Strain Hardening Mechanisms Research Articles

Hierarchical microstructure design to tune the mechanical and corrosion behaviors of a marine structural steel

Marine structural steel with notable strength and ductility is considered as one of the promising construction materials for various offshore applications. However, the harsh marine environment causes a series of issues, e.g., deteriorating ductility and impact toughness, degrading corrosion resistance and inducing stress corrosion cracking. In this work, we propose a novel approach to address these issues by introducing heterogeneous structure and tailoring carbide precipitation in a step quenched and tempered marine structural steel. The step quenched and tempered process not only leads to a heterogeneous structure containing ferrite and tempered martensite zones but also obtains abundant nanoscale VC carbides, resulting in M23C6 refinement. This heterogeneity triggers additional strengthening and strain hardening mechanisms, thereby enhancing strength and ductility. Furthermore, the heterostructure and M23C6 refinement effectively inhibit the microcrack initiation and the formation of galvanic cells during Charpy impact test and electrochemical experiment, respectively, thus improving impact toughness and stress corrosion cracking resistance. This study proposes a hierarchical microstructure design to developing high-performance marine structural steel, which offers an effective avenue for addressing the conflicts between mechanical performance and corrosion resistance.

Mechanical properties variation of samples fabricated by fused deposition additive manufacturing as a function of filler percentage and structure for different plastics

One of the key factors in manufacturing products by fused deposition molding (FDM) or layer-by-layer printing technology is the material intensity of the product. The task of reducing the amount of material required to manufacture a product without significant loss of mechanical properties is one of the most practically important technological tasks. Material saving in FDM printing of products allows to reduce financial costs and increase the speed of manufacturing of the final product without reducing (or not significantly reducing) the quality properties of the product. In our work it is demonstrated that using Combs filling type and materials of poly lactic acid (PLA) and polyethylene terephthalate glycol (PETG) it is possible to achieve material savings of up to 23% at 50% filling for PLA and 17% at 75% filling for PETG without significant reduction of product strength in comparison with other filling types. Exceptions are PLA samples with 100% fill and Lateral fill. Application of Kruskal-Wallis criterion and Dunn’s criterion with Bonferroni multiple comparison correction showed that there were no statistically significant differences within the strength limits of samples made by FDM printing technology from PLA and PETG plastics (p-value = 0.0514), as well as samples with Triangle and Grid filling type (p-value = 1). Based on this result, three groups of samples statistically significantly differing in ultimate strength were identified by methods of hierarchical cluster analysis; in each group (except for group 1, which included samples made of PLA plastic with Lateral filling type and 100% filling), correlation analysis was performed (Spearman correlation was used). The results of the correlation analysis showed a stable average correlation between the percentage of filling, modulus along the secant 0.05–0.2% strain, ultimate strength and strain corresponding to the yield stress. Analysis of the correlation graph showed that the main parameter correlating with all mechanical properties of the specimen is the 0.05–0.2% strain modulus. Based on this conclusion, robust regression equations predicting the 0.05–0.2% strain modulus as a function of the percentage of specimen filling were constructed for the two selected groups. Analysis of the equations showed that in the third group of specimens, the average modulus of 0.05–0.2% strain is more than twice the modulus of 0.05–0.2% strain in the second group. The detected statistical regularities can be explained by the mechanism of strain hardening, the actual value of which depends on the structure of the macrodefect (type of filling), properties and volume of the material (percentage of filling) used in the fabrication of samples using FDM printing technology.

