Laser cladding technology uses high-energy-density laser as the energy source of the process, which can modify and repair the surface of the workpiece, significantly improve the surface mechanical properties of the substrate, and effectively extend the product life cycle. Laser cladding is one of the typical processes for preparing high-entropy alloys. This technology and the addition of appropriate alloying elements can prepare high-entropy alloy coatings with excellent performance. In order to clearly explain the mechanism of enhancing the hardness of laser-clad high-entropy alloy coatings after adding elements, the current research status of the hardness properties of high-entropy alloy coatings prepared by adding common elements in the laser cladding process at home and abroad is first reviewed. Among them, high-entropy alloys have a special “four effects” that promote intermetallic compounds. Their internal microstructure is generally a solid solution phase such as FCC, BCC or HCP, which is usually strengthened by solid solution strengthening, precipitation strengthening and dispersion strengthening. In addition, the laser cladding method will cause the high-entropy alloy coating to cool rapidly, thereby significantly improving the mechanical properties of the alloy. Secondly, the mechanism of the hardness enhancement of high-entropy alloy coatings prepared by laser cladding by two major categories of metal and non-metal elements is analyzed, and the influence of the addition of metal elements and non-metal elements on the hardness of high-entropy alloy coatings is summarized. Finally, effective methods were summarized for improving the hardness performance of high entropy alloy coatings prepared by laser cladding, and its future development was prospected. The research results reveal the theoretical basis for hardness enhancement of laser cladding high entropy alloy coatings, providing a theoretical basis for further development in this field.
With the continuous advancement of science and technology, the application of traditional alloy materials can no longer fully meet the application needs. Therefore, in recent years, alloy materials have become the research focus of the global academic community. Regarding the research and development of new alloy materials, Yeh et al. pointed out that high entropy alloys (HEAS) are composed of 5 or more molar ratios, and there is a high mixing entropy relationship between them. This alloy lattice structure is very complex, with high toughness, high hardness and good corrosion resistance, and can be used to manufacture high-strength metal materials. The atomic percentage of each principal element is controlled to be relatively uniform or close, in order to present it mainly in the form of a relatively simple solid solution multi-principal alloy during the forming process. Cantor et al. also proposed a new term for high entropy alloys. HEAS is mainly a single-phase or dual-phase structure during solidification, which is very different from the reactants generated by the addition of elements in traditional alloy materials. Among them, there are two more typical most densely packed structures, namely face-centered cubic (FCC) and body-centered cubic (BCC). High entropy alloys have attracted much attention due to their unique properties. They have 4 unique effects: 1) Thermodynamically, high entropy effect; 2) Structurally, lattice distortion effect; 3) Kinetics, slow diffusion effect; 4) Performance, “cocktail” effect. Due to the existence of these unique effects, high entropy alloys have special properties that other materials do not have. For example, they have high ductility, excellent corrosion resistance and thermal stability, and have anti-softening and anti-oxidation effects at high temperatures, as well as high hardness and high wear resistance. However, its low
strength performance limits its application in engineering technology. In view of the limitations of its application, a large number of studies have been devoted to expanding its application areas.
Laser cladding is a new type of surface repair and treatment method for metal materials. It can generate a coating composed of molten powder on the surface of metal materials. This coating has the characteristics of uniformity, density, and fine grains, thereby greatly improving its mechanical properties such as surface hardness, wear resistance and corrosion resistance. At the same time, laser cladding technology can generate high temperature and high pressure on the surface of the material, thereby reconstructing and regulating the grain structure, organization and phase change of the material surface, and further improving the material performance. The combination of laser cladding and high entropy alloys has opened up a new application field for this type of alloy. Compared with conventional processes such as vacuum melting, laser cladding has the advantages of small heat-affected zone, compact structure, good mechanical properties and high economic benefits.
With the continuous innovation and gradual improvement of laser cladding technology, the use of laser cladding technology to strengthen the surface of metal materials with lower hardness and prepare high-hardness high-entropy alloy coatings has broad application prospects. This paper systematically summarizes the research status of high-hardness properties of high-entropy alloy coatings prepared by laser cladding with common elements, and provides a reference for the subsequent use of laser cladding to prepare high-hardness high-entropy alloy coatings.
