High Strain Rate Deformation Research Articles

Mantle fragmentation and incomplete core merging of colliding planetesimals as evidenced by pallasites

Main group pallasites were likely formed by the collision of their parent body with a smaller impactor that caused a mixing of mantle and core material from the target and impactor, respectively. In order to better constrain the collision, we present particle size distribution (PSD) analyses of olivines in seven main group pallasites and of the Eagle station pallasite. The PSDs of two fragmental pallasites (Admire and Huckitta) contain linear segments in bi-logarithmic particle number versus size diagrams that are similar to terrestrial cataclasites or impact-related rocks. On the other hand, the PSDs of four angular pallasites only display linear segments above their respective average grain sizes. We argue that fragmental pallasites record brittle rock deformation close to the impact site of the collision, while angular pallasites represent deeper-lying mantle rocks of the target body that were disintegrated by the downward percolation of core metal from the impactor. High strain-rate deformation experiments with the system olivine – FeS melt ± Au melt produced microstructures and PSDs that are broadly similar to these two textural groups. The experiments also suggest that de-localized metal melt percolation and concomitant mantle disintegration as evidenced in angular pallasites is facilitated by weak grain boundaries caused by a small fraction of previously present metal melt in the mantle as opposed to localized diking that is dominant in a melt-free mantle. The former mechanism is expected to prevent efficient core-merging and instead causes mantle-impregnation with metal melt, which could be active when planetesimal mantles were still warm due to short-lived radiogenic heating. In addition to the parent body of the PMG, asteroid 16 Psyche may be an example of this inefficient core-merging mechanism.

High strain-rate deformation analysis of open-cell aluminium foam

This study investigated the high-strain rate mechanical properties of open-cell aluminium foam M-pore®. While previous research has examined the response of this type of foam under quasi-static and transitional dynamic loading conditions, there is a lack of knowledge about its behaviour under higher strain rates (transitional and shock loading regimes). To address this gap in understanding, cylindrical open-cell foam specimens were tested using a modified Direct Impact Hopkinson Bar (DIHB) apparatus over a wide range of strain rates, up to 93 m/s. The results showed a strong dependency of the foam's behaviour on the loading rate, with increased plateau stress and changes in deformation front formation and propagation at higher strain rates. The internal structure of the specimens was examined using X-ray micro-computed tomography (mCT). The mCT images were used to build simplified 3D numerical models of analysed aluminium foam specimens that were used in computational simulations of their behaviour under all experimentally tested loading regimes using LS-DYNA software. The overall agreement between the experimental and computational results was good enough to validate the built numerical models capable of correctly simulating the mechanical response of analysed aluminium foam at different loading rates.

レーザピーニングが浸炭鋼の表面特性および曲げ疲労特性に及ぼす影響

To investigate the effects of laser peening (LP) on the surface and bending fatigue properties of carburized steel (JIS SCM420), surface roughness measurements, residual stress measurements, retained austenite measurements, and four-point bending fatigue tests were conducted. After LP, the compressive residual stress was introduced on the surface, with the effect extending to a depth of more than 1 mm. Due to the high compressive stress, the bending fatigue strength of LP specimen was 1.46 times higher than that of the as-heat-treated specimen even though the surface roughness was increased by LP. Notably, the amount of retained austenite was 10.9％ at the top surface of LP specimen, and high-density twins were observed in the lath martensite structure. This can be attributed to high strain rate deformation by LP.

Dynamic compression behavior of CoCrFeMnNi high-entropy alloy fabricated by direct energy deposition additive manufacturing

High-entropy alloys (HEAs) have attracted significant interest in recent years because of their unique physical properties, which make them suitable for usage in extreme environments. The fabrication of HEAs using metal additive manufacturing (MAM) has shown potential for improving their performance. This study investigates the mechanical properties of CoCrFeMnNi HEA manufactured through direct energy deposition using compression tests conducted at various strain rates ranging from quasi-static to dynamic (10−4 - 4200 s−1). The effect of the strain rate on the mechanical properties was analyzed, and a Johnson-Cook constitutive model was employed to simulate the dynamic compression test using the finite element method. The temperature and strain distributions during high-strain-rate compression were analyzed, reflecting the local temperature rise in the finite element analysis. The comparison of the strain distribution results of the simulation and microstructure after deformation showed that the microstructural behavior was influenced by strain localization, which occurred more prominently at the edge of the material. Because the CoCrFeMnNi HEA had a low stacking fault energy, twin formation was accelerated at high strain rate deformation by easily overcoming the critical twinning stress. The edge region, where strain localization was activated, exhibited a higher twin fraction than the central region (uniform deformation). The partitioned strain in the edge region was absorbed by twin formation, effectively suppressing the formation of the adiabatic shear band. These findings present significant implications for the design and development of MAM-processed HEAs with improved mechanical properties under extreme environments.

