Since the advent in 2004, high-entropy alloys (HEAs) have become an attraction for the world and applied in the extensive engineering fields due to their outstanding properties. These materials are exhibiting their potentiality for challenging conditions where coatings are applied through laser deposition technology. Laser deposition techniques containing laser cladding (LC), laser surface alloying (LSA), and laser surface remelting (LSR) are the modern technologies that can be applied for surface modification through HEAs. Nobler properties of the LC-HEA depositions over the conventional alloys have further explored this field in the last few years. Here, this paper provides a comprehensive state-of-the-art review for LC-HEA-based coatings, focusing on the use of laser deposition technique for HEAs, the effect of laser process parameters on mechanical properties, and microstructural evolution. The mechanical characteristics include but are not limited to hardness, wear, corrosion, and erosion resistance are discussed, with regard to their microstructure and phase composition. Besides, the existing problems, possible future developments regarding the LC-HEA depositions are predicted. Due to excellent mechanical properties and functional potential as well as a variety of element constitutions for HEA designing, laser-assisted depositions containing blended HEAs as feedstock material will have promising applications prospects in the field of material science and engineering.

Laser deposition of high-entropy alloys: A comprehensive review

The role of pin eccentricity in friction stir welding of Al-Mg-Si alloy sheets: microstructural evolution and mechanical properties

High entropy metallic glasses: Glass formation, crystallization and properties

Since the advent in 2004, high-entropy alloys (HEAs) have become an attraction for the world and applied in the extensive engineering fields due to their outstanding properties. These materials are exhibiting their potentiality for challenging conditions where coatings are applied through laser deposition technology. Laser deposition techniques containing laser cladding (LC), laser surface alloying (LSA), and laser surface remelting (LSR) are the modern technologies that can be applied for surface modification through HEAs. Nobler properties of the LC-HEA depositions over the conventional alloys have further explored this field in the last few years. Here, this paper provides a comprehensive state-of-the-art review for LC-HEA-based coatings, focusing on the use of laser deposition technique for HEAs, the effect of laser process parameters on mechanical properties, and microstructural evolution. The mechanical characteristics include but are not limited to hardness, wear, corrosion, and erosion resistance are discussed, with regard to their microstructure and phase composition. Besides, the existing problems, possible future developments regarding the LC-HEA depositions are predicted. Due to excellent mechanical properties and functional potential as well as a variety of element constitutions for HEA designing, laser-assisted depositions containing blended HEAs as feedstock material will have promising applications prospects in the field of material science and engineering. 

Fe-Co-B-Si-Nb amorphous coatings were deposited on 45 medium carbon steel under different scanning speeds by high-speed laser cladding technology. The results showed that the surface roughness of the coating became lower and the surface of the coating became smoother with the increase in scanning speed. In addition, the residual tensile stress and the thermal expansion coefficient decreased with the increase in the scanning speed. When the scanning speed was 80.38 m/min, the residual tensile stress was 106.13 ± 24.03 MPa. The residual stress was reduced by more than 50% compared with the scanning speed of 37.68 m/min. The XRD and TEM observation results of the coating at the scanning speed of 80.38 m/min showed a large volume fraction of amorphous phase and some crystalline dendrites. Under the scanning speed of 80.38 m/min, the cooling rate at the top of the coating could reach 1.08 × 105 °C/s, which greatly increased the amorphous formation ability. The hardness and wear resistance properties of the coating were also tested and analyzed. Better performance of the coatings was obtained when the scanning speed was higher.

Study on microstructure and properties of Fe-based amorphous composite coating by high-speed laser cladding

The influence of Nb doping on the structure evolution, hardness and wear behaviors of laser-cladded (Fe0.25Co0.25Ni0.25(B0.7Si0.3)0.25)100-xNbx (denoted as Nbx hereafter, x = 0, 1, 2, 3 and 4 at.%) high entropy alloy coatings were analyzed. Detailed characterization indicated that the microstructures evolve from face-centered cubic (FCC) and (Fe, Co, Ni)2B phases of the Nb-free coating to FCC, (Fe, Co, Ni)2B and amorphous phases of the Nb-containing coating (1 ≤ x ≤ 2). Notably, the glass formation ability of the coating is enhanced with the addition of Nb. As the Nb content further increases (3 ≤ x ≤ 4), it promotes the fully crystalline structures of body-centered-cubic (BCC), Laves, FCC and (Fe, Co, Ni)2B phases. The average crystalline size of the coating is refined from 3.5 to 1.3 μm as a function of Nb content. The wear resistance of the coatings improves firstly and then aggravates with increasing of Nb content, which corresponds to the variation trend of microhardness and microstrain. The Nb2 coating shows a maximum hardness value of 11.9 Gpa and excellent wear resistance due to the formation of amorphous structure and good elastic properties. The phase prediction in the coating was calculated by thermodynamic parameters. The relationships between the structure and properties of the present coating were elucidated. The new metal-metalloid high entropy metallic glassy coating and superior properties give its good promise for engineering applications.

