Graded Density Impactors Research Articles

Dynamics of stress waves in graded density impactors during the internal ballistic process

Quasi-isentropic loading and unloading, employing graded density impactors (GDIs) as flyers in gas gun-driven plate impact experiments, can provide novel and valuable insights into the equation of state and strength properties of the loaded material. However, the internal ballistic process may lead to spalling or debonding of the GDI due to the intricate interactions between stress waves and interfaces. In this study, the wave propagation in the GDI was analyzed using the multimaterial Lagrangian elastic-plastic model and elastic wave propagation theory. The impact of gradient direction, power-law constant p, and thickness of the first and last layers on the tensile stress was investigated. The outcomes reveal that the mechanism of generating tensile stress varies for two gradient directions. Moreover, adjusting the constant p and the layer thickness may decrease the maximum tensile stress by 74.1% (forward graded) and 95.8% (reverse graded), respectively. The outcomes of this research provide a theoretical and simulation basis for designing and fabricating GDIs to be utilized in quasi-isentropic experiments.

Hydrodynamic simulation of hypervelocity generation by multidimensional graded impactors: Planarity enhancement study

Hypervelocity impact in the universe can be generated by a three-stage gas gun. Achieving the desirable planarity of the flyer enlarges the experimentally effective area of the flyer under the hypervelocity condition. The multidimensional graded density impactor (MDGDI) enhances the planarity of the flyer. In this investigation, a one-dimensional Lagrange elastoplastic hydrodynamic method and a Euler grid finite difference method were used to examine the relationship between the structure of graded density impactors (GDIs) and the planarity of flyers. MDGDIs lead to a deviation of the stress wave produced by the one-dimensional graded density impactor (1DGDI), which offsets the stress disturbance effect, changes the velocity at each particle, and enhances the planarity of flyers. The proportion of flat areas of the flyer increases from 52.70% to 95.71% by adopting MDGDIs. The proportion of flat areas is linear with the wave impedance of the high-impedance layer for 1DGDIs and the wave impedance near the barrel of the high-impedance layer for MDGDIs. This investigation guides the design of GDIs and expands the application of gas gun technology in the field of hypervelocity impact.

(Digital Presentation) Nucleation, Growth, and Grain Structure Control of Electrodeposited Graded Density Alloys

We are developing methods to create thick (mm) electrodeposited films of alloys with graded composition and density. We are designing deposition processes to dynamically control nucleation and growth to investigate correlations between grain structure and dynamic properties. We are also characterizing the properties of the films, both pure and alloyed, as a function of processing conditions.Gold is of interest as a model system in high-pressure and dynamic research because of its inert character, especially because purity, defects, and grain structure can be carefully controlled. Further, because of its potential as a graded density alloy, baseline characterization is important. Gold has complete solid solubility with silver, and similar deposition chemistries. Using several techniques, including control of several process variables, we have produced films of graded composition in silver and gold. To broaden the potential for high-pressure research and other applications, similar techniques can be applied to any material that can be electrodeposited, and we will discuss graded compositions of other materials.The grain structure of the deposit can have an influence on properties, so in addition to composition, we have been researching deposition parameters to control this structure. Controlling nucleation and growth is critical to influencing ultimate grain structure in the deposit, so we have also determined and applied some basic nucleation and growth parameters for our systems. Although there are several well-known aspects, nucleation and growth are some of the most fundamental rate dependent processes in many chemical and physical transformations and remain among the most difficult to measure and predict. Assessing and predicting nucleation and growth, and rate-dependent processes, such as first-order phase transitions, are widely applicable to a multitude of practical applications. In the context of metal deposition for alloy systems, characterizing and modelling nucleation and growth can lead to an ability to design compositionally varied alloys with controlled grain structures. Therefore, we also performed investigations of nucleation and growth in our systems.We present a study of electroplated films with compositional gradients and varied microstructures up to thicknesses of several mm for gold and silver, with discussion of other alloys in development. We relate the composition and grain structure to dynamic response. Constant potential and pulsed plating techniques were used to create a variety of grain structures, including high aspect ratio columnar grains. These structures and their dynamic response were compared. Microscopy was correlated with mechanical characterization of the films at several rates and scales, and impact characteristics are compared and related to growth techniques and their relation to nucleation and growth parameters. With this baseline of properties, we then present initial results of dynamic experiments using graded density impactors, which show good uniformity over large areas.