Shear localization in polycrystalline metal at high-strain rates with dynamic recrystallization: Crystal plasticity modeling and texture effect

Shear localization is an important failure mode, or even the dominant mode in metals at high-strain rates. However, it is a great challenge to accurately predict the occurrence and evolution of shear localization in metals at the high-strain rate deformation. Here, a dislocation-based crystal plasticity constitutive model with a crucial mechanism of shear instability, namely dynamic recrystallization, was developed. The evolution equations of dislocation density and grain size in the process of dynamic recrystallization were proposed and incorporated into the new constitutive model. The threshold of the stored energy in crystals was used as the criterion for the occurrence of dynamic recrystallization. Dynamic compression of a nanograin Cu-Al alloy was performed using the crystal plasticity finite element method based on the new constitutive model, and good agreement of the numerical prediction with the existing experimental data validates the new constitutive model. The results show dynamic recrystallization can be a more dominant mechanism for the occurrence of shear instability than thermal softening. In addition, dynamic tension and shear of the Cu-Al alloys with five typical textures were also simulated, showing that both loading mode and texture can significantly affect the formation of shear localization. This work is helpful for us to understand the role of microstructural evolution in the formation of shear localization at high-strain rates and to design the microstructure for artificially controlling or preventing the formation of shear localization.

Kinking-facilitated nano-crystallization in a shock compressed Mg-1Zn alloy

The local deformation behavior and dynamic recrystallization of a shock compressed Mg-1 Zn alloy was investigated through EBSD and TEM. Since dislocation slipping and twinning were locally suppressed during high strain-rate deformation, a more flexible kinking deformation was activated to adjusted local orientation and facilitate slipping and twinning within the kinks. Meanwhile, due to the slow heat dissipation that resulted in a local temperature elevating, the kink bands were evolved into deformation bands with recrystallized nano-grains. Such a finding provides a new perspective for kinking-facilitated nanocrystallization in Mg alloys and other anisotropic metallic materials.

Temperature dependence of mechanical properties and deformation mechanism of Fe–25Mn–3Al–3Si alloy at high strain rate

High-Mn steels exhibit an excellent combination of strength and ductility owing to the twinning induced plasticity (TWIP) effect or transformation induced plasticity (TRIP) effect, and have become an important material under service conditions of high strain rate and low temperature. However, the mechanical properties and deformation mechanism have not been fully understood during this extreme service conditions. Using impact test on Split Hopkinson Pressure Bar apparatus and microstructure characterization, the relationship among deformation temperature, mechanical properties and the underlying deformation mechanisms of high-Mn TRIP/TWIP steels during high strain rate (HSR) deformation was investigated. Our study shows that no matter the high temperature impact or low temperature impact, the Fe–25Mn–3Al–3Si alloy has good mechanical properties under large impact load and HSR deformation. Moreover, the strain hardening rate of the HSR impact samples at 300 °C are significantly higher than that at −80 °C and room temperature (RT). Detailed transmission electron microscopy results revealed that the strain rate has an important influence on the deformation mechanism of high-Mn TRIP/TWIP steel. Thus, TWIP effect also become the main deformation mechanism of Fe–25Mn–3Al–3Si alloy with a high SFE at 300 °C during HSR deformation. Higher strain rate and lower temperature can promote the formation of ε-martensite twinning (εT) and the occurrence of reversible transformation of ε-martensite ⇆ γ-austenite, which improve great the toughness of the alloy at low temperature (such as −80 °C). The models of reversible transformation are proposed based on the detailed microstructural characterization.

