High Strain Rate Deformation Research Articles

Dynamic formation of gradient structure by microparticle impact — A crystal plasticity material point method study

Metallic materials with unique microstructure can achieve desirable strength-ductility synergy. However, effectively fabricating and precisely controlling microstructure distribution in metals remain challenging. Microparticle impact, the key process of cold spray technique, can lead to a gradient structure in the particle, which may also serve as a promising additive manufacturing technology. To investigate the dynamic formation mechanism of heterogeneous microstructure and its significant influencing factors, the crystal plasticity material point method (CPMPM) is developed, especially for microstructure formation under a high strain rate and large deformation. Our work provides a quantitative analysis of evolution of structural gradient during impact. It is found that decreasing grain size can afford a larger structural gradient and there is negligible influence on the compression ratio of particles. It suggests that microstructure distribution can be tailored by optimizing the impact process without influencing vertical deformation of particles.

Mechanical Responses and Deformation Mechanisms of Low-cost High-entropy Alloy AlCrFe2Ni2 with Heterogeneous Structure Subjected to High Strain Rate Deformation

Mechanical Responses and Deformation Mechanisms of Low-cost High-entropy Alloy AlCrFe₂Ni₂ with Heterogeneous Structure Subjected to High Strain Rate Deformation

Microstructural evolution and dynamic mechanical behavior of PM 2195 Al-Li alloy with resistance to shear instability in high strain rate deformation

The risk of adiabatic shear instability is common in traditional alloys covering high-strength steels, copper alloys and aluminum alloys during high strain rate deformation. However, the research on shear instability of 2195 Al-Li alloy prepared by powder metallurgy (PM) at high strain rates is still blank. The purpose is to explore its microstructure evolution and dynamic compression behavior. In this work, the 2195 Al-Li alloy prepared by PM amazingly demonstrates excellent resistance to shear instability at high strain rates during Split-Hopkinson pressure bar (SHPB) experiment. The microstructural evolution of grains and T1 (Al2CuLi) precipitate phase, and dynamic compression behavior of PM 2195 Al-Li alloy at various strain rates from 3000 s−1 to 9000 s−1 are investigated in detail employing correlative electron-backscatter diffraction (EBSD) and transmission electron microscopy (TEM). During high strain rate deformation, the alloy undergoes grain fragmentation and rotation, increasing dislocation density around the grains. When the strain rate is from 3000 s−1 to 7500 s−1, the Kernel Average Misorientation (KAM) value increases, indicating intensified localized strain. At a strain rate of 9000 s−1, a small number of grains are surrounded by nanocrystals (around 200 nm) that undergo stress release, while the average size of micrometer-sized grains is 0.81 μm. As the strain rate increases, the T1 phases in the alloy extends, vibrates, and fractures before dissolving in the matrix, while high-rate deformation promotes dislocation entanglement and defects, leading to the decomposition or ordering of precipitated phases through rapid plastic deformation. By virtue of energy dissipation strategy via the formation of nanocrystallines and dislocation behavior induced by the dissolution of T1 phases, the "positive-positive" effect is observed. The PM 2195 Al-Li alloy undergoes no shear failure even at the strain rate of 9000 s−1, showing great prospects in future applications in dynamic-loading conditions.

