Decay Of Precursor Research Articles

Влияние температуры на динамический предел упругости и откольную прочность свинцово-висмутового сплава при давлении ударного сжатия до 2.4 GPa

Measurements of the Hugoniot elastic limit and the spall strength of the eutectic alloy Bi – 56.5 mass%, Pb – mass43.5% were carried out at sample temperatures in the range of 20 0C – 109 0C based on registration and analysis of the evolution of shock compression pulses of various amplitudes. It is shown that an increase in the temperature of the samples leads to a decrease in the Hugoniot elastic limit by 25%, and the spall strength of the alloy under study – by 30%, regardless of the strain rate. An increase in the strain rate by two orders of magnitude leads to an increase in the spall strength by about three times. Approximation power-law dependences of the decay of the elastic precursor on the thickness of the samples and the spall strength on the strain rate before fracture at normal and elevated temperatures are constructed.

The effect of temperature on the Hugoniot elastic limit and the spall strength of a lead-bismuth alloy at the pressure of shock compression up to 2.4 GPa

Measurements of the Hugoniot elastic limit and the spall strength of the eutectic alloy Bi --- 56.5 mass%, Pb --- 43.5 mass% were carried out at sample temperatures in the range of 20-109oC based on registration and analysis of the evolution of shock compression pulses of various amplitudes. It is shown that an increase in the temperature of the samples leads to a decrease in the Hugoniot elastic limit by 25%, and the spall strength of the alloy under study --- by 30%, regardless of the strain rate. An increase in the strain rate by two orders of magnitude leads to an increase in the spall strength by about three times. Approximation power-law dependences of the decay of the elastic precursor on the thickness of the samples and the spall strength on the strain rate before fracture at normal and elevated temperatures are constructed. Keywords: lead-bismuth eutectic alloy, shock waves, deformation, temperature, Hugoniot elastic limit, spall strength.

Strength Properties of the Heat-Resistant Inconel 718 Superalloy Additively Manufactured by Direct Laser Deposition Method under Shock Compression

By recording and analyzing complete wave profiles using the VISAR laser interferometer, measurements of the Hugoniot elastic limit and critical fracture stresses were carried out under the spalling conditions of the heat-resistant Inconel 718 alloy, additively manufactured by direct laser deposition, at shockwave loading up to ~6.5 GPa using a light-gas gun. For comparison, similar experiments were performed with the Inconel 718 alloy made by the traditional method of vacuum induction melting. The process of the delay of an elastic compression wave during its propagation through the sample and the dependence of the spall strength on the strain before fracture in the range 105–106 s−1 were investigated. To identify the anisotropy of the strength properties of the material under study, two series of experiments were carried out on loading additively manufactured samples along and perpendicular to the direction of the deposition. The measurements performed showed that the additively manufactured Inconel 718 alloy demonstrates weak anisotropy of strength properties for both the initial and thermal-treated samples. The thermal treatment leads to a noticeable increase in the Hugoniot elastic limit and the spall strength of the samples at low strain rates. For all types of samples, there is an increase in the spall strength with an increase in the strain rate. The spall strength measured for the cast alloy practically coincides with the strength of the as-received additive alloy and is noticeably lower than the strength of the thermal-treated additive alloy over the entire range of the strain rates. The process of the decay of the elastic precursor in the cast alloy occurs much faster than in the additive one, and the minimum values of the Hugoniot elastic limit are measured for thick samples in the cast alloy.

Atomistic simulation of the shock wave in copper single crystals with pre-existing dislocation network

We present molecular dynamics simulations of shock compression and spall fracture in [111] copper single crystals with pre-existing dislocations. Shock waves are simulated for an impact velocity of 50–900 m/s, which is below the Hugoniot elastic limit for pure [111] crystals. A dislocation network is created by carving out interstitial dislocation loops, followed by their interaction and relaxation to the dislocation network. Samples with different dislocation densities are considered. We find that, behind the shock wave, shear stress is relaxed by multiplication of moving dislocations, which increases with piston velocity. Positively oriented dislocation segments move in the direction of the shock wave, while negatively oriented segments move in the opposite direction. Dislocation segments of different orientation interact with and intersect each other behind the shock wave. The dislocation segments moving at supersonic speeds are observed already at the impact velocity of 300m/s. When the shock pulse ends, the subsequent rarefaction wave creates tensile shear stresses which promote the negatively oriented dislocation segments to move along it. Where incident and reflected rarefaction waves contact, the tensile longitudinal stresses cause dislocation multiplication, which results in an increased dislocation density. The time series of dislocation density and stress along the samples are computed during the shock waves propagation and release. An increase in dislocation density behind the shock wave is found to vary quadratically with shock stress, and does not depend on the initial dislocation density. In the region of tensile stresses, an increase in dislocation density is also approximated by a power function of the maximal tensile stresses. The elastic precursor decay is analyzed for different impact velocities and initial dislocation densities. The decay power increases with the impact velocity and the initial dislocation density in the sample. The plastic strain rate is estimated from the decay of elastic precursor, compared to experimental values, and discussed in terms of dislocation dynamics. Spall fracture is observed in samples of various dislocation density at impact velocities of at least 500m/s, corresponding to tensile stresses of about 12GPa, in agreement with the previously published simulation data. The number of voids, their volumes and distribution are studied using the alpha-shape algorithm for free surface extraction. We show that pre-existing dislocations almost do not affect the spall strength, but significantly slow down the spall process compared to defect-free crystals, resulting in a more ductile fracture.