Intermetallic Reaction of the Bonding Interface of TA2/Q235 Explosive Welding Composite

During the explosive welding, the bonding interface of welded materials was fast heated due to high strain rate and drastic plastic deformation. The periodical wave interface, with an amplitude of ~300 μm and a period wavelength of ~800 μm, was identifiable as a uniform wave interface formed in the bonding interface. The details of the formation of melting zone and mixing zone of welding materials at the interface were observed. Combined with the Ti-Fe binary phase diagram and the principle of diffusion welding, the phase composition and evolution process of the melting and mixing zone of the bonding interface were investigated by transmission electron microscopy (TEM) and energy dispersive spectrometer (EDS). Significance of the intermetallic compound was found in the mixing zone and melting zone, which was mainly TiFe, TiFe2, TiO2, Fe2O3 and some other intermetallic oxides. Meanwhile, the phenomenon of the titanium agglomeration and oxygen precipitation was observed in the melting zone. The bonding interface could be determined as a mixing welding of mechanical mixing, melting, diffusion and solidification that occurred in the mixing zone, and melting welding and diffusion welding mainly occurred in the melting region.

Sensitivity of heterogeneous microstructure evolution and preferential variant selection to thermomechanical conditions in a near α titanium alloy

Heterogeneous microstructures such as macrozones and Widmannstätten microstructures are intolerable defects in high-temperature aero-engine parts manufactured using titanium (Ti) alloys. Through thermomechanical operations, microstructure characterization, phase reconstruction, and crystal plasticity finite element method (CPFEM) modeling, a full-route analysis was conducted to uncover the microstructure evolution of the allotropic β and α phases and the variant selection of the β→α phase transformation (PT) of a near-α Ti alloy. With an increase in temperature or decrease in strain rate, the continuous dynamic recrystallization (cDRX) of the β phase gradually prevails over the discontinuous dynamic recrystallization (dDRX) of the α phase. The reconstructed high-temperature β microstructure combing the CPFEM simulation revealed that the prior β grains with the deformation-induced 〈100〉//AD orientation, which are softer and grow preferentially via cDRX to form a simple cubic texture in the β phase. The coarse β phase transforms into the secondary α (αs) phase with a localized phase-transformed texture (the 1̅21̅0//AD and 101̅1//AD components) during the subsequent cooling process. Through the reconstructed β phase and crystallographic analysis of the β→α PT, we found that the low-angle grain boundary (LAGB) in the β phase weakens the anisotropic growth of αs grains and the preferential variant selection of 1̅21̅0/60°αs/αs grain boundary (GB) caused by autocatalytic nucleation. A high strain-rate deformation can increase the intragranular defects (e.g., LAGB) of the β phase, which promote the formation of a basket-weave structure instead of a Widmannstätten structure and weakens the variant selection during β→α PT.

Mechanical property and cellular structure of an additive manufactured FeCoNiCrMo0.2 high-entropy alloy at high-velocity deformation

The microstructural modification for cellular structures can achieve the improvement of dynamic mechanical properties of a selective laser melted FeCoNiCrMo 0.2 high-entropy alloy (SLM-FeCoNiCrMo 0.2 HEA) and can expand its promising applications in the field of high-velocity deformation. In this work, FeCoNiCrMo 0.2 HEAs with cellular structures in different sizes were produced by selective laser melting (SLM) with different process parameters. The dynamic mechanical properties and microstructure of the SLM-FeCoNiCrMo 0.2 HEA were studied. The dynamic mechanical properties of the SLM-FeCoNiCrMo 0.2 HEA increased with decrease of average size of cellular structures, and the values of them were sensitive to strain rates. The energy absorption, compressive strength and yield strength of the SLM-FeCoNiCrMo 0.2 HEAs reached 315.6 MJ/m 3 , 2.2 GPa and 775.6 MPa, respectively at a strain rate of 2,420 s −1 , under the process parameters of laser power and scanning speed of 330 W and 800 mm/s, respectively, where the corresponding average size of cellular structures in the HEAs was 483.6 nm. The value of strain-hardening rate of the SLM-FeCoNiCrMo 0.2 HEA was about 5.1 GPa at a strain level of 0.1, which was much higher than that of the powder-metallurgy FeCoNiCrMo 0.2 HEA. The cellular structure was formed inside the molten pool with segregation of Mo on the boundary. Deformation localization appeared in the cellular structures, forming several deformation bands after high strain-rate deformation. The elemental segregation strengthening and dislocation strengthening are considered to be the main strengthening mechanisms in SLM-FeCoNiCrMo 0.2 HEA.