Simulation Study on Rock Crack Expansion in CO2 Directional Fracturing

In underground construction projects, traversing hard rock layers demands concentrated CO2 fracturing energy and precise directional crack expansion. Due to the discontinuity of the rock mass at the tip of prefabricated directional fractures in CO2 fracturing, traditional simulations assuming continuous media are limited. It is challenging to set boundary conditions for high strain rate and large deformation processes. The dynamic expansion mechanism of the 3D fracture network in CO2 directional fracturing is not yet fully understood. By treating CO2 fracturing stress waves as hemispherical resonance waves and using a particle expansion loading method along with dynamic boundary condition processing, a 3D numerical model of CO2 fracturing is constructed. This model analyzes the dynamic propagation mechanism of 3D spatial fractures network in CO2 directional fracturing rock materials. The results show that in undirected fracturing, the fracture network relies on the weak structures near the rock borehole, whereas in directional fracturing, the crack propagation is guided, extending the fracture’s range. Additionally, the tip of the directional crack is vital for the re-expansion of the rock mass by high-pressure CO2 gas, leading to the formation of a symmetrical, umbrella-shaped structure with evenly developed fractures. The findings also demonstrate that the discrete element method (DEM) effectively reproduces the dynamic fracture network expansion at each stage of fracturing, providing a basis for studying the CO2 directional rock cracking mechanism.

Thermomechanical and damage characterisation of short glass fibre reinforced polyamide 6 and impact-modified polyamide 6 composites

Polyamide 6 and its composites are widely used in engineering applications that are exposed to high strain rate deformation. This paper investigates the thermomechanical properties of two polyamide 6 composites, both reinforced with 30 wt% short glass fibres, and one of which additionally contains an impact modifier, to provide an understanding of the mechanical response over a wide range of strain rates and temperatures. Compression experiments were performed at rates between 2 and 3000 s−1, with high speed optical and infrared cameras to aid interpretation of the rate-dependent failure arising from the formation of adiabatic shear bands. Further high strain rate experiments were performed with ultra-fast X-ray phase-contrast imaging to provide in-situ internal damage evaluation. These data will improve the utilization of these composites and aid in development of advanced thermomechanical models.

Unravelling dynamic recrystallisation in a microalloyed steel during rapid high temperature deformation using synchrotron X-rays

Microstructure evolution during high-strain rate and high-temperature thermo-mechanical processing of a 44MnSiV6 microalloyed steel is investigated using in situ synchrotron high-energy powder X-ray diffraction. The conditions selected replicate a newly developed near solidus high-strain rate process designed for reducing raw material use during the hot processing of steels. High temperatures (exceeding 1300 °C) and high strain rate ε˙ = 9 s-1 processing regimes are explored. The lattice strains and dislocation activity estimated from diffraction observations reveal that the microstructure evolution is primarily driven by dynamic recrystallisation. A steady-state stress regime is observed during deformation, which develops due to intermittent and competing work hardening and recovery processes. The texture evolution during the heating, tension, shear deformation and cooling stages is systematically investigated. The direct observation of phase evolution at high-temperature and high-strain rate deformation enables a comprehensive understanding of new manufacturing processes and provides deep insights for the development of constitutive models for face-centred cubic alloys.

Tungsten heavy alloys for kinetic energy penetrators: a review

Tungsten heavy alloys are preferred materials for kinetic energy penetrators owing to the combination of high density, tensile strength, good impact strength and inert nature. The factors affecting shear band formation in the material and strategies to aid their early emergence by various alloy and process modifications to sustain a self-sharpened head during penetration are the main focus of this review article. Changes in the microstructural features such as tungsten grain size, dihedral angle and contiguity and mechanical properties by the addition of various alloying constituents and post-processing are critically discussed, along with high strain rate deformation behaviour analysis. This article also captures recent developments in the last decade and future prospects in the field of tungsten heavy alloys.

Multiple Deformation Mechanisms in Adiabatic Shear Bands of a Titanium Alloy during High Strain Rate Deformation.

The occurrence of adiabatic shear bands, as an instability phenomenon, is viewed as a precursor to failure caused by instability at high strain rates. Metastable β titanium alloys are extensively utilized due to their excellent mechanical properties, which are often subjected to high strain rate loads in service conditions. Understanding and studying their adiabatic shear instability behavior is thus crucial for preventing catastrophic failure and enhancing material performance. In this study via detailed microstructural analyses in the adiabatic shear region of a Ti-10V-2Fe-3Al alloy subjected to high strain rates, it was observed that α″ martensitic transformation and nano-twinning plus β-to-α phase transformation with α″ martensite as an intermediate phase occurred, in addition to substantial fine grains. The grain refinement mechanisms were mainly related to dynamic recovery dominated by dislocation migration alongside severe plastic deformation.

