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Eutectic high-entropy alloys (EHEAs), as a sub-group of high-entropy alloys (HEAs), are becoming a new research hotspot in the metallic materials community because of their excellent castability, fine and uniform microstructures even in the as-cast state, high strength, and good ductility. Some of the EHEAs have shown promising potentials for industrial applications. Here, the history, interesting solidification microstructure and mechanical properties, and the design strategy of EHEAs are reviewed, and their future prospects are outlined. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Recent studies indicate that eutectic high-entropy alloys can simultaneously possess high strength and high ductility, which have potential applications in industrial fields. Nevertheless, microstructural origins of the excellent strength–ductility combination remain unclear. In this study, an AlCoCrFeNi 2.1 eutectic high-entropy alloy was prepared with face-centered cubic (FCC)(L1 2 )/body-centered-cubic (BCC)(B2) modulated lamellar structures and a remarkable combination of ultimate tensile strength (1351 MPa) and ductility (15.4%) using the classical casting technique. Post-deformation transmission electron microscopy revealed that the FCC(L1 2 ) phase was deformed in a matter of planar dislocation slip, with a slip system of {111} <110>, and stacking faults due to low stacking fault energy. Due to extreme solute drag, high densities of dislocations are distributed homogeneously at {111} slip plane. In the BCC(B2) phase, some dislocations exist on two {110} slip bands. The atom probe tomography analysis revealed a high density of Cr-enriched nano-precipitates, which strengthened the BCC(B2) phase by Orowan mechanisms. Fracture surface observation revealed a ductile fracture in the FCC(L1 2 ) phase and a brittle-like fracture in the BCC(B2) lamella. The underlying mechanism for the high strength and high ductility of AlCoCrFeNi 2.1 eutectic high-entropy alloy was finally analyzed based on the coupling between the ductile FCC(L1 2 ) and brittle BCC(B2) phases. [ABSTRACT FROM AUTHOR]

Eutectic high-entropy alloys (EHEAs), combined with good castability and exceptional properties, are becoming hotpots in the field of metallic materials. It is worth noting that the most reported EHEAs are composed of metallic elements, and the EHEAs containing non-metallic elements have been rarely reported. In this work, four novel EHEAs containing B and Si with face-centered-cubic and M 3 B 2 -type boride were designed and prepared. After a simple annealing, they exhibited high ultimate tensile strength (1048 MPã1140 MPa) and good fracture ductility (4.1%–14.7%). Compared with the as-cast alloys, the fracture ductility of the four annealing EHEAs increases by 3.4–6.1 times, while the ultimate tensile strength shows practically no reduction. The outstanding mechanical properties were attributed to the fragmentation and spheroidization of M 3 B 2 -type boride, which not only reduces the nucleation positions of microcracks, but also widens the distance across which the cracks connect. • By introducing nonmetallic elements, four new EHEAs containing B/Si elements were designed and prepared. • The four EHEAs show good heat modification. By annealing, the serrated boride is fully spheroidized into spherical-shaped. • The four annealing EHEAs showed high UTS (1048∼1140MPa) and good ductility (4.1∼14.7%). • Compared with as-cast, the ductility of the four annealing EHEAs increases by 3.4∼6.1 times, and the UTS almost not lose. [ABSTRACT FROM AUTHOR]

The influences of added Fe element on the microstructures and properties of AlCoCrFeNi high-entropy alloys ( x denoted the atomic fraction of Fe element at 0.2, 0.4, 0.6, 0.8, 1.2, 1.4, 1.6, 1.8, and 2.0) were investigated. When x = 0.2, the alloy exhibited dendrites morphology, but as the Fe content increased, the AlCoCrFeNi HEAs transformed to equiaxed grains morphology. Inside the equiaxed grains and dendritic grains, spinoidal decomposition microstructure could be clearly observed. The microstructures changed from CrNi + B2 + BCC structures to B2 + BCC mixed structures as x exceeded 0.6, the hardness declined from HV637.2 to HV460.2, and the compressive fracture strength showed a slight decrease, whereas the plastic property showed a distinct improvement with the addition of Fe element. The maximum compression strength was 2335MPa when x = 0.2, and the maximum compression ratio was 36% when x = 2.0. The alloys transformed from paramagnetic to ferromagnetic as the content of Fe element increased, and all of the alloys exhibited soft magnetic behaviors. [ABSTRACT FROM AUTHOR]

