High-entropy alloys have become ideal candidates for wear- and corrosion-resistant coatings materials because of their unique structure and excellent physical properties. Owing to their outstanding corrosion resistance and formability, FeCoCrNi high-entropy alloys are used extensively in corrosion-resistant coatings. However, their comparatively low mechanical strength does not satisfy the requirements for wear- and corrosion-resistant coatings. Adding high-melting-point Mo can promote the formation of Mo-rich phases in the alloy, thereby improving its mechanical strength and crevice corrosion resistance. This study aims to investigate the microstructure, forming process, and corrosion resistance of FeCoCrNiMo by fabricating FeCoCrNiMo high-entropy alloy coatings on 45 steel round bars or 316L stainless steel primer using extremely high-speed laser cladding (EHLC) technology. This study is expected to provide some essential technicalities for high-entropy FeCoCrNiMo alloy coatings that can be applied under different and complex wear and corrosion conditions.

In this study, scanning electron microscopy (SEM) in conjunction with backscatter imaging and energy dispersive X-ray spectroscopy (EDS) was employed to characterize the microstructure and composition of the coatings. X-ray diffraction (XRD) was adopted to ascertain the physical phase composition of the coatings, where Co was utilized as the target material. Transmission electron microscopy (TEM) samples of the coatings were prepared using a focused ion beam. The microstructure of the coating was characterized using high-resolution scanning TEM, and microhardness measurements were performed on the polished coating surfaces using a Vickers hardness tester. Electrochemical and neutral salt spray experiments were conducted to evaluate the corrosion resistance of the coatings.

A reduction in the linear velocity from 15 to 5 m/min results in a decrease in the number of cracks in the cladding coated on the 45 steel substrate. Additionally, a transition from reticulated to striated cracks is observed, as shown in Fig. 5. By using 316L stainless steel as a primer and reducing the linear speed to 5 m/min, cold cracks are effectively mitigated, as illustrated in Fig. 7. Therefore, one can reasonably conclude that the heat-affected zone of the 45 steel substrate undergoes a martensitic transformation, which increases the tensile stress within the coating and resultes in reticulated or striped peritectic cracking. Nevertheless, the induction of the 316L primer reveals no alteration in the microstructure of the coatings, which is dominated by the typical lamellar eutectic microstructure, as shown in Fig. 7. In contrast to the microstructure of the coatings created under higher linear velocities, the basic microstructural characteristics remain unaltered. However, a reduction of 15 percentage points is detected in the face-centered cubic (FCC) phase (Fig. 9), which is assumed to have contributed to the modest decrease in the Vickers hardness, although the latter remained at a prominently high level.

This study employed extremely high-speed laser cladding technology to fabricate a FeCoCrNiMo high-entropy alloy coating. Subsequently, a detailed analysis of its microstructure, phase composition, forming process, and corrosion resistance was performed. The findings indicate that the coatings comprise primarily an FCC matrix phase enriched in Fe, Co, and Ni, with a body-centered cubic (BCC) precipitated phase enriched in Mo and Cr. The lower and middle regions of the coating feature columnar crystals of the FCC phase, intersperse with alternating submicron BCC/FCC lamellar eutectic structures among the dendrites. Additionally, the heat-affected zone of the 45 steel substrate undergoes a martensitic transformation, thus increasing the tensile stress within the coating and resulting in reticulated or striped peritectic cracking. Using 316L stainless steel as a primer as well as reducing the line speed effectively mitigates these cracks and maintains high hardness levels. By contrast, the upper region is dominated by equiaxed crystals with similar alternating lamellar eutectic microstructures. Compared with a standard 304 stainless steel coating, the high-entropy alloy coating exhibits a higher self-corrosion potential by 0.130 V, a significantly lower self-corrosion current density by one-sixth, and a 235-fold increase in the coating film resistance, thus suggesting substantially enhanced corrosion resistance. In conclusion, the fine and uniform FCC/BCC lamellar eutectic microstructure at the top of the coating is believed to have contributed significantly in improving the corrosion resistance.