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Phase transition and deformation in cold-rolled and heat-treated Ni 50. 6 Ti 49. 4 alloys are explored by in situ multi-scale measurements with simultaneous X-ray diffraction and optical digital image correlation. Macroscale stress − strain curves, mesoscale strain fields and microscale X-ray diffraction peaks are obtained. Electron backscatter diffraction and transmission electron microscopy are also conducted on samples before and after tension as complements. For cold-rolled samples, since martensite nucleation is strongly associated with the widely distributed preexisting dislocations, homogeneous martensite phase transition takes place, in contrast with the localized mode in heat-treated samples. For localized phase transition, the intense growth of martensite results in clear Lüders bands. The beginning and the end of phase transition plateau on the stress − strain curve are associated with the initiation and complete propagation of Lüders bands. For the homogeneous phase transition, broad growth of martensite leads obscure Lüders bands and uniform strain fields, without any phase transition plateau. Given the initial {110} 〈 110 〉 and {111} 〈 110 〉 sheet textures in the heat-treated samples, different martensite variants with different preferred orientations are activated with respect to the loading directions. [ABSTRACT FROM AUTHOR]

Phase transition and deformation in cold-rolled and heat-treated Ni 50. 6 Ti 49. 4 alloys are explored by in situ multi-scale measurements with simultaneous X-ray diffraction and optical digital image correlation. Macroscale stress − strain curves, mesoscale strain fields and microscale X-ray diffraction peaks are obtained. Electron backscatter diffraction and transmission electron microscopy are also conducted on samples before and after tension as complements. For cold-rolled samples, since martensite nucleation is strongly associated with the widely distributed preexisting dislocations, homogeneous martensite phase transition takes place, in contrast with the localized mode in heat-treated samples. For localized phase transition, the intense growth of martensite results in clear Lüders bands. The beginning and the end of phase transition plateau on the stress − strain curve are associated with the initiation and complete propagation of Lüders bands. For the homogeneous phase transition, broad growth of martensite leads obscure Lüders bands and uniform strain fields, without any phase transition plateau. Given the initial {110} 〈 110 〉 and {111} 〈 110 〉 sheet textures in the heat-treated samples, different martensite variants with different preferred orientations are activated with respect to the loading directions. [ABSTRACT FROM AUTHOR]

A dual-phase high-entropy alloy Fe 50 Mn 30 Co 10 Cr 10 is treated via multi-directional forging (MDF) and subsequent annealing at 600°C–1000°C. Different microstructures with totally different grain sizes and phase ratios are obtained. The MDF increases yield strength and ultimate tensile strength at the expense of ductility. Via annealing the MDF-processed alloy at 1000°C, the full recrystallized sample yields a excellent combination of high strength and good ductility due to the grain boundary strengthening, transformation-induced plasticity and twinning-induced plasticity. The yield strength, ultimate tensile strength and elongation at failure are 16.2%, 14.2% and 35.3% than those of the as-cast sample. The postmortem characterizations of this annealed sample show strong 〈 111 〉 texture in the austenite phase along the loading direction and intense martensite transformation with the gradually disappearance of the annealing twins. [ABSTRACT FROM AUTHOR]

A face-centered cubic (FCC) non-equiatomic high-entropy alloy (HEA), Cr 10 Mn 10 Fe 60 Co 10 Ni 10 , is processed via cold-rolling and subsequent annealing at 800 − 1200 °C with different cooling methods. All the annealing treatments lead to fully recrystallized microstructures with various grain sizes and precipitate types. The mechanical properties under uniaxial tension are obtained along with microstructure characterizations of postmortem samples. The sample annealed at 800 °C followed by furnace cooling yields a reasonable combination of high strength and good ductility. The yield strength and ultimate tensile strength are 43% and 19% higher than those of the sample annealed at 800 °C followed by water quenching, with only a uniform elongation ∼ 6% reduction. The yield strength variations of all the annealed samples are explained with a structure-based strength model. After fracture, all the samples show strong 〈 111 〉 texture along the tensile direction. Besides dislocation slip, deformation twinning and multiple phase transitions contribute to high strength and effective work hardening as additional deformation mechanisms. The newly formed hexagonal close-packed and body-centered cubic phases follow the Shoji–Nishiyama and Kurdjumov–Sachs orientation relations with the FCC matrix, respectively. Given the "treatment method − microstructure − mechanical property" relationship in this study, combining the advantages of the steel and HEAs, this kind of Fe-rich HEA bears great potentials to serve under practical conditions. [ABSTRACT FROM AUTHOR]