Numerical simulation of fracture in Taylor rod impact problem with stochastically distributed inherent damage

High strain rate deformation behavior studies in structures always hold importance in many areas of science and technology. The high strain rate deformation behavior is generally determined by the Taylor rod impact tests. During the impact process, the front portion of the Taylor rod undergoes a large deformation that results in the evolution of the voids leading to damage/fracture in the rod. A process of fracture phenomenon and mechanics of blunt-shaped projectile impacting a rigid target at high-velocity results in different fracture modes viz. mushrooming, petalling, shear cracks, tensile splitting, fragmentation, or mixed modes of failure. The deformation and evolution of these practically observed fracture modes are investigated due to the introduction of stochastically distributed inherent damage in the flat-faced Taylor rod. The process is simulated in the mild steel Taylor rod using continuum damage mechanics. The evolution of the damage and fracture growth has been presented. The process of deformation, the phenomenon of stress propagation, and the effect of stochastically distributed inherent damage on the fracture mode in the Taylor rod have been discussed. As the stresses initially evolve at the outer edge of the Taylor rod, the damage initially grows at the outer edge leading to the tensile splitting and petal formation. It is found that the introduction of stochastically distributed inherent damage and critical damage inside the Taylor rod changes the fracture modes. The results are found to be consistent with the literature.

Correlated structure viscoplastic self-consistent polycrystal plasticity: Application to modeling strain rate sensitive deformation of Ti-6Al-4 V

This paper presents a multi-level viscoplastic self-consistent (VPSC) model for correlated structures (CS-VPSC). A polycrystalline aggregate response is homogenized over inclusions, which are linearized using the generalized affine scheme. Response of the inclusions is that of either a single grain or a correlated structure of multiple single crystals. The single crystal response is a strain rate adjusted power-law viscoplastic formulation, which considers not only the driving force in the glide direction but also the two orthogonal shear and three normal components of the driving force. The resistance to slip is grain size sensitive and based on the evolution of dislocation density. Furthermore, the resistances of prismatic and basal slip systems are adjusted based on the geometry of slip transfer between adjacent correlated structures. The overall model linking the single crystal micro-scale to a correlated structure meso‑scale to an aggregate macro-scale response is also coupled with the implicit finite elements (FE-CS-VPSC). The model is used for modeling strain rate sensitive mechanical response and microstructural evolution of additively manufactured Ti-6Al-4 V (Ti64) alloy in two conditions. As-built and stress-relived (SR) microstructure consisted of correlated α-lath/lamellar structures, while thermo-hydrogen refined microstructure (THRM) consisted primarily of α- and β-globular structures and some correlated α-lath/lamellar structures. Electron backscatter diffraction data, neutron diffraction data, and mechanical data were used to initialize, calibrate, and validate the model. To this end, the flow stress and texture evolution of the materials in tension and compression under quasi-static and high strain rate deformations were predicted using CS-VPSC, while the texture evolution during rolling and impact was simulated using FE-CS-VPSC. Good predictions of the texture evolution and mechanical response including the tension/compression asymmetry allowed us to discuss the roles of correlated structures, slip system activation stresses, size effects, strain partitioning between phases, and slip activities in such predictions.

Experimentally characterising the temperature and rate dependent behaviour of unfilled, and glass microsphere filled, natural rubber

Low-impedance, elastomeric materials are widely used in engineering applications where they are subjected to impact loading leading to high strain rate deformation. The strong rate and temperature dependence that is exhibited by these polymers and their composites provides further motivation for a deeper understanding of this complex behaviour including the interaction between the matrix and filler materials. In this paper, unfilled and glass microsphere filled natural rubbers are used as model materials to better understand the behaviour of low-impedance particulate composites. These materials were characterised experimentally over a range of strain rates and temperatures. The effect of filler volume fraction and mean particle diameter on the rate and temperature dependence of the overall mechanical response was also investigated, as well as damage evolution based on post-deformation analysis of interrupted compression experiments. In this paper, experimental insights and data are presented and discussed, which will influence future constitutive modelling efforts.

