## 3 Results and Discussions  
### Microstructure, Phase Formation, and Phase Composition  
The composition of the FeCoCrNiB\({}_{x}\) (\(x\) = 0.5, 1.0, and 1.5, respectively) mixed powders before the laser cladding, determined by the EDS detector, is shown in Table 2. Although it is difficult to accurately measure the ratio of light element B by the EDS detector (5-10% error of our EDS instrument is considered), the trend of the B ratio can be well specified by EDS. In order to improve the measurement accuracy, the EDS result was collected from many data, and the average ratio was applied to eliminate the error in this study. It can be seen from Table 2 that the composition uniformity was ensured and there was no apparent macroscopic milling loss to the elements during the process of vacuum ball milling. Figure 3 presents the interface microstructure of the polished longitudinal sections of FeCoCrNiB\({}_{x}\) HEA coatings. As can been seen in Figure 3, most of the planar grains were concentrated at the interface between the HEA coating and substrate, and a few of the columnar grains appeared in the middle of the HEA coating. It can be seen from Figure 3a that the coating and substrate were poorly connected and a local loosening phenomenon was observed. As shown in Figure 3b,c, a better metallurgical bonding between the coating and substrate was observed with the increase of B content. This phenomenon was ascribed to the constant increase in B content, which resulted in the increased degree of segregation of the elements in the HEA coating, thereby reducing the solidification temperature of frontier liquids and resulting in easier crystallization. Rapid cooling of alloy elements during the cladding process can increase the nucleation rate markedly and can eventually refine the microstructure of the HEA coating. Figure 4 presents the XRD patterns of the laser-clad FeCoCrNiB\({}_{x}\) (\(x\) = 0.5, 1.0, 1.5) HEA coatings. It was noticed that, in Figure 4a, all coatings displayed a mixture of the major face-centered cubic structure (FCC) solid solution matrix and the secondary boride precipitations. The main boride was the M\({}_{2}\)B phase (M refers to Fe, Co, Cr, and Ni). According to Figure 4a, the phases in the FeCoCrNiB\({}_{0.5}\) coating could be identified as a simple FCC solid solution with diffraction peaks at about 20 = 35.5\({}^{\circ}\), 51.0\({}^{\circ}\), 65.5\({}^{\circ}\), 81.0\({}^{\circ}\), and 96.0\({}^{\circ}\) and a very small quantity of the M\({}_{2}\)B phase with a diffraction peak at \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Alloy Type/Element** & **Fe** & **Co** & **Cr** & **Ni** & **B** \\ \hline FeCoCrNiB\({}_{0.5}\) & 22.64 & 21.92 & 21.95 & 22.36 & 11.13 \\ FeCoCrNiB\({}_{1.0}\) & 19.62 & 19.88 & 20.35 & 19.63 & 20.52 \\ FeCoCrNiB\({}_{1.5}\) & 18.37 & 17.95 & 18.13 & 18.32 & 27.23 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Chemical compositions (in at.%) of FeCoCrNiB\({}_{x}\) (\(x\) = 0.5, 1.0, and 1.5, respectively) mixed powders. Figure 3: Interface microstructure of FeCoCrNiB\({}_{x}\) laser high entropy alloying layer. (**a**) FeCoCrNiB\({}_{0.5}\), (b) FeCoCrNiB\({}_{1.0}\), and (**c**) FeCoCrNiB\({}_{1.5}\). about 57.5\({}^{\circ}\). It also can be seen that the diffraction peaks corresponding to the FCC solid solution of FeCoCrNiB\({}_{1.0}\) were slightly weaker than that of FeCoCrNiB\({}_{0.5}\), whereas the peak at 57.5\({}^{\circ}\) was slightly enhanced, and a new diffraction peak corresponding to the M\({}_{2}\)B phase was observed. For FeCoCrNiB\({}_{1.5}\) coating, some new phases have been identified as the boride precipitations at about 20 = 28.5\({}^{\circ}\), 48.5\({}^{\circ}\), and 87\({}^{\circ}\). This demonstrates that the constituent phase of coating changed from the FCC solid solution to a combination of the FCC solid solution and the M\({}_{2}\)B phase with the increase of B content. Figure 4b shows an enlarged view of the diffraction peak of the FCC solid solution (211) in the FeCoCrNiB\({}_{x}\) HEA coating.