Elsevier

Optics & Laser Technology

Volume 84, October 2016, Pages 23-31
Optics & Laser Technology

Full length article
Laser surface alloying of FeCoCrAlNi high-entropy alloy on 304 stainless steel to enhance corrosion and cavitation erosion resistance

https://doi.org/10.1016/j.optlastec.2016.04.011Get rights and content

Highlights

  • FeCoCrAlNi high-entropy alloy coating has been synthesized by laser alloying.

  • The formation rules of solid solutions in high-entropy alloy were investigated.

  • The microhardness of the coating was ~3 times that of the substrate.

  • The high-entropy alloy coating showed a superior corrosion resistance.

  • The high-entropy alloy coating showed a superior cavitation erosion resistance.

Abstract

FeCoCrAlNi high-entropy alloy coating was synthesized with premixed high-purity Co, Cr, Al and Ni powders on 304 stainless steel by laser surface alloying, aiming at improving corrosion and cavitation erosion resistance. Phase constituents, microstructure and microhardness were investigated using XRD, SEM, and microhardness tester, respectively. The cavitation erosion and electrochemical corrosion behavior of FeCoCrAlNi coating in 3.5% NaCl solution were also evaluated using an ultrasonic vibrator and potentiodynamic polarization measurement. Experimental results showed that with appropriate laser processing parameters, FeCoCrAlNi coating with good metallurgical bonding to the substrate could be achieved. FeCoCrAlNi coating was composed of a single BCC solid solution. The formation of simple solid solutions in HEAs was the combined effect of mixing entropy (ΔSmix), mixing enthalpy (ΔHmix), atom-size difference (δ) and valence electron concentration (VEC), and the effect of ΔSmix was much larger than that of the other factors. The microhardness of the FeCoCrAlNi coating was ~3 times that of the 304 stainless steel. Both the corrosion and cavitation erosion resistance of the coating were improved. The cavitation erosion resistance for FeCoCrAlNi HEA coating was ~7.6 times that of 304 stainless steel. The corrosion resistance was also improved as reflected by a reduction in the current density of one order of magnitude as compared with 304 stainless steel.

Introduction

Conventional alloys are mainly composed of one or two principal elements via the addition of minor elements to improve their properties, such as Fe-, Al-, Mg-, Ni- and Ti-base alloys. The use of only one or two principal elements has restricted the number of alloys that can be studied and utilized. Different from the conventional alloy design concept, Yeh et al. [1] proposed the concept of high-entropy alloys (HEAs) in 2004 for the first time, breaking the bottleneck stage of conventional alloy design concept. HEAs are defined as solid-solution alloys that contain at least five principal elements, but no more than 13 principal elements with the concentrations of each principal element lying between 5 and 35 at%. According to Yeh and Zhang et al. [2], [3], HEAs exhibit simple solid solutions with BCC and/or FCC structure(s), nano-structure or even amorphous, instead of intermetallic compounds or other complex phases, which might be attributed to the effect of high mixing entropy. However, some intermetallic compounds can also form in certain HEAs [4], indicating that the formation of simple solid solutions in HEAs might not solely depend on the high mixing entropy. Then Guo et al. [5], [6] proposed the solid-solution formation rules and improved the theory of HEAs according to the Hume-Ruthery rule. Previous studies have revealed versatile properties of high-entropy alloys, such as high hardness [7], good thermal stability [8], excellent wear and corrosion resistance [9], [10]. Taking the Al0.2Co1.5CrFeNi1.5Ti HEA as an example [11], the hardness of Al0.2Co1.5CrFeNi1.5Ti HEA is similar to that of SUJ2 bearing steel and lower than that of SKH51 high-speed steel, but the wear resistance of the former is 3.6 and 2 times higher than that of the two steels, respectively.

Among the previous HEAs studied, HEAs are usually synthesized by arc melting technology or casting methods [4], [10], [11]. However, the cost of HEAs bulk material may be much higher due to the addition of expensive alloying elements. In view of this point, surface modification technology provides a solution to the problem which could yield a judicious combination of surface and bulk properties, while consuming only a small amount of expensive elements to improve the alloy properties [12]. Recently, it is reported that high-entropy alloy coatings with the thickness of ~1.7 µm were synthesized by magnetron sputtering [13], but the thickness was too thin to meet the mechanical requirements in field applications. Recognizing this, laser surface alloying was attempted because of its unique features such as the rapid melting-solidification process (104–106 K s−1), which contributed to the formation of non-equilibrium phases, a dense coating bonded metallurgically to the substrate and homogeneity in microstructures. In addition, the thickness of the coating can reach the millimeter range [14], [15].

