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Here, novel medium-entropy alloys with chemical compositions of Co17.5Cr12.5Fe55Ni10Mo4C1 and Co17.5Cr12.5Fe55Ni10Mo3C2 (at%) exhibiting excellent tensile properties at both room and liquid nitrogen temperatures have been developed. Precipitation of carbides changes the chemical composition and phase stability of the matrix, resulting in the controlled deformation-induced martensitic transformation from face-centered cubic to body-centered cubic of the alloys. The carbide precipitation, lattice distortion, and martensitic transformation led the alloy to have ultra-high yield strength of ~1 GPa and ultimate tensile strength of ~2 GPa with extra work hardening at liquid nitrogen temperature.

In this work, the mechanical properties and microstructural evolution of a recently developed immiscible medium-entropy alloy (Al15(CuFeMn)85, at%) at 77 K were investigated. Nanoscale deformation twinning was observed in both Cu-rich and Fe-rich grains. Twin-mediated martensitic transformation from Cu-rich β1 phase to β1’ martensite was occurred during tensile deformation at 77 K. The phase transformation into a stronger phase generates high plastic incompatibility between the constituent phases, resulting in high strength and work hardening by hetero-deformation-induced extra strengthening. The measured hetero-deformation-induced stress demonstrates the dominant hetero-deformation-induced strengthening mechanism at 77 K.

Here, a new strategy for designing heterogeneous medium-entropy alloys with light-weight and excellent mechanical properties is proposed. Alx(CuFeMn)100-x (x = 0, 7.5, and 15 at%) alloys were developed by utilizing the immiscible nature of Cu–Fe alloys. The microstructures of the alloys show phase separation into Cu-rich and Fe-rich regions, and the addition of Al transforms the crystal structure from dual face-centered cubic to face-centered cubic and body-centered cubic. Phase separation of the microstructure into two domains enables further dissolution of Al into the matrix. The alloys exhibit high strength because of solid solution strengthening and hetero-deformation induced strengthening caused by heterogeneous microstructures. The presence of nano-scale twins and essential partially recrystallized microstructures also enhances the strength of the alloys. This new type of medium-entropy alloys is expected to expand the design window in physical metallurgy.

We report an analysis of the discontinuous plastic flow of a multiprincipal-element alloy, ${\mathrm{Co}}_{17.5}{\mathrm{Cr}}_{12.5}{\mathrm{Fe}}_{55}{\mathrm{Ni}}_{10}{\mathrm{Mo}}_{5}$ (atomic percent, at. %), in the temperature range of 0.5--4.2 K showing serrated deformation curves. Using the analytical techniques, we studied the statistics of the stress drops associated with the unstable plastic flow. The analysis showed that the complexity and heterogeneity of a discontinuous plastic flow were reduced when the temperature was lowered. This behavior was associated with the effects of dynamic recovery and adiabatic heating on the dislocation-density evolution.

Strain-rate dependence in face centered cubic high entropy alloys still remains controversial despite extensive efforts on this topic. Strain-rate sensitivity reflects underlying thermally-activated deformation and the controversy boils down to the deformation mechanism. There has been disagreement even on the experimental values of the strain-rate sensitivity and activation volume. This study reviewed and analyzed the differences in experimental values and proposed mechanisms to resolve the controversy over the nature of thermal obstacles in CrMnFeCoNi alloy. TEM study on CrMnFeCoNi supports the presence of nanoscale heterogeneity elucidating the nature of rate-controlling mechanism in the alloy.

The nanostructural hierarchy of γ and γ′ phases is characterized in Ni-8.5Al-5.4Ti (at.%) alloy, after double aging treatment at 1000 and 780 °C for 6 and 20 h. Using electron microscopy and atom probe tomography, disordered γ phase inside ordered γ′ precipitate was detected, which underwent plate-like growth during aging treatment. This unique hierarchical structure enabled the simultaneous study of direct (γ′ precipitates/γ matrix) and inverse (γ nanoparticles/γ′ precipitates) order-disorder interfaces in 6 and 20 h double-aged samples. It was found that the compositional interface width, δ, of direct interfaces decreased during aging treatment, whereas the thickness remained almost unaffected for inverse interfaces. The difference in thickening behavior is explained based on the change of the Gibbs-Thomson pressure for γ plates embedded in γ′ and cuboidal γ′ precipitates surrounded by the γ matrix. This inference suggests the possibility of prediction and control of δ(t) variations during heat treatment processes based on the current thermodynamic approach.

