Shock-induced Defects Research Articles

Structural phase transition of YVO4:Pr[formula omitted] caused by shock compression

A pressure-induced transition of YVO4:Pr3+ from the zircon to the scheelite phase was caused by shock compression. Single-crystalline samples were shock compressed at pressures as high as 21 GPa, and the shocked samples were recovered. The recovered samples were characterized using X-ray diffraction analysis, Raman spectroscopy, and photoluminescence (PL) spectroscopy. The sample shocked at 21 GPa was composed of a scheelite phase and a minor amount of a zircon phase, which indicates that the pressure range for the coexistence of these phases caused by shock compression is larger than that for the coexistence of these phases caused by static compression. Although PL measurements were conducted at ambient pressure, no detectable peak was observed for the sample shocked at 21 GPa. Thus, shock-induced defects might result in quenching of the PL peaks.

Experiment and simulation investigations on W/Cu components prepared by strong confinement thermal explosive welding

• A strong confinement thermal explosive welding was proposed for W/Cu composite. • The high-quality W/Cu joint specimen without damage and defects was obtained. • The microstructural evolution mechanism of the bonding interface was revealed. The first-wall materials and components are the most thermally loaded in-vessel components of a fusion machine, which are required to survive at high heat fluxes induced by the intense plasma bombardment. As a candidate material, the W/Cu composite was investigated from the aspects of choosing a feasible and preferable welding method for high performance and high strength. For this, a strong confinement thermal explosive welding technology was proposed to solve the key problems in producing high-quality W/Cu composite material without damage and shock-induced defects. The microstructural evolution of the welding interface was observed and analyzed in the paper. Combining with the numerical simulation, the evolution mechanism of the bonding interface and the changing rule of the grain structure of the material were studied. This comprehensive research allows to provide a new perspective for preparing high-quality W/Cu composite.

Selective excitation of two-wave structure depending on crystal orientation under shock compression

Single crystals subjected to shock compression exhibit responses with distinct two-wave structures for certain crystal orientations. However, little is known to date regarding how the shock response depends on crystal orientation, and especially why the two-wave structure depends on the crystal orientation. In this work, molecular dynamics simulations of shock compressions in copper single crystals are performed to investigate the orientation dependence of shock responses and the corresponding deformation mechanisms. Four copper single crystals with [001], [011], [012], and [123] crystal orientations along the depth direction are investigated. The [011], [012], and [123] crystal orientations of copper single crystals show distinct two-wave structures in their shock responses, while such a two-wave structure in the shock response is not seen for those orientations having a [001] crystal orientation. The potential causes are analyzed by considering the propagation velocities of both elastic and plastic waves. We develop a technique for identifying twin structures in face-centered cubic crystals and this technique can effectively identify the twin structure. The morphology of shock-induced defects (e.g., dislocations and twins) shows the significant dependence of crystal orientation and the mechanisms behind these are discussed in detail. Finally, the Johnson-Cook constitutive model describing dynamic deformations at high temperatures and high strain rates is used to analyze the relationships between the shock responses and microscopic defects. The predictions of the Johnson-Cook constitutive model are consistent with the results of the molecular dynamics simulations.

Nanoscale constraints on the shock-induced transformation of zircon to reidite

In a hypervelocity impact event, the temperatures and pressures generated by the shock waves far exceed the values produced by endogenic processes. The shock-induced processes can modify the distribution of trace elements in zircon grains located in target rocks, potentially affecting the geochemical reliability of zircon, but also providing an opportunity to better understand the mechanisms of shock deformation. The formation of reidite lamellae by the shock-induced phase change of zircon has previously been proposed to be a diffusionless, martensitic transformation, with no associated atomic mobility over nanometre length scales. However, nanoscale characterization of the zircon–reidite interface and a low-angle boundary within the reidite by atom probe tomography, transmission electron microscopy and correlative analytical techniques, shows localised enrichment of particular trace elements (Y, Al, Ca, Be, Mg, Mn, and Ti). These observations indicate the presence of additional short-range diffusional components to explain the local compositional variations observed at the nanoscale for the high-pressure transformation of zircon to reidite lamellae. A new model for this transformation is proposed that consists of two stages: 1) the early stage of the impact event where the shock waves induce defects in the zircon grain and trigger a phase transformation, resulting in trace element segregation by interface migration; and 2) the recovery stage where the trace elements and shock induced defects migrate to areas of lower energy.