Microstructural evolution and modified constitutive model of nanoscale Al2O3 dispersion-strengthened copper under high strain rate deformation

The dynamic mechanical properties and behaviors of an oxide dispersion strengthened copper alloy (ODS Cu) are investigated herein within a strain rate range of 1000–5000 s−1 using a split Hopkinson pressure bar to offer theoretical guidelines for potential applications. The mechanisms of plastic deformation and dynamic recrystallization of ODS Cu were studied by analyzing the microstructure evolution during dynamic compression. The results demonstrate that the strain rate sensitivity of ODS Cu exceeds that of pure Cu, which is beneficial for resisting dynamic high-speed impacts. The accumulation of dislocations and generation of an adiabatic temperature rise during high strain rate deformation promote the splitting of the original grains and the formation of dynamically recrystallized grains. The dominant Goss and cube textures transformed into R-cube textures with increasing strain due to the continuous rotation of subgrains and recrystallized grains. By considering the coupled effects of strain and strain rate, the Johnson-Cook model was modified according to the Voce hardening law. The modified Johnson-Cook model exhibits good agreement with the experimental data across a wide range of strain rates. This study provides insight into the dynamic mechanical behavior of ODS Cu.

Shear localization in gradient high-entropy alloy at high strain rates: Crystal plasticity modeling

Gradient-structured metals have attracted a lot of attention due to their good synergy between strength and ductility. However, whether they can be used as a candidate for effective prevention of shear localization failure during high strain rate deformations is still an open question. Corresponding to the particular mechanisms of dynamic recrystallization and twinning at high strain rates, a physically based constitutive model of crystal plasticity is developed, including a new evolution equation for dislocation density and a twinning model, to investigate the detailed process of shear localization of gradient-structured CoCrFeMnNi high-entropy alloys (HEAs). A physically based strain gradient theory is considered to capture the strengthening effect of gradient structures. The competition between microstructural softening and strengthening effects is quantified to reveal the effects of gradient-structure on shear localization and shear localization can be significantly delayed in gradient-structured HEAs. This study contributes to the understanding of the influence of gradient-structure on shear localization and provides insights for further optimization of the mechanical behavior of gradient-structures at high strain rates to develop strong and ductile metals and alloys for dynamic applications.

High-strain rate deformation of aluminum during the Taylor test

The aim of the study is to develop a methodology for assessing changes in the microstructure of aluminum under dynamic deformation in a rather wide range of the strain rate and strain degree. The distribution of the microstructure and the strength properties in the cross-section of pure aluminum samples (A99) after dynamic deformation according to the Taylor test were studied. The tests were carried out at room temperature using a PG-20 light-gas cannon, at sample throwing speeds of 127 and 165 m/sec. An interference microscope (Leica IM DRM) and a scanning electron microscope (Jeol JSM-6490) were used to study the aluminum microstructure; the microhardness measurements were carried out on an HVS-1000 device to study the uniformity of the strain distribution in samples. It is shown that three characteristic areas can be distinguished in aluminum samples after Taylor test: the elastic deformation zone, the plastic deformation zone, and the zone of severe plastic deformation, which is located in the area of collision of the sample with a steel barrier. It is shown that dynamic deformation reduced the grain structure from 1 – 1.1 mm to 2.5 – 3 μm at high impact velocities. An elongated grain shape is observed in the collision zone. The proposed method provided determination of the critical strain degree necessary for the onset of grain fragmentation and allowed us to explain the formation of zones of weak and severe plastic deformation. It is shown that the critical strain degree corresponding to the beginning of grain fragmentation increases from 0.18 to 0.21 with an increase in the throwing speed of the sample from 127 to 165 m/sec. In the zone of weak deformation, plastic deformation proceeds by intragrain riveting and the initial stages of grain fragmentation. In the zone of severe plastic deformation, a fine-grained microstructure is formed, which leads to an increase in the microhardness of aluminum in accordance with the Hall – Petch equation.