The role of coherent nano precipitates on stacking fault and deformation twin formation during tensile deformation of wire arc additive manufactured nickel-aluminum-bronze

The strain hardening mechanisms and dislocation interactions of a wire arc additively manufactured (WAAM) nickel-aluminum-bronze (NAB) alloy was investigated using multi-scale characterization approach cross-correlating scanning electron microscopy (SEM), SEM-based electron back-scatter diffraction (EBSD), site- and crystallographic orientation-specific focused ion beam (FIB) nanofabrication, scanning transmission electron microscopy (STEM) and STEM-based energy dispersive X-ray spectroscopy (EDS). Flat, rectangular dog bone specimens were strained under uniaxial tension conditions. To understand the micro- and nanometer scale deformation mechanisms throughout the microstructure, tensile test specimens were interrupted near the yield strength and ultimate tensile strength. STEM microstructural characterization revealed that tensile deformation occurs by planar slip and multiple slip bands interacting on different {111} planes as well as the nucleation of dislocations from the interfaces associated with grains and secondary phase particles. To the authors knowledge for the first time the Center of symmetry (COS) analysis revealed that in both samples the shearing of nanoscale precipitates led to the formation of extended stacking faults including a combination of intrinsic stacking faults and 2-layer extrinsic stacking faults. At high strain levels more dislocations and stacking faults were nucleated, as well as intersecting slip bands further facilitating the activation of the stair-rod cross-slip mechanism and thickening of an ESF to create a 3-layer nano-twin. Activated mechanisms of plastic deformation and associated role of the precipitates are discussed with respect to the microscopic strength-plasticity characteristics as well as macroscopic mechanical properties of WAAM NAB alloy.

Influence of pre-torsion strengthening on the fatigue properties of 45 CrNiMoVA ultra-high strength steel

Aiming at the demand for fatigue life improvement of 45CrNiMoVA ultra-high strength steel shaft parts, a pre-torsion strengthening process using a combination of two different angles before and after was proposed based on the mechanisms of strain hardening, fine grain strengthening and dislocation strengthening. Through surface integrity measurement, torsional fatigue test and fatigue fracture characterization of the specimens, the main surface integrity factors affecting the fatigue life of the pre-torsion strengthening were analyzed. The results indicated that the grain size was refined and the plastic deformation was increased by the two-angle pre-torsion strengthening compared with the single-angle pre-torsion strengthening. The obtained surface residual compressive stress σx was the main factor affecting the fatigue life of pre-torsion strengthening, and the change in pre-torsion angle results in a significant secondary strengthening layer in the subsurface layer of the material. In summary, the study of the pre-torsion strengthening process with different combinations of angles has a guiding significance for improving the fatigue life of ultra-high strength steel.

Microstructural origins of enhanced work hardening and ductility in laser powder-bed fusion 3D-printed AlCoCrFeNi2.1 eutectic high-entropy alloys

Limited tensile ductility usually restricts the practical applications of new classes of high-strength materials in many industrial fields. Therefore, in-depth understanding of the work hardening behavior and its underlying plastic deformation mechanism are critical for the newly developed high-entropy alloys (HEAs). In this work, a geometric atomistic model of face-centered cubic (FCC)/ordered body-centered cubic (BCC (B2)) interfaces and the evolution of dislocation substructures have been investigated to explore the microstructural origins of work hardening responses for two additively manufactured AlCoCrFeNi2.1 eutectic high-entropy alloys (EHEAs) with the respective lamellar and cellular microstructures. Unlike the lamellar interphase interfaces with the most classical Kurdjumov-Sachs (KS) FCC-BCC relationship of {111}FCC∥{110}B2〈011〉FCC∥〈111〉B2, the Nishiyama-Wassermann (NW) relationship, namely {111}FCC∥{110}B2〈011〉FCC∥〈001〉B2, is observed to be dominant on the cellular interphase interfaces. Furthermore, our intermittent high-resolution transmission electron microscopy (HR-TEM) results directly show that the deformation of lamellar AlCoCrFeNi2.1 alloy first proceeds with massive stacking faults (SFs) and then dislocation walls developed across phases interfaces, due to the effective dislocation transfer capability of lamellar boundaries. The large uniform elongation of the cellular AlCoCrFeNi2.1 alloy is attributed to the stable and progressive strain-hardening mechanism that is stemmed from the activated multiple slip systems, deformation-induced SF networks, and the associated Lomer-Cottrell locks in the middle and later stages of plastic deformation. Moreover, the nano-bridging of FCC cells in the 3D-printed microstructure provides unique channels for dislocation movement, which offsets the “blocking effect” of cellular boundaries and thus suppresses the pre-mature fracture.