The effect of increasing gas shear on wave structure of thin liquid films

Evolution of thin liquid films sheared by co-current gas stream in a vertical 11.7 mm pipe is studied experimentally using BBLIF technique. The main goal is to investigate the transition of wave patterns due to increase in the gas stream velocity, VG, from 0 to 24 m/s. Apart from relatively weak quantitative changes in the characteristics of the primary waves, replacement of capillary precursor by slow secondary waves is found. The transition is indentified in the range of VG = 8 - 16 m/s for all liquid flow rates. It is observed that the appearing secondary waves may be the main reason of the decay of the capillary precursor. The experimental results are compared to prediction of evolutionary theoretical model, showing qualitative agreement on secondary waves generation, but with no agreement on precursor's disappearance. Introducing artificial perturbations mimicking the action of turbulent pulsations in the gas phase is recommended to improve the model.

Evolution of Shock Waves in Hot-Pressed Ceramics of Boron Carbide and Silicon Carbide

In this paper we studied the evolution of shock compression waves in hot-pressed ceramics based on boron carbide and silicon carbide at a maximum compressive stress of 32 and 34 GPa, respectively, to determine the possible contribution of relaxation processes to the resistance to high-rate deformation. At a change in sample thickness from 0.5 to 8 mm, an appreciable decay of the elastic precursor was observed in boron carbide while an insignificant anomalous growth of the elastic precursor with a sample thickness was observed in the experiments with silicon carbide samples of various thickness. The measured value of the Hugoniot elastic limit of samples with a thickness of 8 mm was σHEL = 17.2 ± 1.3 GPa for boron carbide and σHEL = 15 ± 0.1 GPa for silicon carbide.

The defect evolution in shock loaded tantalum single crystals

The defect structures of three different orientation ([001], [011] and [111]) shocked single crystals of tantalum have been characterised using scanning electron microscopy and transmission electron microscopy. The defect evolution and the response of the single crystals are found to be highly dependent on the orientation of the single crystals and the position in the specimen. Crystal plasticity simulation has been used to calculate the strain tensor in the specimens as a function of position and time. The defect types and distributions are analysed in terms of the shock wave and the lateral and back release waves. Twins at the sample centre and front surface were created by the shock wave front. The twins at the back of the sample close to the side surface are produced by the interaction of the release waves. Twinning area fraction and dislocation density are higher at the impact surface region than at the back surface due to decay of the elastic precursor and the difference in loading duration. Twinning acts as a major deformation mechanism and has a strong influence on the Hugoniot elastic limit (HEL) and spall strength when the loading direction is [011] or [111].

Attenuation of the dynamic yield point of shocked aluminum using elastodynamic simulations of dislocation dynamics.

When a metal is subjected to extremely rapid compression, a shock wave is launched that generates dislocations as it propagates. The shock wave evolves into a characteristic two-wave structure, with an elastic wave preceding a plastic front. It has been known for more than six decades that the amplitude of the elastic wave decays the farther it travels into the metal: this is known as "the decay of the elastic precursor." The amplitude of the elastic precursor is a dynamic yield point because it marks the transition from elastic to plastic behavior. In this Letter we provide a full explanation of this attenuation using the first method of dislocation dynamics to treat the time dependence of the elastic fields of dislocations explicitly. We show that the decay of the elastic precursor is a result of the interference of the elastic shock wave with elastic waves emanating from dislocations nucleated in the shock front. Our simulations reproduce quantitatively recent experiments on the decay of the elastic precursor in aluminum and its dependence on strain rate.

Infrared spectroscopy and 193 nm photochemistry of methylamine isolated in solid parahydrogen.

