The influences of B element on the microstructure and properties of AlCoCrFeNiBx (x=0, 0.1, 0.25, 0.5, 0.75, 1.0, mol ratio) high entropy alloys were investigated. The AlCoCrFeNi high entropy alloy exhibits equiaxed grain morphology, and then turns to dendritic structures when B content x=0.1. The spinodal decomposition microstructure can be clearly observed in equiaxed grains. When x>0.1, both of the dendrite and the spinodal decomposition microstructure gradually disappear, but much borides form instead. The transformation is attributed to the high negative mixing enthalpy of Cr-B and Co-B. The microstructures of AlCoCrFeNiBx high entropy alloys change from B2+bcc structures to B2+bcc+fcc structures, and finally formed B2+bcc+fcc and borides mixing structures with the increased B elements. The hardness HV declines from 4860 to 4607 MPa, then rises to 6157 MPa with the addition of B element. The lowest hardness value is obtained when x=0.1. The compressive fracture strength shows a distinct decrease with B addition. The maximum compression strength is 2227 MPa when x=0.25. But when x reaches 0.75, the samples fracture during the elastic deformation due to the formation of hard and brittle borides. The coercive forces and the specific saturation magnetizations of the alloys decrease as the contents of B element increase. The decreasing coercive forces show a better soft magnetic behavior.

•A new strategy to design eutectic high-entropy alloys was proposed using mixing enthalpy method.•A series of eutectic high-entropy alloys were located and prepared following the method.•Using this new strategy, eutectic compositions can be designed conveniently.Although eutectic high entropy alloys (EHEAs) display homogeneously fine microstructure, excellent castability and promising industrial application potential, how to design eutectic compositions in high entropy alloys (HEAs) remains to be challenging. Here, a novel strategy, specifically, through calculation of mixing enthalpy, was used to locate eutectic compositions in HEAs. As a proof of this concept, a series of EHEAs were located and prepared following the mixing enthalpy method. Using this new strategy, eutectic compositions can be designed conveniently, once one can classify elements into two different groups. This new alloy design strategy can be readily adapted to locate new EHEAs.Download high-res image (277KB)Download full-size image

A series of CoCrFeNbxNi (x values in molar ratio, x = 0, 0.25, 0.45, 0.5, 0.75, 1.0 and 1.2) high entropy alloys (HEAs) was prepared to investigate the alloying effect of Nb on the microstructures and mechanical properties. The results indicate that the prepared CoCrFeNbxNi (x > 0) HEAs consist of a simple FCC solid solution phase and a Laves phase. The microstructures of the alloys change from an initial single-phase FCC solid solution structure (x = 0) to a hypoeutectic microstructure (x = 0.25), then to a full eutectic microstructure (x = 0.45) and finally to a hypereutectic microstructure (0.5 < x < 1.2). The compressive test results show that the Nb0.45 (x = 0.45) alloy with a full eutectic microstructure possesses the highest compressive fracture strength of 2558 MPa and a fracture strain of 27.9%. The CoCrFeNi alloy exhibits an excellent compressive ductility, which can reach 50% height reduction without fracture. The Nb0.25 alloy with a hypoeutectic structure exhibits a larger plastic strain of 34.8%. With the increase of Nb content, increased hard/brittle Laves phase leads to a decrease of the plasticity and increases of the Vickers hardness and the wear resistance. The wear mass loss, width and depth of wear scar of the Nb1.2 (x = 1.2) alloy with a hypereutectic structure are the lowest among all alloy systems, indicating that the wear resistance of the Nb1.2 alloy is the best one.

•A novel AlCrFe2Ni2 alloy was designed and prepared by induction melting.•The alloy possesses FCC+spinodal BCC/B2 composite structures.•The as-cast alloy exhibits a high tensile strength (1437 MPa) and elongation (15.7%).A cost effective Co-free AlCrFe2Ni2 high entropy alloy was designed and prepared based on reported AlCoCrFeNi2.1 eutectic high entropy alloys. The as-cast AlCrFe2Ni2 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.