Advanced interstitial high-entropy alloy engineered for exceptional wear resistance via dual nanoprecipitation-induced heterogeneous structure

Despite achieving notable strength and wear resistance in precipitation-strengthened Cantor high-entropy alloy (HEA), the inherent brittleness of precipitates leads to a loss of ductility. In this study, we propose a novel approach to address this issue by introducing nanosized M23C6-combined with Cr2N, termed dual-nanoprecipitation, and heterogeneous structure in a C and N co-doped interstitial HEA (iHEA). The abundant nanoscale M23C6 and Cr2N particles serve not only as reinforcing agents to enhance strength and wear resistance but also hinder the recrystallization process, resulting in a heterogeneous structure containing recrystallized and non-recrystallized zones. This heterogeneity triggers additional strengthening and strain hardening mechanisms, thereby enhancing the deformability of iHEA. Furthermore, the heterogeneous structure effectively mitigates strain localization on the sliding surface during wear, thus improving tribological properties. By overcoming the challenge of poor ductility while maintaining exceptional strength and wear resistance, the dual-nanoprecipitation-induced heterogeneous structure offers promising avenues for the development of alloys with superior strength-ductility synergy and wear resistance.

Unveiling the Stacking Fault-Driven Phase Transition Delaying Cryogenic Fracture in Fe-Co-Cr-Ni-Mo-C-Based Medium-Entropy Alloy.

In this work, the tensile deformation mechanisms of the Fe55Co17.5Cr12.5Ni10Mo5-xCx-based medium-entropy alloy at room temperature (R.T.), 77 K, and 4.2 K are studied. The formation of micro-defects and martensitic transformation to delay the cryogenic fracture are observed. The results show that FeCoCrNiMo5-xCx-based alloys exhibit outstanding mechanical properties under cryogenic conditions. Under an R.T. condition, the primary contributing mechanism of strain hardening is twinning-induced plasticity (TWIP), whereas at 77 K and 4.2 K, the activation of martensitic transformation-induced plasticity (TRIP) becomes the main strengthening mechanism during cryogenic tensile deformation. Additionally, the carbide precipitation along with increased dislocation density can significantly improve yield and tensile strength. Furthermore, the marked reduction in stacking fault energy (SFE) at cryogenic temperatures can promote mechanisms such as twinning and martensitic transformations, which are pivotal for enhancing ductility under extreme conditions. The Mo4C1 alloy obtains the optimal strength-ductility combination at cryogenic-to-room temperatures. The tensile strength and elongation of the Mo4C1 alloy are 776 MPa and 50.5% at R.T., 1418 MPa and 71.2% in liquid nitrogen 77 K, 1670 MPa and 80.0% in liquid helium 4.2 K, respectively.

Enhancing strength and ductility of extruded Ti-alloy-reinforced Al-Mg-Si composites: Roles of interface nanostructures and chemical bonding

A comprehensive understanding of interface nanostructures and chemical interactions is required for solving the strength-plasticity trade-off dilemma of Al/Ti composites. Continuously Ti-reinforced Al-matrix composites with superior strength and ductility are fabricated via a novel extrusion technique. The unique strain-hardening mechanism of the Al/Ti composite plates at low strains is attributed to the coupling effects of the strong interface constraint potency, the mechanical incompatibility between the soft Al and hard α-Ti, and a large elastic strain of hard α-Ti. Interlocking structures with atomic interdiffusion are observed at the Al/Ti interface. Mechanical and thermal effects induce lattice distortion and atomic superdiffusion, thus resulting in an interfacial intermixing zone with substantial dislocations and misfit strain. These characteristics produce a strong interface constraint effect during tensile, causing superior strain/dislocation hardening and uniform plasticity. Nanoclusters with cubic structures and dispersed dislocations are detected in the three-dimensional (3D) atomic intermixing zone at the Al/TiAl3 interface, accompanied by the aggregation of Mg. The nanostructures with numerous dislocations in the 3D Ti/TiAl3 transition zone feature lattices with partially tetragonal structures and ordered Ti-Al(Si) clusters. Ab initio calculations indicate that the strong chemical interaction between Si and Ti atoms not only improves the interface bonding strength, but also disturbs the lattices and broadens the transition zone. Moreover, the 3D interface transition zones, accompanied by dispersed dislocations, improve the interface constraint potency and hardening capability of the annealed Al/Ti composite plate. This work sheds light on the capability of the extrusion technique to produce fiber-reinforced Al-matrix composites with superior mechanical properties and provides new strategies for improving the chemical bonding strength and interface constraint effect via the interface invasion of specific elements.