The in situ UV photolysis of a precursor molecule trapped in a parahydrogen (pH2) matrix is a simple method used to generate isolated radical photofragments that are well suited for infrared spectroscopic studies. However, for molecules that can dissociate via multiple pathways, little is known about how the pH2 matrix influences the branching among these open pathways. We report FTIR spectroscopic studies of the 193 nm photodecomposition of methylamine (MA, CH3NH2) isolated in pH2 quantum matrixes at 1.8 K. We observe single exponential decay of the MA precursor upon irradiation and the quantum yield for MA photodissociation is measured to be Φ = 0.26(2) consistent with a weak pH2 cage effect. By comparing to gas-phase results, we show the in situ photolysis results in greater production of molecular products (CH2═NH + H2) compared to radical products (CH3NH + H) consistent with the idea of partial caging of the H atom photofragments. The information gained in this work can be used to guide future photolysis studies in pH2 matrixes.

Deformation resistance and fracture of iron over a wide strain rate range

The results of measurements of the decay of an elastic precursor in iron at the distances from 0.13 to 10 mm and the spall strength of the samples with such thicknesses have been compared with similar data for the nanometer-scale samples. The decay has been described by a unique dependence whose differentiation gives the relationship between the initial plastic strain rate in the range from 103 to 109 s−1 and the compression stress in the elastic shock wave from 1.5 to 27.5 GPa. The dynamic breaking strength (spall strength) varies in this range of shock-wave load time from 1.5 to 20 GPa.

Evolution of shock waves in SiC ceramic

The evolution of a shock compression wave in SiC ceramic is measured for determining the possible contribution of relaxation processes to the high-rate straining. No appreciable decay of the elastic precursor and other features of stress relaxation are revealed when the sample thickness changes from 0.5 to 8.3 mm, and the evolution of the compression wave corresponds to a simple wave. The measured values of the Hugoniot elastic limit (σHEL = 8.72 ± 0.17 GPa) and spall strength (σsp = 0.50–0.62 GPa) with allowance for the density of the ceramic are in conformity with the available data.

Preliminary mineralogy and ore petrology of the intermediate-sulfidation Pallancata Deposit, Ayacucho, Peru

Se investiga la compleja mineralogia del Yacimiento de Pallancata (6o productor de plata del mundo) y se establecen las condiciones de formacion (P.T) basadas en la petrologia de las menas comparada con los datos de mineralogia experimental y en la petrografia y microtermometria de inclusiones fluidas en la ganga silicatada, resultando un deposito tipicamente caracterizado como epitermal de sulfuracion intermedia.ABSTRACT:Pallancata is a world-class intermediate-sulfidation epithermal deposit, hosted by upper Miocene volcanics of the south-central Peruvian Andes in a sinuous N70oW, ∼75o SW strike-slip structure, with wide (up to 35 m) pull-apart dilation zones related to bends of the vein strike. The structural evolution of the vein from earlier brecciation to later open space infill resembles the Shila Paula district (Chauvet et al. 2006). Fluid inclusion petrography and microthermometry show that ore deposition is related to protracted boiling of very diluted, mainly meteoric fluids, starting at 250–260 oC, under ∼300 m hydrostatic head. The mineralogical-petrological study reveals a complex sequence of mineralization (eight stages) and mineral reactions consistent with Ag2S enrichment or Sb2S3 depletion, or both, during cooling over the temperature range 250–200 oC: pyrite, sphalerite, galena, miargyrite, pyrargyrite-proustite, chalcopyrite, polybasite-pearceite, argentite (now acanthite), and Au–Ag alloy (“electrum”). This Ag2S enrichment and Sb2S3depletion during cooling may be explained by decay of a Ag-rich galena precursor at deeper levels (Pb2S2–AgSbS2 solid solution), which rapidly becomes unstable with decreasing temperature, producing residual (stoichiometric) PbS and more mobile Ag and Sb sulfide phases, which migrated upward and laterally away from the thermal core of the system. The core is still undisclosed by mining works, but the available geochemical evidence (logAg/log Pb ratios decreasing at depth) is consistent with this interpretation, implying a deeper potential resource. Data from sulfide geothermometry, based on mineral equilibria, document the thermal evolution of the system below 200 oC (stephanite, uytenbogaardtite, jalpaite, stromeyerite, mckinstryite, among others). The end of the most productive stages (3, 4, and 5) is marked by the precipitation of stephanite at temperatures below 197 ± 5 oC, but precipitation of residual silver continues through the waning stages of the hydrothermal system down to <93.3 oC (stromeyerite) or in a supergene redistribution (stage 8, acanthite II).

