## 1 Introduction  
The design concept of the traditional alloy is mainly based on one or two elements, and a small number of other elements are added to optimize the performance. High-entropy alloys (HEAs), first reported in 2004, are novel high-performance metal materials with broad application potential which in recent years have become one of the research hotspots of metallic materials [1, 2]. HEAs containing alloying elements with equal atomic content possess excellent mechanical properties, and the HEA concept represents a significant breakthrough in the compositional and structural design of alloys in physical metallurgy [3, 4]. However, there are still gaps between experimental research and engineering applications. One of the problems is that with a single-phase it has been found challenging to reach an appropriate balance between strength and ductility [5]. Previous mechanical performance results indicated that the HEAs with FCC structures possessed superfluous ductility but low strength, while alloys with BCC structures have good strength but poor plasticity [6]; from the alloy preparation perspective, most HEAs are made by casting. The problem is the formation of as-cast defects, therefore engineering application and development of HEAs are limited [7, 8]. In view of this, [PERSON] et al. [9] designed an AlCoCrFeNi2.1 eutectic high-entropy alloy with a pearlite-like layer structure. In addition, the ordered BCC phase and Laves phase have been the strengthening phase in eutectic HEAs (EHEAs). Through appropriate alloy design, EMEAs with a lamellar structure, usually containing a high strength phase (intermetallic compound of BCC or HCP) and a high plasticity FCC phase [10; 11; 12]. EHEAs with FCC and Laves phases in the form of lamellae have been reported to excellent casting properties and fewer casting defects in the ingot [10; 12]. Moreover, eutectic structures also have advantages of high temperature creep resistance, low-energy phase boundaries, and controllable microstructures [11]. As for Laves phase, the performance of M\({}_{2}\)Ta is superior to that of M\({}_{2}\)Nb which is the strengthening phase in CrCoFeNiNb\({}_{0.65}\) EHEA [11]. Therefore, EHEAs strengthened by M\({}_{2}\)Ta Laves phase are expected to possess ideal mechanical properties. The CrCoNi ternary alloy is a typical medium-entropy alloy (MEA) with FCC structure and the properties of CrCoNi have been reported to be significantly better than those of CrMnFeCoNi [13; 14; 15]. The CrCoNi MEA has been proved to be a single FCC solid solution, none of the composition fluctuations or long-range order are observed [13]. Recently, the study by [PERSON] [12] showed that the CoCrFeNi binary HEA was modified into a eutectic alloy successfully by adding Ta elements. We systematically evaluated the alloying elements in Cr, Co, Ni and found eutectic reactions existed in Cr-Ta [16], Co-Ta [17], and Ni-Ta [18] simultaneously. Luckily, all the eutectic points in binary alloy systems are located approximately at 11% atomic percentage. Accordingly, CrCoNiTax MEAs can be designed into eutectic microstructures. Since the atomic radius of Ta is significantly larger than that of the other three elements--Cr, Co and Ni--the addition of Ta enhances solid solution strengthening and further strengthens the alloy based on the FCC structure. In addition, Ta is a powerful factor for solid solution strengthening and precipitation strengthening in \(\gamma\)/\(\gamma^{\prime}\) structured Ni-based and new Co-based superalloys [19; 20]. Furthermore, the diffusivity of Ta in Ni and Co is relatively low, implying that the CrCoNiTa with lamellar structure should possess great microstructural stability at elevated temperatures. Moreover, in the past few years, the use of computational phase diagram database software to predict the formation of phases has become an important means of alloy composition design and it is very effective in the design of HEAs [11; 21; 22]. Here we proposed to design CrCoNi EMEAs according to binary phase diagrams calculation by Thermo-Calc.