•The eutectic CoFeNi2V0.5Nb0.75 high entropy alloys exhibit excellent thermal stability.•AQ-700 sample exhibits the highest hardness of HV 727.52.•AQ-800 sample shows the best comprehensive mechanical properties.The eutectic CoFeNi2V0.5Nb0.75 high entropy alloys (HEAs) were heated at 500, 600, 700, 800 and 1000 °C, respectively for 6 h and subsequently quenched in the water to investigate their thermal stability and phase transformation at high temperature. The microstructure and mechanical properties of the samples were investigated by scanning electron microscopy, X-ray diffraction, compressive and hardness tests. It was found that the as-cast CoFeNi2V0.5Nb0.75 HEAs showed a eutectic microstructure with alternating fcc solid solution phase and Fe2Nb-type Laves phase. The NbNi4-type intermetallic phase appeared when the heat-treated temperature was higher than 600 °C. With increasing quenching temperature, the volume fraction of the NbNi4-type intermetallic phase increased while that of the eutectic regions decreased. The sample quenched at 800 °C showed the most excellent comprehensive mechanical properties; its fracture strength, yield strength and plastic strain were as high as 2586.76 MPa, 2075.18 MPa and 16.73%, respectively. Moreover, the eutectic CoFeNi2V0.5Nb0.75 HEAs exhibited apparent age hardening, especially quenched at 700 °C, the hardness reached up to the maximum value of HV 727.52.

Woven carbon fibers reinforced Al–Mg (95–5 wt%) matrix composites were successfully prepared by an electromagnetic casting method. In order to improve the wetting behavior, different stabilizers were respectively used in the electroless plating process to coat the fibers with a better Ni/P layer. The lactic acid offered the most outstanding stabilizing effect. The casting results indicated that the contact time between the molten metal and the carbon fibers was vital to the bonding degree: increasing the solidification time significantly reduced the cavity defects which located near the interface; however, an overly long solidification time would result in a tendency to crack in the centers of the woven fibers. The hot-pressing treatment with optimum parameters significantly mitigated the crack and cavity defects which are common in the composite prepared through casting method. The appropriate applied compression ratio turned out to be approximately 6%. Three point bending test showed that the bending strength of the composite had an improvement for 33.6% than that of Al-alloy matrix.

•CoFeNiVMo0.6 and CoFeNi1.4VMo eutectic HEAs were found.•Mo and Ni elements show the inverse effect on the microstructure evolution.•The formation condition of the eutectic HEAs was investigated.A series of CoFeNiVMoy (y = 0–1) and CoFeNixVMo (x = 1–2) high entropy alloys were designed in order to study the effect of Mo and Ni elements on their microstructure evolution and mechanical properties. Results show that for the quinary CoFeNiVMo alloy, by either decreasing the Mo content to 0.6 or increasing the Ni content to 1.4, eutectic microstructures can be obtained. For the CoFeNiVMoy alloys, with increasing Mo content, the volume fraction of the CoMo2Ni-type intermetallic phase increases which results in a decrease of the plastic strain and an increase of the yield strength, and the Vickers hardness shows an approximately linear increase from HV 238.1 to HV 624.6. While for the CoFeNixVMo alloys, with increasing Ni content, the increased FCC solid solution phase leads to an increase of the plastic strain and a decrease of the yield strength. Among these alloys, the CoFeNiVMo0.2 alloy shows the best comprehensive compressive properties: high yield strength of 686.9 MPa and ductility above 80%. In addition, the formation condition of eutectic HEAs was also investigated. It is found that the parameters of valence electron concentration (VEC), atomic size difference (δ), enthalpy of mixing (ΔHmix), entropy of mixing (ΔSmix) and electronegativity (Δχ) cannot effectively design the composition of eutectic HEAs.

•The woven carbon fibers reinforced Al-matrix composites were successfully prepared by a casting process.•The optimization of the technological parameters and the combination mechanism were discovered.•The segregation phenomenon of elements had been properly explained by the mixing enthalpy theory.Woven carbon fibers reinforced Al–Mg (95–5 wt%) matrix composites are successfully prepared through an electromagnetic casting process. The Ni/P coating is obtained using an electroless plating method to offer a better wettability in the casting. The appropriate pre-heating temperature of reinforcement is determined to be 600 K, and the molten matrix temperature should be controlled at approximately, 1013 K. Due to the presence of Ni/P coating, the infiltrated metal is able to spread out on the fiber surface readily. Finally, the accumulation of the metal on the fibers eventually fills all interspaces in the woven fibers. The addition of Mg shows a beneficial effect during the casting process by transferring itself into the coating more readily than Al and by segregating closely around the fibers, which effectively limits the formation of the Al4C3 brittle phase. The mixing enthalpy theory is introduced to explain the distribution of elements in this composite.

