Ultra-high Strain Rate Research Articles

Phase reversion mediated the dual heterogeneity of grain size and dislocation density in an equiatomic CrCoNi medium-entropy alloy

An ultra-high strain rate (104 s-1) dynamic plastic deformation treatment at liquid nitrogen temperature (LNT-DPD) followed by annealing is carried out to obtain dual heterogeneity of grain size and dislocation density in an equiatomic CrCoNi medium entropy alloy (MEA). Such extreme loading conditions resulted in extensive phase transformation in this MEA. Subsequent annealing at 650°C for 1 h further induced reverse phase transformation and partial recrystallization, forming a complex heterogeneous microstructure characterized by nested trimodal grain sizes and partitioned dislocation density. A superior yield strength of ∼800MPa and a good ductility of ∼40% were simultaneously achieved in this heterogeneous alloy. In order to reveal the effects of grain size and dislocation density distributions on the mechanical property improvements, the underlying deformation mechanisms were systematically discussed. High density of geometrically necessary dislocations (GNDs) would be induced in complex heterogeneous structures during tensile deformation due to strain gradients or partitioning between different regions, which would lead to additional strengthening and work hardening. These results provide a novel approach to overcome the strength-ductility trade-off dilemma for FCC-structured MEAs.

Parameter Optimization for Laser Peen Forming on 6005A-T6 Aluminum Alloy Plates to Enhance the Constrained Deformation of Integral Stiffened Plates.

Multiscale parameter optimization for laser peen forming (LPF) on 6005A-T6 aluminum alloy plates was conducted through a combination of simulation and experimentation. By obtaining the optimal parameter, this study aims to explore the constrained deformation and forming laws of the integral stiffened plates. Detailed descriptions were provided regarding the dynamic response process and transient behavior of aluminum alloy plates under ultrahigh strain rates, along with an in-depth analysis of the stress evolution. The results reveal that laser beam diameter and laser beam energy can achieve large range forming, while the number of tracks facilitates the precise deformation adjustment. During the 12-track LPF process, there is an overall upward trend in deformation values accompanied by a dynamic increase in the bend curvature. After static relaxation, the deformation value recovers to 55.2% of the final bending curvature. The chord direction scanning of stiffened plates exhibits a larger bending curvature, indicating its greater forming capacity for large-sized single unfolding direction formation; whereas, the unfolding direction scanning of stiffened plates excels in achieving efficient integrated two-way forming.

Research on Dynamic Mechanical Behavior of Reinforcing Steel Under Ultrahigh Strain Rate

The strain rate effect significantly influences the dynamic mechanical behavior of reinforcing steel under impact loads. To investigate the effect of strain rate on the dynamic mechanical behavior of reinforcing steel, this study conducts dynamic compression tests on four types of reinforcing steel using a Split Hopkinson pressure bar. The results show that there is a significant strain rate effect on the reinforcing steel when the strain rate reaches 103–104 s−1. The experimental results are analyzed using the Johnson–Cook model (J–C model), and the original model is modified based on the error between the model predictions and the experimental results. The results indicate that the modified J–C model, which includes an enhanced strain rate effect term, better predicts the dynamic mechanical behavior of reinforcing steel at ultrahigh strain rates.

Hydrogen susceptibility of Al 5083 under ultra-high strain rate ballistic loading

Abstract The effect of hydrogen on the ballistic performance of aluminum (Al) 5083H131 was examined both experimentally and numerically in this study. Ballistics tests were conducted at a 30° obliquity in accordance with the ballistic test standard MIL-DTL-46027 K. The strike velocities of projectiles were ranged from 240 m s−1 to 500 m s−1 level in the room temperature. Electrochemical hydrogen charging method was utilized to introduce hydrogen into material. Chemical composition of material was analyzed using energy dispersive X-ray (EDX) analysis. Instant camera pictures were captured using high-speed camera to compare H-uncharged and H-charged specimen ballistics tests. The volume loss in partially penetrated specimens were assessed using the 3D laser scanning method. Microstructural examinations were conducted utilizing scanning electron microscopy (SEM). It was observed that with the increased deformation rate, the dominance of the HEDE mechanism over HELP became evident. Furthermore, the experimental findings were corroborated through numerical methods employing finite element analysis (FEM) along with the Johnson–Cook plasticity model and failure criteria. Inverse optimization technique was employed to implement and fine-tune the Johnson–Cook parameters for H-charged conditions. Upon comparing the experimental and numerical outcomes, a high degree of consistency was observed, indicating the effective performance of the model.

