Microstructure and Mechanical Properties Investigation of the CoCrFeNiNbx High Entropy Alloy Coatings

Abstract

In this work, the CoCrFeNiNb x (x: molar ratio, x = 0.45, 0.5, 0.75, and 1.0) high entropy alloy coatings were synthesized on a 304 stainless steel substrate by laser cladding to investigate the effect of Nb element on their microstructure, hardness, and wear resistance. The results indicated that in all of the CoCrFeNiNb x alloy coatings, two phases were found: One was a face-centered cubic (FCC) solid solution phase, the other was a Co 1.92 Nb 1.08 -type Laves phase. The microstructures of samples varied from hypoeutectic structure (x = 0.45 and 0.5) to hypereutectic structure (x = 0.75 and 1.0). The Vickers hardness of CoCrFeNiNb x alloy coatings was obviously improved compared with the substrate. The hardness value of the CoCrFeNiNb 1.0 alloy coating reached to 590 HV, which was 2.8 times higher than that of the substrate. There was also a corresponding variation in wear properties with hardness evolutions. Wherein the hypereutectic CoCrFeNiNb 1.0 alloy coating with the highest hardness exhibited the best wear resistance under the same wear condition, the dry wear test showed the wear mass loss of CoCrFeNiNb 1.0 alloy coating was less than a third of the substrate. The high hardness and wear resistance properties were considered with the fine lamellar eutectic structure and proper combination of FCC and Laves phases.Crystals 2018, 8, 409 2 of 11 superconductivity [24,25], soft magnetism [26,27], etc. As a result, HEAs can be an attractive material for a range of applications. However, the cost of HEAs is higher than that of conventional alloys because of the expensive high-purity raw materials. To reduce the cost, the HEA coatings obtained by laser cladding with advanced properties continue to be in great demand. At present, laser cladding as a surface coating method has been the subject of extensive research [28,29]. Advantages of laser cladding are the integrity of the fusion bond between the cladding coating and substrate, smaller thermal effects on the substrate, high process flexibility, high working speeds, and environmental protection. Moreover, the rapid solidification of laser cladding results in the finer microstructure and extended solid solution, in addition to the high-performance surface with enhanced properties. Stainless steels are widely used in industry and daily life due to their high corrosion, oxidation resistance, and workability. However, its low hardness and wear resistance are not ideal for practical applications. Therefore, the enhancement of hardness and wear resistance of the 304 stainless steels is quite necessary. Recently, there has been a growing interest in the fabrication of surface layers of HEAs on stainless steel [30][31][32]. The hardness and wear resistance of stainless steel can be improved by coating HEA on its surface. For example, FeCoCrAlNi HEA coating [33] prepared by laser cladding on 304 stainless steel possessed high hardness, which was~three times higher than that of the substrate. FeNiCoSiCrAlTi HEA coating [34] prepared by laser cladding on Q235 steel showed ...

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