## 3 Results and Discussion
### Mechanical Properties  
Usually, the strength and plasticity of metals are challenging to obtain simultaneously. It is always desirable that a material be both strong and plastic, especially in advanced fields such as aerospace. How to increase the strength and plasticity of a material at the same time, or how to greatly increase the strength of a material without decreasing its \begin{table}
\begin{tabular}{c c c c c c} \hline
**Composition** & **Phase** & \(\Delta S_{mix}\)**/J mol\({}^{-1}\) K\({}^{-1}\) & \(\Delta H_{mix}\)**/kj mol\({}^{-1}\) & \(\Omega\) & \(\delta\)**/\%** & **VEC** \\ \hline CrCoNi & FCC & 9.13 & \(-\)4.79 & 1.91 & 1.35 & 8.33 \\ X = FCC & FCC & 9.64 & \(-\)5.92 & 1.63 & 2.14 & 8.30 \\ Ta\({}_{0.1}\) & FCC+Laves & 10.02 & \(-\)7.06 & 1.42 & 2.76 & 8.24 \\ Ta\({}_{0.3}\) & FCC+Laves & 10.84 & \(-\)10.66 & 1.02 & 4.12 & 8.03 \\ Ta\({}_{0.4}\) & FCC+Laves & 11.07 & \(-\)12.13 & 0.91 & 4.55 & 7.97 \\ Ta\({}_{0.5}\) & FCC+Laves & 11.24 & \(-\)13.47 & 0.83 & 4.90 & 7.89 \\ Ta\({}_{0.7}\) & FCC+Laves & 11.44 & \(-\)15.52 & 0.74 & 5.39 & 7.71 \\ X = Laves & Laves & 11.46 & \(-\)16.56 & 0.69 & 6.05 & 7.37 \\ \hline \end{tabular}
\end{table} 
Table 2: Entropy of mixing (\(\Delta H_{mix}\)), enthalpy of mixing (\(\Delta S_{mix}\)), \(\Omega\), atomic size difference (\(\delta\)), valence electron concentration (VEC) of the CrCoNiTa\({}_{\rm x}\) alloy system. plasticity, is a very challenging subject. The \(\gamma\) phase with FCC structure usually has good plasticity while the hard and brittle Lave phase with HCP structure is just the opposite of \(\gamma\) phase, having high strength and hardness. Therefore, the lamellar structure, containing both the highly plastic \(\gamma\) phase and the high-strength Laves phase, is promised to have both high strength and high plasticity simultaneously. The load of 1000 g for hardness measurement has been selected to obtain the composite hardness. Figure 6 shows the variation of hardness with Ta content in the CrCoNiTa\({}_{\rm x}\) alloys, the hardest alloys is 632 HIV when the x value reaches 0.7. According to the previous results, the increasing Ta addition causes the increasing volume fraction of Laves phase. The Laves phase in alloy is a hard and brittle phase of HCP structure, therefore with bigger volume fraction of Laves phase, comes higher hardness. To dig deeper into the correspondence between the microstructure evolution with Ta addition and the mechanical properties, nanoindentation was performed to analyze the mechanical properties of each phase in the alloys. Finally, the nano-hardness and elastic  
Figure 4: Relationship between \(\Omega\) and atomic size difference \(\delta\) indicating the evolution of FCC, eutectic, and Laves regions. Figure 5: The phase formation of FCC, eutectic, Laves on VEC versus Ta content in CrCoNiTa\({}_{\rm x}\). modulus of each phase of different microstructures, namely primary phase and eutectic structures were characterized. The typical load-displacement curve of \(\gamma\) phase of Ta\({}_{0.1}\) and Ta\({}_{0.3}\), Laves phase of Ta\({}_{0.5}\) and Ta\({}_{0.7}\) in Figure 7a, eutectic in Ta\({}_{0.3}\) Ta\({}_{0.4}\) and Ta\({}_{0.5}\) in Figure 7b. The detailed results were shown in Figure 7c,d. As shown in Figure 7a,c, the displacement curve of nanoindentation keeps decreasing, suggesting that the nano-hardness of the primary phase increases as Ta addition increased, from \(\gamma\) phase (3.81-5.23 GPa) to eutectic (9.01 GPa) and then to (15.73-16.59 GPa). The elastic modulus of each phase has the same trend as the nano-hardness variation. There was an important phenomenon, the nano-hardness of \(\gamma\) phase was evidently increased from 3.81 GPa (Ta\({}_{0.1}\)) to 5.23 GPa (Ta\({}_{0.3}\)), this was a convincing evidence that solid solution strengthening in \(\gamma\) phase was significantly enhanced. As shown in Figure 7b,d, the nano-hardness and elastic modulus of eutectic firstly increased and then decreased.