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We report the mechanical and microstructural characteristics of a medium-entropy alloy, Co17.5Cr12.5Fe55Ni10Mo5 (atomic percent, at%) at cryogenic temperatures, down to a record low temperature of 0.5 K. The alloy exhibits excellent strength and ductility combined with a high strain-hardening rate in the entire temperature range investigated. Its property profile, including the yield strength, ultimate tensile strength, strain hardening capability, and absorbed mechanical energy, is better than those of most alloys and HEAs used in cryogenics. Within the interval of extremely low temperatures considered (0.5–4.2 K), the alloy exhibits several unusual features, including anomalies of the temperature dependence of the yield strength and tensile ductility, discontinuous plastic deformation (DPF), and a change in the propensity for the deformation-induced martensitic transformation. While the occurrence of these effects in the same temperature interval may be fortuitous, we hypothesize that they are interrelated and provide a tentative explanation of the observed phenomena on this basis.

Measuring true stress–strain curve over a large-strain-range is essential to understand mechanical behavior and simulate non-linear plastic deformation. The digital image correlation (DIC) technique, a non-contact full-field optical measurement technique, is a promising candidate to obtain a long-range true stress–strain curve experimentally. This paper proposes a method for measuring true stress–strain curves over a large-strain-range during tensile testing using DIC. The wide-strain-range true stress–strain curves of dual-phase and low carbon steels were extracted on the transverse direction in the neck region. The axial strain on the neck section was estimated by averaging the inhomogeneous deformation on the cross-section of the tensile specimen. The true stress was calculated from the engineering stress and the cross-sectional area of the neck. The validity of the proposed method was assessed by comparing the experimental load–displacement responses during tensile testing with the finite element method (FEM) simulation results. The stress and strain on the neck section estimated using the FEM and DIC, respectively, were proven to satisfy the uniaxial condition and successfully obtained. The experimental results agree well with the FEM results. The proposed concept can be applied to various deformation modes for accurately measuring long-range true stress–strain curves.

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.

Constitutive modeling of CoCrFeMnNi high-entropy alloy (HEA) at cryogenic temperature (77 K) and room temperature (293 K) has been investigated. The effect of temperature on deformation behavior such as twinning, forest hardening, and back stress hardening has been established. The enhanced ductility and strength of CoCrFeMnNi HEA at 77 K are due the combination of sub-grain structure, twinning, and dislocations. This phenomenon is explained in terms of quantitative values of twin volume fraction, inter-twin spacing, and dislocation density. The isotropic kinematic constitutive model is constructed with a critical twinning stress parameter to obtain the criteria for twinning initiation. The developed finite element model simulation results at 77 K and 293 K are in good agreement with the experimental data. The model displays a smooth increase in the twin volume fraction until fracture point (maximum twin fraction region). Also, different modeling parameters are obtained for each temperature to account for the changing deformation behavior.

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.

Here, a novel alloy design strategy for improving corrosion resistance of Al15(CuFeMn)85 (at%) immiscible dual-phase medium-entropy alloy is proposed and verified experimentally. The corrosion resistance of the alloy significantly enhances with the addition of Cr. During the corrosion of the Cr-containing alloy, three different oxide layers of Cu-, Cr-, and Fe-rich oxide layers with a few nm thickness are sequentially formed as a layered structure. Such tri-layer oxide enhances corrosion resistance by shielding the surface of the matrix, and the corrosion resistance of the alloys is comparable to that of austenitic stainless steels.

Selective laser melting (SLM) offers unprecedented advantages in fabrication of metals and alloys with complex geometry and unique microstructural features with hierarchical heterogeneity. The SLM process also induces a unique cell structure with high dislocation density and solute segregation at cell boundaries. Here, we propose an innovative utilization of unique dislocation network to achieve superior mechanical properties through metastability engineering of ferrous-medium entropy alloy (FeMEA). While the high dislocation density at cell boundaries contributes to the improvement of yield strength as additional barriers of dislocation movement, the solute segregation at cell boundaries can beneficially control the phase instability of matrix in materials produced by SLM. Our results demonstrate that solute segregation at cell boundaries decreases the face-centered cubic phase stability in the matrix and activates transition of the deformation mechanism from slip to metastable plasticity (i.e., transformation-induced plasticity). Furthermore, the high density of dislocation at cell boundaries also has an effect on not only yield strength enhancement but also controlling kinetics of metastable plasticity, and it beneficially contributes the high ductility of the SLM-processed FeMEA. This work presents a new microstructural design strategy for beneficially customizing the material performance of high-quality products based on SLM-driven metastability engineering of metallic materials.

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.

A breakthrough in the strength and ductility trade-off is crucial for the development of advanced metallic materials. Herein, we present a novel heterostructured composite composed of immiscible magnesium (Mg) and ferrochrome (FeCr) with a 3D interconnected morphology and synthesized by liquid metal dealloying. Soft Mg and hard FeCr zones mutually interlock with each other. This unique interpenetrating-phase configuration leads to a significant alternation of their intrinsic mechanical properties, especially in the soft Mg zone. It causes a strong forest hardening effect, resulting in a high initial dislocation density, and the surrounding hard zones create hydrostatic pressure at the soft zone under tension. The measured yield strength of the composite is close to the upper rule of mixture while its tensile elongation is larger than that of the mixture. These outstanding mechanical properties originate from the synergetic interaction between the soft and hard zones through the immiscible interface zone. The current 3D interconnected heterogeneous composite acts a guideline for the design of advanced materials possessing physical properties beyond expectations.

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.

Ferrous medium-entropy alloys (FeMEAs) are coming into attention these days for their excellent mechanical properties. Most of the FeMEAs developed so far form metastable face-centered cubic (FCC) matrix, and “metastability engineering” that utilizes deformation-induced martensitic transformation (DIMT) from FCC to body-centered cubic (BCC) as a method to enhance work hardenability has been the key to the exceptional mechanical behaviors. However, the FeMEAs have a significant weakness: low yield strength compared with high tensile strength and ductility. In this study, partial recrystallization is presented as a solution to the current drawback of the FeMEAs. A Co18.5Cr12Fe55Ni9Mo3.5C2 (at. %) FeMEA was annealed at 800 °C for 10 and 30 min and partially recrystallized microstructures with relatively coarse non-recrystallized grains that contain profuse mechanical twins and ultrafine recrystallized grains were attained. In addition, nanosized Cr-rich M23C6-type and Mo-rich M6C-type carbides were precipitated during the annealing. The partially recrystallized FeMEA showed a yield strength of ∼1.07 GPa, significantly enhanced from ∼600 MPa of the recrystallized counterpart. Dislocation strengthening, precipitation strengthening, grain boundary strengthening, and twin boundary strengthening led to the improved strength of the partially recrystallized FeMEA. Back stress hardening owing to the heterogeneity also contributed to the high strength and work hardenability. Moreover, the transformation-induced plasticity effect from the FCC-to-BCC DIMT activated by BCC nucleation at defects within the non-recrystallized grains effectively enhanced the work hardenability, leading to ∼1.34 GPa of tensile strength and ∼30% of elongation. This study provides an insight to optimize the microstructure and corresponding mechanical properties of metastable metallic materials.