Y. M. Gupta

Phase Diagram of Hexahydro-1,3,5-trinitro-1,3,5-triazine Crystals at High Pressures and Temperatures

Raman spectroscopy and optical imaging were used to determine the phase boundaries between various hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) polymorphs. Experiments were performed on single crystals at pressures up to 8.0 GPa and temperatures ranging from room temperature to 550 K. Several distinct pressure regions were found in the RDX response at elevated temperatures: (i) melting of alpha-RDX followed by decomposition, below 2.0 GPa, (ii) decomposition of alpha-RDX between 2.0 and 2.8 GPa, (iii) irreversible transformation of alpha- and gamma-RDX to epsilon-RDX between 2.8 and 6.0 GPa, and (iv) decomposition of gamma-RDX above 6.0 GPa. A triple point between the alpha-, gamma-, and epsilon-RDX was found at 3.7 GPa and 466 K. The alpha-gamma phase transition was confirmed to occur at the same pressure, approximately 3.7 GPa, regardless of temperature, in the range of 295-460 K. Furthermore, it was determined that epsilon-RDX (i) has limited chemical stability under the pressure and temperature conditions where it is produced and (ii) decomposes according to the autocatalytic rate law. The findings reported here have provided new information about the response of RDX crystals at high pressures and temperatures.

Nonlinear anisotropic description for the thermomechanical response of shocked single crystals: Inelastic deformation

A nonlinear anisotropic continuum framework for describing the thermoelastic-plastic response of single crystals shocked along arbitrary orientations has been developed. Our modeling approach incorporates nonlinear elasticity, crystal plasticity, and thermodynamic consistency within an incremental tensor formulation. Crystal plasticity was incorporated by considering dislocation motion along specified slip planes. The theoretical developments presented here are sufficiently general to also accommodate other types of inelastic deformation mechanisms. As representative applications of the theoretical developments, numerical simulations of shock wave propagation in lithium fluoride (LiF) and copper single crystals are presented and compared to wave profile data for several crystal orientations. Simulations of shock wave propagation along low-symmetry directions, where data are not available, are also presented to examine the propagation of quasilongitudinal and quasishear waves in crystals undergoing elastic...

Dynamic tensile response of Zr-based bulk amorphous alloys: Fracture morphologies and mechanisms

Plate impact experiments were conducted to examine the dynamic tensile response of Zr-based bulk amorphous alloys (BAAs) having a nominal composition of Zr56.7Cu15.3Ni12.5Nb5.0Al10.0Y0.5. The experimental configuration used in our work permitted soft recovery of the samples to allow a careful examination of the fractured samples along with real-time measurements of the sample free-surface velocity (FSV) histories. Tensile loading was preceded by elastic compressive loading to peak stresses in the 3.6 to 6.0 GPa range. Tensile damage in the recovered samples was examined using optical and electron microscopy. The microscopy results showed that the BAA samples exhibit a brittle behavior (as a glass) at the macroscopic level and a ductile behavior (as a metal) at the microscopic level; in addition, corrugations and bumps are observed at the nanoscale. The observed fracture morphologies are related to three key features present in our spall experiments: preceding compressive stress (3.6–6.0 GPa), high tensile loading rate (∼106/s), high mean tensile stress (∼2.3 GPa); and are intrinsically related to the amorphous glassy structure of the BAAs (free volume content). We propose that the compressive stress depletes the free volume content. With increasing compressive stress, the available free volume decreases causing a suppression of shear stresses during tension. Thus, the mean tensile component becomes more dominant at higher stresses. Consequently, the observed surface morphology results from brittle cleavage, causing an increased damage localization in the recovered samples spalled at higher stresses. These observations support the inferences made from measurements of FSV histories. The high tensile loading rate is proposed to be responsible for cracking by multiple shear band propagation and interception, rendering the observed serrated surface morphology. Finally, we proposed that the corrugations are created due to a succession of arrest and propagation of mode I cracks. A subsequent dilatation, due to the effect of the tensile mean stress, caused the corrugations to evolve to bump-type features with sizes in the range of 10–100 nm. Our proposed mechanisms, although qualitative, constitute a systematic attempt to provide an explanation for the fracture morphologies observed in spalled BAA samples.Plate impact experiments were conducted to examine the dynamic tensile response of Zr-based bulk amorphous alloys (BAAs) having a nominal composition of Zr56.7Cu15.3Ni12.5Nb5.0Al10.0Y0.5. The experimental configuration used in our work permitted soft recovery of the samples to allow a careful examination of the fractured samples along with real-time measurements of the sample free-surface velocity (FSV) histories. Tensile loading was preceded by elastic compressive loading to peak stresses in the 3.6 to 6.0 GPa range. Tensile damage in the recovered samples was examined using optical and electron microscopy. The microscopy results showed that the BAA samples exhibit a brittle behavior (as a glass) at the macroscopic level and a ductile behavior (as a metal) at the microscopic level; in addition, corrugations and bumps are observed at the nanoscale. The observed fracture morphologies are related to three key features present in our spall experiments: preceding compressive stress (3.6–6.0 GPa), high tensile...