Modeling atomically mixed graded density impactors

Graded density impactors (GDIs) are multi-material composite impactors used in gas gun experiments to tailor the drive conditions imparted to a sample test material. Previous graded density impactors generally rely on thin, but discrete, layers of different materials. The thinner and the greater number of layers will result in smoother compression. Taken to the limit of very thin layers would be pure material 1 at one surface, such as the front surface of an impactor, smoothly transitioning at the atomic scale to pure material 2 on the back surface. Such an impactor can initially shock, then smoothly compresses a material during a dynamic experiment. This type of experiment can serve to explore a larger region of thermodynamic space than a single or even multi-shock experiments. An overview of how graded density impactors are made is reviewed and sample results are given. A strategy for modeling these kinds of impactors is presented. The length scales of constituent mixing are given from the experimental build through electrochemical-deposition. Equation of state models for pure constituents and their subsequent mixtures are presented. It is demonstrated that the time scales for pressure and temperature equilibration, for atomically mixed GDIs, are short enough to be a justifiable closure for the resulting multiphase flow. Furthermore, we present simulation results of dynamic shock followed by a ramp compression, utilizing a silver/gold graded density impactor, onto a tantalum sample.

Shock-ramp analysis test problem

Quasi-isentropic (ramp) compression is now a well-established experimental method and so are the analysis techniques to give Lagrangian sound speed, pressure, and density along the sample material's isentrope. A shock followed by ramp compression is a natural extension to investigate, for example, shock melt and refreeze on compression, or isentropes of states off the Hugoniot or principal isentrope. In practice, graded-density impactors produce initial shocks, compression by shaped laser pulses may be unable to produce a smooth pressure increase from zero, and incidental perturbations on the drive pulse may also give rise to shocks, so robust shock-ramp analysis methods will be needed. Appropriate analysis methods are needed for shock-ramp experiments, based on those for quasi-isentropic compression, and these require validation. This paper describes three different analyses of a shock-ramp test problem, including an assessment of their estimated errors. The methods tested were based on hydrodynamic characteristics or integration backward in space. All methods gave the known Lagrangian sound speed to within ∼1%, and pressure and volume to within less than 2% and 1%, demonstrating that the analysis methods of isentropic compression experiments can be confidently extended to the analysis of shock and ramp compression.

Development of semiempirical equation of state of binary functionally graded materials and its influence on generation of ramp compression: Comparison with bilayer graded density impactors

Ramp wave study involving graded impactors with discrete or continuous variation of material density is of immense interest for investigation of shock induced structural changes in solids. Hydrodynamic simulation of ramp wave generated by continuously changing functionally graded materials (FGM) requires accurate knowledge of its equation of state (EOS) parameters. However, theoretical as well as experimental EOS data for the same are not readily available. Current work is an attempt towards development of semiempirical EOS model for binary FGM (bFGM) wherein the density is a continuous function of position, either linearly or quadratically. This has been achieved by deriving analytical functions for density dependence of hugoniot parameters using the recently proposed kinetic energy average (KEA) model as well as commonly employed interpolation based methods. Six bFGMs with Al, Mg, and paraffin as low-impedance components and Cu and W as high-impedance ones are considered for the present study. It is shown that shock impedance of linear and quadratic bFGM can be expressed as superlinear/superquadratic functions of position. Subsequently, hydrodynamic simulation results of impact loading of thus constructed bFGMs on ultrathin Ta target are reported. The time profile of target pressure displayed signatures of quasi-isentropic compression (shock ramp). Ramp profiles obtained in our simulation are compared with those generated by recently reported [Phys. Rev. B 99, 214105 (2019)] alternate material bilayer graded density impactor (blGDI) with programmable layer thicknesses. Time profile of ramp pulse is shown to have direct correlation with spatial profile of shock impedance. Influence of associated physical parameters, such as impedance of front layer material, impedance ratio of two components, impact velocity, and mixture model of EOS on shock-ramp adiabats are explored in great detail. The merits of using low and high impedance material as one component of bFGM or blGDI in lowering target heating and enhancing peak ramp pressure are investigated theoretically. Study reveals that paraffin based bFGM with quadratic density function produces the lowest temperature and entropy.

