To study the effect of Al content on the microstructure and mechanical properties of high entropy alloy CoCrCu_0.5FeTi_0.5Al_x, CoCrCu_0.5FeTi_0.5Al_x(x=0, 0.2, 0.4, 0.6, 0.8, 1.0) is melted by vacuum arc furnace. X-ray diffractometer is used to measure the crystal structure of the alloy, scanning electron microscope is used to observe the microstructure of the alloy, and Vickers microhardness tester and universal testing machine are used to test the microhardness and compressive mechanical properties of the alloy. The results show that CoCrCu_0.5FeTi_0.5Al_x is composed of FCC phase and BCC phase. With the increase content of Al, the hardness increases gradually. When x=0, the minimum hardness is 323.61 Hv, and when x=1, the maximum hardness is 764.91 Hv. The plasticity of the alloy first decreases and then increases, and the compressive strength first increases and then decreases. When x=0, its plasticity is the best with plastic deformation of 32.08%, and when x=0.2, its plasticity is the worst with plastic deformation of 5.99%. When x=0.8, the compressive strength and plasticity are the best, which are 1993 MPa and 11.43%.

High entropy alloys are emerging field in recent years. Different from traditional alloys, they are generally composed of five or more major elements, with the content of each principal element ranging from 5% to 35% (atomic fraction). Many elements are disordered but have simple phase structure. High entropy alloys have obvious advantages and huge development space. Refractory metal based refractory high entropy alloys have great concern in recent years. Refractory metal alloy with three or more high entropy is called refractory high entropy alloys. Due to the high melting point of refractory metal, the refractory high entropy alloys show good high temperature mechanical properties, high temperature oxidation resistance and corrosion resistance, which are welcomed by the mass and expected to replace the traditional high temperature alloy. In this paper, the preparation method, phase structure, mechanical properties, oxidation resistance and corrosion resistance of refractory high entropy alloys are described in detail. Finally, the development of refractory high entropy alloys are prospected.

With the in-depth study of HEAs, the rule and mechanism of the influence of the main elements of HEAs on the properties of HEAs established by experiments have a large amount of work and great uncertainty.But it can be solved through first principle calculation based on density universal funcition theory. The microstructure and properties of materials can be discussed in terms of atoms and molecules.Therefore, the CASTEP method based on plane wave pseudopotentiality was adopted to calculate CoCu_xFeNi(x=0.5,1.0,1.5) HEAs and CoCu_xFeNi(x=0.5,1.0,1.5) HEAs by first principle in this study, and the crystal structure and mechanical properties were studied. The results show that with the increase of Cu content, the lattice constant increasedbut the density and the binding energy decreased. The volume modulus K, shear modulus G, elastic modulus E and the hardness of CoCu_0.5FeNi HEAs were higher than that of other CoCu_XFeNi HEAs, but the thermodynamic stability was poor.However, with the increase of Co content, the lattice constant decreased, but the density increased and the binding energy increasedcontinuously. The volume modulus K, shear modulus G, elastic modulus E and the hardness of Co_1.5CuFeNi HEAs with higher Co content were all higher than that of other CoCuFeNi HEAs. But their binding energy was higher and stability was poor under thermodynamic conditions.

With the continuous development of science and technology, traditional alloys can not meet the requirements of modern industrial development, so a new alloy material with excellent properties is urgently needed to meet the needs of industrial development. High entropy alloys are proposed on the basis of traditional alloys, which have better properties than traditional alloys, so the research on high entropy alloys have great value. High-entropy alloys are composed of five or more principal elements, each of which has an atomic fraction greater than 5% and less than 35%. The research status and progress of bulk high entropy alloys, thin film high entropy alloys, filamentous high entropy alloys and powdery high entropy alloys were reviewed in this paper.

In this paper, the single target power adjustable characteristics of multi target magnetron co-sputtering technology were adopted. (AlCrMoNbZr)_1-x-yN_yO_x high entropy alloy coatings were deposited on Zr-4 substrates by adjusting O₂ flow rate. X-ray diffraction (XRD), scanning electron microscope (SEM), atomic force microscope (AFM), nano-indentation and electrochemical workstation were used to characterize the microstructure, morphology, nano-hardness and corrosion resistance of the (AlCrMoNbZr)_1-x-yN_yO_x high entropy alloy coating with different O₂ flow. The results showed that the phase structure of the coating was completely transformed from fcc to bcc and maintained a stable bcc structure with the increase of O₂ flow from 0 to 15 mL/min. The coating nano-hardness decreased from 22 to 7 GPa with the increase of O₂ flow. The (AlCrMoNbZr) _1-x-yN_yO_x high entropy alloy coating with O₂ flow of 10 mL/min exhibited excellent corrosion resistance, and its corrosion current density was about 6 times lower than that of the (AlCrMoNbZr)N coating with O₂ flow rate of 0 mL/min.

