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• The spall strength of Al 0.1 CoCrFeNi HEA is improved via pre-strain. • Pre-strain smears elastic–plastic transition upon shock loading. • Pre-strain affects the spatial features of damage evolution in Al 0.1 CoCrFeNi HEA. The effects of pre-strain on spallation damage of an Al 0.1 CoCrFeNi high-entropy alloy are investigated with plate impact loading. During the pre-strain process, Kink bands and dislocations are generated in the microstructures. Pre-strain smears elastic–plastic transition upon shock loading and improves spall strength via dislocation strengthening. Although the level of pre-strain does not affect the ductile damage mode, it has a pronounced effect on damage evolution. [ABSTRACT FROM AUTHOR]

We investigate the onset of detwinning in magnesium alloy Mg-3Al-1Zn under continuous loading with real-time in situ synchrotron X-ray diffraction. Detwinning of the { 10 1 ¯ 2 } extension twins activated both by tension parallel to and compression perpendicular to the c -axis fibers is explored. The experimental results reveal that detwinning of the { 10 1 ¯ 2 } extension twins occurs immediately upon unloading, regardless of whether the twins are activated by tension parallel to or compression perpendicular to the c -axis fibers. [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]

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]

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. [Display omitted] • Impact spall experiments on CrCoNi combined with MD simulations. • The highest spall strength of CrCoNi among HEAs/MEAs ever reported. • Inter-granular to intra-granular damage transition with increasing impact velocity. • Nano deformation twinning, and dislocation-tangling induced intra-granular damage. [ABSTRACT FROM AUTHOR]

Ballistic impact experiments are conducted on a CrMnFeCoNi high-entropy alloy (HEA) using spherical steel projectiles, to investigate the penetration behavior in a wide range of impact velocities (∼ 800 to 2300 m s−1). Penetration processes are captured by high-speed camera. Postmortem samples are characterized via three-dimensional laser scanning, scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy and microhardness tests. Both depth and diameter of impact crater show a nearly linear increase with increasing impact velocity. Penetration induces increased hardness; hardness is the same near the crater bottom and the sidewall, and the peak hardness (∼ 400 Hv 1.0) is independent of impact velocity. The penetration-induced hardening is caused by multiple deformation defects including dislocations, twins and kink bands (KBs). Quantitative analysis further reveals that KB boundaries have a 5–50° misorientation angle, and their density decreases with increasing misorientation angle. KB boundary density is higher near the crater bottom than near the crater sidewall, while this trend is inverse for twinning boundary density. A finite element method model based on measured static and dynamic mechanical properties reproduces experimental observations and is used for interpreting deformation mechanisms. [Display omitted] • Both crater depth and diameter increase linearly with increasing impact velocity. • More kink bands (KBs), but less twins are observed near crater bottom than near sidewall. • KB boundary density decreases with increasing boundary misorientation from 0 to 50°. • Simulation interprets well the different deformation mechanisms in different regions. [ABSTRACT FROM AUTHOR]

Taylor impact experiments are conducted on a typical face-centered cubic single-phase high-entropy alloy (HEA) Al 0.1 CoCrFeNi, to investigate its dynamic severe plastic deformation and gradient structure. Parameters of the Johnson–Cook constitutive model are obtained, and finite element modeling based on this model is consistent with the experimental observations. A bulk gradient structure with deformation gradient is generated directly via the Taylor impact. The deformation mechanisms include the {111}〈112〉 deformation twins and kink bands with the 〈110〉 or 〈112〉 rotation axes, and vary considerably along the impact direction. Subsequent annealing at 800∘C and the impact-induced gradient in the degree of re-crystallization lead to a new bulk gradient structure with gradients in grain size and proportion of the non-recrystallized region, providing a novel approach to developing bulk gradient structured materials. • Obvious deformation gradient in Al 0.1 CoCrFeNi is generated via Taylor impact. • The bulk gradient structure with gradients in grain size is obtained by annealing. • Numerical simulations are established for Taylor impact of Al 0.1 CoCrFeNi. • {111}〈112〉 deformation twins and kink bands with 〈110〉 or 〈112〉 rotation axes are activated. [ABSTRACT FROM AUTHOR]

Ballistic impact experiments are conducted on a cast Mg-6Gd-3Y-0.5Zr alloy using spherical projectiles under a wide range of impact velocities. Penetration processes are captured by high-speed camera. Postmortem target samples are characterized via three-dimensional laser scanning, metallographic microscopy, scanning electron microscopy and electron backscatter diffraction. Shear bands and twinning dominate the deformation of Mg-6Gd-3Y-0.5Zr near the crater, and texture changes due to compression caused by cavity expansion. For instance, a finite element model based on Johnson–Cook constitutive model and Mie–Grüneisen equation of state is established. Four dimensionless quantities related to craters and projectiles are defined. An analytical model based on dynamic cylindrical cavity expansion model is applied considering projectile deformation, and consistent well with experimental and simulated results over the entire incident velocity range. [Display omitted] • Ballistic impact experiments are conducted on cast Mg-6Gd-3Y-0.5Zr alloy. • Shear bands and twins dominate deformation of Mg-6Gd-3Y-0.5Zr near the crater. • Finite element model is established and consistent with experimental results. • A cylindrical cavity expansion model is derived considering projectile deformation. [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]

• Deformation dynamics of Mg alloy is investigated with in situ X-ray multiscale diagnostics. • Deformation induced by twinning has a slowing effect on strain localization. • Strain localization in the as-received and pre-twinned samples exhibit differences. We investigate the deformation dynamics in pre-twinned Mg-3Al-1Zn alloy under uniaxial compression based on in situ synchrotron X-ray diagnostics. Stress-strain curves, strain field mapping and X-ray diffraction are achieved simultaneously. The same measurements are conducted on the as-received Mg alloy for comparison. Massive { 1 0 1 ¯ 2 } – { 1 0 1 ¯ 2 } double twinning is observed in preexisting twins of the pre-twinned specimen, and abundant { 1 0 1 ¯ 2 } extension twins are activated in the as-received specimen. The { 1 0 1 ¯ 2 } extension twinning in the as-received specimen can rapidly boost strain hardening rate by slowing down the process of strain localization. The { 1 0 1 ¯ 2 } – { 1 0 1 ¯ 2 } double twinning does not lead to significant reduction in the rate of strain localization, and thus the corresponding increase in strain hardening rate is modest. [ABSTRACT FROM AUTHOR]