Free Surface Velocity Histories Research Articles

Effects of microstructures on dynamic deformation and spallation damage of high-entropy alloy Al0.3CoCrFeNi under plate impact loading

Plate impact experiments are conducted on high-entropy alloy Al0.3CoCrFeNi with different microstructures to investigate the effects of grain size, precipitates and heterogeneous structure on dynamic response. Free surface velocity histories are obtained. All initial and postmortem samples are characterized with transmission electron microscopy and electron backscatter diffraction. Nanoscale L12 precipitates result in a considerable increase in dynamic yield stress. Spall strength is higher for the heterogeneous structure, the large grain size (200 μm) and small grain size (63 μm) with L12 precipitates by ∼4%, 19% and 10% than for the small grain size (77 μm) without precipitates. After shock compression, dislocation slip, stacking faults, the Lomer-Cottrell locks and nanotwins are found in the samples without the L12 precipitates. L12 precipitates increase the energy barrier and inhibit deformation twinning. Spall damage is ductile in Al0.3CoCrFeNi. For the homogeneous structures, intragranular voids are predominant. Voids are preferentially distributed in the fine-grain domains or at the fine-grain–coarse-grain domain boundaries of the heterogeneous structure.

Effect of grain size on spall fracture of CrCoNi medium-entropy alloy under Taylor-wave loading

The effect of grain size on spall properties of medium-entropy alloy CrCoNi under Taylor-wave loading is investigated. Three different grain sizes, 300 μm, 20 μm, and 3 μm, are explored. The free surface velocity histories are measured to obtain such quantities as dynamic yield strength and spall strength. With decreasing grain size, there is a slight decrease in spall strength, but a significant increase in damage rate. The shock-recovered samples are characterized with scanning electron microscopy and electron backscatter diffraction. In coarse-grained samples, voids tend to nucleate within grain interiors, while in fine-grained samples, voids nucleate preferentially at grain boundaries (GBs). Molecular dynamics simulations show that the nucleation of intergranular or intragranular voids is dominated by GB–slip and slip–slip intersections, respectively. The higher GB density results in more extensive GB–slip intersections, consistent with the experimental results.

Anisotropic dynamic response of AlSi10Mg fabricated via laser powder bed fusion under plate impact

The effects of structural anisotropy on dynamic yield and spallation damage of an AlSi10Mg alloy manufactured via laser powder bed fusion are investigated via plate impact experiments along the build direction (BD) and the transverse direction (TD). Free surface velocity histories are measured to deduce its dynamic mechanical properties. The as-received and postmortem samples are characterized with scanning electron microscopy, metallograpic microscopy, electron backscatter diffraction and synchrotron X-ray computed tomography. The Hugoniot elastic limit for loading along TD is about 40% higher than that along BD, given the initial 〈100〉∥BD texture. For spallation, voids prefer to nucleate at melt pool boundaries. Spall strength for loading along TD is about 18% higher than that along BD, as a result of higher critical stress for void nucleation and less nucleation sites due to the melt pool morphology. The damage evolution features are also correlated well with melt pool morphology and grain morphology within the pool interior, demonstrating the “ductile” interpool and “brittle” intrapool fracture modes for loading along BD and TD, respectively.

Examination of machine learning method for identification of material model parameters

In this work, we compare two methods of using artificial neural network (ANN) to determine the optimal parameters of material model by results of high-speed plate impact experiments. The first ANN is trained to determine the free surface velocity history based on the impact parameters and material model parameters; this ANN is used as a fast emulator of model to speed-up the statistical Bayesian calibration of the model parameters. The second ANN directly determines the material model parameters based on the impact parameters and the free surface velocity history, because this ANN is trained by inverse data set; this approach for inverse problem is proposed for the first time and can be more preferable for fast-track expert systems in engineering applications. Both approaches are successfully used to determine the optimal parameters of material model in the case of impact of copper and aluminum plates at different temperatures and impact velocities with experimental data from the literature. A simple relaxation plasticity model with a constant relaxation time is used in the present research as an example; more accurate coincidence with the experimental data and wider applicability of the ANN-based approach can be expected for a more realistic plasticity model.