Femtosecond laser irradiation of olivine single crystals: Experimental simulation of space weathering

Space weathering is one of the most common surface process occurring on atmosphere-free bodies such as asteroids and the Moon. It is caused mainly by solar wind irradiation and the impact of micrometeoroids. In order to simulate space weathering effects, in particular those produced by hypervelocity impacts, we produced microcraters via ultra-short (∼100 fs) laser irradiation of crystallographically oriented slices of forsterite-rich (Fo94.7) olivine. The main advantages of the application of a femtosecond laser radiation to reproduce the space weathering effects are (1) the high peak irradiance (1015 W cm−2), which generates the propagation of the shock wave at the nanosecond timescale (i.e., timescale of the micrometeoroid impacts); (2) the rapid transfer of energy to the target material, which avoids the interaction of laser light with the developing vapor plume; (3) a small laser beam, which allows the effects of a single impact to be simulated. The results of our spectroscopic and electron microscopic investigation validate this approach: the samples show strong darkening and reddening of the reflectance spectra and structural damages similar to the natural microcraters found on regolith grains of the Moon and asteroid 25143 Itokawa. Detailed investigations of several microcrater cross-sections by transmission electron microscopy allowed the detection of shock-induced defect microstructures. From the top to the bottom of the grain, the shock wave causes evaporation, melting, solid-state recrystallization, misorientation, fracturing, and the propagation of dislocations with Burgers vectors parallel to [001]. The formation of a short-lived vapor plume causes the kinetic fractionation of the gas and the preferential loss of lighter elements, mostly magnesium and oxygen. The high temperatures within the melt layer and the kinetic loss of oxygen promote the thermal reduction of iron and nickel, which leads to the formation of metallic nanoparticles (npFe0). The final stage of the microcrater formation is the cooling of the melt layer that results in its partial crystallization and the formation of olivine crystals with a palisade-like texture.

Refractive index and phase transformation of sapphire under shock pressures up to 210 GPa

Under shock pressures up to 210 GPa, we measured the refractive index of sapphire at a wavelength of 1550 nm by performing plate impact experiments in order to investigate its refractive-index change behaviors and phase transitions along the Hugoniot state. There were two discontinuities in the refractive index at ∼65 to 92 GPa and ∼144 to 163 GPa, respectively. Moreover, above the Hugoniot elastic limit, the pressure dependence of the refractive index was divided into three segments, and there were large differences in their pressure-change trends: the refractive index decreased evidently with pressure in the first segment (∼20 to 65 GPa), remained nearly constant from ∼92 to ∼144 GPa in the second segment, and obviously increased with pressure in the last segment (∼163 to 210 GPa). Our first-principles calculations suggest that the observed discontinuities were closely related to the corundum-Rh2O3(II) and Rh2O3(II)-CaIrO3 structural transitions, and the shock-induced vacancy point defects could be one factor causing these great discrepancies in pressure-change trends. This work provides sapphire refractive-index information in a megabar-pressure range and clear evidence of its shock structural transitions. This not only has a great significance for the velocity correction of laser interferometer experiments and the analysis of sapphire high-pressure properties but also indicates a possible approach to explore the shock transitions of transparent materials.

A molecular dynamics study of the shock-induced defect microstructure in single crystal Cu

Molecular dynamics simulations of shock compression are carried out over a range of pressures (from 22GPa to 75GPa) to investigate the shock-induced defect microstructure in single crystal Cu. The results show that under increasing shock pressure, we can detect three distinct phases of the defect regimes for single crystal Cu: dislocations→stacking faults→twins. We show the evolution processes of different defect microstructures. According to the analyses of these evolution processes, the formation mechanisms of various defect microstructures are studied. When the shock pressure is less than 30GPa, the motion of dislocations is a significant mechanism in single crystal Cu under the shock compression. The motion between various dislocations and loops widely exists on the (111) close-placed plane. The interesting dislocation loops are obtained at a relatively low pressure, and a new mechanism of nucleation and development of the dislocation loop is proposed. When the shock pressureis greater than 30GPa, a large number of intersecting stacking faults are formed. On the (001) surface, stacking faults align along the [220] and [2¯20] directions at exactly 90 degrees, and they are present roughly in the same proportion. The stacking fault spacing decreases with the increase of shock pressure, and shock-induced plasticity increases as a function of shock strength. The structure of the simulation, based on the topology, matches extremely well with that found in recent transmission electron microscopy studies of single crystal Cu recovered from laser shock experiments. At a relatively high pressure (61GPa), we find that the stacking faults and twins exist together in single crystal Cu. It is shown that two types of stacking faults with vertical directions are the competitive mechanism. When one of them dominates, twins will be formed from this type of stacking fault. Thus, the directions of twins are randomly distributed in the initial stage. Rotation and combination of twins are significant mechanisms, and these mechanisms lead to the final formation of twins. At 75GPa, twins elongate along [12¯1] and [1¯2¯1¯] directions on the (1¯01) surface. The configuration and direction of the twins are in agreement with experimental results.