In situ scanning electron microscope (SEM) and electron back scattering diffraction (EBSD) are used to investigate detwinning deformation of pre-twinned titanium under uniaxial tension. An electro-polishing procedure is optimized to prepare SEM speckles on the Ti samples with minimum interference with EBSD characterization. Strain fields are obtained from the SEM images via digital image correlation. The experimental results suggest that the rate of detwinning decreases with increasing plastic strain, and detwinning deformation is governed by the Schmid's law. Strain localization mainly occurs in the detwinning regions during the early deformation. As deformation proceeds, localized plastic strain tends to accumulate rapidly around the { 10 1 ‾ 2 } extension twins with a low Schmid factor. However, the strain values inside the { 10 1 ‾ 2 } extension twins are relatively low, regardless of their Schmid factors and the applied tensile strain level. [ABSTRACT FROM AUTHOR]

Dynamic compression experiments are conducted on a heterogeneous-structured CrMnFeCoNi high-entropy alloy (HEA) at different strain rates (1000–5000 s−1) and temperatures (123–293 K) with a split Hopkinson pressure bar system. Yield strength and strain hardening rate increase considerably with decreasing temperature at 5000 s−1 (low-temperature strengthening), but are insensitive to strain rate at 173 K, which can be well described by the Zerilli–Armstrong model. Compared to typical aluminum alloys and homogeneous-structured CrMnFeCoNi HEA, the heterogeneous-structured HEA exhibits higher thermal sensitivity and strain hardening rate. Microstructural characterizations show that dislocation density in the fine-grained domains increases more drastically than in the coarse-grained domains, inducing high deformation gradient between the coarse- and fine-grained domains. At 123 K, nano-scale primary and secondary deformation twins form in grain interior, along with a great number of stacking faults on twin boundaries; the twin width and spacing in coarse grains are considerably higher than in fine grains. In addition, the twin density at 123 K is significantly higher than that at 293 K. Overall, the increased deformation twinning and deformation gradient contribute to pronounced strengthening and strain hardening with decreasing temperatures in the heterogeneous-structured HEA under dynamic compression. [Display omitted] • Dynamic compression responses of CrMnFeCoNi HEA are investigated under multiple low temperatures. • Strain localization occurs preferentially in fine grains near normal grain boundaries. • Twin width and spacing in coarse grains are significantly higher than those in fine grains. • With decreasing temperatures, twin density increases, leading to increasing strain hardening. • Increased local deformation gradient at low temperatures contributes to higher HDI strengthening. [ABSTRACT FROM AUTHOR]

Dynamic equal channel angular pressing (DECAP) and heat treatment are applied to pure Ti to achieve a strength-ductility synergy. DECAP induces a complex multiscale twin structure in Ti including four primary twin types, secondary twins and tertiary twins with multiple variants, and an increased yield strength (58% higher than the as-received Ti). Subsequent annealing at 200 ℃ and 400 ℃ leads to improved ductility and strength. The yield strength (σ y) and total elongation (ɛ t) for the Ti sample processed with DECAP and 400 ℃ annealing are 555 MPa and 30%, respectively, and its σ y ɛ t product is ∼ 45% higher than the as-received Ti. DECAP and subsequent annealing lead to a superior strength-ductility synergy in twinned Ti, providing a novel approach to produce high strength and ductility materials via dynamic deformation and subsequent heat treatment. [ABSTRACT FROM AUTHOR]

The {332} 〈 113 〉 twinning in a β -type Ti-25Nb alloy is investigated quantitatively at 10% compression strain over a wide range of strain rate from 1 0 − 2 s−1 to 1 0 6 s−1, using material testing system, split Hopkinson pressure bar, and gas gun loading. Electron backscatter diffraction data are collected from the postmortem samples to obtain activated twinning systems and twinning strain tensors. With increasing strain rate, yield stress and twin density increase, and twin thickness decreases. At low strain rate, only the twinning systems with high Schmid factor can be activated to form thick twins. However, at medium and high strain rates, certain twinning activation deviates from the Schmid law, and twin thickening is relatively insensitive to Schmid factor. Twinning strain is the highest for the matrix grain orientations near 〈 114 〉 , but approximately zero for the orientations near 〈 111 〉. Contribution of twinning to plastic strain at medium strain rate is significantly higher than those at low and high strain rates. [ABSTRACT FROM AUTHOR]