Two-stage heat dissipation in plastic deformation of metals under ultra-high strain rate deformation

Understanding plastic heating at high strain rates is essential for designing material behavior. We employ molecular dynamics to analyze plastic heating and introduce a refined Taylor-Quinney coefficient (TQC) calculating approach. Our results reveal a two-stage heating behavior in high-strength systems, a relaxation stage characterized by rapid heating, stress relaxation, and increasing TQC, followed by a steady-state stage marked by linear temperature increase, constant stress, and stable TQC. The dislocation movement may behave as a plastic heating converter. During the relaxation stage, defect/dislocation nucleation dominates, leading to an increase in TQC as defect density rises upon yielding. Despite an increasing portion of plastic work being stored in defects, the data suggest that dislocations primarily act as sources of heat. In the steady-state stage, an unusual scenario emerges where defect density decreases as plastic deformation proceeds, yet TQC continues to increase due to the release of stored energy. This phenomenon is potentially explained by the annihilation of dislocation dipoles moving towards each other on the same slip plane. Additionally, TQC remains relatively insensitive to variations in temperature and strain rate over a broad range, challenging prior assumptions and expanding the understanding of plastic heating mechanisms.

Shock-induced phase transition and damage in nano-polycrystalline graphite affected by grain boundaries

Dynamic structural response of nano-polycrystalline graphite under shock compression is investigated using molecular dynamics (MD) simulations. Hugoniot data shows that the structural transition is activated at shock pressure P∼30 GPa (experimental range, 20–50 GPa), resulting in the formation and extension of hexagonal diamond nuclei along grain boundaries, embedded incoherently among thin-graphite grains. As P increases from 130 GPa, the structure starts to liquefy, accompanied by a decrease in shear stress τ from approximately 5.3 GPa, and completely liquefies at P∼250 GPa (melting pressure of graphite, 180–280 GPa) and τ ∼ 0 GPa. In ultrahigh-pressure region, a two-wave structure is generated consisting of an elastic shock wave and a phase transition wave, and when the piston velocity exceeds 5.2 km/s, the latter wave can catch up with the elastic one, eventually becoming a single over-driven wave. During the relaxation of compressed nano-polycrystalline graphite, void nucleation inside the sample induces the initiation of visible cracks when piston velocity is higher than 1 km/s. At low piston velocities, the cracks propagate gradually along grain boundaries due to shear-slip effects. While at high piston velocities, direct spall of the nano-polycrystalline graphite makes it into multiple fragments by ultrahigh strain rate tensile forces. This study provides a useful guide to the structural transition and dynamic damage evolution of nano-polycrystalline graphite under shock compression.

A dynamic response prediction of ultra-high strain rates of composite materials based on a surrogate model

Based on the propagation of laser-induced tensile waves in materials and the principle of material ply cracking, nondestructive testing technology for measuring the interfacial bonding force of composite materials can be developed. The study of the ultra-high strain rate dynamic response model of composite materials under the action of laser shock waves is crucial to this technique. Therefore, this paper proposes a method for constructing a dynamic response prediction model for composites based on data and a physical model driven by comprehensively considering the strain rate effect of materials. The constructed model can accurately predict the peak velocity and duration of the initial pulse of the velocity curve under the condition of ultra-high strain rate. The relative error between the predicted and simulated peak values is within 3%, and the relative error between the predicted and simulated initial pulse durations is within 5%. In addition, laser impact experiments were conducted on specimens of various thicknesses to validate the accuracy of the predicted data at ultra-high strain rates. The results show that the relative errors between the predicted and experimental peak values are all within 2%, and the relative errors of the initial pulse duration are within 9%.

Study on crystal to amorphous transition of nickel-based alloy processed by ultra-short pulsed laser shock peening

The main purpose was to investigate the process of crystal to amorphous transition in nickel-based alloy subjected to ultra-short pulsed laser shock peening. A molecular dynamics model of laser shock peened nanopolycrystalline nickel-based alloy was established using LAMMPS. The influence of shock velocity on the amorphous transformation in nickel-based alloy was analyzed. The results indicated that ultra-short pulsed laser shock peening with ultra-high strain rate plastic deformation promoted the increase and accumulation of defect for nickel-based alloy, contributing to the formation of amorphous structures.