Effects of vacancy point defects and phase transitions on optical properties of shocked Al2O3

The velocity interferometer system for any reflector (VISAR) and pyrometric measurements in dynamic highpressure experiments require the use of an optical window, and Alumina (Al2O3) or sapphires is often considered as a window material due to its high shock impedance and excellent transparency. Consequently, understanding the characteristics of its transparency and refractive index change under shock loading is crucial for explaining such experimental data. Experimental studies indicate optical transparency loss in shocked Al2O3. The mechanisms for the phenomenon are some interesting issues. A first-principles study suggests that shock-induced VO+2 (the +2 charged O vacancy) defects in Al2O3 could be an important factor causing the transparency loss. Recently, the red shift of the extinction curve (i.e., the wavelength dependence of the extinction coefficient) with increasing shock pressure has been observed. It is needed to ascertain whether this behavior is also related to shock-induced vacancy point defects. In addition, up to now, information about Al2O3 refractive index at a wavelength of 532 nm under strong shock compression (the optical source wavelength in VISAR measurement is usually set at 532 nm) has been unknown, and neither the effects of structural transitions nor vacancy point defects on the refractive index of shocked Al2O3 are determined. Here, to investigate the above-mentioned questions, we perform first principles calculations of optical absorption and refractive index properties of Al2O3 crystal without and with VO+2 and VAl3 (the -3 charged Al vacancy) defects in a pressure range of 180 GPa (the calculations in CASTEP are carried out by the plane-wave pseudo potential method in the framework of the density functional theory). Our absorption data show that the observed optical extinction in shocked Al2O3 cannot be explained by only considering pressure and temperature factors, but shock-induced VO+2 should be an important source for this behavior. On the basis of these results, we may judge that 1) the transparency loss explanation for shocked Al2O3 in the view of vacancy point defects is reasonable; 2) the absorption extinction should dominate the extinction phenomenon observed in shocked Al2O3. Our calculations find that high-pressure structural transition in Al2O3 causes an obvious enhancement of its refractive index. The refractive index decreases with increasing shock pressure in corundum and Rh2O3 regions, and decreases slightly below 172 GPa and increases slowly above 172 GPa with increasing shock pressure in CalrO3 region. The VO+2 and VAl3 defects in Al2O3 have apparent influences on the shock pressure dependence of its refractive index. These results mean that the information about Al2O3 refractive index under strong shock loading cannot be obtained simply by extrapolating its low pressure data. Our prediction could be of importance for future experimental study and new window-material development.

Amorphization and nanocrystallization of silicon under shock compression

High-power, short-duration, laser-driven, shock compression and recovery experiments on [001] silicon unveiled remarkable structural changes above a pressure threshold. Two distinct amorphous regions were identified: (a) a bulk amorphous layer close to the surface and (b) amorphous bands initially aligned with {111} slip planes. Further increase of the laser energy leads to the re-crystallization of amorphous silicon into nanocrystals with high concentration of nano-twins. This amorphization is produced by the combined effect of high magnitude hydrostatic and shear stresses under dynamic shock compression. Shock-induced defects play a very important role in the onset of amorphization. Calculations of the free energy changes with pressure and shear, using the Patel-Cohen methodology, are in agreement with the experimental results. Molecular dynamics simulation corroborates the amorphization, showing that it is initiated by the nucleation and propagation of partial dislocations. The nucleation of amorphization is analyzed qualitatively by classical nucleation theory.