Non-equiatomic Co17.5Cr12.5Fe55Ni10Mo5 (Mo5) and Co18Cr12.5Fe55Ni7Mo7.5 (Mo7.5) medium-entropy alloys were synthesized by vacuum induction melting, cold rolling, and subsequent annealing treatment at various temperatures (900–1200 °C) and they were investigated to exploit the precipitation strengthening in addition to solid solution strengthening of alloys. The effect of annealing temperature and Mo content on the microstructure and mechanical properties are systematically analyzed. From the microstructural analysis, a Mo-rich μ phase is observed in the face-centered cubic (fcc) matrix. Increasing the Mo content and low annealing temperature enhance the formation of μ phase, which is consistent with the thermodynamic calculation results. The formation of μ phase effectively enhances the strength of the Mo7.5 alloy by precipitation strengthening, and suppression of grain growth and recrystallization by Zener pinning effect. These lead to superior combination of tensile strength as high as 1100 MPa and large ductility. Our results provide insights not only into μ-phase strengthening of fcc-structured alloys, but also into the future development of high-performance MEAs.

Ultrafine-grained materials with nanotwins are expected to produce a remarkable combination of strength and ductility. In the present study, ultrafine-grained CoCrNi medium-entropy alloy with nanotwins is fabricated by high-pressure torsion followed by annealing; and investigated for cryogenic tensile properties. The alloy exhibits superior cryogenic tensile properties with a tensile strength of ~2 GPa and tensile strain of ~27%. The cryogenic tensile strength of ultrafine-grained sample increased by 67% as compared to the cryogenic tensile strength of coarse-grained sample due to fine grain size, annealing nanotwins, residual dislocation density, and strong temperature dependence of yield strength.

A newly developed precipitation-hardened high entropy superalloy was produced using double vacuum melting, and its microstructural evolutions were examined after homogenization, hot rolling, annealing, and aging. The precipitation behaviour of the samples was studied using two different heat treatment cycles (with and without solution treatment). Microstructural observations using scanning electron microscope and transmission electron microscopy showed that the direct-aged samples consisted of nano-sized γ′ precipitates in γ matrix, while for the solution-aged samples, reprecipitation of the Laves phase occurred in the interior of the grain and at the grain boundaries. The sluggish diffusion present in the proposed alloy is what mainly accounts for the smaller than 20 nm of γ′ precipitates formed after about 18 h of aging. The strengthening effects of precipitation on the mechanical properties of the annealed and aged conditions were studied using a universal testing machine under a tensile load. The room temperature tensile strength and total elongation of the direct-aged samples were as high as 1310 MPa and 32%, respectively, which represent a promising combination of strength and ductility for industrial applications. In addition, the direct-aged specimens exhibit significant enhancements of 150% and 46% for yield and tensile strengths, respectively, as compared to the annealed condition. These results confirm that the precipitation-strengthening mechanism is an effective way of increasing the strength of face-centered cubic high entropy alloys.

Order-disorder coherent interfaces determine the microstructure and mechanical properties of precipitation-hardened high-temperature alloys. The characteristics of these interfaces can be defined by a compositional width, δ, and structural width, δ′. The latter, which can be considered as the width of the ordered part of the interface, can play an important role in high-temperature mechanical behavior of precipitation-hardened alloys. This is due to the fact that diffusion in the ordered part of the interface is generally much slower than diffusion in the disordered phase, thus hindering the solid-state diffusion-based phenomena. Here, we investigate the order-disorder interface in a Ni-19Al (at.%) alloy as a model alloy for Ni-based superalloys using atomic-resolution scanning transmission electron microscopy and three-dimensional atom probe tomography. Then, we employ thermodynamic modeling to describe the interplay between the structural and compositional interface widths in binary Ni-Al and in ternary Ni-Al-Cr and Co-Al-W systems. We introduce the δ′/δ ratio as a critical parameter that varies significantly in different alloys. Our findings offer a general pathway to control the δ′/δ ratio of interfaces, which in turn affect the high-temperature properties of precipitation-hardened alloys.