Modeling of strain hardening behaviors of 6061 aluminum alloy considering strain rate and temperature effects

This paper focuses on the plastic strain hardening behaviors of 6061 aluminum alloy (AA6061), considering its dependence on strain rate and temperature. Quasi-static tensile tests were conducted over a wide temperature range (20 °C-300 °C), and the digital image correlation technique was used to capture the deformation process of specimens. Additionally, the uniaxial compressive mechanical response of AA6061 was tested utilizing quasi-static compression and split Hopkinson pressure bar experiments over a wide range of strain rates (0.001 s−1–6000 s−1). Experimental results showed that the flow stress decreased and increased nonlinearly with temperature and strain rate, respectively. A modified Johnson–Cook (MJC) model was developed by incorporating a temperature-dependent strain softening term, along with corrections to the temperature and strain rate product terms, to reproduce the nonlinear dependence. Furthermore, the MJC model was proven to be able to accurately reproduce the deformation of the specimens tested in Taylor impact experiments (strain rates up to 105 s−1), suggesting its effectiveness for interpolating and extrapolating stress-strain relationships. This model provides applicable modeling for materials with nonlinear rate sensitivity and temperature sensitivity, and it is expected to be applied in engineering problems involving ultra-high strain rates.

Surface gradient structures in single-crystal nickel alloy induced by ultrasonic-assisted high-speed grinding

The gradient structures in surface layer of single-crystal nickel alloy (SX alloy) were produced (i.e., nanograin, submicron grain and coarse single crystal of bulk material) by means of ultrasonic-assisted high-speed grinding (UAHSG). The surface layer with a grain size gradient (from 29.7 to 111.8 nm) along depth from the treated surface featured impressive nano-indentation hardness (up to 12.1 GPa) beyond the initial SX alloy. The coupled effects of high rotation speed of abrasive grain (100 m/s) and the severe ultrasonic vibration impact (up to 4.3 times per contact cycle) induced large strain (up to 6.67) and ultrahigh strain rate (∼109 s−1), resulting in the superiority of such a structure gradient. This study suggests a new approach of UAHSG that can not only achieve high machining efficiency, but also make nickel alloy stronger by tailoring the surface layer gradient structures.

Terahertz characterization of combined pressure-shear shock loaded aromatic polyurea

Realistic shock loading scenarios imply combined, multiaxial stress states, where the mechanically induced energy competes for failing the material intrinsically due to cohesive failures. Under shock loading, failure encompasses various modes, including spallation, shear bands, microcracking, morphological alteration, phase transformation, and plastic deformation. However, elucidating failure in high strain loading conditions challenges the state-of-the-art characterization due to the disparity interfacing different loading and measurement techniques. This research used a novel spectro-mechanical characterization setup, integrating bulk terahertz spectroscopy synchronized in real-time with mixed-mode laser-induced shock waves. The latter was used to submit polyurea elastomers to concurrent pressure and shear waves by saddling a thin polymer sheet on the hypotenuse of a glass prism. Hence, the laser-induced pressure shock waves underwent simultaneous mode conversion at the glass-polyurea interface at the hypotenuse of the glass prism, effectively submitting the sample to tunable amplitude pressure-shear shock waves. The terahertz spectroscopy, operating in the right-angle reflection configuration, was modified to probe predetermined spots on the polyurea samples at different stress states (adjusted by changing the illumination energy) in a three-step sequence: before, during, and after shock loading. The shock-loaded polyurea samples were also characterized using the optical and scanning electron microscopes, revealing compression-shear and tensile-shear failure modes, including surface depression, nucleation of micro-voids and microcracks, and formation of small planes. The induced stress state based on the illumination energy was quantified using an accompanying finite element simulation, resulting in completing the quasi-isentropic behavior of polyurea elastomers, which was found to be in good agreement with previous experimental data from the plate impact experiments and the predictions based on the Lennard-Jones potential. Time domain analysis of terahertz data, using temporal shifting, hysteresis, and dynamic time warping based on the sum of the Euclidean distances between signal pairs, affirmed the microscopy observations while shedding insights on the elastic and plastic deformations and the reversible conformational changes. Irreversible changes to the macromolecule were also detected using the terahertz data spectral analysis. The current results establish a new experimental framework for fundamentally characterizing polymers and their composites at ultrahigh strain rate loading conditions.