Adiabatic shear instability in a titanium alloy: Extreme deformation-induced phase transformation, nanotwinning, and grain refinement

Increasingly harsh service conditions place higher requirements for the high strain-rate performance of titanium alloys. Adiabatic shear band (ASB), a phenomenon prone to dynamic loading, is often accompanied by catastrophic damage. Yet, it is unclear how the internal nanostructures are related to shear instability. Here we report detailed microstructural evolution in the ASB of a titanium alloy via in-depth focused ion beam (FIB), transmission Kikuchi diffraction (TKD), and high-resolution transmission electron microscope (HRTEM) analyses, with the deformation instability phenomenon discussed from the energy perspective. The ASB interior undergoes multifaceted changes, namely deformation-induced beta-to-alpha transformation and deformation-induced martensitic transformation to form substantially refined and heterogeneous structures. Meanwhile, two types of extremely fine twins are identified to occur within both nano-sized martensite and alpha phase. The critical plastic work representing the onset of adiabatic shear instability and dynamic equilibrium is observed to be constant for a specific structure in the same deformation mode. The energy analysis could be extended to other materials subjected to high strain-rate dynamic deformation.

Charpy impact behaviors of metastable β-Ti alloys: Transformation induce plasticity versus twining induce plasticity

TRansformation Induced Plasticity (TRIP) and TWinning Induced Plasticity (TWIP) have been successfully proposed to overcome the trade-off between strength and ductility of metastable β Ti alloys under static tensile deformation. However, whether they can be directly extended to impact deformation is still controversial due to the high strain rate and the existence of notch. In this work, Ti-4.7Al-6.26Mo-2.85Cr (TRIP Ti alloy) and Ti-8.5Cr-1.5Sn alloy (TWIP Ti alloy) were selected to compare the Charpy impact deformation behaviors and the corresponding deformation mechanisms. It was found that, under Charpy impact deformation, the primary deformation mechanism of TRIP Ti alloy was dislocation slip due to the massive stress-induced martensite (SIM) were inhibited at high strain rate; whereas, in the case of TWIP Ti alloy, the activity of stress-induced deformation twins (SIDTs) was promoted at high strain rate, which resulted in the intersection of different DTs and the formation of secondary DTs. The formation of the hierarchical DTs structures in TWIP Ti alloy not only effectively released the stress concentration to delay the crack initiation, but also remarkably increased the crack propagation path to enhance crack propagation resistance. Specifically, compared with TRIP effect, it seems as though TWIP effect was more efficient at accommodating the imposed high strain rate deformation. These findings enrich the understanding of TRIP and TWIP under impact deformation, and provide references for the design of metastable β titanium alloy with high impact toughness.

Thermomechanical Treatment Performance and Flow Stress Model of a Fe–Mn–Al–C Low‐Density Steel

The flow stress curves of Fe‐31.7Mn‐11.5Al‐0.9C low‐density steel are obtained by a hot‐compression test with a Gleeble‐3500 thermomechanical simulator. It is shown in the results that the dynamic recovery and dynamic recrystallization (DRX) of δ‐ferrite occur preferentially under the same deformation condition, inducing a “yield point‐like” phenomenon in the true stress–strain curves. The Zener–Hollomon parameter is introduced to fit the characteristic parameters used to establish the flow stress model according to Arrhenius hyperbolic sine relationship and Avrami equation, and the DRX activation energy of austenite is 414.12 KJ mol−1. The hot‐processing maps at different true strains are simulated based on the dynamic material model to determine the suitable hot‐rolling process and the optimal hot‐working process selection of high strain rate and high deformation temperature combination, 1100–950 °C/1–0.2 s−1, is recommended.