The effects of loading on spall strength of a mild steel are investigated systematically under the flat-top impact loading, including the effect of tensile stress history and the effect of shock-induced microstructure. Peak stress, strain rate, pulse duration, spall strength and tensile stress history are obtained directly or indirectly from free-surface velocity measurements, and the recovered samples are characterized with electron backscatter diffraction analysis. For different tensile stress histories, spall strength depends on peak stress or strain rate, depending whether spall occurs on the plateau or rising edge of a tensile pulse. Pulse duration has an effect on whether spall occurs (incipient spallation) but not spall strength. Low shock stresses induce small microstructure change (dislocations) which has weak effects on nucleation and propagation of brittle cleavage cracks, while high shock stresses lead to deformation twins (in addition to dislocations) which act as the source of crack nucleation. A model is proposed to predict the spall strength of the mild steel under arbitrary loading conditions (peak stress below the phase transition). [ABSTRACT FROM AUTHOR]

Abstract Deformation twinning in a mild steel is investigated under quasi-isentropic compression (IC) and shock compression (SC) to about 12 GPa, as regards the effects of pulse duration (plateau width) and strain rate (rising edge slope), and associated twin strengthening. The pulse duration ranges from 80 ns to 790 ns; and two rise times are explored, approximately 30 ns for SC and 300 ns for IC. Free-surface velocity histories are measured to obtain strain and strain rate. The postmortem samples are characterized with electron backscatter diffraction, and the yield strengths of the samples pre-deformed by impact are examined with a materials testing system. For SC, twin density and size increase with pulse duration up to 580 ns. At longer pulse durations, the increase in twin density is stagnated as a result of deviatoric stress relaxation, while twin size continues growing. For IC (410 ns), twin density and size are much larger than the SC counterpart, as a result of shallower rising edge. Increased deformation time can compensate the effects of reduced strain rate or applied stress for deformation twinning. Twin strengthening effect is strong in postmortem samples, depends on twin density instead of area fraction, and follows the empirical Hall–Petch relationship. [ABSTRACT FROM AUTHOR]

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]

In situ synchrotron x-ray imaging and diffraction are used to investigate deformation dynamics in two kinds of additively manufactured AlSi10Mg with different pore configurations. The pre-existing pore configurations are characterized with synchrotron x-ray computed tomography. The specimens are subjected to tensile loading along the build direction and the longitudinal direction. Bulk stress–strain curves (macroscale), strain fields (mesoscale) and x-ray diffraction patterns (microscale) are obtained simultaneously. Scanning electron microscopy characterization is performed as a complement. For the small-pored AlSi10Mg, preferred orientations of pores lead to anisotropic ductility, and only ductile dimples are observed. For the large-pored AlSi10Mg, ductility loss is significant due to its higher porosity, and no remarkable anisotropy is found; both ductile and brittle fracture features are identified. For both types of AlSi10Mg, strain localization occurs around the pores. At the lattice scale, the Al matrix deforms plastically but the Si phase is still strained elastically, and thus, significant stress concentration occurs in the Si phase for the small-pored AlSi10Mg, while stress is more evenly partitioned between the Al matrix and the Si phase for the large-pored AlSi10Mg. [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]

Texture-induced anisotropic phase transformation of a rolled NiTi sheet was investigated via uniaxial tension and cyclic loading using in situ laboratory X-ray diffraction. The tensile tests were carried out along rolling direction (RD) and transverse direction (TD), which respectively have the highest and lowest phase transformation plateau strain. The initial preferred orientations of the rolled sheet (single austenite phase) concentrate on the ζ and γ fibers, i.e. <110>∥ND and <111>∥ND (normal direction). For loading along RD, six favorably oriented martensite variants form and lead to the formation of the new texture component {111}<112>, whereas along TD, three favorably oriented martensite variants form, making the initial {110}<110> texture component stronger. Besides, the Lüders band can be clearly observed in the first cycle of cyclic loading, during which the favorably oriented martensite variants are firstly induced, followed by the less-favorably oriented martensite variants. The martensite transformation continues after the phase transformation plateau via the propagation of the favorably and less-favorably oriented martensite variants. At high cycles, Lüders band cannot be observed due to remained traces of martensite and grain refinement. [ABSTRACT FROM AUTHOR]