Development of a velocity measurement method for a microparticle projectile and high-speed impact testing of metallic materials for grain refinement

Since grain refinement, such as the nano-twinning and nano-crystallization of metallic materials, can improve mechanical properties, surface modification to generate nano-structures is an in-demand technology. This study aims to conduct ultrahigh velocity impact testing using microparticle projectiles to generate indentation craters via impact. Due to the resulting extreme plastic deformation with high strain rate, grain refinement is expected to occur. This study conducts the laser-induced particle impact test (LIPIT), in which laser ablation induces supersonic ejection of a microparticle, resulting in the generation of an impact crater. We first propose a method for measuring particle projectile velocity using a mechanoluminescent (ML) material. A particle collision against the ML material emits light as a result of the mechanical stimulus. The light is detected to determine the timing of particle arrival, thus, the particle projectile velocity is obtained. This measurement method is verified using a high-speed video camera. Next, LIPIT is applied to pure copper (Cu) and pure iron (Fe), thus producing nano-structures including twinning around the impact indentation craters. To clarify this mechanism, a molecular dynamics (MD) simulation is conducted to reveal the formation of nano-twinning around the impact craters. In addition, nano-indentation testing is conducted, indicating plastic hardening around the impact craters unlike the craters obtained using quasi-static indentation. This indicates that plastic deformation due to LIPIT improves mechanical properties, owing to the extreme plastic strain and strain rates involved. Therefore, our LIPIT has potential as a new surface modification technique to induce extreme plastic deformation with ultrahigh strain rates, resulting in grain refinement.

Impact and adhesion mechanics of block copolymer micro-particles with a silicon substrate

Deformation of a two-phase block copolymer (BCP) during high velocity impacts is studied experimentally and theoretically with an aim to use this material in cold spray (CS) additive manufacturing. Micron scale (10–20 μm) spherical particles of polystyrene-block-polydimethylsiloxane (PS-b-PDMS) are impacted on a silicon substrate by using a laser-induced projectile impact test (LIPIT) setup with impact velocities in the range of 50–600 m/s. Experiments indicate that polymer particles adhere to the substrate when their impact velocities fall within the range of 140–500 m/s. A constitutive model that accounts for the effects of both strain rate and temperature on the mechanical behavior of such materials is developed. A critical energy release rate function which depends on the surface temperature and rate of separation is formulated and used in a cohesive zone model (CZM) to model bonding of the BCPs on the substrate. The model parameters are calibrated by comparing the deformed and computed deformed particle shapes and coefficient of restitution values of the rebounding particles. Simulations show that the particles experience ultra-high strain rates (>104 s−1), large deformation, and temperature elevation due to plastic dissipation and interfacial friction. The outer rim of the contact interface is predicted to experience temperature levels above the glass transition temperature of the PS-domain of the BCP. Bonding is correlated with increase of contact area, plastic dissipation and temperature rise in the interface.

Dynamic shear behavior of forged CrMnFeCoNi high entropy alloy using hat-shaped specimens over a wide range of temperatures and strain rates

Dynamic shear deformation generally occurs in machining, punching and other manufacturing processes, so it is of key significance to study the shear properties of materials under high strain rate for guiding machining and realizing the application of materials. In this work, the dynamic shear behavior of forged CrMnFeCoNi high entropy alloys at different temperatures and strain rates was studied by using hat-shaped specimens. Dynamic loading at a wide temperature range (-100–650 °C) and ultra-high strain rate (5 × 105 s−1) was achieved. To investigate the influence of specimen geometry on shear properties, the shear widths (the horizontal width between the upper hat portion and the lower brim portion of the specimen) were set as 50 μm,100 μm, 200 μm and 400 μm, respectively. The results showed that the adiabatic shear was a precursor to failure, and the resistance to adiabatic shear localization of this alloy was related to the strain rate and geometric effect. The shear strength decreased gradually with the increase of temperature. The microstructure of the shear zone was characterized. The central ASB was composed of nanograins with the size of 200–300 nm, containing nano-twins and dislocations. The hardness of the central ASB was 70 % higher than that of the matrix due to the strengthening effect of fine grains. The grains in the transition region were deformed along the shear direction to form slender sub grains containing a large number of dislocations and nano-twins, resulting in strain hardening. The cracks propagated along the ASB, and the ductile fracture characterized by dimples was the main fracture mode.