A series of CoFeNiVNb ( x = 0-1) high-entropy alloys (HEAs) were synthesized to investigate the alloying effect of the Nb element on the microstructures and properties. It was found that the microstructures change from initial single-phase FCC solid solutions ( x = 0), to a hypoeutectic microstructure (0 < x < 0.75), then to a fully eutectic microstructure ( x = 0.75), and finally to a hypereutectic microstructure (0.75 < x < 1). The eutectic microstructure consists of the FCC solid solution phase and the FeNb-type Laves phase. Nb0 ( x = 0) and Nb02 ( x = 0.2) alloys exhibit excellent ductility, the compressive plastic elongation of which can reach up to 80 % without fracture. With increasing Nb content, increased brittle Laves phase leads to the decrease of the plastic strain and the increase of the yield strength, and the Vickers hardness shows an approximately linear increase from HV 133.4 to HV 687.7. Besides, the CoFeNiVNb HEAs exhibit excellent soft magnetic behavior, in which the Nb0 alloy shows the maximum saturation magnetization ( M) of 70.33 emu/g, and minimum coercivity ( H) of 1.62 Oe. [ABSTRACT FROM AUTHOR]

Most previous researches focused on small casting ingots prepared by arc melting, when studying high-entropy alloys. Large sized ingots were also necessary in exploring the existence of volume effects in the multi-principal element alloys. During the experiments, a large sized CoCrFeNiTi0.5 alloy casting ingot was prepared by a medium frequency induction melting furnace. A slight volume effect occurred, reflecting mainly in the growth of crystalline grains and the increase of alloy hardness in the ingot. To investigate the effect of annealing temperature on microstructure and properties of CoCrFeNiTi0.5 alloy, several samples taken from the ingot were annealed at 600 °C, 700 °C, 800 °C and 1000 °C respectively for 6 h. Almost no effects were found to the crystalline structure and elemental distribution when the samples were annealed below 1000 °C. The crystalline structure of CoCrFeNiTi0.5 alloy was composed of one principal face-centered cubic (FCC) solid-solution matrix and a few intermetallic phases in the form of interdentrite. Dendrite contained approximately equivalent amount of Co, Cr, Fe, Ni and a smaller amount of Ti. When annealed below 1000 °C, the interdendrite stayed in (Ni, Ti)-rich phase, (Fe, Cr)-rich phase and (Co, Ti)-rich phase. After 1000 °C annealing, (Co, Ti)-rich phase disappeared, while (Ni, Ti)-rich phase and (Fe, Cr)-rich phase grew. The microhardness of the as-cast CoCrFeNiTi0.5 alloy was 616.80 HV and the macrohardness was 52 HRC. The hardness of the samples stayed generally unchanged after annealing. This indicated a high microstructure stability and excellent resistance to temper softening that the CoCrFeNiTi0.5 alloy exhibited. [ABSTRACT FROM AUTHOR]

Abstract: The AlCrFeNiMo x (x =0, 0.2, 0.5, 0.8, 1.0) high entropy alloys were produced by vacuum arc melting, and their microstructure and mechanical properties were investigated. AlCrFeNi, AlCrFeNiMo0.2, and AlCrFeNiMo0.5 alloys contained two body centered cubic (BCC) phases which were AlNi-type intermetallic compound and FeCr-type solid solution. The diffraction peaks of the two BCC phases separated with the increase of Mo content while Mo element preferred to dissolve into FeCr-type solid solution. When the value of x was higher than 0.5, the FeCrMo-type σ phase appeared and replaced the FeCr-type solid solution. The AlCrFeNiMo0.2 had the highest fracture strength of 3222MPa and plastic strain of 0.287. The hardness increased obviously from HV472.4 to HV911.5 with the addition of Mo element. The solid-solution strengthening of the BCC matrix and the σ phase hardening were the two main factors that strengthened the alloys. The AlCrFeNiMo0.2 high entropy alloy possesses the excellent mechanical properties which imply its potential application in industrial areas. [Copyright &y& Elsevier]

Highlights: [•] The AlCoCrFeNiV x high entropy alloys were prepared by arc melting. [•] The compressive strength and plastic strain was as high as 3297.8MPa and 26.8%. [•] V element can reduce elements segregation and homogenize the microstructure. [•] A transition from ferromagnetic to paramagnetic happens in this alloy system. [Copyright &y& Elsevier]