The electrical conductivity of Al2O3 under shock-compression.

Sapphire (Al2O3) crystals are used below 100 GPa as anvils and windows in dynamic-compression experiments because of their transparency and high density. Above 100 GPa shock pressures, sapphire becomes opaque and electrically conducting because of shock-induced defects. Such effects prevent temperature and dc conductivity measurements of materials compressed quasi-isentropically. Opacities and electrical conductivities at ~100 GPa are non-equilibrium, rather than thermodynamic parameters. We have performed electronic structure calculations as a guide in predicting and interpreting shock experiments and possibly to discover a window up to ~200 GPa. Our calculations indicate shocked sapphire does not metallize by band overlap at ~300 GPa, as suggested previously by measured non-equilibrium data. Shock-compressed Al2O3 melts to a metallic liquid at ~500 GPa and 10,000 K and its conductivity increases rapidly to ~2000 Ω−1cm−1 at ~900 GPa. At these high shock temperatures and pressures sapphire is in thermal equilibrium. Calculated conductivity of Al2O3 is similar to those measured for metallic fluid H, N, O, Rb, and Cs. Despite different materials, pressures and temperatures, and compression techniques, both experimental and theoretical, conductivities of all these poor metals reach a common end state typical of strong-scattering disordered materials.

Laser compression of nanocrystalline tantalum

Nanocrystalline tantalum (grain size ∼70nm) prepared by severe plastic deformation (high-pressure torsion) from monocrystalline [100] stock was subjected to shock compression generated by high-energy laser (∼350–850J), creating pressure pulses with initial duration of ∼3ns and amplitudes of up to ∼145GPa. The laser beam, with a spot radius of ∼1mm, created a crater of significant depth (∼135μm). Transmission electron microscopy revealed few dislocations within the grains and an absence of twins at the highest shock pressure, in contrast with monocrystalline tantalum. Hardness measurements were conducted and show a rise as the energy deposition surface is approached, evidence of shock-induced defects. The grain size was found to increase at a distance of 100μm from the energy deposition surface as a result of thermally induced grain growth. The experimentally measured dislocation densities are compared with predictions using analyses based on physically based constitutive models, and the similarities and differences are discussed in terms of the mechanisms of defect generation. A constitutive model for the onset of twinning, based on a critical shear stress level, is applied to the shock compression configuration. The predicted threshold pressure at which the deviatoric component of stress for slip exceeds the one for twinning is calculated and it is shown that it is increased from ∼24GPa for the monocrystalline to ∼150GPa for the nanocrystalline tantalum (above the range of the present experiments). Calculations using the Hu–Rath analysis show that grain growth induced by the post shock-induced temperature rise is consistent with the experimental results: grains grow from 70 to 800nm within the post-shock cooling regime when subjected to a laser pulse with energy of 684J.

Shear-induced ringwoodite formation in the Martian shergottite Dar al Gani 670

Shock-produced melt veins in the Martian shergottite Dar al Gani 670 crosscut large olivine crystals. The upper part of one of these crystals appears to be sheared off and displaced along the shock vein. From the olivine–vein interface small lamellae of ringwoodite grow into the host crystal. The ≤1–3µm wide and up to 20µm long lamellae consist of small bands and blocks and are orientated along specific crystallographic orientations. Texture and composition, i.e., more Fe-rich than the host olivine, indicate that lamellae formed via incoherent diffusion-controlled growth. It is suggested that a combination of high particle velocities and shock-induced defects lead to enhanced diffusion rates. In addition, shearing caused grain size reduction allowing rapid Fe–Mg interchange and induced lattice defects serving as nucleation sites for ringwoodite. Crystallographic orientation of ringwoodite lamellae indicates that during shock deformation the [001]{hk0} slip system was activated in olivine. Natural high-pressure phases in Martian meteorite allow to constrain phase transitions taking place in the inaccessible Earth's mantle. High-pressure shear instabilities of olivine at subduction zones in 400–700km depth are considered being responsible for deep earthquakes. At such p–T-conditions, breakdown of olivine results in formation of ringwoodite filled micro-anticracks which interact with each other finally leading to catastrophic shear failure. Our results strongly suggest that shearing itself contributes to a runaway process of enhanced ringwoodite formation and, thus, reinforces catastrophic material failure that may result in deep earthquakes.