Dual protection against electrochemical corrosion for AA5083 Al alloys: Synergistic action of superhydrophobicity and strain-strengthening

A new laser compound process, namely, laser shock peening of the surface to obtain the strain-strengthening layer (under the ultrahigh strain rates of >107 s−1), followed by laser surface texturing to achieve the superhydrophobicity on the strengthened surface, is proposed to provide dual protection against electrochemical corrosion for AA5083 Al alloys. The surface morphology, microstructure, residual stress and surface wettability of AA5083 Al alloy after laser compound processing were investigated. The corrosion experiments were performed by an electrochemical station. Results indicate that LSP-induced strain-strengthening layer with grain refinement and high-density dislocations accompanied by compressive residual stress on the surface. Subsequent LST produced surface superhydrophobic structures with an SCA of 155.95°. The electrochemical corrosion experiments displayed that the superhydrophobic surface works as the first protection to prevent the corrosive medium from adsorbing. With the failure of the defense function of superhydrophobic structures, the following refined grains and high-density dislocations in the strain-strengthening layer are preferred to generate protective passive films. Besides, compressive residual stress retards the permeation of Cl− anions, greatly delaying the breakdown of passive films. For the Al alloy immersed in 3.5 % NaCl solution for 48 h, the LSP-LST specimen exhibited the lowest Icorr and the highest Ecorr and Epit, with the value of −1.304 V, −0.662 V, and 50.71 μA/cm2, which exhibited the greatest corrosion properties.

Microstructural Study of Cold-Sprayed CoCrFeNiMn High Entropy Alloy

The rapid development of cold spraying technology for additive manufacturing of engineering components has made it a viable option for developing thick deposits from high-entropy alloys (HEAs). The microstructure of cold-sprayed CoCrFeNiMn deposit was investigated in this study using electron backscattered diffraction, scanning electron microscopy, and finite element analysis (FEA). The limited studies on the impact deformation behavior of the HEA during cold spraying, limiting our understanding of impact phenomena, and interactions between the HEA particles under ultra-high strain rate deformation motivated this study. From the microstructural characterization, heterogeneous microstructure appears to be formed in the cold-sprayed HEA deposit, comprising of equiaxed ultrafine grains at the particle–particle interfacial regions and coarse grains at the particle interiors. The FEA reveals large strain (> 250%) and temperature (> 90% of the alloy solidus temperature), mainly at the splat’s interfaces. Adiabatic shear instability and rotational dynamic recrystallization resulting from heat accumulation and high strain are believed to be responsible for these observations during the ultra-high strain rate deformation of the HEA. The large deformation and grain refinement experienced by the HEA resulted in greater deposit hardness when compared with the sprayed powder, with the nanohardness increasing from 1.16 GPa in the powder to 5.14 GPa in the deposit. This study explores and provides an understanding of the deformation behavior of the HEA and the resulting microstructure during cold spraying.

Fatigue crack initiation and growth of laser shock peened 2Cr13 martensitic stainless steel as a function of the coverage layer

Fatigue properties and plastic deformation-induced gradient nanostructures of 2Cr13 martensitic stainless steel subjected to laser shock peening (LSP) were investigated. Combining the residual stresses and microstructural evolution of 2Cr13 martensitic stainless steel before and after three-point bending fatigue test, the influences of LSP on fatigue crack initiation and growth are systematically analyzed. Results showed that massive LSP treatments with one and two coverage layers caused obvious improvements of 13% and 37.6% in the fatigue crack initiation life, 19.3% and 34.9% in the fatigue lives of 2Cr13 stainless steel, respectively. Typical in-depth microstructures for two kinds of LSP-treated specimens were described by transmission electron microscopy characterization. Results showed that dynamic recrystallization (DRX) process at room temperature may be activated and accelerated by LSP-induced atom collision, and then a nucleation equation by ultrahigh-strain-rate plastic deformation was developed, which depended strongly on LSP-induced ultrahigh strain rate. Moreover, the basic improvement mechanism of LSP on fatigue properties was analyzed in detail.