Investigation on dynamic deformation behavior of 7A75 aluminum alloy over wide strain rates and constitutive model establishment

The aluminum alloy components on automobile often experience high strain rate deformation during accidents. The dynamic deformation behavior and constitutive model are crucial for analysis of vehicle body collisions and failures. In this work, the 7A75 aluminum alloy plate was heat treated according to standard body manufacturing procedures. Quasi-static and dynamic tensile experiments were conducted on the aged 7A75 alloy in a wide strain rate range from 0.001 s−1 to 500 s−1. The dynamic deformation behavior of the alloy was revealed, and the constitutive models were established. It was found that the 7A75 alloy has strong strain rate sensitivity, and its mechanical properties can be significantly improved with the increase of strain rate. As the strain rate increases, the necking instability strain shifts towards higher values, which leads to a larger uniform deformation ability and fracture elongation. This behavior results in a better energy absorption capacity at high strain rates, thus providing a good collision protection property. The constitutive models, including Johnson-Cook (J-C) and Zerilli-Armstrong (Z-A) types, were established and compared. Note that the conventional J-C and Z-A models exhibit severe prediction distortion. Therefore, a modified J-C model incorporating strain rate compensation was introduced, which decrease the errors by 149% and achieves accurate predictions of both dynamic and static deformation behaviors over a wide range of strain rates.

Shear localization-induced amorphization in nanocrystals during high strain rate deformation

Shear localization is the predominant form of failure of metals at high strain rates where amorphization often occurs. However, the mechanism and role of amorphization in shear localization are not yet clearly understood. In this work, a series of shear deformations of nanocrystals with different grain sizes at high strain rates are investigated using molecular dynamics simulations. The results show that amorphization is not the result of thermal melting, but a new plastic deformation mechanism in the process of shear localization, which serves as a complement to dislocation and grain boundary to support plastic deformation at a high strain rate. The critical shear strain at which shear localization and amorphization occur is greater for small-grained specimens than for large-grained specimens. This can be attributed to the fact that small-grained specimens have more defects and slip systems that can be activated, which cause amorphization to occur later. The strain rate has a significant influence on shear localization and amorphization. The processes of shear localization and amorphization become more complicated at extremely high strain rates than at relatively low strain rates.

A novel mathematical-based approach to predict the high-speed deformation behavior of cryodeformed materials with varying stacking fault energies

Cryodeformed (CR) materials have complex microstructures containing nano-twins, dislocation clusters, tangles, and others, depending on the stacking fault energy (SFE). Such unique microstructure makes them suitable for several engineering applications under a high strain rate environment. The high-speed deformation (HSD) behavior of CR materials is difficult to understand due to their complex microstructure. The present work is focused on developing a novel mathematical modeling method to understand the high strain rate deformation of CR materials with varying SFE. The model equation correlates the microstructure-based variables to deformation-based variables like stress, strain, and strain rate. High-speed deformation was conducted using a crossbow test. Transmission electron microscopy (TEM) was conducted on these materials to validate the model output.

Enhanced solid particle erosion resistance of nanocrystalline Ni-W coatings through multilayer approach