The influence of oxygen content on the α to ω phase transformation and shock hardening of titanium

In this study the influence of alloy chemistry on the propensity of omega phase formation in two titanium alloys during shock loading is examined. The effect of peak shock stress on the phase stability and substructural evolution of high purity and A-70 (3700ppm oxygen) titanium was probed utilizing velocity interferometer system for any reflector and post shock substructural analysis. While in the high purity titanium the alpha to omega phase transformation was found to occur at 10.4GPa, no transformation was observed in the A-70 material for stresses up to 35GPa. Transmission electron microscopy analysis and neutron diffraction of shock-recovered samples confirmed these results and probed the details of twin and dislocation structures. Debye temperature data are also presented and the Debye-Waller temperatures for the alpha and omega phases in the high purity titanium are calculated. Finally, the compressive, quasistatic reload behaviors of both high purity and A-70 titanium are compared to the as-annealed counterparts at comparable strains and discussed in terms of the phase transformation and the shock-induced defect generation.

Chemical alteration and REE mobilization in meteorites from hot and cold deserts

The effects of terrestrial weathering on REE mobilization are evaluated for a variety of uncommon meteorites found in Antarctica and in hot deserts. The meteorites analyzed include 7 non-cumulate eucrites, 10 shergottites, 3 nakhlites, 2 lunar meteorites, 4 angrites, 10 acapulcoites, 1 winonaite, and 1 brachinite. In-situ concentration measurements of lanthanides and selected other minor and trace elements were made on individual grains by secondary ion mass spectrometry (SIMS). In Antarctic meteorites, oxidation converts Ce 3+ to Ce 4+, which is less soluble than the trivalent REE, resulting in Ce anomalies. The mineral most affected is low-Ca pyroxene. However, not all grains of a given mineral are, and distinct analyses of a single grain can even yield REE patterns with and without Ce anomalies. The effect is most pronounced for Antarctic eucrites in which Ce anomalies are observed not only in individual minerals but also in whole rock samples. Although Ce anomalies are observed in meteorites from hot deserts as well, the most characteristic signs of chemical alteration in this environment are a LREE enrichment with a typical crustal signature, as well as Sr, Ba and U contaminations. These can modify the whole rock REE patterns and disturb the isotope systematics used to date these objects. The LREE contamination is highly heterogeneous, affecting some grains and not others of a given mineral (mainly olivine and low-Ca pyroxene, the two minerals with the lowest REE concentrations). The major conduit for REE movement is through shock-induced cracks and defects, and the highest levels of contamination are found in altered material filling such veins and cracks. Meteorites that experienced low shock levels and those that are highly recrystallized are the least altered.

Shock-Induced Defects in Bulk Materials

ABSTRACTIn this paper examples of the shock-induced defects produced during shock compression which correlate with microstructure / mechanical property changes induced in materials due to shock prestraining are discussed. The characteristics of the shock impulse(peak shock pressure, pulse duration, and rarefaction rate) imparted to the material under investigation and the shock-induced defects produced in numerous metals and alloys are compared with their deformation behavior at ordinary rates of deformation. Examples of the range of defects observed in shock-recovered metals and alloys, include: dislocations, deformation twins, point defects, and residual metastable remnants from pressure-induced phase transformations. Results concerning the influence of interstitial content on the propensity of ω-phase formation and its structure in high-purity and A-70 Ti are presented. The influence of shock-wave deformation on the phase stability and substructure evolution of high-purity (low-interstitial) titanium and A-70 (3700 ppm oxygen) titanium were probed utilizing real-time velocity interferometry (VISAR) and “soft” shock-recovery techniques. Suppression of the α-ω pressure-induced phase transformation in A-70 Ti, containing a high interstitial oxygen content, is seen to simultaneously correspond with the suppression of deformation twinning.