Dual-phase Al0.5CoCrFeMnNi high-entropy alloy consisted of face-centered cubic (FCC) and body-centered cubic (BCC) phases exhibited enhancement of both strength and strain hardening ability at 77 K. It resulted from back stress hardening in high work hardening due to large strength difference of two constituent phases with decreasing temperature.

International audience; In a bid to improve the mechanical properties of high-entropy alloys, particularly, their strain hardening capability, we adapted the time-proven concept of ‘twinning engineering’, developed in the context of TWIP steels, to twinning-assisted high-entropy materials. The strategy chosen involved a two-step thermomechanical processing that consisted of low-temperature pre-straining and subsequent annealing. This approach was trialled on CoCrFeMnNi as an exemplary high-entropy alloy. The annealing conditions selected ensured that the deformation twins generated under low-temperature deformation were retained, whilst the dislocation density was recovered. The viability of this strategy was convincingly confirmed for room temperature deformation of the alloy. A constitutive model accounting for the effect of the pre-straining induced deformation twins was proposed. It was shown to provide a reliable description of the low-temperature and room-temperature deformation behavior of CoCrFeMnNi when deformation twins are involved.

In the present study, V10Cr15Mn5Fe35Co10Ni25 (at%) high-entropy alloy (HEA) of a single phase face-centered cubic structure with various grain sizes was fabricated. The influences of grain size on the work-hardening behavior and deformation mechanisms were investigated. The fine-grained and coarse-grained samples showed different work hardening behaviors during room temperature tensile deformation. Microstructural analysis revealed the presence of a high-density tangled dislocation structure without any mechanical twinning in the fine-grained sample, while mechanical twinning was observed to be the additional deformation mechanism in the coarse-grained sample.

Laser similar welding of cast and rolled high-entropy alloys (HEAs) was performed using the cantor system (Co0.2Cr0.2Fe0.2Mn0.2Ni0.2). As the welding velocity was increased from 6 to 10 m min−1, the shrinkage voids, primary dendrite arm spacing, and dendrite packet size decreased, thus improving the mechanical properties of the cast and rolled HEA welds. The cast HEA welds showed tensile properties comparable to those of the base metal (BM). In all the specimens fracture occurred near the heat-affected zone and BM at 298 K. However, the rolled HEA welds showed lower tensile strength than the BM, and fracture occurred in the weld metal (WM). This can be attributed to the larger dendrite packet size of the WM than the grain size of the BM. In addition, the tensile properties of the specimens at the cryogenic temperature were superior to those observed at 298 K, regardless of the cast and rolled HEA welds. This is because the formation of deformation twins and dislocations was predominant at 77 K. Therefore, the laser similar welds of cast and rolled HEAs are suitable for cryogenic applications.

High-entropy alloys (HEAs) are a newly emerging class of materials that show attractive mechanical properties for structural applications. Particularly, face-centered cubic (fcc) structured HEAs and medium-entropy alloys (MEAs) such as FeMnCoNiCr and CoNiCr alloys, respectively, which exhibit superior fracture toughness and tensile properties at liquid nitrogen temperature, are the potential HEA materials available for cryogenic applications. Here, we report a ferrous Fe60Co15Ni15Cr10 (at%) MEA exhibiting combination of cryogenic tensile strength of ∼1.5 GPa and ductility of ∼87% due to the multiple-stage strain hardening. Astonishingly, detailed microstructural observations at each stage reveal the sequential operation of deformation-induced phase transformation from parent fcc to newly formed bcc (body-centered cubic) phases. No compositional heterogeneity is observed at phase boundaries, indicating diffusionless phase transformation, as confirmed by atom probe tomography. The transformation to bcc phase occurs predominantly along grain boundaries (GBs) at the early stage of plastic deformation. Simultaneously, numerous deformation-induced shear bands (SBs) having stacking faults associated to the Shockley partial dislocations and thin hcp plates, form within fcc grains. Further deformation leads to the intense nucleation and growth of the bcc phase at the intersections of SBs within fcc grains. These micro-processes consecutively enhance the strain hardening rate, which play a key role in the multiple strain hardening behavior. The in-situ neutron diffraction studies make it clear that the martensite formation and the concurrent load partitioning between the fcc and bcc phases play an important role in the increase in strength. Furthermore, replacing high-cost alloying elements cobalt and nickel with iron, as well as introduction of metastability-engineering at liquid nitrogen temperature, distinguishes the new ferrous MEAs from previously reported equiatomic HEAs. This result underlines insights to provide expanded opportunities for the future development of HEAs for cryogenic applications.

