## 3 Results and Discussion
### Mechanical Properties
On one hand, the Ta\({}_{0.4}\) has high uniformity of structure, on the other hand, lamellar spacing in Ta\({}_{0.4}\) is much smaller. Therefore, nano-hardness and elastic modulus of eutectic of Ta\({}_{0.4}\) alloy among all alloys were the highest. The compression stress-strain curves are shown in Figure 8a, 0.2% yield stress and fracture strain are also presented in Figure 8b. Ta\({}_{0.1}\) alloy did not fracture, which was similar to FCC phase, the alloy shows high ductility and low strength. When the Ta addition x varies from 0.3 to 0.7, the fracture properties firstly increased and then decreased, Ta\({}_{0.7}\) alloy has the highest hardness and 0.2% yield strength, Ta\({}_{0.4}\) alloy exhibits the strongest compression strength up to 2502 MPa with 20.6% compression plasticity. The mechanical properties results show that Ta\({}_{0.4}\) alloy possesses both high ductility and high strength. The plastic strain of the alloy was significantly reduced from 33.4 to 6.5% due to the increasing volume fraction of the brittle and hard Laves phase. Nanoindentation results show the nano-hardness of \(\gamma\) phase in Ta\({}_{0.3}\) is higher than that in Ta\({}_{0.1}\), EDS results also represent that more content of Ta dissolved into the \(\gamma\) phase. In addition, Ta exhibits large negative mixing enthalpy, the mixing enthalpy for Ta-Cr, Ta-Co, Ta-Ni are \(-7\), \(-24\) and \(-29\) kJ mol\({}^{-1}\), respectively [22]. Therefore, Ta causes an increase in the lattice strain, distortion and strengthens the \(\gamma\) FCC phase. Since the Laves phase is a hard and brittle intermetallic phase, it strengthens the alloy at the expense of ductility. The nano-hardness of Laves phase in Ta\({}_{0.5}\) and Ta\({}_{0.7}\) is very close. In fact, the coarse Laves phase formed in hyper-EMEAs can significantly damage the boundary strength [31]. Therefore, the plasticity and strength of Ta\({}_{0.5}\) and Ta\({}_{0.7}\) decreased sharply. Figure 6: Hardness of CrCoNiTa alloys. Nevertheless, the state of the lamellar structure in the eutectic alloy is also one of the important factors in the performance of the alloy. The relationship between yield strength and lamellar spacing has been successfully established according to a Hall-Petch equation. For the EMEA, the equation is as follows:  
\[\sigma_{\text{EMEA}}=\ \sigma^{*}+\frac{k}{\sqrt{\lambda}} \tag{7}\]  
Figure 8: (**a**) Compression engineering stress-strain curves; (**b**) compression strength, 0.2% yield strength and fracture strain of CrCoNiTa\({}_{\text{x}}\) alloys. Figure 7: Typical load-displacement curves (**a**), hardness and elastic modulus (**c**) of primary phase in Ta\({}_{0.1}\), Ta\({}_{0.3}\), Ta\({}_{0.4}\), Ta\({}_{0.5}\) and Ta\({}_{0.7}\), typical load-displacement curves (**b**), hardness and elastic modulus (**d**) of eutectic in Ta\({}_{0.3}\), Ta\({}_{0.4}\) and Ta\({}_{0.5}\). where \(\sigma^{*}\) is a frictional stress, \(k\) is the Hall-Patch slope, and \(\lambda\) is the lamellar spacing. Eutectic in Ta\({}_{0.3}\), Ta\({}_{0.4}\) and Ta\({}_{0.5}\) is chosen to ensure accuracy. Figure 9 shows the mechanical properties of HEA with a typical eutectic structure and compares them with the results of this work, (CoFeNiV\({}_{0.5}\)Nb\({}_{x}\)[30], Co2 Mo\({}_{x}\)Ni2 VW\({}_{x}\)[32], CrCoFeNiNb\({}_{x}\)[11], CoCrFeNiTa\({}_{x}\)[12], CoFeNiVMo\({}_{x}\)[33]), alloys with full eutectic microstructure are described as \"eutectic\". Compared with other alloys, EMEA CrCoNiTa\({}_{0.4}\) possesses relatively good compression strength (2502 MPa) and good plastic strain (20.6%) compared with EMEA CoCrFeNiTa\({}_{x}\) (compression strength 2377 MPa, plastic strain 17.5%).