Mechanical and Electrochemical Properties Comparison of Additively Manufactured Ti-6Al-4V Alloys by Electron Beam Melting and Selective Laser Melting

This work involves additively manufactured Ti-6Al-4V alloys, which are widely used in automobile, biomedical, and aircraft components for a comparison of the microstructure–properties relationship between electron beam melted (EBM) and selective laser melted (SLM) alloys after hot isostatic pressing treatment. We carried out microstructural, mechanical, and electrochemical measurements on both alloys. They showed comparable α and β phase contents with slightly higher lattice parameters in the EBM sample compared to the SLM. The EBM sample showed higher yield strength and uniform elongation due to the activation of multistage defects-driven strengthening and strain hardening mechanisms. Cracking during the tensile test nucleated mainly at the α phase near high-mechanical mismatch α/β interfaces. This mechanism was consistent with the reported generation of hetero-deformation-induced strengthening and strain hardening. Both alloys showed similar electrochemical behavior, but the SLM sample was more susceptible to corrosion than the EBM alloy.

Deformation twinning and the role of stacking fault energy during cryogenic testing of Ni-based superalloy 625

Ni-based superalloys play a crucial role in various high-temperature applications, where their exceptional mechanical properties and resistance to corrosion are highly desirable. However, their response to low temperatures, especially concerning strain hardening, microstructural evolution, and deformation mechanisms, requires further scrutiny. In this study, we investigate the influence of temperature on the stacking fault energy (SFE) and its implications on deformation twinning in Alloy 625. Uniaxial tensile tests are performed at 298 K, 173 K and 77 K. The study reveals a notable increase in strain hardening at intermediate strain levels, suggesting the activation of a secondary deformation mechanism. To gain deeper insights, crystal plasticity-based simulations using the DAMASK framework are employed, complementing the experimental outcomes. Deformation twins are consistently observed at all temperatures, albeit with a small volume fraction and thickness. The critical strain for twinning decreased with decreasing temperature. Based on the numerous literature studies, experimental and computational observations, the SFE of the material is estimated to be constant over the studied temperature range.

Elucidating the tensile work hardening behaviour of precipitate containing Al0.3Co1.5CrFeNi1.5Ti0.2 high entropy alloy

A significant amount of work has been performed, since the last decade, on designing novel high entropy alloy (HEA) compositions. However, the effort on concluding a universal mechanism on the strain hardening behavior of these alloys with precipitates is limited. The current study is designed with an aim to comprehend the mechanical behavior of Al0.3Co1.5CrFeNi1.5Ti0.2 HEA in solution treated and aged conditions. The results show that the peak aged condition of sample contains two types of precipitates – B2 and L12 (γ′) of approximately 800 nm and 40 nm size, respectively. They are found to be responsible for a significant increase in strength in aged samples than solution treated ones. Deformation was found to occur mainly by planar slip on the {111} primary slip planes. Noticeable strain hardening behavior was attributed to the formation of stacking faults (SFs) and Lomer-Cottrell (LC) locks, inhibiting the dislocation motion. Numerous paired dislocations formed due to inhibition of slip by the ordered γ′ precipitates, coexist with a few Orowan loops in the peak-aged alloy after deformation. However, the primary strengthening mechanism was found to be the shearing of γ′ precipitates by partial dislocations. Shearing of nanosized γ′ precipitates by partial dislocations is energetically favourable for the system than shearing by full dislocations. Overall, this study enhances the understanding of the precipitation strengthening and strain hardening mechanisms in high entropy alloys.