Refractory metal high entropy alloy HfNbTaTiZr with ultrafine grained structure and grain size of ≈80 nm was processed by high pressure torsion. The development of microstructure, lattice defects and mechanical properties with increasing strain was examined. Grain refinement of HfNbTaTiZr alloy deformed up to the equivalent strain e ≈ 50 resulted in a significant enhancement of strength while keeping sufficient ductility. However, further straining e > 100 led to a decrease of strength and the loss of ductility due to the decomposition of solid solution facilitated by vacancies introduced by severe plastic deformation.

In the present work, a solid solution strengthening effect in high-entropy alloys (HEAs) was studied by investigating mechanical characteristics of single crystalline CoCrFeMnNi HEA and a corresponding model was developed. (100) and (110) oriented single crystals of the CoCrFeMnNi alloy were grown and their single crystallinity was identified using X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and electron backscatter diffraction (EBSD) analyses. The mechanical testing of the single crystalline CoCrFeMnNi alloy was performed and the critical resolved shear stress (CRSS) was obtained. A solid solution strengthening modeling based on the lattice friction stress and an intrinsic residual strain of HEAs was performed. The solid solution strengthening effect of HEAs was verified using the model proposed.

In this work, the mechanical characteristics and microstructural evolution of Al0.5CoCrFeMnNi high-entropy alloy (HEA) were studied after annealing at various temperatures (1000, 1100, and 1200 °C). X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy analyses were performed to reveal the phase and microstructural variations. The mechanical properties related to different microstructures of the alloy were characterized using tensile testing with digital image correlation. Annealing at lower temperatures led to a higher fraction of B2 phase and finer grain size of FCC (face-centered cubic) phase. A good combination of strength and ductility in this alloy was attributed to the ductile FCC matrix and hard secondary B2 phase. The alloy showed the active evolution of deformation twinning due to the low stacking fault energy when Al was added to CoCrFeMnNi to make the HEA. However, for alloy annealed at lower temperatures, twinning activity was suppressed by the smaller size of grains and depletion of Al content in the FCC matrix. The correlation between the microstructure and mechanical properties was also explored using a simple composite model.

In this work, the effects of homogenization annealing temperature of Al 0.5 CoCrFeMnNi HEA on cracking during cold-rolling were investigated. AlNi B2 phases were formed by low-temperature homogenization and affected the cracking phenomenon during cold-rolling. X-ray diffraction, microstructure, and composition analyses and thermodynamic calculations were conducted to identify the crystal structure of the alloy after the homogenization annealing treatment. The phase fraction and hardness of the homogenized alloy confirmed that the formation of the AlNi B2 phase induces cracking during cold-rolling of the Al 0.5 CoCrFeMnNi alloy. High-temperature annealing for homogenization of the alloy is recommended to prevent cracking during cold-rolling.

In this work, compaction behavior of CoCrFeMnNi high-entropy alloy powders with various particle sizes and size distributions, produced by water atomization, was investigated experimentally and theoretically. Theoretical modeling was employed using a pressure-dependent yield function in associated with a phenomenological constitutive model. Results for the quantitative densification behaviors from the experimental and theoretical analyses are in good agreement. We found that the size and size-distribution of the powder particles are important factors in the tap density as with conventional powder compaction. The compact density of large powder particles with coarse dendrite arm spacing is high due to low deformation resistance and low strain hardening (i.e., low evolution of dislocation density).

