## 3 Results and Discussion  
### Microstructure and Phase Composition  
The XRD patterns of the FeCrCoNiTiAl\({}_{0.6}\) HVOF coating and the laser remelted HVOF coating are shown in Figure 1. The HVOF coating exhibits a BCC-type phase. After laser remelting, the coating exhibits two kinds of BCC-type phases. Between the two BCC-type phases, BCC1 phase keeps the same with the HVOF coating and BCC2 phase has a smaller lattice constant. The phase change is mainly attributed to the characteristic of laser remelting process. During the laser remelting process, the HVOF coating was heated to a very high temperature, then cooled down at a high speed (generally \(10^{5}\) K/s [27]). However, the solidification rate in this process was much slower than thermal spraying (about \(10^{7}\) K/s [28]). In this case, the elements in the coating migrated drastically due to the thermal effect. This resulted in that the single BCC phase changed to two BCC phases. Meanwhile, a small amount of aluminum oxide phase was detected as a result of oxidation behavior. Figure 2 shows the BSE SEM micrographs of the cross-section of the FeCrCoNiTiAl\({}_{0.6}\) HVOF coating and the laser remelted HVOF coating at low magnification. The HVOF coating consists of plenty of micro-pores resulting from HVOF spray methods. In contrast, the laser remelted HVOF coating is compact and the typical lamellar structure of thermal sprayed HVOF coating disappeared. The pre-existing pores and micro-cracks were remarkably eliminated. Figure 1: X-ray diffractometer patterns of high-velocity-oxygen-fuel (HVOF) sprayed coating and the laser remelted HVOF coating. A high-magnification BSE SEM micrograph of the coatings (Figure 3) shows that some spherical particles of the powder still exist in the HVOF coating and exhibits a dendritic crystal structure. This is because the particles had a high in-flight velocity and the heating time was very short during the HVOF spraying process, resulting in that the particles were not fully melted and retained the characteristics of the HEA powder. Moreover, the spherical particles were reserved and some voids and lamellar structure appeared in the HVOF coating. After laser remelting, the coating was fully crystallized due to the fact that the HVOF coating received vast heat then was fully melted during laser remelting. In this process, the buoyancy effect promoted the gas bubbles to escape from the coating, which favored the elimination of pre-existing pores [29]. It is noted that the microstructure of the laser remelted HVOF coating consists of dendrites of a BCC1 phase (Area 2) and an interendritic BCC2 phase (Area 1), as wel as oxide (Area 3). The dendritic structure is similar to the structure in HVOF coating. In our former investigation, we found that the HVOF coating consisted of two BCC phases that showed different compositions but exhibited just one set of peaks in XRD analysis due to the similar lattice parameters [26]. During laser remelting, the HVOF coating was fully melted and then cooled down rapidly. In this process, the elements in the coating were distributed differently in dendrite and inter-dendrite regions. The migration of elements led to the phase change, which means that the lattice parameters of the BCC solid solution in HVOF coating changed and the two BCC phases can be differentiated in XRD analysis. Figure 4 shows the EDS mapping result of the laser remelted HVOF coating. The result shows that all elements can be detected in both dendrite and inter-dendrite regions. However, the elements are distributed non-uniformly, and the dendrite is rich in Al and Ni, while the inter-dendrite is rich in Fe and Cr. In contrast, Cr and Co are distributed more evenly in both regions. The compositions in different areas are investigated by EDS, which are shown in Table 1. A similar microstructure was reported in the literature [23; 30]. Figure 3: High-magnification BSE SEM micrograph of the cross-section of the coating.