A cost effective Co-free AlCrFe 2 Ni 2 high entropy alloy was designed and prepared based on reported AlCoCrFeNi 2.1 eutectic high entropy alloys. The as-cast AlCrFe 2 Ni 2 alloy showed a tensile yield strength of 796 MPa, an ultimate strength of 1437 MPa, and an elongation of 15.7%, which was superior to most high entropy alloys and was even comparable with the Ti-based ultrafine-grain alloy. The alloy consisted of noodle-like FCC phases, disordered BCC (A2) phases, and ordered BCC (B2) phases. The excellent mechanical properties of the alloy were attributed to the spinodal decomposition of BCC phases and the composite effect of the softer FCC and harder BCC phases. [ABSTRACT FROM AUTHOR]

The present work aimed to develop a novel medium entropy alloy (MEA) with superior mechanical properties and biocompatibility for the metallic-bioimplant application. A series of (TiZrNb 0.7) 100-x O x (x = 0, 0.5, 0.75 at. %) MEAs were carefully designed and prepared. The microstructure, mechanical properties, corrosion resistance, and biocompatibility were investigated in detail. Single-phase body-centered cubic (BCC) structural (TiZrNb 0.7) 100-x O x MEAs consisted of as-cast dendritic morphology. Interestingly, the yield strength and ductility of (TiZrNb 0.7) 100-x O x MEAs were improved simultaneously with their O concentration. The (TiZrNb 0.7) 99.25 O 0.75 MEA had optimal mechanical properties, exhibiting a yield strength of 911 ± 11 MPa, a fracture elongation of 16.1 ± 0.7%, and an elastic modulus of 70.5 ± 1.2 GPa. According to the potentiodynamic polarization and cell culture experiment, the (TiZrNb 0.7) 99.25 O 0.75 MEA exhibited superior corrosion resistance and biocompatibility, equivalent to the commercially pure titanium (CP-Ti). These findings provide a paradigm for achieving outstanding comprehensive properties in MEA via O doping. • A series of (TiZrNb 0.7) 100-x O x (x = 0, 0.5, 0.75 at.%) MEAs were designed and fabricated. • The (TiZrNb 0.7) 99.25 O 0.75 MEA has a combination of high strength, plasticity, and low modulus. • The (TiZrNb 0.7) 99.25 O 0.75 MEA exhibites superior corrosion resistance at different pH values (7.4, 6, and 5). • The (TiZrNb 0.7) 99.25 O 0.75 MEA has better biocompatibility than CP-Ti in vitro. [ABSTRACT FROM AUTHOR]

Eutectic high entropy alloys (EHEAs) have garnered much research attention due to their excellent mechanical properties, superior castability, good oxidation resistance, and stable microstructure in a wide range of temperatures. Inspires from mature Ni and Co-based superalloys, a novel Al 20 Co 36 Cr 4 Fe 4 Ni 36 EHEA was designed and prepared to obtain good mechanical properties at both room temperature and high temperatures in this study. The novel EHEA consists of a L1 2 phase and a B2 phase with a superior ultra-tensile strength of 1005 ± 40 MPa and a ductility of 8.8 ± 0.4% at room temperature. The engineering compressive stress–strain curves at 1073 K indicated that the current EHEA performs a high strength of 321 ± 9 MPa without fracture even at a high temperature of ~0.6 T E , which is even superior to some refractory high entropy alloys (RHEAs). The hot deformation behavior and microstructure evolution of the new EHEA was investigated at the temperature range of 1073 K to 1273 K and various strain rates from 10−3 s−1 to 10−1 s−1. The constitutive equation that describes the correlations between the flow stress, strain rate, and temperatures was constructed. Continuous dynamic recrystallization (CDRX) was the primary dynamic recrystallization (DRX) mechanism in both the L1 2 and B2 phases during hot deformation. However, the proportion of dynamic recrystallization grains in the L1 2 phase is higher than that in the B2 phase according to the high angle grain boundaries (HAGBs) under the same deformation condition, suggesting that the CDRX kinetics was faster and stronger in the L1 2 phase. The present study provided a new thought for designing EHEAs and an insight into the hot deformation behavior of EHEAs with lamellar morphologies. • This work proposed a novel Al 20 Co 36 Cr 4 Fe 4 Ni 36 EHEA with a good strength-ductility combination in a wide range of temperature. • CDRX was the primary dynamic recrystallization mechanism in both L1 2 and B2 phases during hot deformation. • Dynamic recrystallization of L1 2 phases is prior to that of B2 phases. [ABSTRACT FROM AUTHOR]