In this study, a suitable method to simulate pipe-manufacturing processes (i.e., bending, unbending, and flattening) was developed to reproduce the process and the mechanical properties of the pipe. To verify the reliability of the strain distribution and plastic deformation behavior predicted by the proposed method, experimentally-derived strain by digital image correlation (DIC) and finite element analyses were employed. Hardness and tensile tests were performed on bent-flattened specimens. The finite element method confirmed that the proposed method can predict the yield strength of pipes.

In this work, the effects of strain rate on mechanical deformation and microstructural evolution of CoCrFeMnNi high-entropy alloy (HEA) under quasi-static and dynamic compression were investigated. The HEA exhibited high strain-rate sensitivity values (m = 0.028) of yield strength under quasi-static conditions. In particular, due to the viscous drag effect, the variation of yield strength with strain rate under dynamic compression was much larger than that under quasi-static compression. Microstructural analysis using electron backscatter diffraction shows profuse twinning under both conditions. The dynamically deformed specimens exhibited strongly localized deformation regions (i.e., adiabatic shear bands). The process of dynamic compressive behavior in this HEA is competitive between hardening by dislocation and twinning, and thermal softening. To analyze numerically the flow behavior of the HEA under dynamic conditions, the modified Johnson-Cook model considering adiabatic temperature rise was employed. The modified Johnson-Cook model offered good agreement with experimental results regarding dynamic flow curves of this HEA.

After annealing the Zr-2.5wt%Nb pressure tube material at 700 ∘ C for 24h, the largest grain aspect ratio reduced from 3.1 to 1.8, and the average minimum grain size increased from 0.2 μ m to 0.9 μ m . In addition, Kearns’ factors characterizing texture along specific directions became more anisotropic by the annealing. While the annealed microstructure resulted in the decreases of all compressive strengths along three directions—transverse, axial, and radial, the anisotropy in the yield strengths displays uneven changes. The ratio of the transverse and axial strengths remains nearly constant. On the other hand, the ratio of the radial and the axial becomes lower. In the study, the origin of the changes in the strength anisotropy was investigated by the crystal plasticity finite element method (CPFEM). To consider the alteration of grain morphology, the grain shape model is included in the material constitutive model using a well-known Hall–Petch type relation. Through iteratively comparing the calculated yield strengths to the compressive yield strengths, the best-estimated critical resolved shear stresses (CRSSs) of the deformation systems were obtained without and with the grain model. In the latter case, the dependencies of CRSSs on the grain size were found as well. Although the effect of the grain model on the yield strengths was apparent, its influence on determining the anisotropic ratio was insignificant. On the other hand, changing texture information led to a quite definite variation in the yield anisotropy. It implies that the decrement in the ratio of the radial and the axial yield strengths could be caused mainly by the texture modification after annealing. In summary, the anisotropy in the yield strengths of the Zr-2.5wt% pressure material is mainly controlled by the texture and partly affected by the grain morphology.

In the work reported here, the mechanical properties of a newly developed Fe-Ni-Co-Mn-Ti-Si medium-entropy alloy with heterogeneous microstructure were investigated. High yield strength (~930 MPa), high ultimate tensile strength (~1100 MPa), and good ductility (~30%) were achieved using hetero-deformation-induced strengthening and high plastic-incompatibility between mechanically induced-martensite and the austenitic matrix.

In the present study, high temperature (700–1100 °C) tensile deformation behavior of hot-rolled CrMnFeCoNi high-entropy alloy was investigated. A single face-centered cubic phase was retained in the hot deformed specimens under different deformation conditions. The flow behavior was significantly influenced by temperature and microstructure evolution. A strong temperature dependence of yield stress was observed with a drastic drop in yield strength from 413 MPa at 700 °C to 94 MPa at 900 °C. A profound increase in ductility was observed above 700 °C and a perceptive decrease in ductility was observed above 900 °C. Electron backscatter diffraction analysis indicates incomplete dynamic recrystallization at 700 °C leading to a significant difference in the flow behavior.