Correlation between grain size and dynamic response of NiTi alloy during intense shock-induced multi-spallation

This work committed to performing non-equilibrium molecular dynamics simulations to explore the influence of grain size on multi-spall performance of nanocrystalline nickel-titanium (NiTi) alloy. By comparing the calculated Hugoniot relationship with present experimental results, the Finnis-Sinclair (FS) type potential used in this work was verified to be suited for reproducing the shock behavior of NiTi alloy at high strain rate. During shock wave evolution, a single plastic wave was clearly observed in the sample with grain size of 5 nm, while there was a more pronounced elastic-plastic double wave structure at larger grain size. According to the free surface velocity history, the predicted spall strength exhibited a decreasing tendency and subsequently increased when the grain size decreased from 50 to 5 nm, indicative of a critical grain size (10 nm). Correspondingly, there were multiple spall planes appearing in the sample, the grain size of which was not smaller than the critical one. Contrarily, a single spall plane was observed at 5 nm. By counting the void distribution statistic, we found that as the grain size increased, the void-nucleation behavior changed from the intergranular to intergranular/transgranular coexisting pattern. In consequence, more nucleation sites were provided to promote an increase in the number of voids. Interestingly, for intergranular, there were much more voids involved in spallation at smaller grain size, which was attributed to grain boundary effect. The results demonstrated that void evolutions rarely occurred at the grain boundaries parallel to the loading direction. Moreover, when the grain size was larger than 5 nm, the B19 phases were found to be produced around those potential void-nucleation sites. After void nucleation, these B19 phases decreased till disappearance, accompanied by the proliferation of a large number of [100] dislocations. Such dislocations accumulated in the region without voids could block the void growth in this region, which was expected to be the reason for the formation of multiple spall planes. On the contrary, the absence of dislocations at 5 nm was inclined to the fact that the space for dislocation emission and aggregation was preoccupied by nucleated voids.

Destruction of a magnesium alloy film in the condensed state by an ultrashort laser-driven shock wave

Laser-driven shock wave phenomena in a sub-micrometer Mg–4Al–2Zn alloy film are studied using spectral interferometry with spatial and temporal (1 ps) resolution. Upon irradiating the film through a glass substrate by 500 fs laser pulses, the ultrashort elastic compression pulses with the peak stress up to 4.6 GPa at a propagation distance of 0.5 μm were generated. Depending on the laser fluence, either spall fracture near the rear surface in the solid state or cavitation near the metal–glass interface in the liquid state was observed. The spall strength of the solid Mg alloy and the upper limit of the cavitation threshold in the melt at the strain rate of ∼109 s−1 were extracted from the free surface velocity history. The depth of fracture initiation was retrieved from the instant of the spall pulse exit, and the thickness of the molten layer was estimated to be 100–160 nm depending on laser fluence. The investigation of the residual morphology by scanning electron and atomic force microscopies revealed the presence of melting and nucleation within the irradiated area. The experimental findings are of interest for predicting the behavior of magnesium alloys in the condensed state at extremely high strain rates, for studying the physics of metastable states and for simulating the interaction of ultrashort laser pulses with thin film materials.

Unraveling Anisotropy in Crystalline Orientation under Shock-Induced Dynamic Responses in High-Entropy Alloy Co25Ni25Fe25Al7.5Cu17.5.

Shock-induced plastic deformation and spall damage in the single-crystalline FCC Co25Ni25Fe25Al7.5Cu17.5 high-entropy alloy (HEA) under varying shock intensities were systematically investigated using large-scale molecular dynamics simulations. The study reveals the significant influence of crystalline orientation on the deformation mechanism and spall damage. Specifically, the shock wave velocities in the [110] and [111] directions are significantly higher than that in the [001] direction, resulting in a two-zone elastic-plastic shock wave structure observed in the [110] and [111] samples, while only a single-wave structure is found in the [001] sample. The plastic deformation is dominated by the FCC to BCC transformation following the Bain path and a small amount of stacking faults during the compression stage in the [001] sample, whereas it depends on the stacking faults induced by Shockley dislocation motion in the [110] and [111] samples. The stacking faults and phase transformation in the [001] sample exhibit high reversibility under release effects, while extensive dislocations are present in the [110] and [111] samples after release. Interestingly, tension-strain-induced FCC to BCC phase transformation is observed in the [001] sample during the release stage, resulting in increased spall strength compared to the [110] and [111] samples. The spall strength estimated from both bulk and free surface velocity history shows reasonable consistency. Additionally, the spall strength remains stable with increasing shock intensities. The study discusses in detail the shock wave propagation, microstructure change, and spall damage evolution. Overall, our comprehensive studies provide deep insights into the deformation and fracture mechanisms of Co25Ni25Fe25Al7.5Cu17.5 HEA under shock loading, contributing to a better understanding of dynamic deformation under extreme environments.

