by converting the discontinuous shock wave to a steep gradient to maintain the continuity hypothesis. This causes an inaccurate prediction of fracture. This work presents a hybrid shock code PAGOSA with FLIP+MPM to accurately predict and explore the recompression of spall with 3D effects induced by double-shock waves. This algorithm seamlessly couples a finite different method and the particle FLIP+MPM method, in which a mapping between Eulerian grid and marker particles is constructed.  The failure particles are removed in an amazing way. We first validate the capabilities of PAGOSA with FLIP+MPM to predict different fracture/fragmentation cases by solving benchmark problems and comparing with the analytical solutions or the experimental results. The convergences and the infinity-norm errors are also investigated. Subsequently, the 3D effects on the numerical results are demonstrated by simulating the spallation in material. Some factors that affect the spallation with 3D effects are discussed. Finally, the recompressions of spall in material with/without 3D effects are systematically simulated and analyzed. The conditions that induce the recompression of spall are explored. Numerical results show that PAGOSA with FLIP+MPM can accurately predict different fracture cases, that the 3D effects play an important role in the fracture/fragmentation, and the recompression of spall is very sensitive to the occurrence conditions, shows great differences when considering 3D effects in real applications. Moreover, the presented algorithm is not limited to the finite different method presented here, but can be easily extended to other grid-based techniques employed for fracture in materials.   

Platinum equation of state to greater than 2 terapascals: experimental data and analytical models.

In order to develop a new high impedance standard for shock physics applications, we investigated the equation of state (EOS) and shock compression of bulk platinum. We used canonical ab initio molecular dynamics (AIMD) simulations, we developed a SESAME-style EOS and we validated it with experimental shock data to 2177 GPa on Sandia's Z machine. The theoretical and AIMD results are in excellent agreement with experiments.   

Experimental Observations of Laser-Driven Tin Ejecta Microjet Interactions

The study of high-velocity particle-laden flow interactions has broad applicability to fields ranging from planetary formation [1] to cloud interactions [2]. Ejecta microjets are micron-scale jets formed by strong shocks interacting with imprinted surfaces to generate streams of particles traveling at more than several kilometers per second. As such, ejecta microjets offer a methodology to study particle interactions at extreme velocities. We present the first time-sequences of x-ray radiography images of two interacting tin ejecta microjets from experiments performed on the OMEGA EP laser [3]. We observe that jets emerging from a shock pressure of 11.7 GPa pass through each other unattenuated, whereas jets emerging from a shock pressure of 116.0 GPa have five times greater densities and interact strongly, forming a cloud around the center-point of interaction. Radiation hydrodynamics simulations of particle-stream collisions capture many of the observed interaction behavior characteristics, but are unable to capture the full spread of the cloud formed. 

 

[1] M. Lambrechts et al., A&A 627, A83 (2019).

[2] T. Matsumoto et al., The Astrophysical Journal 801, 77 (2015).

[3] A. M. Saunders et al., PRL 127, 155002 (2021).

    

Exploring the phase state of ejected particles of cerium

Over the past decade and a half, the team has been developing an understanding of the equation of state for cerium.  The experimental emphasis has been to explore the dynamic compression states of this complex material.  There have been numerous shock experiments which had provided a deep understanding of dynamic properties across phase boundaries using gas gun driver systems.  The team has published multiple papers showing how cerium transforms across phase boundaries and its dynamic response.  In this paper, we continue this theme by presenting x-ray diffraction data on the ejected mass coming from machined grooves across the sample.  Additionally, data of the ejected areal density, will also be presented from 9 GPa to 25 GPa. These in material shocked states ranges from slightly below the melt to well into the liquid state.  The experiments were performed Argonne's Advanced Photon Source utilizing the Dynamic Compression Sector.   

Investigation of dislocation density evolution during simulated metallic microparticle impacts

Accurately modeling the rate-dependent plastic deformation of metals across a large

range of strain rates requires a comprehensive accounting of dislocation motion and evolution.

The newly developed analytical dislocation evolution model of Hunter and Preston, accounts for

a wide range of dislocation annihilation and nucleation mechanisms including: network storage,

Frank-Read sources, cross-slip, double cross-slip, mobile-immobile annihilation, grain boundary

storage, grain boundary nucleation, and shock induced nucleation. In conjunction with the mean

first passage time flow stress model, this new model is able to capture the plastic behavior of

polycrystalline FCC metals across a large range of loading regimes from quasi-static to shock

loading environments. These models have been recently implemented in Los Alamos National

Laboratory’s hydrodynamics research code, FLAG. Data from quasi-static, Hopkinson bar, and

flyer plate experiments are used to create material parameter sets. Then continuum scale

microparticle impacts are simulated in FLAG and compared to experimental observations.   

Additively Manufactured Granular and Binder Metamaterial's Response To Shock

Homogenous mixtures were created using sucrose crystals with characteristic lengths of 180 μm to 250 μm and ultraviolet sensitive resin.  A single stage light gas gun was utilized in a uniaxial plate impact configuration to drive a shock through the mixtures at particle velocities ranging from 20 m/s to 200 m/s.  A photo doppler velocimetry system (PDV) was used to measure the transmitted particle velocity and infer the state of the system during loading.  Initially the sucrose crystals were arranged in simple orientations so that the dynamic response of individual grains within the resin could be determined.  A genetic algorithm, coupled with a Lagrangian hydrocode, was used in order to suggest crystal arrangements that may influence bulk wave behavior.  The crystal geometries were physically realized with a laser sintering technique. 

The initial results indicate some control over the bulk behavior.