Photodissociation of N-methylformamide isolated in solid parahydrogen

We report FTIR studies of the 193 nm photodecomposition of N-methylformamide (NMF) isolated in solid parahydrogen (pH(2)) matrices at 1.9 K. By studying the detailed photokinetics we can distinguish between primary and secondary photoproducts. We observe single exponential decay of the NMF precursor upon irradiation and identify three competing primary dissociation channels: HCO + NHCH(3); H + CONHCH(3); and CO + CH(3)NH(2) with branching ratios of 0.46(7):0.032(8):0.51(6), respectively. Two of the primary photoproducts (NHCH(3) and CONHCH(3)) are observed for the first time using IR spectroscopy and assigned via ab initio calculations of the vibrational frequencies and intensities of these radicals. The dominant radical formation channel HCO + NHCH(3) is consistent with efficient C-N peptide bond fission at this wavelength and escape of the nascent radical pair from the pH(2) solvent cage. The significant branching 0.51(6) measured for the molecular channel CO + CH(3)NH(2) is unexpected and raises important questions about the details of the in situ photochemistry. Starting from the NMF precursor, we observe and characterize spectroscopically a wide variety of secondary photoproducts including CH(2)NH, HCN, HNC, HNCO, CH(3)NCO, CH(4), and NH(3).

High strain rate deformation and fracture of the magnesium alloy Ma2-1 under shock wave loading

This paper presents the results of measurements of the dynamic elastic limit and spall strength under shock wave loading of specimens of the magnesium alloy Ma2-1 with a thickness ranging from 0.25 to 10 mm at normal and elevated (to 550°C) temperatures. From the results of measurements of the decay of the elastic precursor of a shock compression wave, it has been found that the plastic strain rate behind the front of the elastic precursor decreases from 2 × 105 s−1 at a distance of 0.25 mm to 103 s−1 at a distance of 10 mm. The plastic strain rate in a shock wave is one order of magnitude higher than that in the elastic precursor at the same value of the shear stress. The spall strength of the alloy decreases as the solidus temperature is approached.

Investigation of incomplete fusion in 20Ne + 55Mn reactions at 3–8 MeV/nucleon

The excitation functions of radio-nuclides have been measured for the 20Ne + 55Mn system in the energy range ≈51–164 MeV by employing the recoil catcher technique followed by off-line gamma-ray spectrometry. It has been observed that some of the residues produced directly via complete and/or incomplete fusion (ICF) give rise to independent yield while some of the residues are produced in the decay of higher charge precursor isobars through β+ and/or electron capture and give rise to cumulative yield. Independent yields of radio-nuclides 71, 70As, 69, 67Ge and 67, 66Ga have been separated out from the measured cumulative yields. The measured excitation functions are then compared with statistical model code PACE-2. It has been observed that excitation functions of the measured complete fusion channels are in good agreement with theoretical predictions. However, a significant enhancement in the measured excitation functions for the residues, produced in the break-up of the projectile into α-clusters, has been observed. This enhancement may be attributed to the ICF process. Moreover, ICF-fraction has also been calculated by dividing the ICF-cross-section to the total fusion cross-section and found to increase with the projectile energy as well as with the entrance channel mass-asymmetry of the interacting partners.

Optical precursors in the singular and weak dispersion limits

The description of the precursor fields in a single-resonance Lorentz model dielectric is considered in the singular and weak dispersion limits. The singular dispersion limit is obtained as the damping approaches zero and the material dispersion becomes increasingly concentrated about the resonance frequency. The algebraic peak amplitude decay of the Brillouin precursor with propagation distance z&gt;0 then changes from a z&#x2212;1/2 to a z&#x2212;1/3 behavior. The weak dispersion limit is obtained as the material density decreases to zero. The material dispersion then becomes vanishingly small everywhere and the precursors become increasingly compressed in the space-time domain immediately following the speed-of-light point (z,t)=(z,z/c). In order to circumvent the numerical difficulties introduced in this case, an approximate equivalence relation is derived that allows the propagated field evolution due to an ultrawideband signal to be calculated in an equivalent dispersive medium that is highly absorptive.

Influence of radioactive filiation on measured release curves

Perturbation of measured release curves of radioactive nuclei by the decay of precursor nuclei in the target is investigated. The target is described by a simple model taking into account the direct production of the nucleus of interest by the reaction but also allowing decay of the isobaric precursor to take place specifically during diffusion or effusion. Their respective contributions to the observed ion currents can be enough different to lead to uncorrect analysis of the experimental release curves. As an example we discuss release curves of the pairs of neutron-rich isotopes ( Rb m 86 , 91Rb) and ( Cs m 130 , 139Cs) produced by proton-induced fission at the IRIS facility, Gatchina.