We investigate deformation and damage of a Zr-based bulk metallic glass (BMG) and its Ta particle-reinforced composite (MGMC) under impact loading, as well as quasi-static tension for comparison. Yield strength, spall strength, and damage accumulation rate are obtained from free-surface velocity histories, and MGMC appears to be more damage-resistant. Scanning electron microscopy, electron back scattering diffraction and x-ray computed tomography, are utilized for characterizing microstructures, which show features consistent with macroscopic measurements. Different damage and fracture modes are observed for BMG and MGMC. Multiple well-defined spall planes are observed in BMG, while isolated and scattered cracking around reinforced particles dominates fracture of MGMC. Particle–matrix interface serves as the source and barrier to crack nucleation and propagation under both quasi-static and impact loading. Deformation twinning and grain refinement play a key role in plastic deformation during shock loading but not in quasi-static loading. In addition, 3D cup-cone structures are resolved in BMG, but not in MGMC due to its heterogeneous stress field. [ABSTRACT FROM AUTHOR]

Duplex stainless steel (DSS) with pronounced microstructural anisotropy, is shock-loaded along normal direction, transverse direction and rolling direction to investigate the effects of structural anisotropy on high strain rate deformation and damage. Hugoniot elastic limit (HEL) and spall strength are obtained from free surface velocity histories. The recovered samples are characterized with electron backscatter diffraction and x-ray computed tomography. HEL shows negligible anisotropy and is dominated by the weaker austenite phase. Spall strength shows anisotropy for incipient spallation but not for full spallation. Plastic deformation and damage depend on the mismatch in sound speed and strength between ferrite and austenite. Damage nucleates at phase boundaries, and propagates as cleavage cracks in the textured ferrite; the propagation is blocked by austenite. These factors coupled with different stress states induced by different impact velocities contribute to DSS's anisotropic or isotropic response. [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]

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]

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]

We investigate spall damage of a mild carbon steel under high strain-rate loading, regarding the effects of peak stress, strain rate, and pulse duration on spall strength and damage, as well as related microstructure features, using gas gun plate impact, laser velocimetry, and electron backscatter diffraction analysis. Our experiments demonstrate strong dependences of spall strength on peak stress and strain rate, and its weak dependence on pulse duration. We establish numerical relations between damage and peak stress or pulse duration. Brittle and ductile spall fracture modes are observed at different loading conditions. Damage nucleates at grain boundaries and triple junctions, either as transgranular cleavage cracks or voids. [ABSTRACT FROM AUTHOR]

We investigate dynamic fracture or spallation of a ternary bulk metallic glass, Zr 50 Cu 40 Al 10 , under high strain-rate (4–5×10 5 s −1 ) loading. Both incipient and full spall are achieved. Free-surface velocity histories and microstructure features of the recovered samples, such as necking, softening, microvoids, and rounded cups/cones, indicate exceptional ductility in deformation and fracture of this glass. Softening/necking is attributed to decreased glass transition temperature with increasing tension, and rounded cups/cones, to localized shear banding, void formation, and their interactions. [ABSTRACT FROM AUTHOR]

Shock compression and spallation of a low-cost cobalt-free medium-entropy alloy (MEA), Fe 40 Mn 20 Cr 20 Ni 20 (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 Fe 40 Mn 20 Cr 20 Ni 20 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 m s−1 here), nanoscale deformation twinning becomes a key deformation mechanism for Fe 40 Mn 20 Cr 20 Ni 20 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 Fe 40 Mn 20 Cr 20 Ni 20 MEA, void coalescence is accomplished by intragranular shear deformation bands and cracks, which contributes to its high ductility. • Hugoniot EOS of Fe 40 Mn 20 Cr 20 Ni 20 is obtained. • Fe 40 Mn 20 Cr 20 Ni 20 MEA exhibits good balance between spall strength and ductility. • Nanoscale deformation twinning occurs at high-velocity impact. • Voids nucleate at grain boundaries, and coalesce via intragranular fracture. [ABSTRACT FROM AUTHOR]