Numerical and experimental studies on specimens with integrated pulse-shaper used for the instrumented Taylor impact test to measure stress-strain curves at high rates of strain

In the previous studies, an instrumented Taylor impact test was established to measure the stress-strain curve at an ultrahigh strain rate. To calculate the axial stress distribution in the specimen, a bi-linearity of the internal force distribution in both the elasticity- and plasticity-dominant regions was assumed. However, the unavoidably generated excessive local deformation (ELD) near the impact surface results in early unloading and a strong nonlinear distribution of the internal force in the region near the impact surface. Consequently, the measurable stress-strain curve was limited to 0.1 of the axial strain. On the other hand, the pulse-shaping technique, which is widely used in split Hopkinson pressure bar tests, can be employed to increase the strain until the stress-strain curve is measurable through Taylor impact tests because the short rise time is considered to be a possible reason of ELD generation. For the first time, this study introduced the pulse-shaping technique to the instrumented Taylor impact test to expand the measurability of the stress-strain curve to a higher strain range. To suppress the ELD, the dimensions of the pulse shaper are determined through finite element analyses after a discussion of the mechanism to suppress ELD. Then, the proposed method with a newly designed specimen is validated conducting the real Taylor impact test.

On adiabatic shear instability in impacts of micron-scale Al-6061 particles with sapphire and Al-6061 substrates

Impact mechanics of micron-scale, Al-6061 particles on sapphire and Al-6061 substrates were investigated experimentally and through simulations. Particle impact experiments were carried out by using the laser induced projectile impact test (LIPIT) and the coefficient of restitution was evaluated for impact velocities in the range of 50–950 m/s. Bonding was observed only in Al-on-Al impacts with impact velocities over 810 m/s. A continuum model of the impacts was implemented by using commercially available finite element software. The bilinear Johnson–Cook model was implemented to model the material flow effects in the ultra-high strain rate (UHSR) regime. A cohesive zone model was used for the bonding along the particle–substrate contact interface. Heat generation due to plasticity and friction, and material damage were also included in the simulations. A quantitative measure of adiabatic shear instability (ASI) was implemented into the modeling framework to search for material points that experience drop of strength during loading. A small volume of the material internal to the particle was found to experience ASI in Al-on-sapphire impacts, but this was not observed in Al-on-Al impacts. However, if the flow rule is switched to one that neglects the effects of the UHSR-regime, ASI can be predicted in regions of the particle where the finite element mesh deforms excessively. It is recommended that any study searching for ASI in high-velocity impact simulations should assess the mesh distortion carefully and use a flow rule that includes UHSR effects.

Dynamic compression behavior of TiZrNbV refractory high-entropy alloys upon ultrahigh strain rate loading

In this study, the dynamic compressive response behavior of a body-centered cubic (BCC) single-phase TiZrNbV refractory high-entropy alloy (RHEA) was investigated under impact at speeds of 313–1584 m s–1 using two-stage, gas-gun-driven, high-speed plate-impact experiments; recovery sample analysis; and theoretical calculations. The strain rate and pressure were approximately 107 s−1 and 5.07–29.37 GPa, respectively. The results showed that the TiZrNbV RHEA had a Hugoniot elastic limit of 4.12–5.86 GPa and a spall strength of 1.84–2.03 GPa. The initial yield strength of the alloy showed a strong strain-rate dependence and could be described by the modified Zerilli–Armstrong model, while the phonon-damping effect was the main reason for its high strain-rate sensitivity. Microstructural analysis showed that the dynamic deformation of the TiZrNbV RHEA was controlled by the dislocation slip, dislocation proliferation, intersection of the deformation bands, and grain refinement. The analysis also showed that the intergranular, transgranular, and mixed-type cracks dominated the spall failure of the material. The dynamic Hall–Petch effect and pinning from the lattice distortion led to high dynamic yield strength. The critical strain rate for the phonon drag effect was positively related to the relative atomic mass and local strain field of the metals. Within the experimental loading range, the RHEA showed good structural stability, and simultaneously, the theoretical calculation method for the equation of state based on a cold-energy mixture could accurately predict its shock-response behavior. The valence-electron concentration (VEC) had a direct effect on the shock-compression properties of the HEAs; higher VEC implied more difficulty in compressing the HEAs. The findings of this study provide insights into understanding the mechanical response characteristics of RHEAs under extreme conditions such as high-speed impact and ultrahigh strain-rate loading.