Search

Abstract Plate impact experiments are conducted on a rolled titanium alloy Ti-6Al-4V to investigate the effects of structural anisotropy on dynamic deformation and damage, with the loading axis being parallel to the normal direction or transverse direction, referred to as LA ∥ ND and LA ∥ TD, respectively. Free-surface velocity histories are measured to evaluate the mechanical properties; the Hugoniot elastic limits for the LA ∥ ND and LA ∥ TD loading are 3.38 GPa and 3.75 GPa, respectively; the spall strengths are 4.54–4.71 GPa for the LA ∥ ND loading, and 4.69–4.91 GPa for the LA ∥ TD loading, for the impact velocities explored. The postmortem samples are characterized with scanning electron microscopy, electron backscatter diffraction and synchrotron X-ray computed tomography. The { 10 1 ¯ 2 } extension twins are activated near spall cracks and their number increases with increasing impact velocity. Microvoids nucleate at both grain (in the matrix) and phase boundaries, and grain boundary nucleation is more likely to occur given the small volume fraction of the β -phase. Cracks for the LA ∥ ND loading are generally perpendicular to the shock direction, while they are serrated and distributed more discretely for the LA ∥ TD loading. The damage degree for the LA ∥ ND loading is larger than for the LA ∥ TD loading, consistent with the corresponding spall strengths. At incipient spall, the average size of voids/cracks is larger for the LA ∥ ND loading than for the LA ∥ TD loading. Compared with the LA ∥ ND loading, small voids/cracks are more isotropic and large voids are more anisotropic for the LA ∥ TD loading. [ABSTRACT FROM AUTHOR]

Three texture-free CoCrNi-based medium-entropy alloys (MEAs) with similar grain sizes, CoCrNi, (CoCrNi) 94 Al 3 Ti 3 and (CoCrNi) 90 Al 5 Ti 5 , 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 L 1 2 precipitates and micron sized body-centered-cubic phase are found in (CoCrNi) 94 Al 3 Ti 3 and (CoCrNi) 90 Al 5 Ti 5 , 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) 94 Al 3 Ti 3 and (CoCrNi) 90 Al 5 Ti 5 , 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) 94 Al 3 Ti 3 and (CoCrNi) 90 Al 5 Ti 5 with small additions of Al and Ti. For spallation, damage is ductile in nature for CoCrNi and (CoCrNi) 94 Al 3 Ti 3 , but brittle for (CoCrNi) 90 Al 5 Ti 5 . Under similar shock stress, the spall strength of (CoCrNi) 94 Al 3 Ti 3 is comparable to that of CoCrNi, ∼ 150% higher than that of (CoCrNi) 90 Al 5 Ti 5 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. [ABSTRACT FROM AUTHOR]

Crystallographic orientation evolution of metals under dynamic loading conditions is of considerable interest, but rarely explored in simulations at atomistic scales. Here we present a methodology for atomic-scale orientation mapping, with atomic positions as input. The rotation matrix representing the orientation of a crystallite consisting of a central atom and its nearest neighbors, and corresponding Euler angles, are calculated, which are used for orientation analysis and visualization. As application cases, we investigate orientation evolution related to grain boundary migration, deformation twinning, deformation within grain interior, partial grain rotation, and grain refinement in representative FCC, BCC and HCP metals under dynamic loading conditions including shock loading. [ABSTRACT FROM AUTHOR]

Dynamic split Hopkinson pressure bar experiments with in situ synchrotron x-ray imaging and diffraction are conducted on a rolled magnesium alloy at high strain rates of ~5500 s −1 . High speed multiscale measurements including stress–strain curves (macroscale), strain fields (mesoscale), and diffraction patterns (microscale) are obtained simultaneously, revealing strong anisotropy in deformation across different length scales. {10 1 ¯ 2} extension twinning induces homogenized strain fields and gives rise to rapid increase in strain hardening rate, while dislocation motion leads to inhomogeneous deformation and a decrease in strain hardening rate. During the early stage of plastic deformation, twinning is dominant in dynamic compression, while dislocation motion prevails in quasi-static loading, manifesting a strain-rate dependence of deformation. [ABSTRACT FROM AUTHOR]