Directed energy deposition additive manufacturing of functionally graded Al-W composites

Directed energy deposition using laser heat sources (DED-L) is a metal additive manufacturing method that can be used to fabricate functionally graded materials by feeding multiple powder streams directly into a laser-generated melt pool. The composition can be spatially varied by independent control of the materials feed rates. DED-L additive manufacturing is used in this work to create continuously graded Al/W composites that span 0–55 vol% W in Al over length scales from 0.6 to 1.2 mm with individual layers on the order of 0.1 mm and powder particle sizes on the order of 10 µm. The obtained composition and thickness ranges are comparable to those typically achieved using conventional powder metallurgy approaches and enables fabrication of composites to be used as a high impedance graded density impactor (GDI) in dynamic ramp compression experiments to study matter under extreme conditions. DED-L additive manufacturing of a GDI presents an opportunity to reduce GDI fabrication time to less than a day compared to many weeks by conventional methods. It is also possible to scale the deposition by DED-L to essentially unlimited dimensions in the lateral directions, whereas conventional methods become limited. • Functionally graded Al-W composites were fabricated by directed energy deposition. • Gradients and length scales comparable to those obtained by conventional methods. • Enables use as graded density impactors in dynamic ramp compression experiments. • Reduced graded density impactor fabrication time from many weeks to less than a day. • Potentially enables 2D or 3D spatial control over composite gradients.

Estimates of Ta strength at ultrahigh pressures and strain rates using thin-film graded-density impactors

We present results from an experimental technique used to estimate the strength of Ta at extreme pressures (150 GPa) and strain rates (${10}^{7}\phantom{\rule{0.28em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$). A graded-density impactor (GDI) was fabricated using sputter deposition to produce an approximately $40\ensuremath{-}\ensuremath{\mu}\mathrm{m}$-thick film containing alternating layers of Al and Cu. The thicknesses of the respective layers are adjusted to give an effective density gradient through the film. The GDIs were launched with a 2-stage light gas gun, and shock-ramp-release velocity profiles were measured over timescales of $\ensuremath{\sim}10$ ns. Results are presented for the direct impact of the film onto LiF windows, which allows for a dynamic characterization of the GDI, as well as from impact onto thin ($\ensuremath{\sim}40\ensuremath{\mu}\mathrm{m}$) sputtered Ta samples backed by a LiF window. The measurements were coupled with mesoscale numerical simulations to infer the strength of Ta, and the results agree well with other high-pressure platforms, particularly when strain-rate, microstructural, and thermodynamic-path differences are considered.

Application of Al-Cu-W-Ta graded density impactors in dynamic ramp compression experiments

Graded density impactors (GDIs) are used to dynamically compress materials to extreme conditions. Two modifications to a previously developed Mg-Cu-W GDI are made in this work before using it in a dynamic compression experiment: Mg is replaced with Al and a Ta disk is glued to the back. The Mg phase is replaced by Al because FCC Al remains solid to higher pressure along its Hugoniot compared to Mg. The addition of the Ta disk creates a constant particle velocity regime and facilitates a definition of peak pressure states. Microstructure analysis, profilometry, and ultrasonic C-scans of the Al-Cu-W GDI all confirm excellent uniformity. We evaluated signal variation in the radial direction of a dynamically compressed Al-LiF bilayer target to evaluate the contribution of spatial nonuniformity to errors. Velocity traces from five photon Doppler velocimetry (PDV) probes located at different radial distances from the center of the target varied at most by 1.1% with a root mean square of 0.3% during the compression ramp, demonstrating low PDV measurement error over a relatively large experimental area. The experimental PDV data also agrees well with 1D simulations that use inputs from predictive characterization models developed for the material properties resulting from tape casting, laminating, and powder consolidation processes. Low measurement error during quasi-isentropic compression, leading to better precision, ensures a robust platform to reach extreme compression and low-temperature recovery states and facilitates discovery via synthesis, quenching, and preservation of new high-pressure phases.

Research on phase transition and strength under ramp compression with graded density impactor

In our work, graded density impactors fabricated from 8 to 40 layers, are specifically designed to generate desired strain rates (on the order of 105~106 s-1) and thermodynamic path(shock loading-ramp loading-release). And experiments on phase transition and strength for metals (bismuth, LY12 Al) have been performed with light gas gun to peak pressure between 30 GPa and 50 GPa. Particle velocity at sample/window interface in these experiments are simultaneously traced by a distance interferometer system for any reflector, and a wave profile analysis is employed to explore the solidification transition and strength behaviour along elevated isentrope.