Strength-ductility materials by engineering a coherent interface at incoherent precipitates.

In the quest for excellent light-structural materials that can withstand mechanical extremes for advanced applications, design and control of microstructures beyond current material design strategies have become paramount. Herein, we design a coherent shell at incoherent precipitates in the 2195 aluminum alloy with multi-step metastable phase transitions. A high local strain rate via a neoteric deformation-driven metallurgy method facilitated the diffusion of Li. The original T1 (Al2CuLi) phases were transformed into coherent-shell (Li-rich) irregular-coated incoherent-core (Al2Cu) precipitates. The ultimate tensile strength and elongation reached 620 ± 18 MPa and 22.3 ± 2.2%, exhibiting excellent strength-ductility synergy. Grain boundaries, dislocation, solid solution atoms, and precipitates all contributed to the yield strength of the materials, among which precipitates occupied a dominant position, contributing approximately 56.07%. A new "incoherent-coherent interact" strain-hardening mechanism was also clarified, which was believed to be promoted in other heat-treatable alloy systems, especially with multi-step metastable phase transitions.

Enhancing strength and ductility in the nugget zone of friction stir welded 7Mn steel via tailoring austenitic stability

The martensite often appears in the nugget zone (NZ) of friction stir welding (FSW) 7 wt.% Mn steel due to low austenite stability, deteriorating ductility and toughness. In this work, a 7 wt.% Mn steel was subjected to FSW, and preheating was used to tailor the austenitic stability to greatly improve the strength–ductility combination of the NZ. The austenitic deformation behavior and strain hardening mechanism in the NZ were systematically investigated. The microstructure of the as-welded NZ was composed of ultrafine blocky ferrite, austenite, and small amounts of martensite, whereas the as-preheated NZ contained ultrafine blocky ferrite and austenite, and the concentration of Mn in austenite was increased from 8.4 wt.% to 10.7 wt.%. This enhanced the austenitic stability, resulting in a significant increase in the volume fraction of austenite in the as-preheated NZ from 37.3% to 66.4%. The product of strength and elongation (PSE) in the as-preheated NZ increased dramatically from 42.6 GPa% to 67.1 GPa%, depending on a persistent high strain hardening rate (SHR). Multiple strain-hardening mechanisms were revealed. The austenite with enhanced stability can provoke sustained transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) effects, and massive dislocation multiplication occurs during tension, resulting in strong strain hardening.

Influence of Strain Rate on Metallurgical and Mechanical Properties of Friction Stir Spot Welded Aluminium Joints

Nowadays, the substitution of copper with aluminium is widely pursued in order to save weight and material costs, for battery components and wire connectors. Additionally, cost reductions can be further enhanced with effective reduction of energy consumption through efficient manufacturing. Therefore, friction stir spot welding as a solid-state welding technique is a potential choice with low energy demands and high joining performances. However, the joining of aluminium and its alloys with solid-state welding techniques is still a challenging task due to a persistent and chemically stable aluminium oxide layer formed at the sheets prior to the welding, due to the reaction between aluminium and atmospheric oxygen. In this paper, the influence of strain rate induced during friction stir spot welding process on the metallurgical, mechanical and electrical properties of friction stir spot welding of AA 5754-H111 was studied. The strain rate was calculated according to the rotational speed of the tool and the effective (average) radius and depth of the stir zone. It was observed that the specimens welded with a lower strain rate endured a 15 % higher average strain failure load compared to the specimens welded at a higher share rate. The microhardness profiles of the specimens obtained at low strain rates imply strain hardening mechanisms in the weld zone, while the microhardness of specimens welded at high strain rates expressed thermal softening. It was also found that the friction welded sheets, regardless of the strain rate, show increased electrical resistance compared to the base material, however, it decreases with an increase in strain rate. Microstructural analysis reveals a stress-induced metallurgical transformation in the narrow zone around the weld-faying interface.