In this study, CoCrFeNiTa x (x value in molar ratio, x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.75) high entropy alloys were prepared to investigate the alloying effects of Ta on the microstructures and mechanical properties. With the addition of Ta element, the microstructure changed from an initial single FCC solid solution (x = 0) to a hypoeutectic microstructure (x = 0.1–0.3), then to a fully eutectic microstructure (x = 0.4) with a mixture of FCC and Co 2 Ta-type Laves phases, and finally to a hypereutectic microstructure (x = 0.5–0.75). The Ta-free (CoCrFeNi) alloy showed high ductility but low strength. With the addition of Ta, the yield strength and hardness of the CoCrFeNiTa x alloys increased but decreased in ductility. Therein, the fully eutectic CoCrFeNiTa 0.4 alloy displayed a high fracture strength of 2293 MPa with a compression plasticity of 22.6%. The regular/uniform ultrafine lamellae structure was the primary reason for the remarkable properties of the CoCrFeNiTa 0.4 eutectic high entropy alloys. [ABSTRACT FROM AUTHOR]

Eutectic high entropy alloys (EHEAs) hold promising industrial application potential, but how to design EHEA compositions remains challenging. In the present work, a simple and effective strategy by combining mixing enthalpy and constituent binary eutectic compositions was proposed to design EHEA compositions. This strategy was then applied to a series of (CoCrFeNi)M x (M = Nb, Ta, Zr, Hf) HEAs, leading to the discovery of new EHEAs, namely, CoCrFeNiNb 0.45 , CoCrFeNiTa 0.4 , CoCrFeNiZr 0.55 and CoCrFeNiHf 0.4 . The microstructure of these new EHEAs comprised of FCC and Laves phases in the as-cast state. The experimental result shows that this new alloy design strategy can be used to locate new EHEAs effectively. [ABSTRACT FROM AUTHOR]

Dislocation-twin interactions play important roles in tuning the plastic deformation of materials. Here, we report the great influence of interstitial oxygen atoms on dislocation-twin interactions in NiCoCrO medium entropy alloy through in-situ transmission electron microscopy mechanical testing, atomic resolution energy-dispersive X-ray spectroscopy characterizations, and integrated differential phase-contrast scanning transmission electron microscopy imaging. The spreading lattice distortion in medium-entropy alloy was found to enable large oxygen interstitial solubility. Oxygen atoms occupied both the tetrahedral and octahedral interstitial sites, and tended to be enriched at defects such as twin boundaries and stacking faults. Oxygen-enriched regions near the twin boundary hindered dislocations from direct transmitting or cross-slip into another twin plane during plastic deformation, resulting in significant dislocation entanglement, partial dissociation of dislocations, and the deviation of slip directions. Our results give insight into the strengthening mechanisms associated with the synergetic effects of twin boundary and interstitial atoms. [Display omitted] • Massive interstitial oxygen doping was achieved in NiCoCr-based HEA. • Oxygen atoms occupy both octahedral and tetrahedral interstitial sites. • The segregation of oxygen atoms affects dislocation-twin interaction. [ABSTRACT FROM AUTHOR]

In this paper, Co 2 Mo x Ni 2 VW x (x = 0.5, 0.6, 0.8, 1.0, 1.5, and 1.75 in molar ratio) eutectic high entropy alloys (EHEAs) were designed to achieve a combination of strength and ductility. The effects of Mo and W elements on the microstructures and mechanical properties of the alloys were investigated, and results showed that by decreasing the Mo and W contents to 0.8 in Co 2 Mo x Ni 2 VW x alloy, a fully eutectic microstructure was obtained. By increasing the Mo and W contents to 1.75, a primary BCC solid solution phase plus eutectic microstructure were obtained. The eutectic microstructure consisted of a mixture of FCC solid solution phase and μ phase. The Co 2 Mo 0·8 Ni 2 VW 0.8 alloy with the largest volume fraction of eutectic microstructure exhibited the highest compression strength of 2364 MPa, while the Co 2 Mo 1·75 Ni 2 VW 1.75 alloy with the largest volume fraction of BCC phase exhibited the highest yield strength of 1607 MPa. [ABSTRACT FROM AUTHOR]