Generation of ramp waves using variable areal density flyers

Ramp loading using graded density impactors as flyers in gas-gun-driven plate impact experiments can yield new and useful information about the equation of state and the strength properties of the loaded material. Selective Laser Melting, an additive manufacturing technique, was used to manufacture a graded density flyer, termed the “bed-of-nails” (BON). A 2.5-mm-thick $$\times $$ 99.4-mm-diameter solid disc of stainless steel formed a base for an array of tapered spikes of length 5.5 mm and spaced 1 mm apart. The two experiments to test the concept were performed at impact velocities of 900 and 1100 m/s using the 100-mm gas gun at the Institute of Shock Physics at Imperial College London. In each experiment, a BON flyer was impacted onto a copper buffer plate which helped to smooth out perturbations in the wave profile. The ramp delivered to the copper buffer was in turn transmitted to three tantalum targets of thicknesses 3, 5 and 7 mm, which were mounted in contact with the back face of the copper. Heterodyne velocimetry (Het-V) was used to measure the velocity–time history, at the back faces of the tantalum discs. The wave profiles display a smooth increase in velocity over a period of $$\sim \!\!2.5 \, {\upmu } \hbox {s}$$ , with no indication of a shock jump. The measured profiles have been analysed to generate a stress vs. volume curve for tantalum. The results have been compared with the predictions of the Sandia National Laboratories hydrocode, CTH.

Microstructure and Properties of Aluminum-Copper Composites Prepared by Hot-Pressure Sintering

Aluminum (Al) and Copper (Cu) have been used in graded density impactors. Al-Cu composites with different compositions have been prepared by hot pressing sintering. Intermetallic compounds were produced when the temperature was above 723 K, while they can’t be found at the temperature of 723 K. The relative density of Al-Cu composites from pure Al to pure Cu consolidated at 723K all exceeded 98.5%. The optimum sintering conditions were then determined to sintering temperature of 723 K, uniaxial pressure of 100 MPa, and duration time of 2 h. Al and Cu were mixed homogeneously and well compacted, and no obvious pores were found. The thickness of diffusion layer between Al and Cu was very thin, only about 0.6 μm. The wave impedance values increased almost linearly from 16.98×106to 40.66×106kg/m2·s with increasing Cu content in Al-Cu composite from 0 to 100 vol.%, which was well consistence with the values calculated according to the rule of mixtures. The elastic modulus of different compositions matched well with the theoretical model of Voigt and Reuss. Highly densified Al-Cu composites without intermetallic compounds were successfully prepared using hot-pressure sintering, which was the basis of fabricating Al-Cu system graded density impactors.

Modelling the equation of state of a graded density impactor

There is a requirement for off-Hugoniot data to validate material and equation of state models. One technique is to use projectiles with graded shock impedance layers in a gas gun to impact a target and apply a ramp loading. To accurately design a pressure profile to be applied to the target the impactor must be accurately characterised and modelled. Modelling also provides an assessment of whether lateral rarefactions will hinder the experiment, and increases confidence in the analysis of the results obtained. This paper considers several methods for calculating the equation of state of layered mixtures for application to the modelling of graded density impactors.

Fabrication and application of high impedance graded density impactors in light gas gun experiments

Recent advances in Graded Density Impactor fabrication technique have increased the maximum achievable pressure in gas gun quasi-isentropic experiments to 5 Mbars. In this report, we outline the latest methodologies and applications of Graded Density Impactors in experiments at extreme conditions. These new Graded Density Impactors are essentially metallic discs made of nearly one hundred layers of precisely mixed Mg, Cu, and W. The density gradients in these impactors are specifically designed to generate the desired thermodynamic path required for each experiment. We carried out a number of experiments at various pressures using these Graded Density Impactors. These experimental results and their simulations will be presented here.