In the present work, the mechanical properties of a newly developed metastable 65Fe-15Ni-8Co-8Mn-3Ti-Si (at%) ferrous medium-entropy alloy were investigated at 298 K and 77 K. The short-time annealing followed by aging treatment is employed to gain heterogeneous microstructure containing fine- and coarse-grains decorated with nanometric precipitates. The yield strength of the alloy enhances substantially from 0.9 GPa at 298 K and 1.3 GPa at 77 K in the annealed state, respectively, to 1.1 GPa and 1.5 GPa after the aging treatment, while the total elongation sustains more than 20%. This extraordinary improvement results from the synergistic effect of hetero-deformation-induced strengthening, mechanical nano-twins, and martensitic phase transformation along with precipitation strengthening of the well-distributed nano-scale Fe2SiTi and Ni3Ti precipitates during tensile deformation of the aged alloy. Therefore, the corresponding ferrous MEA can be considered as a promising candidate for ultra-high-strength components at extreme service conditions.

A strategy to improve the tensile properties of Co17.5Cr12.5Fe55Ni10Mo5 (at%) medium-entropy alloy through high-pressure torsion and subsequent annealing is presented in this work. Microstructural study revealed that the high-pressure torsion process led to the formation of fine grains (≤~1 μm) and profuse nano-scale Mo-rich μ-phase precipitates. And resultantly, the yield strength of the alloy was tuned from ~400 MPa to ~1 GPa, while preserving reasonable uniform elongation over 15%. The combination of strengthening from grain refinement and precipitation contributed to the excellent strength, while the post-HPT annealing provided substantial ductility.

In this work, a high-entropy alloy of equiatomic CrMnFeCoNi was processed by vacuum induction melting followed by rolling. After cold rolling, annealing for one hour was conducted during which the annealing temperature was varied from 650 to 1000 °C. This was done to investigate the effect of microstructure on tensile properties and stretch formability (defined as the capability of materials to undergo plastic deformation under biaxial stretching). The strengthening effect of partial recrystallization, with a remaining small fraction of sigma phase particles, led to improved yield and tensile strengths while minimizing the loss of ductility. Fully recrystallized microstructure resulted in a slight increase in ductility, and a considerable decrease in strength, during a tensile test. On the other hand, the results for stretch formability suggest that partial recrystallization had exactly the opposite results. In this regard, the present results raise a new issue to consider when utilizing partial recrystallization for improvement of mechanical properties.

Herein, the deep drawability and deep drawing behavior of an equiatomic CoCrFeMnNi HEA and its microstructure and texture evolution are first studied for future applications. The CoCrFeMnNi HEA is successfully drawn to a limit drawing ratio (LDR) of 2.14, while the planar anisotropy of the drawn cup specimen is negligible. The moderate combination of strain hardening exponent and strain rate sensitivity and the formation of deformation twins in the edge region play important roles in successful deep drawing. In the meanwhile, the texture evolution of CoCrFeMnNi HEA has similarities with conventional fcc metals.

In the present work, the deformation mechanisms of the CoCrFeMnNi high-entropy alloy at 77 K were investigated using thermal activation analyses. The strain rate jump test was performed to estimate strain rate sensitivity and the activation volume of the alloy. Transmission electron microscopy analyses were performed to identify the evolution of twins at low temperatures. The insensitivity of activation volume to strain observed in the CoCrFeMnNi alloy at 77 K was different from the observed increase in the activation volume with strain at room temperature, which occurred due to the shearing of nanoscale inhomogeneities, such as co-clusters and short-range orders.

CoCrFeMnNi high-entropy alloys containing 1 at% carbon (C-HEAs) were additively manufactured using selective laser melting (SLM). Superior tensile properties of the as-built C-HEA (to previously reported ones) were achieved by utilizing multiple strengthening mechanisms (i.e. solid solution, grain refinement, dislocation density, and nano-precipitation) by this new manufacturing method. In particular, the SLM process could allow to maximize the strengthening effect of carbon addition to HEAs via finely distributed nano-carbides at the boundaries of solidification cellular structure. This work will open a new window to utilize the SLM process for enhanced mechanical properties of HEAs with great potential. CoCrFeMnNi high-entropy alloys containing 1 at% carbon were successfully produced by selective laser melting, and the alloys exhibited much better tensile performance than additive manufactured high-entropy alloy previously reported.