Heterostructure manipulation of Ti2AlNb alloy through directional induction heating: investigation of multi-scale deformation mechanisms in the O phase

In this study, a heterostructure of Ti2AlNb alloy (Ti-22Al-26Nb) was fabricated using directional heating treatment (DHT) technology. The microstructure of the alloy was examined before and after DHT treatment using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The primary objective of the study is to investigate the deformation mode and strain hardening mechanism in the multi-scale O phase. The results indicate that DHT treatment leads to a significant refinement in the grain size of the alloy, resulting in a 14.2% increase in ultimate tensile strength (UTS) and a 29.2% increase in elongation. However, the treatment also causes a decrease in yield strength. The soft phase region, characterized by the microstructure with micron-sized O phase precipitates, and the hard phase region, characterized by the microstructure with submicron-sized O phase precipitates, exhibit different modes of strain adjustment. To mitigate the internal stress arising from the deformation mismatch between the soft and hard phases, the micron-sized O phase activates slip on the {110} and {010} planes during the tensile process at room temperature. Additionally, twinning behavior was observed on the {110} plane. On the other hand, the submicron O-phase activates slip on the {041} plane and occurs a crystal rotation of approximately 90° along the 〈012〉 and 〈100〉 orientations. These high-angle grain boundaries (HAGBs), which form continuously during deformation, effectively store dislocations and contribute to a continuous strain hardening effect in the alloy. Additionally, the alloy exhibits a high strain hardening ability due to the accumulation of geometrically necessary dislocations caused by the strain gradient resulting from the uneven plastic deformation of the soft and hard phase components, as well as the potential hetero deformation-induced (HDI) hardening. Ultimately, a Ti2AlNb alloy with a favorable balance of UTS and elongation was achieved.

Deformation behavior of TiB reinforced Ti and Ti6Al4V composite particles under in-situ microcompression

To reveal the role of nano-TiB whiskers in micromechanical behaviors of two common titanium composite particles (TCPs), this paper presents experimental investigation on the elastic and plastic deformation of micron-sized Ti-TiBw and Ti6Al4V-TiBw particles. Scanning electron microscope with in-situ uniaxial compression experiment of single TCP was performed within a diameter range of ∼1–5 μm. This study enables detailed insights into the elastic and plastic deformation and geometrical shape evolution of TiBw reinforced TCPs. It is found that, when the particle size decreases, the flow stress and contact yield stress in microcompression for both TCPs exceed the yield strength of bulk composite pillars with identical content of TiB. The contact yield stress is as high as ∼1.5 GPa and ∼1.3 GPa for the smallest Ti-TiBw (D∼1.45μm) and Ti6Al4V-TiBw (D∼1.78μm) particles measured, respectively. In elastic-plastic and fully plastic regime, the steady plastic flow for both particles can be maintained over ∼2 GPa till the engineering compressive strain of εE ∼ -0.4. Compared with bulk composites, higher strain hardening rate of ∼34.2 and ∼37.5 GPa for Ti-TiBw and Ti6Al4V-TiBw particles demonstrate remarkable pinning capacity induced by nano-TiBw network. No structural damage and microcracks was observed in surface morphology of severely deformed Ti6Al4V-TiBw particles, showing distinctive microscale plasticity. The deformation and strain hardening mechanism for such composite particles have been discussed in terms of grain size, aspect ratio of TiBw and the network architecture.

A quantitative insight into strain hardening behavior of typical Hadfield steel under dynamic load

In this paper, the critical threshold associated to the activating of strain hardening capacity with respect to typical Hadfield steel Mn13Cr2 was explored quantitatively utilizing a Separated Hopkinson Press Bar (SHPB). The strain hardening behavior of the Mn13Cr2 under dynamic load was investigated, systematically, and the strain hardening mechanisms involved in different plastic deformation conditions were illuminated. Results indicated the overall plastic deformation process under different dynamic loads could be divided into three stages, including the (LHS), the (HHS) as well as the parabolic hardening stage (PHS). The critical transformation strength (CTS) from LHS to the HHS showed a trend of increasing first and then decreasing with the shock pressure increasing from 0.1 MPa to 0.8 MPa, and the maximum hardening effect with respect to the studied Mn13Cr2 steel could be activated at dynamic load threshold of 0.6 MPa with max CTS of ∼493.6 MPa. Dislocation-dominated strengthening mechanism and C–Mn atomic dipoles strengthening mechanism induced by dynamic strain aging were responsible for the excellent strain hardening capability when the impact load was increased above 0.6 MPa.

