## 3 Results and Discussions
### Microstructure, Phase Formation, and Phase Composition
In the long strip structure of the M2B phase, a large amount of B was enriched, followed by Cr and F. Moreover, the atomic ratio of Cr to B was about 2:1. With the increase of the content of B, the M2B phase separated from the intergranular structure. The M2B phase was mainly composed of Fe and Cr borides, and the Cr from the interendritic structure was segregated gradually to the M2B phase. Figure 5: Scanning electron microscope (SEM) images of the microstructures of the laser-clad FeCoCrNiBx HEA coatings. (**a**) FeCoCrNiB0,5, (**b**) FeCoCrNiB1,0, and (**c**) FeCoCrNiB1.5. Figure 6: Energy dispersive spectroscopy (EDS) maps of the laser-clad FeCoCrNiBx HEA coatings. (**a–f**) FeCoCrNiB0,5, (**g–l**) FeCoCrNiB1,0, and (**m–r**) FeCoCrNiB1.5. [PERSON] et al. [3] proposed that HEAs should contain at least five elements and that the entropy of mixing is the main factor that promotes the formation of a multi-component solid. The largest atomic radius difference among the Fe, Co, Cr, Ni, and B elements could be calculated by the following formula [28]:  
\[\Delta R_{max}=\max(R_{i}-R_{a})/R_{a}, \tag{1}\]  
where \(R_{a}\) is the average atomic radius and \(R_{i}\) represents the atomic radius of any element. According to the formula of the Gibbs free energy [29]:  
\[\Delta G_{min}=\Delta H_{mix}-T\Delta S_{mix}, \tag{2}\]  
where \(\Delta G_{mix}\) is the change in Gibbs free energy before and after the phase change, \(\Delta H_{mix}\) is the mixing enthalpy, \(\Delta S_{mix}\) is the mixing entropy, and \(T\) is the absolute temperature. The mixing entropy \(\Delta S_{mix}\) can be calculated by the following formula:  
Figure 3:0  
where \(R\) is constant gases and \(R=8.31\)J\(\cdot\)K\({}^{-1}\)-mol\({}^{-1}\) and \(C_{i}\) is the molar fraction of each element [29]. According to Equation (2), the high \(\Delta S_{mix}\) can significantly lower the free energy of solid solution with multi principal elements, thus lowering the tendency to order and segregate, which consequently allows the solid solution to more easily form and be more stable than intermetallics or other ordered phases during the solidification of alloys [15]. However, the kinetic effects by laser rapid solidification also play a more important role on the phase formation in the coating. [PERSON] et al. [30] indicated that not all HEAs only form solid solution phases during solidification, and small amount of compound phases are also separated from some HEA precipitates. Based on Equation (2), whether the alloy solidifies into a simple solid solution or into a compound is determined commonly by the \(\Delta H_{mix}\) and the \(\Delta S_{mix}\). It is noted that [PERSON] et al. [31] reported that the FeCoCrNiB\({}_{x}\) may form the \(\Delta H_{mix}\) of binary boride. As shown in Table 4, Fe and Cr borides had large \(\Delta H_{mix}\), whereas Co and Ni borides had small \(\Delta H_{mix}\). Therefore, the contents of Fe and Cr in the produced borides (M\({}_{2}\)B) were high, whereas those of Co and Ni were relatively low. These results are consistent with those of the energy spectrum measurement.