Nb doping in laser-cladded Fe25Co25Ni25(B0.7Si0.3)25 high entropy alloy coatings: Microstructure evolution and wear behavior

The effects of the B/Si ratio on the glass formation, structure and properties evolutions for the high-entropy Fe25Co25Ni25(BxSi1-x)25 (x = 0.5, 0.6, 0.7, 0.8) coatings were investigated. The crack-free high-entropy glassy coatings were synthesized by using the optimized three-step broadband laser cladding process. The microstructure of the coatings consists of face-centered cubic (FCC), (Fe, Co, Ni)2B and amorphous phases for 0.5 ≤ x ≤ 0.6. Further the increase of the B/Si ratio (0.7 ≤ x ≤ 0.8), it promotes new tiny precipitation of (Fe, Co, Ni)3B phase. The average crystalline size and volume fraction of interdendrite regions are depending on B content. The variations in the mechanical properties of the coatings correspond to its structural evolutions. The hardness increases almost linearly from 5.24 GPa of the Fe25Co25Ni25(B0.5Si0.5)25 coating to 8.39 GPa of the Fe25Co25Ni25(B0.8Si0.2)25 coating. The wear resistance is also proportional to the x content. The mechanisms of the glass formation ability and the relationships between structure and properties of the present coatings were discussed. This combination of these excellent properties gives the new high-entropy glassy coating good prospect for the potential engineering applications.

Effect of B/Si ratio on structure and properties of high-entropy glassy Fe25Co25Ni25(BxSi1-x)25 coating prepared by laser cladding

Thermally induced microstructure evolution and effects on the corrosion behaviors of AlFeSi metallic glass coatings

Thermal sprayed aluminum coatings are widely scalable to corrosion protection of the offshore steel structure. However, the corrosion rate of the Al coating increases considerably due to the severe marine environment. It has remained a challenge to improve the corrosion resistance and protective ability of Al coatings. The superhydrophobic surface provides a potential way to improve the corrosion resistance of metal materials. Hence, the development of superhydrophobic Al coatings with superior corrosion resistance is of great interest. In this work, the feasibility of the preparation of superhydrophobic Al coatings on a steel substrate was explored. First, Al coatings were prepared onto the steel substrate by the arc-spraying process, followed by ultrasonic etching with 0.1 M NaOH solution, and afterward passivated using 1% fluorosilanes. The effects of the etching time on morphology, contact angle, and corrosion resistance of the Al coatings were evaluated. The schematic model of the fluorosilane passivation process on the Al coating surface was provided. The micro/nanoscale surface structure of the low-surface-energy fluorosilanes promotes the wetting angle of 153.4° and a rolling angle to 6.6°, denoting the superhydrophobic properties. The superhydrophobic Al coating surface displays excellent self-cleaning performance due to its weak adhesion to water droplets. The corrosion current density of the superhydrophobic Al coating (1.36 × 10-8 A cm-2) is 2 orders of magnitude lower than that of the as-sprayed Al coating (1.18 × 10-6 A cm-2). Similarly, the charge-transfer resistance is found to be 12 times larger for the superhydrophobic Al coating and the corresponding corrosion inhibition efficiency reaches 98.9%. The superhydrophobic Al coating displays superior corrosion resistance and promising applications in a marine corrosion environment.

Formation and Properties of Superhydrophobic Al Coatings on Steel.

Pure Al coating and Al2O3 particles reinforced Al composite coatings were prepared by arc-spraying. The feedstocks were pure Al wires and Al-based cored wires containing 10, 20, 30 vol.% Al2O3, respectively. The evolutions of microstructure, porosity and microhardness of the coatings with different Al2O3 contents were investigated. Microstructural analysis shows that the addition of Al2O3 particles improves density and decreases the porosity of the coating. The average hardness of the composite coatings varies from 39 to 416.5 HV0.1 as Al2O3 content increases from 0 to 30 vol.%. However, the microhardness of the Al matrix in the coatings increases first and then decreases as a function of Al2O3 contents. The tribocorrosion tests were performed under potentiodynamic polarization conditions in 3.5 wt.% NaCl solution. The cathodic shift of open circuit potential and the increase in current density are induced by sliding wear. Compared with pure Al coating, the Al2O3 particles reinforced composite coatings display a lower wear rate and coefficient of friction. The Al-20 vol.%Al2O3 composite coating shows superior resistance to tribocorrosion. The tribocorrosion mechanism of the composite coatings is delamination coupling of corrosion wear.

Yunyun Ge

Papers

Nb doping in laser-cladded Fe25Co25Ni25(B0.7Si0.3)25 high entropy alloy coatings: Microstructure evolution and wear behavior

Effect of B/Si ratio on structure and properties of high-entropy glassy Fe25Co25Ni25(BxSi1-x)25 coating prepared by laser cladding

Thermally induced microstructure evolution and effects on the corrosion behaviors of AlFeSi metallic glass coatings

Formation and Properties of Superhydrophobic Al Coatings on Steel.

Microstructure and Tribocorrosion Behavior of Al 2 O 3 /Al Composite Coatings: Role of Al 2 O 3 Addition