Effect of minor elements Al and Ti on dynamic deformation and fracture of CoCrNi-based medium-entropy alloys

Three texture-free CoCrNi-based medium-entropy alloys (MEAs) with similar grain sizes, CoCrNi, (CoCrNi)94Al3Ti3 and (CoCrNi)90Al5Ti5, are investigated under plate impact loading. Free surface velocity histories are obtained along with microstructure characterizations of postmortem samples. The influences of the addition of minor elements Al and Ti on the microstructures, dynamic mechanical properties and underlying deformation and damage mechanisms of MEAs are elucidated. Compared to the single-phase CoCrNi MEA, spherical nanosized L12 precipitates and micron sized body-centered-cubic phase are found in (CoCrNi)94Al3Ti3 and (CoCrNi)90Al5Ti5, respectively. The addition of minor elements Al and Ti leads to an increase in the dynamic yield stress approximately by 70% (to 1.0 GPa) and 130% (to 1.4 GPa) for (CoCrNi)94Al3Ti3 and (CoCrNi)90Al5Ti5, respectively, due to the solid solution strengthening and precipitate strengthening. Multiple deformation mechanisms are observed. Planar dislocation glide and stacking faults are the dominant deformation mechanisms for all the MEAs. However, deformation twinning is deactivated in (CoCrNi)94Al3Ti3 and (CoCrNi)90Al5Ti5 with small additions of Al and Ti. For spallation, damage is ductile in nature for CoCrNi and (CoCrNi)94Al3Ti3, but brittle for (CoCrNi)90Al5Ti5. Under similar shock stress, the spall strength of (CoCrNi)94Al3Ti3 is comparable to that of CoCrNi, ∼ 150% higher than that of (CoCrNi)90Al5Ti5 because intergranular brittle fracture activation does not require substantial plastic deformation. The minor addition of Al and Ti in medium-entropy alloys significantly affects their dynamic mechanical properties, deformation and damage mechanisms.

Impact response of metastable dual-phase high-entropy alloy Cr10Mn30Fe50Co10

Plate impact experiments are conducted on a metastable dual-phase high-entropy alloy (HEA) Cr10Mn30Fe50Co10 with different matrix grain sizes and phase fractions. Free surface velocity histories are obtained along with microstructure characterizations. Postmortem samples are characterized with transmission electron microscopy and electron backscatter diffraction. Multiple deformation mechanisms are observed, including dislocation slip and stacking fault in the face-center cubic (FCC) phase, dislocation slip and deformation twinning in the hexagonal close-packed (HCP) phase, and the FCC to HCP transformation. For spallation, damage is ductile in nature. Despite different microstructures, micro-voids prefer to nucleate at high-angle grain boundaries within the FCC phase, leading to similar spall strength. Upon shock loading, a dynamic plastic deformation partitioning behavior among the two phases is observed; at high shock stress, denser dislocations in the HCP phase result in increased fractions of voids around the FCC/HCP phase interface and within the HCP phase during spallation damage.