Electrodeposited nanocrystalline Ni-W alloy coatings are projected as potential alternatives for hard chrome (HCr) coatings owing to their comparable hardness and superior corrosion resistance. However, the limited strain hardening ability of the nanocrystalline Ni-W coatings manifests in their poor resistance towards solid particle erosion involving a high strain-rate (>103 s−1) deformation. In the present study, a novel microstructural engineering strategy has been implemented to enhance the strain hardening ability of the nanocrystalline Ni-W coatings without compromising on their strength/hardness. Ni-W multilayer coatings composed of alternatively stacked soft (2.6 GPa) and hard (8.5 GPa) Ni-W layers were developed using the pulse reverse electrodeposition technique. Further, to investigate the effect of individual layer thickness (λ), multilayer coatings with different λ were deposited. Solid particle erosion testing using the angular SiO2 particles showed that the multilayer coatings (irrespective of λ) exhibit superior erosion resistance compared to the homogeneous nanocrystalline Ni-15W at both oblique (θ = 30°) and normal (θ = 90°) impact conditions. The post-erosion microstructural analysis of the multilayer coatings revealed that the hard Ni-W layers undergo uniform strain (ε > 50 %) without cracking due to the constrained co-deformation imposed by the surrounding soft Ni-W layers. Therefore, the enhanced erosion resistance of the multilayer coatings is attributed to the effective dissipation of impact energy through co-deformation of the individual hard and soft Ni-W layers.

Evolution of interfacial phases between Al alloy and high entropy alloy during annealing

While the interfacial phases between Al and high-entropy alloy (HEA) share the same types as that between Al and steel, it has been found previously that the thermal stability of the Al-HEA interfacial phases is significantly higher than that of the Al-steel interfacial phases. To further elucidate the reason for the exceptional thermal stability of Al-HEA interfacial phases, this study investigated the evolution of the interfacial phases formed between Al alloy and FeCoCrNiMn HEA during annealing. An Al-FeCoCrNiMn thin film diffusion couple was extracted from a friction stir lap welding (FSLW) joint by focused ion beam and then annealed at 400 °C. An amorphous layer is formed initially at the interface due to the high strain rate deformation during FSLW. The amorphous layer remained even after prolonged annealing at 400 °C (∼0.72 Tm of Al alloy) for 60 min with very limited thickening (from 50 nm to 55 nm), showing remarkable thermal stability at the relatively high temperature. Al13Fe4-type intermetallic compound (IMC) with the Fe site occupied by Fe, Co, Cr, Ni and Mn first nucleates at the interface between Al alloy and amorphous layer. With the annealing time extended to 180 min, the amorphous layer is partially transformed into Al5Fe2-type IMC. The rate constant for the thickening of the reaction layer at the Al/FeCoCrNiMn interface is 0.03 μm∙min1/2, which is much smaller than (only 0.02 times) that at the Al/steel interface under similar annealing condition. The high thermal stability of Al-HEA interfacial layer is thus attributed to the slow kinetics in crystallization of the initially formed amorphous layer, the barrier effect of amorphous layer for interdiffusion, and the reduced diffusivity inside of IMCs caused by significant fluctuations in lattice potential energy due to high-density doping with HEA elements. This finding provides new insights into the fabrication of thermally stable Al-HEA hybrid structures suitable for service conditions involving high-temperature exposure.

Microstructural evolution of grain refinement in superalloy Inconel 718 during low temperature and slow strain rate hot compression

This study explores the plastic deformation mechanism and microstructural evolution of as-homogenized and as-forging Inconel 718 alloy through low temperature and slow strain rate hot compression deformation. The stress-strain curves of Inconel 718 alloy were analyzed by modulating the deformation temperature, strain rate, and strain, and the extreme grain refinement process from cast billets (with abnormally coarse grains) to the final compression sample was revealed. The results show that the true stress levels are lower during both low temperature and slow strain rate hot compression, relative to high temperature and fast strain rate deformation. For the as-homogenized Inconel 718 sample, the same grain refinement as the high temperature and fast strain rate (i.e. forging deformation) was achieved by the low temperature and slow strain rate compression deformation. For Inconel 718 samples after forging, uniform, equiaxed, fine grains with an average size of 2.9 μm are obtained at lower deformation temperatures and slower strain rates. Based on these results this low temperature and slow strain rate metallic forming process demonstrates the potential for the application of plastic deformation in controlling grain size, morphology, and distribution, providing a direct, economical, and efficient approach to achieving fine equiaxed grain structures in as-forging metallic materials.