In situ synchrotron x-ray imaging and diffraction are used to investigate deformation of a rolled magnesium alloy under uniaxial compression at room and elevated temperatures along two different directions. The loading axis (LA) is either perpendicular or parallel to the normal direction, and these two cases are referred to as LA ⊥ 〈 c 〉 and LA ∥ 〈 c 〉 loading, respectively. Multiscale measurements including stress–strain curves (macroscale), strain fields (mesoscale), and diffraction patterns (microscale) are obtained simultaneously. Due to initial texture, { 10 1 ¯ 2 } extension twinning is predominant in the LA ⊥ 〈 c 〉 loading, while dislocation motion prevails in the LA ∥ 〈 c 〉 loading. With increasing temperature, fewer { 10 1 ¯ 2 } extension twins are activated in the LA ⊥ 〈 c 〉 samples, giving rise to reduced strain homogenization, while pyramidal 〈 c + a 〉 slip becomes readily activated, leading to more homogeneous deformation for the LA ∥ 〈 c 〉 loading. The difference in the strain hardening rates is attributed to that in strain field homogenization for these two loading directions. [ABSTRACT FROM AUTHOR]

In situ synchrotron x-ray imaging and diffraction are used to investigate anisotropic deformation of an extruded magnesium alloy AZ31 under uniaxial compression along two different directions, with the loading axis (LA) either parallel or perpendicular to the extrusion direction (ED), referred to as LA ∥ ED and LA ⊥ ED, respectively. Multiscale measurements including stress–strain curves (macroscale), x-ray digital image correlation (mesoscale), and diffraction (microscale) are obtained simultaneously. Electron backscatter diffraction is performed on samples collected at various strains to characterize deformation twins. The rapid increase in strain hardening rate for the LA ∥ ED loading is attributed to marked { 10 1 ¯ 2 } extension twinning and subsequent homogenization of deformation, while dislocation motion leads to inhomogeneous deformation and a decrease in strain hardening rate. [ABSTRACT FROM AUTHOR]

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 Al 0.1 CoCrFeNi 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. [Display omitted] • CoCrFeNi HEA demonstrates an excellent spall strength-ductility combination. • Nano-twin bundles, rather than individual nano-twins, apper at u f = 901 m s−1. • Voids nucleate at grain boundaries, especially at grain boundary triple junctions. • Heterogeneous-structured dimples reveal ductile damage in CoCrFeNi HEA. [ABSTRACT FROM AUTHOR]

Explosion-welded Ti/Al plates are characterized with energy dispersive spectroscopy and x-ray computed tomography, and exhibit smooth, well-jointed, interface. We perform dynamic and quasi-static uniaxial tension experiments on Ti/Al with the loading direction either perpendicular or parallel to the Ti/Al interface, using a mini split Hopkinson tension bar and a material testing system in conjunction with time-resolved synchrotron x-ray imaging. X-ray imaging and strain-field mapping reveal different deformation mechanisms responsible for anisotropic bulk-scale responses, including yield strength, ductility and rate sensitivity. Deformation and fracture are achieved predominantly in Al layer for perpendicular loading, but both Ti and Al layers as well as the interface play a role for parallel loading. The rate sensitivity of Ti/Al follows those of the constituent metals. For perpendicular loading, single deformation band develops in Al layer under quasi-static loading, while multiple deformation bands nucleate simultaneously under dynamic loading, leading to a higher dynamic fracture strain. For parallel loading, the interface impedes the growth of deformation and results in increased ductility of Ti/Al under quasi-static loading, while interface fracture occurs under dynamic loading due to the disparity in Poisson's contraction. [ABSTRACT FROM AUTHOR]