Exploding wire method for the characterization of dynamic tensile strength of composite materials

In the presented study a wire electric explosion method is adopted for the shock loading test of wound T700/LY113 carbon/epoxy composite. The advantage of the proposed test method is a possibility to reach strain rate ∼104 s−1, exceeding upper limit of split Hopkinson pressure bar, but matching lower limits of plate impact test. Compared to the latter test method, a promising feature is a capability to measure in-plane tensile strength of the laminated composite. To implement the proposed method, test samples were fabricated by winding of carbon fiber layers over a polymethyl methacrylate (PMMA) cylinder. Electric explosion of wire inserted in the channel inside the cylinder generates a shock wave, which propagates towards the outer composite shell, and free surface velocity history is recorded using a laser interferometer. It allows estimation of failure strain and tensile strength of the composite in circumferential direction. However, incident shock wave and reverberations are also critical for out-of-plane strength, including delamination, so finite element analysis is used to elaborate what type of failure mode is critical.

Spall response of medium-entropy alloy CrCoNi under plate impact

Dynamic mechanical properties of high-entropy alloys (HEAs) or medium-entropy alloys (MEAs) under high strain rate loading are of increasing interest for their promising applications in dynamic extreme environments. Here, spall damage of a coarse-grained CrCoNi MEA with a face-centered-cubic structure is firstly investigated by combining plate impact experiments, and molecular dynamics (MD) simulations. Yield strength, spall strength and spall-induced re-acceleration at different impact velocities are derived from free-surface velocity history measurements. In addition, soft-recovered samples after spallation are characterized with scanning electron microscopy and electron backscatter diffraction. The CrCoNi MEA exhibits the highest spall strength (around 4 GPa) among the MEAs/HEAs ever reported, with similar or better impact ductility compared to the reported MEAs/HEAs. With increasing impact velocity, the dominant damage mode of the CrCoNi MEA undergoes a gradual transition from intergranular damage, to mixed intergranular and intragranular damage, and then to intragranular damage. MD simulations reveal multiple deformation mechanisms (dislocation slip, stacking faults and twinning) for the CrCoNi MEA under impact loading, and show consistent spall damage mode transition with the experimental observations. The excellent combination of spall strength and ductility for the CrCoNi MEA is mainly attributed to the pronounced intragranular void nucleation followed by ductile void growth and coalescence, ensuring broad application prospects of the CrCoNi MEA in impact scenarios.

Shock compression and spall damage of dendritic high-entropy alloy CoCrFeNiCu

Plate impact experiments are conducted on a dendritic dual face-center cubic phase high-entropy alloy (HEA) CoCrFeNiCu, consisting of Cu-lean dendritic (DR) and Cu-rich interdendritic regions (ID). Free surface velocity histories are obtained along with microstructure characterizations. The Hugoniot equation of state and spall strength at different shock stresses are determined. Dislocation slip is the main deformation mode for CoCrFeNiCu HEA, and the dislocation density of the Cu-rich interdendritic regions increases more significantly during shock compression. Both ductile and brittle damage modes are observed. With increasing impact velocity, the Cu-rich interdendritic regions with severe strain localizations and more defects provide more damage nucleation sites. The spall strength increases firstly, reaches its maximum at a peak shock stress of 5.8 GPa, and then decreases. Molecular dynamics simulations reveal that the abnormal dependence of spall strength on peak shock stress is a result of strain localization and thermal softening in the Cu-rich regions.

Spall damage in laser-powder-bed-fusion manufactured Ti-6Al-4V: Mechanisms and microstructure effects

Spall damage of Ti-6Al-4V alloy fabricated via laser powder bed fusion (LPBF) is investigated via plate impact along the build direction (BD) and the transverse direction (TD), as regards its underlying mechanisms and microstructure effects. Conventional hot-rolled Ti-6Al-4V alloys processed with annealing and quenching are also examined for comparison. Hugoniot elastic limit (HEL) and spall strength of the LPBF-fabricated samples are derived from free-surface velocity histories, and are higher for the TD loading than for the BD loading. The LPBF-fabricated alloy has a lower HEL but similar spall strength compared to the conventionally fabricated alloys. Quantitative analysis of recovered samples reveals that, the number of cracks is smaller and the mean crack length is larger in the LPBF-fabricated and quenched hot-rolled samples containing α′ martensite laths than the annealed hot-rolled sample containing equiaxed α grains. For the LPBF-fabricated samples, the TD loading induces cracks of similar length along α′ grain boundaries (GBs) and α′{0001} planes, but much longer cracks along prior β GBs compared to the BD loading. Anisotropy in mechanical properties and damage is due to columnar prior β grains, morphological and crystallographic texture of α′ laths.