Impact behaviour of Al 2014 wrought alloy using Split-Hopkinson pressure bar (SHPB): Its influence on mechanical and microstructural properties

The deformation behaviour of Al 2014 alloy under high strain rate dynamic load experienced in the actual service condition of aero-structural components is scarce in the literature. Therefore, Split Hopkinson Pressure Bar (SHPB) based high strain rate compressive behaviour (dynamic loading) of Al 2014 is investigated in this work. The wrought (WAR) form of Al 2014 alloy has been chosen to examine its deformation behaviour with varying high strain rate compressive loading, generated through 4 different pressure ranges (1 bar, 1.5 bar, 1.75 bar, and 2 bar) at striker bar end in SHPB technique. These pressure ranges induced high strain rates 5536/s, 6236/s, 6576/s, and 7783/s in separate WAR specimen, correspondingly, which were recorded as flow stress curves. Also, the WAR Al 2014 alloy was heat treated with standard Tempered-6 (T6) condition to homogenize its microstructure. The T6 condition was exposed further with the same pressure values, which induced high strain rates 5303/s, 6139/s, 6977/s, and 7861/s in T6 specimens. The SHPB-based flow stress responses for both WAR and T6 conditions were compared to understand high strain rate deformation behaviours. The WAR specimen exposed at 7783/s strain rate has shown superior Peak Stress (P.S., ∼765 MPa) and %contractionP.S. (∼15%) compared to its T6 counterpart (at 7861/s). Prior to the dynamic studies, the quasi-static responses were also explored through tensile, compressive and hardness measurements (with WAR and T6) to realize the overall materials' behaviour. These dynamic responses under high strain rate compressive loading were correlated with in-process microstructural evolution, explored through SEM and XRD techniques. Exhaustive EBSD analysis quantified the excellent peak stress and % contraction in WAR conditions due to presence of huge amount of low angle grain boundaries (∼85%), ultrafine grains, relatively textured microstructure ({111} 〈100〉, {101} 〈001〉, etc), and significant strain hardening phenomena, whereas large amount of high angle grain boundary (∼88%), equiaxed and uniform microstructure contributed for higher hardness (∼22%) and tensile strength (∼2%) in T6 conditions.

High strain rate induced shear banding within additively manufactured AISI 316L

The effects of post-processing via cold/hot rotary swaging, combined with high strain rate deformation, on formation of shear bands and deformation induced martensite within AISI 316L steel prepared by Selective Laser Melting are investigated. The as-built sample exhibited randomly oriented grains and residual porosity, which was reduced/eliminated by rotary swaging and subsequent high strain rate deformation. Hand in hand with significant changes in grain morphology after swaging (grain elongation and refinement, especially under cold conditions) and high strain rate processing (formation of shear bands and deformation induced martensite), the combined deformation imparted substantial hardening and resulted in microhardness increase by more than 20 %, primarily due to the occurring martensitic transformation. The structure changes were observed especially for the cold processed sample, due to aggravated plastic flow and hindered movement of generated partial dislocations.

High-speed deformation behaviour of 45° extruded magnesium alloy under quasi-in-situ conditions

In this study, quasi-in-situ electron backscatter diffraction was employed to investigate the microstructural evolution and deformation behaviour of ZK60 magnesium alloy with an initial 45° extruded state orientation under two high-speed impacts. The experimental results demonstrated that the deformation of the alloy mainly depended on slip and [Formula: see text] tension twinning. With the increase of deformation, the [Formula: see text] tension twins nucleated continuously, and there were stretching twin variants that were 60° away from the original stretching twins. In addition, compared with the conventional orientation (extrusion direction), 45° samples were easier to activate various slips under high-strain rate deformation conditions, especially ⟨c + a⟩ slips.