Impact fracture of single-crystal Si is critical to long-term reliability of electronic devices and solar cells for its wide use as components or substrates in semiconductor industry. Single-crystal Si is loaded along two different crystallographic directions with a split Hopkinson pressure bar integrated with an in situ x-ray imaging and diffraction system. Bulk stress histories are measured, simultaneously with x-ray phase contrast imaging (XPCI) and Laue diffraction. Damage evolution is quantified with grayscale maps from XPCI. Single-crystal Si exhibits pronounced anisotropy in fracture modes, and thus fracture strengths and damage evolution. For loading along [1 1 ¯ 0] and viewing along [001], ( 1 ¯ 1 ¯ 0 )[1 1 ¯ 0] cleavage is activated and induces horizontal primary cracks followed by perpendicular wing cracks. However, for loading along [01 1 ¯ ] and viewing along [111], random nucleation and growth of shear and tensile-splitting crack networks lead to catastrophic failure of materials with no cleavage. The primary-wing crack mode leads to a lower characteristic fracture strength due to predamage, but a more concentrated strength distribution, i.e., a higher Weibull modulus, compared to the second loading case. Moreover, the sequential primary cracking, wing cracking and wing-crack coalescence processes result in a gradual increase of damage with time, deviating from theoretical predictions. Particle size and aspect ratios of fragments are discussed with postmortem fragment analysis, which verifies fracture modes observed in XPCI. [ABSTRACT FROM AUTHOR]

Shock response of Cu/graphene nanolayered composites is investigated with molecular dynamics simulations, including deformation and spall damage of Cu, and delamination of the nanolaminates, as well as wrinkling, fracture and perforation of graphene, for parallel and normal shock loading. The Cu (111)/graphene interface is the source of dislocations in Cu and barrier to their propagation, and nucleation sites follow the Moiré pattern. Direct transmission of dislocation across graphene is not observed. Spallation occurs at the Cu/graphene interface with a reduced tensile strength. Under strong shock loading, graphene forms wrinkles and folds upon compression, and fractures upon release and tension for parallel shocks, while it is perforated by Cu atoms for normal shock loading via defect formation in graphene at high temperatures. [ABSTRACT FROM AUTHOR]

Dynamic tensile experiments are conducted on 15% and 30% in weight percentage B 4 C/Al composites with a split Hopkinson tension bar, along with high-speed synchrotron x-ray digital image correlation (XDIC) to map strain fields at μm and μs scales. As manifested by bulk-scale stress–strain curves, a higher particle content leads to a higher yield strength but lower ductility. Strain field mapping by XDIC demonstrates that tension deformation and tensile fracture, as opposed to shear and shear failure, dominate deformation and failure of the composites. The fractographs of recovered samples show consistent features. The particle-matrix interfaces are nucleation sites for strain localizations, and their propagation and coalescence are diffused by the Al matrix. The reduced spacing between strain localization sites with increasing particle content, facilitates their coalescence and leads to decreased ductility. Designing a particle-reinforced, metallic-matrix composite with balanced strength and ductility should consider optimizing the inter-particle distance as a key parameter. [ABSTRACT FROM AUTHOR]

We investigate deformation and damage of a magnesium alloy, AZ91, under high strain rate ( ∼ 10 5 s − 1 ) loading via planar impact. The soft-recovered specimens are examined with electron back-scatter diffraction (EBSD). EBSD analysis reveals three types of twinning: { 10 1 ¯ 2 } extension, { 10 1 ¯ 1 } contraction, and { 10 1 ¯ 1 } – { 10 1 ¯ 2 } double twinning, and their number density increases with increasing impact velocity. The extension twins dominate contraction and double twins in size and number. Dislocation densities of the recovered specimens are evaluated with x-ray diffraction, and increase with increasing impact velocity. X-ray tomography is used to resolve three-dimensional microstructure of shock-recovered samples. The EBSD and tomography results demonstrate that the second phase, Mg 17 Al 12 , plays an important role in both deformation twinning and tensile cracking. Deformation twinning appears to be a common mechanism in deformation of magnesium alloys at low, medium and high strain rates, in addition to dislocation motion. [ABSTRACT FROM AUTHOR]

Dynamic compression experiments are conducted on micron-sized SiC powders of different initial densities with a split Hopkinson pressure bar. Digital image correlation is applied to images from high-speed X-ray phase contrast imaging to map dynamic strain fields. The X-ray imaging and strain field mapping demonstrate the degree of heterogeneity in deformation depends on the initial powder density; mesoscale strain field evolution is consistent with softening or hardening manifested by bulk-scale loading curves. Statistical analysis of the strain probability distributions exhibits exponential decay tail similar to those of contact forces, which are supposed to lead to the grain-scale heterogeneity of granular materials. [ABSTRACT FROM AUTHOR]