Deformation and damage of equiatomic CoCrFeNi high-entropy alloy under plate impact loading

High-entropy alloys (HEAs) are considered as potential structural materials for aerospace and defense applications where impacts are recurrently encountered. The dynamic mechanical properties and the underlying deformation and damage mechanisms are significant for safety assessment and structural design optimization, but are underinvestigated. In this work, two types of plate impact experiments, i.e., shock compression and spallation, are performed on typical quaternary CoCrFeNi HEA (at%), to investigate its dynamic mechanical properties and microscopic deformation/damage mechanisms. Free-surface velocity histories are measured to evaluate the mechanical properties and damage processes, including the Hugoniot elastic limit (HEL; ∼0.8 GPa), spall strength (∼3.2 GPa) and pullback rates. The spall strength of the CoCrFeNi HEA is higher than those of most medium- and high-entropy alloys ever reported, except for the Al0.1CoCrFeNi HEA. The deformed samples are characterized with scanning electron microscopy, electron backscatter diffraction and transmission electron microscopy. Shock-induced dislocation slip and deformation twinning dominate plastic deformation. With increasing impact velocity, dislocation density increases significantly and twin bundles appear instead of individual twins. For incipient spallation, voids nucleate preferentially at grain boundaries, especially at grain boundary triple junctions. Damage in the CoCrFeNi HEA is ductile in nature.

Shock compression and spallation of a medium-entropy alloy Fe40Mn20Cr20Ni20

Shock compression and spallation of a low-cost cobalt-free medium-entropy alloy (MEA), Fe40Mn20Cr20Ni20 (at%), with a face-centered-cubic structure are investigated via plate impact experiments, to reveal its dynamic mechanical properties and corresponding microscopic deformation/damage mechanisms. The Hugoniot equation of state, yield strength, spall strength and pullback rate are obtained from free-surface velocity histories. Post-deformation samples are characterized with transmission electron microscopy, scanning electron microscopy and electron backscatter diffraction. The Fe40Mn20Cr20Ni20 MEA exhibits a good balance in spall strength and ductility (low pullback rate) at a relatively low cost, compared to several other types of medium/high entropy alloys and steels. At sufficiently high impact velocity (e.g., 500 ms−1 here), nanoscale deformation twinning becomes a key deformation mechanism for Fe40Mn20Cr20Ni20 MEA in addition to dislocation slip. During spallation, voids (i.e., damage) nucleate preferentially at grain boundary triple junctions, and grow isotropically with increasing loading. Owing to fine grains and strong plastic deformation capability of the Fe40Mn20Cr20Ni20 MEA, void coalescence is accomplished by intragranular shear deformation bands and cracks, which contributes to its high ductility.

Deformation and damage of heterogeneous-structured high-entropy alloy CrMnFeCoNi under plate impact

Plate impact experiments are conducted on a hot-rolled, heterogeneous-structured, high-entropy alloy CrMnFeCoNi consisting of two types of domains: the UR domains containing unrecrystallized stretched grains and the FR domains containing fine recrystallized grains. Free surface velocity histories are obtained along with microstructure characterizations. Shock-induced deformation twinning is activated firstly in the UR domains, and the dislocation density of the FR domains increases more significantly during shock compression. Under similar shock stress, spall strength is the highest for loading along the rolling direction as a result of the texture-induced highest longitudinal sound velocity. Voids prefer to nucleate at the triple junctions of high angle grain boundaries in the FR domains or around the UR–FR domain boundaries. Compared to the homogeneous (annealed) structure, the heterogeneous structure leads to a negligible increase in spall strengths because of grain refinement in the FR domains and relatively severe plastic deformation around the UR–FR domain boundaries.