Dynamical evolution of the Brillouin precursor in Rocard-Powles-Debye model dielectrics

When an ultrawide-band electromagnetic pulse penetrates into a causally dispersive dielectric, the interrelated effects of phase dispersion and frequency dependent attenuation alter the pulse in a fundamental way that results in the appearance of so-called precursor fields. For a Debye-type dielectric, the dynamical field evolution is dominated by the Brillouin precursor as the propagation depth typically exceeds a single penetration depth at the carrier frequency of the input pulse. This is because the peak amplitude in the Brillouin precursor decays only as the square root of the inverse of the propagation distance. This nonexponential decay of the Brillouin precursor makes it ideally suited for remote sensing. Of equal importance is the frequency structure of the Brillouin precursor. Although the instantaneous oscillation frequency is zero at the peak amplitude point of the Brillouin precursor, the actual oscillation frequency of this field structure is quite different, exhibiting a complicated dependence on both the material dispersion and the input pulse characteristics. Finally, a Brillouin pulse is defined and is shown to possess near optimal (if not optimal) penetration into a given Debye-type dielectric.

Nucleogenic iron studied by emission Mössbauer spectroscopy in Co-metal

Results obtained by means of the emission Mössbauer spectroscopy in metallic cobalt are reported. The Mössbauer line of the 14.4-keV energy connecting the first excited state of the stable 57Fe nucleus with its ground state was used. Radioactive 57Co was used as the precursor of the above nuclear state. It was dissolved in the natural metallic cobalt with the concentration of about 40 at. ppm including nucleogenic iron generated during decay of the cobalt precursor. Mössbauer spectra were collected in the temperature range between room temperature (RT) and 1075 K with the sample kept under vacuum. A transition from the low temperature hexagonal phase to the face centered cubic high temperature phase at about 690 K has no influence on the iron magnetic hyperfine field arising due to the ferromagnetic ordering of the host. On the other hand, the electron charge density on the iron nucleus has some relatively narrow maximum in the vicinity of the transition temperature. There is some discontinuity in the recoilless fraction as well indicating that the high temperature cubic phase provides somewhat stronger bonds for the isolated iron impurity. The anharmonic behavior of the lattice vibrations could be seen in the cubic phase well above the transition point. No measurable electric quadrupole interaction was found in the hexagonal phase. The evolution of the magnetic hyperfine field with the temperature is reasonably described by the spin wave formalism provided strong magnon–magnon scattering is allowed for. On the other hand, charge density on the iron nucleus follows thermal expansion except some singularity in the vicinity of the transition point.

Stoichiometric production of hydrogen peroxide and parallel formation of ferric multimers through decay of the diferric-peroxo complex, the first detectable intermediate in ferritin mineralization.

The catalytic step that initiates formation of the ferric oxy-hydroxide mineral core in the central cavity of H-type ferritin involves rapid oxidation of ferrous ion by molecular oxygen (ferroxidase reaction) at a binuclear site (ferroxidase site) found in each of the 24 subunits. Previous investigators have shown that the first detectable reaction intermediate of the ferroxidase reaction is a diferric-peroxo intermediate, F(peroxo), formed within 25 ms, which then leads to the release of H(2)O(2) and formation of ferric mineral precursors. The stoichiometric relationship between F(peroxo), H(2)O(2), and ferric mineral precursors, crucial to defining the reaction pathway and mechanism, has now been determined. To this end, a horseradish peroxidase-catalyzed spectrophotometric method was used as an assay for H(2)O(2). By rapidly mixing apo M ferritin from frog, Fe(2+), and O(2) and allowing the reaction to proceed for 70 ms when F(peroxo) has reached its maximum accumulation, followed by spraying the reaction mixture into the H(2)O(2) assay solution, we were able to quantitatively determine the amount of H(2)O(2) produced during the decay of F(peroxo). The correlation between the amount of H(2)O(2) released with the amount of F(peroxo) accumulated at 70 ms determined by Mössbauer spectroscopy showed that F(peroxo) decays into H(2)O(2) with a stoichiometry of 1 F(peroxo):H(2)O(2). When the decay of F(peroxo) was monitored by rapid freeze-quench Mössbauer spectroscopy, multiple diferric mu-oxo/mu-hydroxo complexes and small polynuclear ferric clusters were found to form at rate constants identical to the decay rate of F(peroxo). This observed parallel formation of multiple products (H(2)O(2), diferric complexes, and small polynuclear clusters) from the decay of a single precursor (F(peroxo)) provides useful mechanistic insights into ferritin mineralization and demonstrates a flexible ferroxidase site.