In order to obtain high performance coating material, MoFeCrTiWAlNb_x (x=1, 1.5, 2, 2.5, 3) high-melting-point high-entropy alloy coatings were prepared by laser cladding on W₆Mo₅Cr₄V₂AlA tool steel. The effect of Nb on the microstructure and properties of high-melting-point high-entropy alloy coatings was investigated by X-ray diffractometer (XRD), scanning electron microscope (SEM), microhardness tester and wear resistance tester. The results show that the microstructure of the coating consists of BCC phase, MC phase and a small amount of Laves phase, including the first eutectic and eutectic microstructure. With the increase of Nb content, the number of the first eutectic carbides decreases, but the size increases, and the particles phase change irregularly. The volume fraction of the eutectic microstructure increases with the BCC phase increasing and the (MC)_e phase decreasing, and the eutectic microstructure gradually changes from small irregular particles + rod dendrites to bulk particles + network dendrites. The hardness of the coatings decreases gradually with the wear resistance increasing. The wear mechanism mainly consists of the adhesive wear and the abrasive wear in the coatings.

The Al_xCoCrFeNi high entropy alloy films with different Al contents (x=0 -20at%) were prepared by DC magnetron sputtering technique in this paper. The effect of Al content on the microstructure and mechanical properties of Al_xCoCrFeNi high entropy alloy films was investigated. The results show that the addition of Al element weakens (200) diffraction peak, and the Al_xCoCrFeNi film is consisted of a uniform fcc-structured solid solution with a (111) preferential growth orientation. Due to the solid solution strengthening effect, the hardness of Al_xCoCrFeNi film is 1-2 GPa higher than that of CoCrFeNi film. When the Al content is 0.8, the Al_xCoCrFeNi film presents the finest columnar crystals. Under the strengthening effect of the Hall-Petch relationship, the hardness of Al_xCoCrFeNi film remarkably improves with the maximum value of 18.7 GPa.

The Al_xCrFeCoCuNi high entropy alloys (HEA) were prepared by laser cladding. Al contents were adjusted for further investigating the effect on wear resistance of sample. The microstructure, chemical composition and phase analyses of the cladding layer were studied by scanning electron microscopy, EDS analysis, and X-ray diffraction measurement. The result shows that, Al_xCrFeCoCuNi HEA samples consist of the cladding zone, bounding zone and heat affected zone. The bounding between cladding layer and the substrate of a good combination; the cladding zone is composed mainly of axis crystal and columnar crystals. The Al_xCrFeCoCuNi HEA coating phase structure simple (FCC and BCC structure) due to high-entropy effect. The surface microhardness of Al_xCrFeCoCuNi HEA samples up to 758 Hv, about 3 times as the substrate, and the hardness increases with increasing Al content. The high Al content gives a large improvement in wear resistance due to its high hardness. So the coating can play a good protective role on substrate AISI 1045 steel.

A high entropy alloy CuCoCrNiFe was treated at 850 ℃ for 5 h by solid carburization. SEM, EDS, EPMA and XRD were employed for microstructural characterization and phase indentification. The measurement of hardness of the carburized sample was performed.The results show that after carburization, a large number of carbides precipitate in the sample. Near the sample surface, the carbides are fine and aggregated. As the distance from sample surface increasing, the intensiveness of carbides gets lower, whereas the size increases.The hardness of sample surface rises to 240 HV, which has been improved greatly compared to that of the substrate.

Research progress and some achievements of high-entropy alloys with multi-principal elements today are summarized.Firstly,the broad design principles,four effects and several preparation methods of this new kind alloy are covered.Then major research areas of high-entropy alloys are introduced. Besides,the problems existed in application process and relevant achievements are described in detail.At last,according to the characteristics of high-entropy alloys,probable applications fields are presented.

Different content of AlxCoCrFeNiCu2 high-entropy alloy were prepared by vacuum arc melting furnace and the effect of Al content on microstructure and mechanical properties was investigated. The results showed that the as-cast high-entropy alloys are mainly composed of simple BCC and FCC solid solution. The AlxCoCrFeNiCu2(x=1,2,3) alloy phase structure is transformed from BCC to FCC with the increase of Al contents. When x=4,5 ,the content of BCC solid is increased. With the addition of Al content, the hardness of the alloy is improved. The Al4CoCrFeNiCu2 has the highest hardness. Al3CoCrFeNiCu2 appeares the best mechanical properties. Keyword : High-entropy alloy; Microstructure; Strength; Plasticity

The microstructure of AlxFeCoCrNiCu (x=0.25, 0.5, 1) with different Al content have been studied. Their electrochemical properties are compared with 304 stainless steel and AlxFeCoCrNiCu high-entropy alloys. The results show that the structure of prepared high-entropy alloy is gradually changed from a single FCC phase to FCC phase and BCC phase with the addition of Al. With the addition of aluminum from x=0.25 to 1.0, the hardness of the alloys increase from 165 to 485 HV Correspondingly. The polarization curves show that alloys has the better corrosion resistance than 304 stainless steel in 0.5 mol/L H2SO4 solution and 1 mol/L NaCl solution, and meanwhile Al0.5FeCoCrNiCu high-entropy alloy has the best comprehensive corrosion resistance.

Crystalline structure of AlCrTaTiNi high entropy alloys (HEAs) with lower resistance were prepared by a four targets magnetron sputtering method in pure argon atmosphere. Four-point probe (FPP), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used for characterization of electrical property and microstructure. The results show that the grain size is directly related to the power. The grain size is biggest while the power is 80 W, meanwhile the electrical resistivity is the lowest, about 160 . Moreover, various cross section morphologies have been evidenced by SEM shows that crystalline structure is not columns.