An extensively investigated high-entropy alloy (HEA), Al 0. 1 CoCrFeNi , is processed by dynamic equal channel angular pressing (D-ECAP) and subsequent annealing. Severe shear deformation and ultrahigh strain rate give rise to simultaneous activation of the { 111 } 〈 112 〉 macro deformation twins and kink bands with the 〈 110 〉 or 〈 112 〉 rotation axes. Secondary kink bands are also observed in primary kink bands. With increasing annealing temperature, the deformation twins and kink bands both become less or disappear. The yield strength is increased by ∼ 255% solely by D-ECAP compared to the as-cast HEA. A reasonable combination of high strength and good ductility is achieved through annealing twins in recrystallized grains via annealing the D-ECAP-processed alloy at 900 °C. It is the first observation on kink bands within a face-centered cubic HEA under various loading conditions. The strengthening in these D-ECAP-processed samples before and after annealing is discussed in terms of grain boundary strengthening and dislocation strengthening. [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]

Plate impact experiments are conducted on a representative, low-alloy, 24CrNiMo steel (containing lath martensite substructures) fabricated via laser powder bed fusion and tempering, to investigate its spall strength and damage mechanism within a peak stress range of 5–11 GPa. The Hugoniot elastic limit (2.4 GPa) and spall strengths (3.9–4.2 GPa) are obtained from free surface velocities, and show negligible anisotropy. The microstructure before and after impact loading are characterized by electron backscatter diffraction and scanning electron microscopy. The length, width and orientation angle of cracks are quantified. Spall cracks are large in quantity, but small in mean propagation distance and opening displacement, especially at high peak stress. Cracks develop mainly along the {100} cleavage planes of laths of the same variant rather than along melt pool boundaries. The mismatch between the {100} cleavage planes across the ∼ 60 ° 〈 111 〉 lath boundaries leads to high resistance of fine laths against crack propagation. The fine lath substructure is beneficial to the dynamic mechanical properties of low-alloy steels. • LPBF-fabricated 24CrNiMo exhibits isotropic and high spall strengths. • Spall damage accumulates via nucleation of microcracks. • Cleavage occurs along a {100} plane of laths of the same variant. • Dense lath boundaries promote crack nucleation but restrain crack extension. [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]

High strain-rate (up to 6600 s−1) and quasi-static compression tests are conducted on a powder-sintered porous titanium with different pore sizes (mean: 30 μ m and 120 μ m). In situ X-ray imaging is implemented to characterize the pores-scale deformation dynamics. The yield strength as a function of strain rate exhibits two stages of rate sensitivity, and the transition occurs at ∼ 1600 s−1. X-ray images show that pore compaction and strain localizations occur preferentially at pores oriented perpendicular to the loading direction under quasi-static loading, but become more random under high strain-rate loading as a result of higher driving force and plastic deformation nucleation rate. The more homogeneous nucleation of plastic deformation contributes to the increased rate sensitivity beyond ∼ 1600 s−1. At the same strain rate, the yield strength of porous Ti as well as strain field homogeneity decreases significantly with increasing pore size. The small pore spacing in fine-pored Ti reduces the degree of stress concentrations around pores. Therefore, the higher stress concentrations in coarse-pored Ti lead to an earlier yield of matrix around pores and thus a lower bulk yield strength. • Two stages of rate sensitivity reported firstly for porous Ti. • Yield strength decreases with increasing pore size. • XDIC reveals rate- and pore-size-dependent deformation heterogeneity. • Increasingly homogeneous deformation leads to strain rate effect. • Stress concentration contributes to pore size effect. [ABSTRACT FROM AUTHOR]