Excellent dynamic properties and corresponding deformation mechanisms in a microband-induced plasticity steel with dual-heterogeneous structure

A dual-heterogeneous structure with dual phases and heterogeneous grains was designed in a high Mn microband-induced-plasticity (MBIP) steel. The heterogeneous structured sample exhibits similar dynamic shear yield strength, while exhibiting enhanced dynamic uniform shear strain and dynamic shear toughness by 36 % and 47 %, respectively, as compared to the homogeneous structured sample. The strain hardening mechanisms contributing to the excellent dynamic shear properties in the heterogeneous structured sample have been revealed. Firstly, the superior hetero-deformation-induced hardening capability of the heterogeneous structured sample arises from a higher density of geometrically necessary dislocations induced in each phase. Secondly, a pronounced strain rate effect is observed in body-centered cubic (BCC) grains and face-centered cubic (FCC) ultra-fine grains, leading to an elevated level of strain rate sensitivity. Finally, the MBIP effect is also observed in FCC coarse grains under dynamic loading. Moreover, the MBIP effect in FCC ultra-fine grains is more likely to occur at high strain rates due to the emergence of single-wall domain boundaries, which is not observed during quasi-static tensile testing. The present results provide new routes for designing impact-tolerant structures in advanced metals and alloys.

Achieving excellent mechanical properties in a dual-phase FeCrNi medium entropy alloy through athermal transformations and dislocation structures

In this study, a face-centered cubic (FCC) + body-centered cubic (BCC) dual-phase Fe40Cr40Ni20 (at.%) medium-entropy alloy (MEA) with outstanding mechanical properties was developed. Deformation and strain-hardening mechanisms of the MEA were investigated using a transmission electron microscope. In the FCC phase, dislocation slip was the main deformation mechanism, and twinning appeared at the high-stress stage. Dislocation tangle induced by multi-slip positively contributed to the improvement of strain hardening ability. In the BCC phase, deformation mainly depended on dislocation slip, stress-induced martensitic transformation, and twinning. ω particles resulting from martensitic transformation strongly inhibited dislocation motion. Deformation twins and dislocation structures enhanced the strain-hardening ability by reducing the dislocation mean free path. The BCC/FCC interface also contributed to strain hardening ability by hindering the dislocation movement. The combined action of multiple mechanisms led to the high strain-hardening rate of the MEA.

Mechanical characteristics and microstructure of saturated sand under monotonic and cyclic loading conditions

To gain a deeper understanding regarding the intrinsic properties and correlations of macro mechanical responses, including critical states and large flow deformations under monotonic and cyclic loading conditions, with a view to revealing the fundamental characteristics of internal forces and deformations in granular materials. A series of undrained triaxial compression, triaxial extension and cyclic triaxial numerical tests are performed using discrete element method (DEM) software PFC3D. Under both monotonic and cyclic loading conditions, a comparable deformation mechanism of strain softening and strain hardening is observed, and the effective stress skeleton curves are in good agreement. As the progression from high stress ratio to the critical stress ratio, flow deformation and strain hardening are observed in saturated sand, where the mechanical coordination number gradually increases and the sliding ratio, suspended particle ratio decrease. The fabric anisotropy tends to increase in the flow deformation stage, decreases slightly in the strain hardening stage but remains highly anisotropic, and decreases rapidly in the stress decay (unloading) stage. As the cycle number increases after liquefaction of the sample, the percentage of deviatoric strain in the flow deformation stage to the total unidirectional shear strain increases, while the percentage in stress decay (unloading)/hardening stages decreases, which means that the strain development of samples is more fully developed in the flow deformation stage. The results serve as a reference for studying soil dilatancy and modifying constitutive models.