International audience; The contribution of kinematic hardening to the overall strain hardening of a high entropy alloy (HEA) was investigated for the first time ever. It was assessed for a single-phase equiatomic CoCrFeMnNi alloy based on a study of the Bauschinger effect. The observed occurrence of high back stresses signifies substantial kinematic hardening. We attribute it to the low probability of cross-slip in this HEA which, however, rises in the process of straining. A model that captures the mentioned effects was proposed and validated.

Deformation-induced twinning has been a notable example of overcoming the strength/ductility trade-off dilemma as a strengthening mechanism. By borrowing this concept from the area of TWIP steels, we designed a thermomechanical treatment for a CoCrFeMnNi high-entropy alloy to improve its mechanical characteristics. We used pre-straining at 77 K to introduce deformation-induced twins in the microstructure of the alloy, and then recovered it by annealing at 773 K, while avoiding recrystallization. The deformation-induced twins generated by pre-straining at 77 K were retained after this heat treatment, whilst partial recovery of dislocations occurred. As a result, the room-temperature mechanical properties of the alloy, including its strain hardening ability, were improved substantially.

New AlxCo50−xCu50−xMnx (x = 2.5, 10, and 15 atomic %, at%) immiscible medium-entropy alloys (IMMEAs) were designed based on the cobalt-copper binary system. Aluminum, a strong B2 phase former, was added to enhance yield strength and ultimate tensile strength, while manganese was added for additional solid solution strengthening. In this work, the microstructural evolution and mechanical properties of the designed Al-Co-Cu-Mn system are examined. The alloys exhibit phase separation into dual face-centered cubic (FCC) phases due to the miscibility gap of the cobalt-copper binary system with the formation of CoAl-rich B2 phases. The hard B2 phases significantly contribute to the strength of the alloys, whereas the dual FCC phases contribute to elongation mitigating brittle fracture. Consequently, analysis of the Al-Co-Cu-Mn B2-strengthened IMMEAs suggest that the new alloy design methodology results in a good combination of strength and ductility.

The nature of obstacles to dislocation motion in CoCrFeMnNi alloy was analyzed using the thermally activated deformation analyses at low temperatures. The strong temperature dependence of yield stress and small activation volume in CoCrFeMnNi favor the dislocation glide over the obstacles with high friction stress. The activation volume of CoCrFeMnNi alloy (10–100 b3) in this study is much smaller than those of conventional FCC metals (102~103 b3), but close to those observed in BCC metals (8–100 b3) and HCP metals (5–100 b3). The increase of the activation volume with strain supports overcoming the nanoscale inhomogeneity such as co-clusters and/or short range orders as the rate controlling mechanism. The transition of dislocation structure from planar array to cell structure at 20% strain in CoCrFeMnNi reported in the literature can be attributed to the prevalent shearing of nanoscale inhomogeneity with strain.

In this study, we achieved hetero-structuring of multi-phase (CuFe)100-xMnx (x = 15, 20, and 33 at%) medium-entropy alloys having excellent strength-ductility synergy by systematic investigation of microstructural and mechanical responses. The significant hetero-deformation-induced strengthening of the alloys is attributed to the high plastic incompatibility between Cu-rich and Fe-rich phases from thermally- and deformation-induced martensitic phase transformation.

A deformation-induced phase transformation in a face-centered cubic (FCC) Co 20 Cr 26 Fe 20 Mn 20 Ni 14 high-entropy alloy during cryogenic high-pressure torsion (HPT) discovered in this work is reported. Thermodynamic calculations prove that in the Co 20 Cr 26 Fe 20 Mn 20 Ni 14 alloy, a hexagonal close-packed (HCP) phase is stable at low temperatures. Microstructural and compositional analyses suggest that a diffusionless deformation-induced FCC-to-HCP phase transformation occurred during cryogenic HPT.