TEM study of shock metamorphism in quartz from the Sedan nuclear test site

Nuclear explosions represent a useful simulation of meteorite impacts. Stresses of tens of gigapascals are produced during detonation and stress pulses can last 0.1 ms or longer, which is 3–4 orders of magnitude longer than laboratory experiments. Samples studied here were taken from the ejecta of the Sedan nuclear test site and consist of a coarse-grained granodiorite containing quartz, plagioclase, K-feldspar, cordierite and hornblende. Shocked quartz grains were selected optically in thin section and studied by TEM. TEM reveals the presence of planar deformation features (PDFs), which consist of multiple sets of planar amorphous lamellae with the crystallographic orientations {10¯12}, {10¯12}, and rarely {10¯11}. Some lamellae exhibit a substructure which consist of amorphous sublamellae organized ‘enéchelon’. No high-pressure phases were detected along the transformation lamellae. Some lamellae show evidence of beginning recrystallization, which probably occurred during cooling of the hot ejecta. The shock-induced defects present at the Sedan site are very similar to those seen in laboratory shock recovery experiments and also to those present at meteorite impact craters (e.g., Meteor Crater or Ries Crater). This suggests that shock-induced amorphization in quartz is not strongly dependant on shock pulse duration.

Effects of shock-induced defect density on flux pinning in melt-textured YBa2Cu3O7−δ

To introduce a high density of homogeneously distributed defects in YBa2Cu3O7−δ (Y123), melt textured samples were shock-compacted at pressures up to 12.6 GPa (126 Kbar) at orientations favorable to slip along the basal planes.1 Shock compressing melt-textured Y123 results in a nearly uniform detect density which is two to three orders of magnitude higher than in unshocked melt-textured material. However, the intergranular critical current density in bulk samples (Jcb) decreases by two orders of magnitude in the as-shocked state. This decrease in Jcb is attributed to microfractures. However, if the shocked disk is annealed in O2 then ground, sieved, and magnetically aligned, Jc for H τ c-axis (Jcab) is enhanced two to three times over the unshocked value. This indicates that the increase in dislocations density does increase flux-pinning.

Effects of shock-induced defects and subsequent heat treatment on flux pinning in melt-textured YBa 2Cu 3O 7−δ

Shock compression of melt-textured YBa 2Cu 3O 7−δ (Y123) results in a nearly uniform defect density which is two to three orders of magnitude higher than unshocked melt-textured material. The crystallographic alignment of the sample relative to the shock wave is critical because slip is limited to the basal plane in Y123. In the experiments the specimens were cut so that their effective c-axis was about 30° from the shock direction. Specimens were recovered intact from high shock pressures. The superconducting order parameter is decreased in the as-shocked state due to disorder of the crystalline lattice, predominantly of the Cu-O chains. A 450°C anneal in oxygen decreases the random atomic disorder, while retaining a large defect density. The defects remaining after a 450°C anneal enhance the intragranular critical current density by a factor of two to three over the unshocked initial value, depending on temperature and applied magnetic field. This shock process could be scaled to larger sizes using chemical explosives. These results show that shock-induced defects enhance J c's in oxygen-annealed melt-textured Y123. The maximum enhancement is expected to occur for shock pressures of about 5 GPa. Determinations of how the degree of mosaic spread and the size and distribution of Y 2BaCuO 5 affect the distribution of defects remain to be investigated.

Microstructures of AIPO4 subjected to high shock pressures

Berlinite single crystal specimens were shocked to peak pressures 12 and 24 GPa. Specimens were placed in an Al capsule to minimize shock-wave reflections at interfaces between specimen and capsule. Shock pressures were achieved with a 6.5-m-long two-stage gun. The shock-induced microstructures in recovered specimens were then investigated by Transmission Electron Microscopy. In the sample shocked at 12 GPa, the prominent shock-induced defects are dislocations and basal a glide appears to be the only glide system activated. In contrast, the sample shocked at 24 GPa exhibits no dislocations. The material is partially converted into an amorphous phase occurring under the form of thin amorphous lamellae parallel to the }10 $$\bar 1$$ n{ planes (n=0, 2, 3, 4). This microstructure is very similar to the one observed in experimentally shocked quartz.

Microstructural defects in experimentally shocked diopside: A TEM characterization

Transmission electron microscopy was used for characterizing the defect microstructure induced by shock experiments in a single crystal of diopside. The shock-induced defects found in the crystal can be divided in four distinct types: 1) A high density and pervasive distribution of dislocations in glide configuration (glide systems (100)[0 2) Mechanical twin lamellae, mostly parallel to (100), the (001) twin lamellae are less abundant. li]3) 4) Heterogeneously distributed tiny molten zones (3 to 20 μm size) which, after cooling, appear as a glass with a chemical composition very close to the one of the original diopside.