High entropy alloy has attracted increasing attentions. However, to enhance the alloy strength often leads to impairment of the ductility, or vice versa. Here we reported a heat treatment approach on AlCrFeNi2Ti0.5 high entropy alloy, which can elevate the strength and ductility simultaneously. An ingot of AlCrFeNi2Ti0.5 weighing 2.5 kg was firstly fabricated by medium frequency induction melting. Then samples from the same height of the bulk ingot were annealed for 6 h at 600, 700, 800 and 1000 °C, respectively. After 1000 °C annealing, an optimal microstructure was obtained by using our approach which can make some precipitation particles distribute homogeneously in the dendrite interior while keep the interdendrite structure as a single solid solution phase. The mechanical test on this AlCrFeNi2Ti0.5 alloy sample showed that, the compressive fracture strength σbc was increased by about 600 MPa and the plastic strain εp was doubled, compared with those of the as-cast sample. Our approach can be readily adapted to large-scale industrial production of high entropy alloys with high strength and ductility by proper annealing treatment.

CrFeNiV0.5Wx and CrFeNi2V0.5Wx (x = 0.25, 0.5, 0.75, and 1.0) high-entropy alloys were prepared by vacuum arc melting. The effects of W element on the microstructures and mechanical properties of these alloys were investigated. The experimental results indicated that the CrFeNiV0.5Wx alloys were composed of σ, FCC, and BCC phases. Although the microstructures of the CrFeNi2V0.5Wx alloys were still constituted by FCC, BCC, and σ phases, the volume fraction of the FCC phase increased significantly. Dendrite morphology was also observed in the CrFeNi2V0.5Wx alloys. With the addition of W element, the hardness of the CrFeNiV0.5Wx alloys declined from 869 to 633 HV, while the hardness of the CrFeNi2V0.5Wx alloys increased from 226 to 305 HV. Moreover, the CrFeNi2V0.5Wx alloys exhibited better compressive ductility than the CrFeNiV0.5Wx alloys. This study was the first known incidence in which the FCC phase increased in the HEAs with a decrease of the valence electron concentration (VEC) value (i.e., the FCC phase of the CrFeNiV0.5Wx alloys increased with the addition of the BCC-structured W elements).

•The large sized as-cast CoCrFeNiTi0.5 high entropy alloy ingot (Φ70 × 150 mm) was fabricated.•Slight volume effect was discovered in the as-cast CoCrFeNiTi0.5 alloy.•CoCrFeNiTi0.5 HEA ingot exhibited a high microstructure stability and excellent resistance to temper softening.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.

In an electromagnetic field, the morphology of a binary faceted-faceted (FF) Ni31Si12-Ni2Si eutectic microstructure and the alloy’s mechanical properties were investigated. Hardness experiments demonstrated that the solidified ingots were significantly strengthened, and the hardness was improved to 63.1 and 786.6 on the Rockwell hardness C and Vickers hardness scales, respectively. Tests of friction and wear in stirred FF eutectic alloys showed excellent anti-fatigue and anti-adhesion wear performance. Alloy changed from an anomalous microstructure to a refined quasi-regular structure, and there was an increase in the lamellar microstructure fraction. The formation process of the refined quasi-regular microstructure and the resulting mechanical properties were investigated.

Low-melting-point alloys have an extensive applications in the fields of materials processing, phase change energy storage, electronic and electrical automatic control, continuous casting simulation, welding, etc. Specifically, the eutectic compositions make up a large number of low-melting-point alloys that are exploited because of their desirable features like single melting peaks, excellent operational reliability, and casting fluidity. However, the fundamental physicochemical properties from the current available literature on low-melting-point multi-component eutectic alloys (LMP-MCEAs) are rather rare and lowly accurate, including the exact melting temperatures and compositions, constituent phases, microstructures and morphologies, melting enthalpies, specific heats, densities, and so on. This lack of information seriously limits the development and application of low-melting-point multi-component eutectic alloys. In this paper, the low-melting-point multi-component eutectic alloys composed of Bi, Cd, Sn, Pb, and In elements synthesized by high vacuum induction melting and fundamental data were investigated by scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and density analysis instrument. Most of the LMP-MCEAs with complex eutectic morphology structures and XRD diffraction patterns could be explained with the fact that they were three-phase eutectic alloys with mixed growth way. Generally, LMP-MCEAs present an extremely low melting point between 48.3 and 124 °C and high density between 8 and 10 g/cm3.