In this work, the mechanical characteristics of high-entropy alloy Co20Cr26Fe20Mn20Ni14 with low-stacking fault energy processed by cryogenic and room temperature high-pressure torsion (HPT) were studied. X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) analyses were performed to identify the phase and microstructure variation and the mechanical properties characterized by Vickers hardness measurements and tensile testing. Cryogenic HPT was found to result in a lower mechanical strength of alloy Co20Cr26Fe20Mn20Ni14 than room temperature HPT. Microstructure analysis by SEM and TEM was conducted to shed light on the microstructural changes in the alloy Co20Cr26Fe20Mn20Ni14 caused by HPT processing. Electron microscopy data provided evidence of a deformation-induced phase transformation in the alloy processed by cryogenic HPT. Unusual softening phenomena induced by cryogenic HPT were characterized by analyzing the dislocation density as determined from X-Ray diffraction peak broadening.

The enhancement of strength in materials by conventional strengthening mechanism is always accompanied by loss of ductility due to the reduced strain hardenability, leading to a strength–ductility trade-off. In this paper, we engineered Al0.3CoCrNi medium-entropy alloy to contain multi-phase hierarchical microstructure and demonstrated a high yield strength of ∼1 GPa, a high tensile strength of ∼1.2 GPa, with a uniform elongation and total elongation of ∼28% and ∼39%, respectively. The multi-phase hierarchical microstructure is developed by a simple processing route of cold rolling followed by annealing. The superior combination of strength and ductility is primarily attributed to the generation of back stress, high strain hardening rate, and formation of deformation twins.

We engineered an Al0.3CoCrNi medium-entropy alloy with heterogeneous grain structure and nanoscale precipitation through thermo-mechanical processing route, with an aim to achieve a remarkable combination of cryogenic yield strength and ductility. The alloy exhibited an exceptional combination of high cryogenic yield strength of ∼1.1 GPa and high uniform elongation of ∼37%.

In this study, the superplasticity of nanostructured V10Cr15Mn5Fe35Co10Ni25 (at%) high-entropy alloy processed by high-pressure torsion was investigated using high-temperature tensile testing in the temperature range of 873–1073 K and strain rate range of 5.0✕10−4 to 1.0✕10−2 s−1. The alloy exhibited extreme elongation at these elevated temperatures, with the greatest elongation of 770% at 973 K without any necking or a notable cavity in the fracture area. Other impressive achievements were also recorded (700% elongation at 1073 K and 3.3✕10−3 s−1 and 600% elongation under other conditions). The equiaxed microstructure was maintained in both the deformed and undeformed regions of the tensile specimen, demonstrating that grain-boundary sliding is the dominant mechanism of superplasticity.

A constitutive model based on the dislocation glide and deformation twinning is adapted to face-centered cubic high-entropy alloys (HEAs) as exemplified by the CrMnFeCoNi system. In this model, the total dislocation density is considered as the only internal variable, while the evolution equation describing its variation during plastic deformation is governed by the volume fraction of twinned material. The suitability of the model for describing the strain hardening behavior of HEAs was verified experimentally through compression tests on alloy CrMnFeCoNi and its microstructure characterization by electron backscatter diffraction and X-ray diffraction using synchrotron radiation. [GRAPHICS] . IMPACT STATEMENT We adopted a constitutive model based on dislocation density and twin volume fraction evolution, to analyze the deformation behavior of the high-entropy alloy CrMnFeCoNi theoretically.

Ultrafine-grained (UFG) Ti for improved mechanical performance as well as its surface modification enhancing biofunctions has attracted much attention in medical industries. Most of the studies on the surface etching of metallic biomaterials have focused on surface topography and wettability but not crystallographic orientation, i.e., texture, which influences the chemical as well as the physical properties. In this paper, the influences of texture and grain size on roughness, wettability, and pre-osteoblast cell response were investigated in vitro after HF etching treatment. The surface characteristics and cell behaviors of ultrafine, fine, and coarse-grained Ti were examined after the HF etching. The surface roughness during the etching treatment was significantly increased as the orientation angle from the basal pole was increased. The cell adhesion tendency of the rough surface was promoted. The UFG Ti substrate exhibited a higher texture energy state, rougher surface, enhanced hydrophilic wettability, and better cell adhesion and proliferation behaviors after etching than those of the coarse- and fine-grained Ti substrates. These results provide a new route for enhancing both mechanical and biological performances using etching after grain refinement of Ti.