Spall damage of two cast TiNb binary alloys, Ti 75 Nb 25 and Ti 50 Nb 50 , is investigated under Taylor-wave loading at different impact velocities. The dynamic yield stresses are similar for both alloys (∼ 1.2 GPa). The incipient spall strength of Ti 75 Nb 25 (3.2 GPa) is considerably lower than that of Ti 50 Nb 50 (4.3 GPa). The damage in Ti 75 Nb 25 is ductile in nature, and abundant { 332 } 〈 113 〉 deformation twins are activated. For Ti 50 Nb 50 , only a small amount of { 112 } 〈 111 〉 deformation twins are observed, and cracks are along grain boundaries or along the {112} planes in a grain interior, leading to the typical brittle spall failure. Spall strength, and deformation and damage modes of TiNb binary alloys depend on chemical compositions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: Fracture initiation toughness, fracture energy, fracture propagation toughness, and fracture velocity are key dynamic fracture parameters. We propose a method to simultaneously measure these parameters for mode-I fractures in split Hopkinson pressure bar (SHPB) testing with a notched semi-circular bend (SCB) specimen. The initiation toughness is obtained from the peak load given dynamic force equilibrium. A laser gap gauge (LGG) is developed to monitor the crack surface opening displacement (CSOD) of the specimen, from which the fracture velocity and the fracture energy can be calculated. The feasibility of this methodology for coarse-grained solids is demonstrated with the SHPB-SCB experiments on Laurentian granite. [Copyright &y& Elsevier]

[Display omitted] • In situ CT maps 3D pore structures. • Pore morphology evolution quantified by pore network model. • Throat number and size decrease from the elastic stage. • Pore orientation affects stress concentration and its collapse. In situ , synchrotron-based micro computerized tomography is used to investigate three-dimensional (3D) deformation dynamics of open-cell porous titanium under uniaxial compression. The 3D pore structures are captured, segmented, and transformed into separated pores (spheres) connected by throats (bars) according to the pore network model (PNM). Pore and throat size distributions are quantified via the PNM, while pore shape and orientation distributions, via gyration tensor analysis. After yield, the mean values of equivalent diameter, sphericity and aspect ratios of the pores decrease as deformation progresses, indicating increased pore anisotropy. However, the number and equivalent diameter of the throats decrease even at the elastic stage, and the average diameter decreases approximately linearly with increasing bulk strain. The collapse of individual pores is tracked, and the pores with their longest axes aligned perpendicular to the loading direction are prone to compaction compared to those aligned parallel to the loading direction, because of different stress concentrations. Microstructure-based finite element (FE) analysis reproduces the overall deformation characteristics observed in the experiments. The deformation mechanisms and FE model may be useful for guiding material assessment and design related to porous Ti. [ABSTRACT FROM AUTHOR]

We investigate deformation dynamics in a textured magnesium alloy under uniaxial tension and compression along two different directions. The loading axis (LA) is either parallel or perpendicular to the normal direction (ND), referred to as LA ∥ ND tension/compression and LA ⊥ ND tension/compression, respectively. Stress − strain curves are measured simultaneously with X-ray diffraction and strain field mapping via synchrotron X-ray digital image correlation. Massive { 10 1 ̄ 2 } extension twins are observed in the LA ∥ ND tension and LA ⊥ ND compression specimens. The growth rate of tension-induced twins is lower than that of compression-induced twins, likely because the pyramidal 〈 c + a 〉 slips accommodate partial deformation along the c -axis in the LA ∥ ND tension specimen. Both the tension- and compression-induced deformation twins can slow down the process of strain localization, and thus an increase in strain hardening rate as well as the anisotropy in plastic deformation. The plastic strain ratios depend on strain localization and the relative orientation of the loading direction with respect to the basal texture, and are much larger in tension than in compression. [ABSTRACT FROM AUTHOR]