Raman Spectroscopy of High-Pressure-High-Temperature Polymorph of Hexahydro-1,3,5-trinitro-1,3,5-triazine (ε-RDX)

Raman spectroscopy was used to determine the vibrational structure and the stability of the high-pressure-high-temperature (HP-HT) polymorph of RDX after it had been quenched to room temperature. Although this polymorph has limited chemical stability under high pressure and temperature, we show that it is chemically and structurally stable from 0.6 GPa to at least 20 GPa at room temperature. Below 0.6 GPa, it readily converts to the alpha-polymorph. Pressure dependence of the vibrational structure of the HP-HT polymorph was measured and compared with the vibrational structures of other known RDX polymorphs: alpha, beta, and gamma. In contrast with previous suggestions, our data indicate that the HP-HT polymorph can have a different structure than the beta-polymorph. This finding supports the recent suggestion that the HP-HT polymorph should be given a separate designation, epsilon-RDX. Furthermore, symmetry correlation analyses of Raman spectra indicate that the HP-HT polymorph (epsilon-RDX) may assume the space group isomorphous with the C(2v)[C(1)(4)] point group and with molecules adopting the pseudo-AAA conformation.

Influence of grain size on the tensile response of aluminum under plate-impact loading

Plate-impact experiments were performed to examine the influence of grain size on the dynamic tensile (or spall) behavior of shocked polycrystalline aluminum. Ultrapure and commercially pure 1050 aluminum plates were cold rolled to 80% strain and heat treated under predetermined conditions to produce recrystallized samples with average grain sizes varying between 49 and 453μm. Well-characterized samples were subjected to plane wave loading at peak compressive stresses of 4 and 21GPa, and free-surface velocity profiles were obtained using velocity interferometry. At 4GPa, the observed pullback velocity, a characteristic feature of the spall response, was similar for different grain sizes of 1050 and ultrapure Al, suggesting that the preferential failure mode is intragranular. At 21GPa, the spall response (i.e., the pullback velocity and the signal structure) depended on the alloy content; the pullback velocity of ultrapure Al increased with increase in grain size, while it remained constant for 1050 Al. In...

Anisotropic material model and wave propagation simulations for shocked pentaerythritol tetranitrate single crystals

An anisotropic continuum material model was developed to describe the thermomechanical response of unreacted pentaerythritol tetranitrate (PETN) single crystals to shock wave loading. Using this model, which incorporates nonlinear elasticity and crystal plasticity in a thermodynamically consistent tensor formulation, wave propagation simulations were performed to compare to experimental wave profiles [J. J. Dick and J. P. Ritchie, J. Appl. Phys. 76, 2726 (1994)] for PETN crystals under plate impact loading to 1.2 GPa. Our simulations show that for shock propagation along the [100] orientation where deformation across shear planes is sterically unhindered, a dislocation-based model provides a good match to the wave profile data. For shock propagation along the [110] direction, where deformation across shear planes is sterically hindered, a dislocation-based model cannot account for the observed strain-softening behavior. Instead, a shear cracking model was developed, providing good agreement with the data ...