Eutectic high entropy alloys (EHEAs) with superior mechanical properties and good castability are one of the most promising candidates among high entropy alloys (HEAs) for engineering applications. In this study, a new Co-free Al 0.75 CrFeNi EHEA with a low density of ~ 6.56 g/cm3 was designed and prepared, the solidification process and ambient deformation behavior of the alloy were investigated in detail. Al 0.75 CrFeNi was composed of a disordered body-centered cubic (BCC) phase and an ordered body-centered cubic (B2) phase, which grew alternately to form a beautiful sunflower-like eutectic morphology. The alloy exhibited excellent mechanical properties with a high yield strength (σ 0.2) of 1482 ± 56 MPa, fracture strength (σ bc) of 3639 ± 190 MPa, and elongation to fracture of ~37.8% ± 1.5%. The high strength and low density of the alloy lead to a superior specific yield strength of ~ 226 KPa•m3/kg. During deformation, dislocation gliding in the BCC phase dominated the plastic deformation in the early stage. Subsequently, the dislocation activated in the B2 phase when the strain was further increased. Dislocation interactions and microstructural features, such as grain boundaries and substructures, in different stages generated the good balance between the strength and the ductility of the alloy. • A Co-free Al 0.75 CrFeNi EHEA with a beautiful sunflower-like eutectic morphology was designed. • The EHEA exhibits a high yield strength of 1482 ± 56 MPa, and a good ductility of ~37.8% ± 1.5%. • The dislocation glide mainly governed the plastic deformation of the EHEA during deformation. [ABSTRACT FROM AUTHOR]

Eutectic high-entropy alloys (EHEAs) have drawn extensive attention due to their excellent castability and remarkable combination of strength and plasticity. In this study, a new Fe 2 Ni 2 CrMo x (x = 0.25, 0.50, 0.75, 1.0, 1.25, 1.50) EHEA family was designed and prepared, with the dependence of microstructural and mechanical evolution on the Mo addition investigated. As the increase of Mo addition in the Fe 2 Ni 2 CrMo x EHEA, the microstructure evolves from hypoeutectic (x = 0.25) to near-eutectic (x = 1.25) and then to hypereutectic (x = 1.50), where the stacked lamellae consist of a face-centered cubic (FCC) phase and a topological close-packed (TCP) σ phase. Among all the compositions, the Fe 2 Ni 2 CrMo 1.5 EHEA exhibits the most excellent comprehensive mechanical properties, with the yield strength, fracture strength and compressive plasticity being 1503 MPa, 2199 MPa and 17.6%, respectively. • The near-fully eutectic high entropy alloy Fe 2 Ni 2 CrMo 1.25 is designed with FCC phase and σ phase. • The eutectic phase formation criteria, the effect of ΔH mix , VEC, and δ r is more applicable for Fe 2 Ni 2 CrMo x EHEAs. • The strength and hardness of Fe 2 Ni 2 CrMo x EHEA family are improved with the addition of Mo. [ABSTRACT FROM AUTHOR]

Tribological behavior of the model AlCoCrFeNi 2.1 eutectic high-entropy alloy (EHEA) sliding against different counterfaces including Al 2 O 3 , Si 3 N 4 , SiC ceramics and GCr15 steel at room temperature (RT), and particularly SiC at elevated temperatures were investigated. Our EHEA was found to maintain high hardness (＞197 HV) up to 900 °C. The RT tribology tests show that the friction and wear of the EHEA are slightest when against SiC. The wear rates of the EHEA increases monotonously from RT to 900 °C, but the friction coefficient drops sharply (＜0.35) when the temperature exceeds 600 °C. This is attributed to the enhancing in the oxidative wear and the formation of deformed fine-grained layer beneath the worn surface. Image 1 • Tribological behavior of the AlCoCrFeNi 2.1 EHEA against different counterfaces was investigated. • The EHEA/SiC tribo-pair shows the optimal compatibility at RT. • The high hardness of over 197 HV of the EHEA could be maintained up to 900 °C, superior to the Inconel 718 superalloy. • The EHEA exhibits good wear resistance over a wide temperature range from RT to 900 °C. [ABSTRACT FROM AUTHOR]