Molecular dynamics study on the shock induced spallation of polyethylene

Macroscopic experimental results of the plate impact tests of polymers are generally interpreted using the free surface approximation and the acoustic approximation. However, their validity over a range of shock pressures has not been thoroughly investigated yet. We conducted molecular dynamics simulations of plate impact tests of polyethylene to obtain molecular-level insights on those two common approximations associated with the interpretation of shock pressure and spall strength. Our results revealed that the free surface approximation could slightly underpredict the shock pressure in the polymer. The spall strength computed from the free surface velocity history can be significantly smaller than the actual tensile stress in the region of spallation.

Dependence of spallstrength on temperature, grain size and strain rate in pure ductile metals

When a shockwave, which can be generated by high velocity impact or explosive detonation, reflects from the free surface of a metal, it usually creates tensile stress inside the metal. While the tensile stress is large enough, voids nucleation, growth and coalescence happen inside the metal, causing the metal to spall. As one of the main contents of the spallation damage research, the spallation strength, which is often characterized by features of the free surface velocity history measured in spallation experiments, represents the maximum tensile stress that the material can withstand, and is actually a complex interaction among several competing mechanisms. Optimizing the spallation strengths of metals is important for their applications in the aerospace, automotive, and defense industries, and can be achieved by using the advanced manufacturing strategies, if we can know better the meaning and present analytic model of the spallation strength of metal. A large number of experiments show that the spallation strength of ductile metal is strongly dependent on the tensile strain rate, grain size and temperature of material. Based on the analysis of early spallation evolution and influence of grain size and temperature on the material, a simple analytic model of spallation strength is presented in this paper, which takes into account the effects of strain rate, grain size and temperature in materials. The applicability of this model is verified by comparing the calculated results from the model with the experimental results of spall strength of typical ductile metals such as high purity aluminum, copper, and tantalum.

Effects of shock-induced phase transition on spallation of a mild carbon steel

Effects of shock-induced phase transition (PT) on spallation of a mild carbon steel are investigated via plate impact experiments at impact velocities of 0.433–1.163 kms−1. The free surface velocity history is recorded to deduce spall strength and wave propagation/interaction within the sample. Postmortem samples are characterized with scanning electron microscopy and electron backscatter diffraction. Detailed wave analysis indicates that the PT affects wave propagation and interaction. Such wave propagation and interaction finally lead to the change of spall strength and spall plane position for different stress ranges. In the peak shock stress range of 13–17 GPa (slightly above the PT stress), the release fan of the PT wave accelerates the free surface but is not involved in the major spallation, giving rise to an overestimation of spall strength by the conventional acoustic method. However, at higher shock stress above PT (>17 GPa), the release fans of the PT wave interact with each other and induce secondary spallation, and the acoustic method is applicable in such cases; the major spall plane moves to the sample central region, and multiple secondary spall planes appear. In addition, the reverse PT upon release facilitates nucleation and growth of deformation twins which subsequently act as nucleation sits of voids during spallation. With increasing shock stress, the spall damage exhibits mode transitions from brittle damage (transgranular cleavage cracks), to mixed brittle and ductile damage, and to ductile damage (void nucleation and growth).

Molecular Dynamics Simulations of Shock Propagation and Spallation in Amorphous Polymers

Abstract We conducted large-scale molecular dynamics (MD) simulations of shock wave propagation and spallation in amorphous polyurethane and polyurea. First, we computed the shock Hugoniot of the polymers using the multiscale shock technique and compared them with available experimental data to establish the upper limit of the shock pressure that can be accurately modeled using a non-reactive interatomic force field. Subsequently, we simulated shock wave propagation in the polymers, varying the shock particle velocity from 0.125 km/s to 2 km/s. A remarkable similarity in the shock behavior of polyurethane and polyurea was observed. The spall strength of each sample was computed by two methods: (a) the indirect method (based on the free surface velocity history)—accessible in experiments and (b) a direct method (based on the atomic stresses in the region of spallation)—accessible only through MD. The results reveal that the tensile strength computed from the indirect method is consistently smaller than the value obtained from the direct method. Moreover, the strength computed from the indirect method shows a noticeable agreement with the fracture nucleation stress. Our results provide novel molecular-level insights into the spallation mechanisms of amorphous polymers, which could facilitate the design of polymers for structural barrier applications.