Investigation into thermodynamic behavior of LA103Z Mg[sbnd]Li alloy during turning based on modified Johnson–Cook model

The LA103Z MgLi alloy is a promising lightweight material for aerospace applications due to its superior performance. Its machining involves a complex thermal–mechanical coupling process during which the material undergoes high strain rate deformation. Thus, determining the precise constitutive model for accurately modeling the cutting is extremely important. To analyze the material flow behavior, Split Hopkinson Pressure Bar (SHPB) tests were conducted under strain rates of 1200–5000 s−1. By observing the microstructural characteristics of the material, adiabatic temperature rise (ATR) is found to be the dominant factor affecting the flow stress. Accordingly, a modified Johnson–Cook (J–C) constitutive model was derived by considering the effects of ATR on thermal softening and strain rate strengthening terms. The modified model was validated by SHPB measure data, the average relative error is 3.08 %. The finite element (FE) model was established based on the modified J–C model to predict the turning behaviors at various parameters. The predicted values coincide well with measured results, which proving the high-precision of FE model. The analysis results of the thermodynamic behaviors under different cutting parameters of MgLi alloy shows that the influences of cutting speed on temperature, and depth of cut on cutting forces (main cutting force Fc and feed force Ff) are the most significant. However, the effect of cutting speed on the cutting force is not evident due to the effect of temperature rise. The reasonable turning conditions of the large depth of cut and feed rate, and small cutting speed was suggested for studied LA103Z MgLi alloy to consider the machining quality and efficiency.

Особенности высокоскоростной деформации состаренных сплавов

The sliding of an ensemble of edge dislocations under high-speed deformation (high strain rate deformation) of an aged binary alloy is theoretically analyzed. An analytical expression of the dependence of the dynamic yield strength on the dislocation density is obtained. The conditions under which this dependence is nonmonotonic and has a minimum and maximum are obtained. The minimum occurs during the transition from the dominance of dynamic braking (drag) of dislocations by point defects to the dominance of braking by other dislocations (Taylor hardening). The position of the maximum corresponds to the value of the dislocation density, at which their contribution to the formation of the spectral gap becomes dominant.

High strain rate deformation of conductive materials with use of pulse magnetic fields

The study of metal deformation processes under extreme impacts is relevant due to the need to predict the behavior of materials in operational and critical modes using various deformation models. In this case, the key factor is to expand the range of experimentally determined/achievable parameters for their verification. The high energy density of the magnetic field and a reliable and simple technique for monitoring its parameters make it possible to use the magnetic pulse method of generating controlled pressure pulses of microsecond duration with an amplitude of ∼1 GPa. A classification of methods for high strain rate deformation of conductive materials by magnetic-pulse loading is carried out and features of magnetic-mechanical. An analysis of the experimental results showed the applicability of the magnetic-pulse method for studying plastic deformation processes with strain rates up to 104 1/s and more and the possibility to formulate requirements for the magnetic system and pulse current sources.

A MODEL OF MICROSTRUCTURE EVOLUTIONUNDER HIGH- STRAIN RATE DEFORMATION OF COPPER

A physical model of high-strain rate deformation of metallic materials has been developed to describe the evolution of microstructure during the process. The model is based on representations of the mechanisms of grain boundary sliding, strain-induced grain growth and dynamic fragmentation of coarse grains developed in the framework of the theory of nonequilibrium grain boundaries. These representations are modified for the description of high-speed flow and supplemented by a model of dynamic recrystallization. It is shown that during high-strain rate deformation of metallic materials, grain boundary sliding dominates in the material regions with a fine-grained submicron structure and the material deforms in the superplasticity mode. In regions of material with larger grain size, grain volume deformation dominates, accompanied by fragmentation and formation of fine-grained structure with submicron grain size. The transformation of the structure from fine-grained to coarse-grained occurs due to strain-induced grain growth. Transformation of structure from coarse grain to fine grain occurs due to dynamic fragmentation. The transition from grain boundary sliding to grain volume deformation occurs at grain size corresponding to the optimum value for the course of dynamic recrystallization. The cyclic change of deformation mechanisms (from grain boundary strain-induced to grain volume deformation) is caused by alternation of the processes of deformation-stimulated grain growth and dynamic fragmentation. The alternation of the above cycles of grain boundary and intragranular deformation in different regions of the material makes it possible to maintain a high strain rate and suppress the processes of both local hardening and local softening. The high strain rate is achieved at the expense of a high strain of the regions in which grain sliding dominates. In the regions dominated by intragranular deformation, the structure is “prepared” for superplastic flow.