Time-Resolved Spectroscopic Measurements of Shock-Wave Induced Decomposition in Cyclotrimethylene Trinitramine (RDX) Crystals: Anisotropic Response

Plate impact experiments on the (210), (100), and (111) planes were performed to examine the role of crystalline anisotropy on the shock-induced decomposition of cyclotrimethylenetrinitramine (RDX) crystals. Time-resolved emission spectroscopy was used to probe the decomposition of single crystals shocked to peak stresses ranging between 7 and 20 GPa. Emission produced by decomposition intermediates was analyzed in terms of induction time to emission, emission intensity, and the emission spectra shapes as a function of stress and time. Utilizing these features, we found that the shock-induced decomposition of RDX crystals exhibits considerable anisotropy. Crystals shocked on the (210) and (100) planes were more sensitive to decomposition than crystals shocked on the (111) plane. The possible sources of the observed anisotropy are discussed with regard to the inelastic deformation mechanisms of shocked RDX. Our results suggest that, despite the anisotropy observed for shock initiation, decomposition pathways for all three orientations are similar.

Real-time microstructure of shocked LiF crystals: Use of synchrotron x-rays

We describe the use of a third generation synchrotron facility to obtain in situ, real-time, x-ray diffraction measurements in plate impact experiments. Subnanosecond duration x-ray pulses were utilized to record diffraction data from pure and magnesium-doped LiF single crystals shocked along the [111] and [100] orientations. The peak stresses were 3.0 GPa for the [111] oriented LiF and between 3.0 and 5.0 GPa for the [100] oriented LiF. For these stresses, shock compression along [111] results in elastic deformation and shock compression along [100] results in elastic-plastic deformation. Because of the quality of the synchrotron x-ray pulses, both shifting and broadening of the diffraction data were obtained simultaneously. As expected, shifts for elastic compression and elastic-plastic compression in shocked LiF were consistent with uniaxial and isotropic lattice compression, respectively. More importantly, diffraction patterns from crystals shocked along [100] exhibited substantial broadening due to e...

Compressive shock wave response of a Zr-based bulk amorphous alloy

Plane shock wave experiments were performed at peak stresses up to 13 GPa on Zr-based bulk amorphous alloy (BAA) samples. A velocity interferometer was used to measure the particle velocity history either at the impact surface or at the rear surface of the BAA samples. From the measured particle velocity histories, the Hugoniot elastic limit (HEL) was determined to be 7.1±0.3 GPa, corresponding to an elastic strain of approximately 4%. For experiments in which the peak stress exceeded the HEL, a clear two-wave structure consisting of an elastic precursor followed by a plastic wave was observed. Measurements of the transmitted wave profiles, along with direct determination of the longitudinal stress and particle velocity at the impact surface, suggest that the shear strength of the Zr-based BAA is reduced as it is shocked above the elastic limit.

Chemical Changes in Liquid Benzene Multiply Shock Compressed to 25 GPa

Shock wave experiments utilizing stepwise-loading, with peak stresses ranging between 4 and 25 GPa, were performed to examine the dynamic high pressure response of liquid benzene at thermodynamic conditions not attainable in single shock experiments. Time-resolved Raman spectroscopy was used to monitor the molecular and chemical changes on sub-mus time scales. Up to 20 GPa, the Raman modes showed pressure-induced shifting and broadening but no indication of a chemical change. At 24.5 GPa, however, the Raman modes become indistinguishable from an increasing background within 40 ns after the sample attained peak pressure, indicating a chemical change. A thermodynamically consistent equation of state (EOS) was developed to calculate the relevant thermodynamic variables in multiply shock compressed liquid benzene. Idealized molecular configurations were used in combination with the thermodynamic quantities in the shocked state to calculate the intermolecular separation between benzene molecules and to ascertain the likelihood of pi-orbital overlap. These idealized calculations show that sufficient energy and pi-orbital overlap exist in multiply shock compressed liquid benzene to permit intermolecular bonding at 24.5 GPa. Analysis of the Raman spectra, using the thermodynamic and intermolecular separation calculations, suggests that benzene undergoes polymerization through cycloaddition reactions. The rapid rate of polymerization is attributed to the benzene remaining in a liquid state on the sub-mus experimental time scale. The results from the present work demonstrate the importance of time, pressure, temperature, and phase in chemical changes associated with pi-bonded molecules.