Though the microstructure and mechanical properties of the AlCoCrCuFeNi HEAs system have been investigated [1], the rules of phase formation and cavitation erosion resistance have not been reported. It can be concluded that cavitation erosion is formed under the interaction of mechanical, chemical and electrochemical processes during water hammer effect. In most cases, the synergistic effect between the mechanical and electrochemical processes plays an important role in contributing to the total mass loss [16], [17]. Therefore, only the material with both good mechanical property and good corrosion resistance may serve well under hydraulic condition. Laser surface modification of stainless steel using various elements (Co, Ni, Mn, C, Cr, Mo and Si) and alloys or compounds (AlSiFe, NiCoCrB and Si3N4) was reported in a number of studies by Kwok et al. [18]. The highest increase in cavitation erosion resistance could reach 12 folds. In comparison, laser surface melting of 316 L stainless steel could only bring about minimal increase in cavitation erosion resistance (20%). The increase was much higher by laser surface alloying [18]. NiCrSiB has been employed for laser surface alloying of stainless steel, aiming at improving the cavitation erosion resistance [19]. The resistance of laser surface-alloyed stainless steel using NiCrSiB was improved by factors of 7.4. Zhang et al. [20] have investigated the corrosion resistance of FeCoNiCrCu HEA coating prepared by laser technique. The NiCrSiB coating was selected for the comparison of the corrosion resistance. It was found that the FeCoNiCrCu coating was easier to be passivated and had a wider passive region and higher corrosion resistance compared to the NiCrSiB coating. The improvement in corrosion resistance of the HEA coating could be attributed to its structure and the formation of very stable passive films. However, the total content of Ni, Cr and Co in the FeCoNiCrCu coating was only about 60% that of the NiCrSiB coating. Therefore, the HEA coating has lower cost in preparation. In the present study, we have selected another high-entropy alloy system, FeCoCrAlNi, as a cavitation erosion resistant coating on 304 stainless steel. Compared with AlCoCrCuFeNi system, Cu was removed as it would lead to serious segregation and deteriorate corrosion resistance [21], and Fe in the substrate participated in the formation of solid solution during laser irradiation to form the FeCoCrAlNi HEA coatings. The results of the present study on rules of phase formation, microhardness, cavitation erosion and electrochemical corrosion resistance would provide essentials for further research and applications of HEA coatings.

Section snippets

Material and specimen preparation

As-received 304 stainless steel in the form of plate with dimensions of 40 mm×20 mm×10 mm was used as the substrate material. The nominal chemical composition in wt% is: 0.08 C; 19 Cr; 11 Ni; 1.0 Si; 2.0 Mn; 0.03 S; 0.035 P and Fe balance. The substrate was ground clean with 600 grade SiC paper to remove surface oxides or contaminants and then sandblasted to increase surface roughness for enhancing powder adhesion. Equiatomic ratios of the Co, Cr, Al, Ni powders of 99.9% purity and particle size

Selection of processing parameters

When a material is irradiated with a laser beam, the specific energy density absorbed by the material surface depends on a multitude of factors, which can be classified into materials properties and laser processing parameters [24]. With proper selection of the materials and laser processing parameters, crack and porous free alloying coatings with good metallurgical bonding to the substrate could be achieved. The group of materials properties includes the reflectivity of the surface to the

Conclusions

  • (1)

    With optimum process parameters, FeCoCrAlNi HEA coating has been synthesized by laser surface alloying of equiatomic ratios of Co, Cr, Al, Ni powders on 304 stainless steel, with good metallurgical bonding between the coating and the substrate.

  • (2)

    FeCoCrAlNi HEA coating exhibits a single BCC solid solution. The formation of simple solid solutions in HEAs is the combined effect of ΔSmix, ΔHmix, δ and VEC, and the effect of ΔSmix may be large enough to dominate the formation of simple solid solutions

Acknowledgments

The authors gratefully acknowledge to the financial support for this research from National Natural Science Foundation of China (Grant No. 51271126) and the Natural Science Foundation of Liaoning Province of China (Grant No. 2013020101).

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