Interactions between slip bands (SBs) and interfaces in a compressed duplex stainless steel (DSS) are investigated regarding the roles of grain boundaries (GBs) and phase boundaries (PBs) in plastic deformation. Digital image correlation based on scanning electron microscopy, and electron backscatter diffraction are utilized to characterize SBs in ferrite and austenite phases, and the interactions between interfaces and SBs: slip transmission, slip blockage and non-intersection. Most ferrite SBs have lower local strains than austenite SBs, and exhibit weak interactions with interfaces. For austenite GBs and PBs, the strain gradient across interfaces with slip transmission is much lower than that across interfaces with slip blockage. Moreover, GBs/PBs following the twin/Kurdjumov–Sachs relationship exhibit a higher probablity of slip transmission, but a considerably lower probability of slip blockage, compared to general GBs/PBs. This is attributed to the different geometries of slip systems for different types of interfaces. Therefore, twin GBs and PBs following the Kurdjumov–Sachs relationship play a lesser role in restraining plastic flow and promoting the strength of DSS than general GBs and PBs. In addition, the effect of loading orientation is minor for the SB-GB interactions, but pronounced for SB-PB interactions. [ABSTRACT FROM AUTHOR]

We investigate deformation twinning in a highly textured magnesium alloy plate of Mg-3.1%Al-0.9%Zn-0.4%Mn under edge-on impact with in situ , synchrotron-based, ultrafast X-ray diffraction measurements, and corresponding stress states are simulated with the finite element method. Deformation twinning and its anisotropy under the triaxial stress condition are explained by a statistical analysis of resolved shear stress. The critical resolved shear stress criterion is applicable under complicated stress conditions induced by high strain rate impact loading. Three typical simple stress conditions are further explored as verification and application cases: uniaxial-stress, uniaxial-strain, and plane-stress. Extension twinning in the magnesium alloy is prone to occur for impact loading applied perpendicular to the crystallographic c -axis, regardless of the exact stress conditions. [ABSTRACT FROM AUTHOR]

The influence of second-phase boron particles (up to 0.15 wt%) on impact response of aluminum is investigated under high strain rate gas gun loading. The Hugoniot elastic limit (∼ 0.11 GPa) shows a negligible dependence on boron particle content up to 0.15 wt%. The second-phase particles reduce considerably spall strength of aluminum, and such reduction is more pronounced at high boron content. Spall strength undergoes a slight decrease at high impact velocities. Scanning electron microscopy and synchrotron X-ray computed tomography analyses indicate a competition between microvoid nucleation and microvoid growth. The damage features and spall strength characteristics can be attributed to the weak particle–matrix interface. [ABSTRACT FROM AUTHOR]

Gas-gun experiments are carried out to study the mechanical properties and fracture behavior of a textured Mg–3Al–1Zn alloy under high strain rate. The impact direction (ID) is either perpendicular or parallel to crystallographic {0001} planes, referred to as ID ∥ 〈 c 〉 and ID ⊥ 〈 c 〉 , respectively. Shock compression and spallation are achieved with different specimen-to-flyer plate thickness ratios. The Hugoniot elastic limits and spall strengths are estimated as 0.32–0.35 GPa and 0.9–0.92 GPa, respectively, showing weak anisotropy. The deformed specimens are characterized with scanning electron microscope and X-ray computed tomography. Abundant { 10 1 ̄ 2 } extension twins are activated and the strain hardening rate exhibits a sharp increase and then decrease for the ID ⊥ 〈 c 〉 specimen, while a small number of twins are observed and the hardening rate decreases monotonically for the ID ∥ 〈 c 〉 specimen. As a result of the anisotropic distribution of the β -Mg 17 Al 12 phase, cracks are generally distributed in the mid-part and connected together for the ID ⊥ 〈 c 〉 case, while the distribution of cracks in the ID ∥ 〈 c 〉 specimen is relatively discrete along the ID and parallel to the rolling plane. [ABSTRACT FROM AUTHOR]

Deformation twinning is investigated in a polycrystalline mild steel under shock compression to about 12 GPa. The free surface velocity history is measured to calculate twinning stress. The postmortem sample is characterized with electron backscatter diffraction (EBSD). We provide a method to calculate the activated twinning systems in all twinned grains from EBSD data. Twinning is completely activated and suppressed for grain size above 36 μ m and below 4 μ m , respectively. For intermediate grain sizes (14– 30 μ m), twinning tends to occur in the grain orientations near 〈 101 〉 , consistent with the Schmid factor analysis. Almost all the twins nucleate on the twinning systems with the largest or second largest Schmid factors. The twinning stress is 1.16 GPa, and critical resolved shear stress for twinning is 0.47 GPa. Twinning in weakly textured sample can induce a strong 〈 001 〉 fiber texture, as a result of its following the Schmid's law. [ABSTRACT FROM AUTHOR]