Computational design and simulation of the Mg-Cu system graded density impactors for complex loading experiments

Two types of Mg-Cu composition system graded density impactors used for complex loading (shock loading and quasi-isentropic compression) are designed by the elastic-plastic hydrodynamic method in this paper. Mixtures of metal powders in the Mg-Cu system are cast into a series of 17 and 25 uniform compositions ranging from 100% Mg to 100% Cu. The graded density impactors are launched to the stationary 10 μm aluminum film and 12 mm LiF window targets by a two-stage light-gas gun in the National Key Laboratory for Shock Wave and Detonation Physics Research, Institute of Fluid Physics, CAEP, and the resulting wave profiles are measured with the DISAR system. Hydrodynamic simulation results are perfectly consistent with the experiments. Our work in this paper will set up a foundation for further research of controllable loading/releasing routes and rate experiments in the future.

Characterizations of Mg-W System Graded-Density Impactors for Complex Loading Experiments

We designed an Mg-W functionally graded-density materials system with two different sequences of density–thickness distributions and fabricated disk-like pellets with the graded-density materials by elaborate stacking of different metal powder layers, followed by a one-step hot-pressure sintering process. The microstructures of the sintered pellets were characterized using a scanning electronic microscope and a scanning acoustic microscope. The pellets were used as graded-density impactors (GDIs) to test their capabilities to produce complex particle-velocity wave profiles along a specified thermodynamic path. The wave profile was obtained from reverse-impact experiments using a two-stage light gas gun in which the GDI flyer impacted a lithium fluoride window. The particle-velocity history at the impact surface of the LiF window was deduced from the interference fringe numbers of the distance interferometer system for any reflector records. For GDIs consisting of nine composition layers of graded-density materials, the particle velocity profiles displayed combined shock loading and quasi-isentropic compressing and release processes. The impactor, consisting of 12 composition layers, produced wave profiles that contained shock loading, quasi-isentropic reloading, controlled releasing, and recompressing and released processes. The wave profiles simulated using a hydrodynamic computer model developed in house agree well with the experimental results as well as other theoretical results.

Characteristics analysis of Isentropic Compression Experiments (ICE)

Several new techniques for high-pressure (quasi-) isentropic compression of materials are becoming available: pulsed-power-driven magnetic pressure, graded-density impactors on gas-guns, and laser drive. These experiments are analysed to give sound speed as a function of particle velocity, which may then be integrated to pressure and volume, giving data for a complete isentrope with one shot. However, to do this the experimentally-measured free-surface - or window-interface - velocities must be corrected to give the in-situ material velocities in the absence of the surface or interface. This may be done by either backward integration (integrate the equations of motion in space rather than time using surface velocity vs time as the boundary conditions) or by a characteristics method. This presentation describes the characteristics method and its application to AWE experiments on the isentropic compression of lead. The experiment has produced results consistent with pre-existing experimental data within errors in sound speed and pressure of ∼4%. Iterative corrections to in-situ velocity, using the EOS calculated during the previous step, reduce - or do away with - any dependence of the results on an assumed EOS. Simultaneous characteristics calculations of the input pressure-time are used to estimate the errors.

Tape Casting Technique for Fabrication of Graded-Density Impactors for Tailored Dynamic Compression

AbstractRecently, the use of graded density impactors for dynamic compression experiments has received increasing interest. These gas gun experiments have demonstrated complex loading paths which can last microseconds, and may be capable of bridging the timescales of existing static and dynamic compression experiments. A tape casting technique has been developed for fabrication of the impactors. In the technique, a series of tapes were prepared in the Mg-Cu system with compositions ranging from 100% Mg to 100% Cu. The tapes were characterized for their layer thickness, density, and sound wave velocity. Impactors were fabricated by punching individual layers from the tapes, stacking and laminating them, removing the organic matrix, and hot-pressing the laminated structure. The density profile is determined by the order in which the tapes are stacked in the laminate and is therefore highly flexible. The resultant thickness and average density of the impactors is consistent with the data for the individual layers. Impactors were characterized for uniformity by ultrasonic C-scan and white light interferometry. Dynamic compression experiments were performed on a two-stage helium gas gun using the graded density impactors. Results will be presented and shown to agree well with hydrocode modeling.

Dynamic quasi-isentropic loading of tungsten

Abstract Experiments have been performed on smooth bore guns to obtain quasi-isentropic compression in tungsten to stresses approaching 250 GPa. Quasi-isentrophic loading was introduced in the material either by using graded density or layered impactors. Results of these experiments indicate that the quasi-isentrope lies above the Hugoniot up to ∼140 GPa, contrary to initial expectations. These results suggest that the dynamic yield strength of the material is higher for the relatively slower rates of isentropic loading.