1 Analysis of the strengthening mechanism of coating hardness performance
1.1 Preparation of high entropy alloy coating by laser cladding
Laser cladding is a commonly used method for manufacturing high entropy alloys. Laser cladding technology uses high-power laser to irradiate the surface of the processed material, melt the irradiated area and quickly solidify it to form a coating or composite material, and achieve good bonding performance between the substrate and the coating through laser. Its advantage is that it can modify and repair the surface of the material, significantly improve the surface properties of the base layer, increase the hardness and wear resistance of the substrate, and improve its service life. Due to the rapid heating and cooling of laser cladding, there is a large temperature gradient inside it, so the growth of the microstructure has a certain directionality to a certain extent. In addition, laser cladding technology has the advantages of concentrated energy action area, small dilution rate, no change in the inherent properties of the substrate, and high forming accuracy. Therefore, it has been widely used in the field of surface modification such as complex parts. Common cladding methods are shown in Figure 1, mainly including coaxial powder feeding, pre-set powder feeding, off-axis powder feeding, wire feeding and other methods. The schematic diagram of its working principle is shown in Figure 2. It is mainly composed of a laser system, a control system, and a powder feeding system. The use of laser cladding technology to prepare high entropy alloys has many advantages. It can effectively control the composition ratio and microstructure of the alloy, enhance the uniformity and stability of the material, and improve the hardness and wear resistance of the coating. It can effectively reduce the interface and grain boundary between materials, improve their corrosion resistance and oxidation resistance; the laser power and scanning speed can be finely adjusted to achieve an efficient and accurate preparation process, greatly reducing the preparation cost and energy consumption. Ren et al. prepared a high-hardness NbMoTaWTi high-entropy alloy coating on TC4 alloy, in which the hardness of the coating was 350% higher than that of the substrate (72HV), and the wear rate was reduced by 121.9% compared with the substrate. Guan et al. prepared a FeCoNiAlCu dual-phase high-entropy alloy coating on a 38CrMoAl substrate. This dual-phase structure helps the coating have good strength and toughness. At the same time, the high-temperature wear resistance of the coating is also significantly improved. Shi et al. prepared NiCoCrMnFe high entropy alloy coating on the surface of H13 die steel, and the microhardness and wear resistance of the high entropy alloy were significantly improved.
1.2 Four major effects of high entropy alloys
The design concept of high entropy alloys is relatively special, which makes its performance have some characteristics different from traditional alloys. Based on the summary of the basic laws and characteristics of high entropy alloys, some researchers proposed four core effects that are different from traditional alloys, which are manifested in thermodynamics, kinetics, organizational structure and performance. These effects make high entropy alloys have excellent performance. Figure 3 illustrates the mechanism of these four core effects and their mutual relationship with microstructure and mechanical properties.
1.2.1 High entropy effect
The most important characteristic in high entropy alloys is the high entropy effect, which is also the most basic and most iconic effect. The relationship between the number of phases P and the number of components C can be obtained through the principle of Gibbs phase law, as shown in formula (1) P=C +1-F in the figure.
Where: P is the number of phases; F is the maximum number of thermodynamic degrees of freedom; C is the number of components.
From formula (1), it can be seen that as the alloy component C increases, the number of phases P of the alloy will increase accordingly. However, the formation law of high entropy alloys is contrary to this. In thermodynamics, the relationship between Gibbs free energy G, absolute temperature T, mixing entropy S and mixing enthalpy H is shown in formula (2) ΔGmix=ΔHmix-TΔSmix.
From formula (2), it can be seen that as the mixing entropy increases, the mixing enthalpy will decrease, and the Gibbs free energy will also decrease, so that the compatibility between the elements is improved, and it is easier to form face-centered cubic and body-centered cubic structures, thereby limiting the formation of intermetallic compounds. This situation is called the “high entropy effect”. Generally speaking, the strength of the BCC phase is higher than that of the FCC phase. This is because the BCC phase has a higher Peierls stress, that is, the atoms in the BCC unit cell need a greater force to move dislocations in the plane.
Degrees of freedom refer to the number of independent changes that a system can make under the premise that the number of phases remains unchanged at equilibrium. In a five-component alloy system at a given pressure, the number of phases in equilibrium is at most 1 more than the number of components, so the maximum number of equilibrium phases is only 6. As shown in Figure 4, the number of phases in the system is always less than the maximum number specified by the Gibbs phase. This makes it easier for high-entropy alloys to form single-phase solid solutions.
1.2.2 Lattice distortion
According to existing research, it is found that choosing a reasonable high-entropy alloy system can improve the hardness properties of the coating to varying degrees. According to the lattice distortion effect, when the components are evenly mixed, each element is the main element, and there is no doping of any trace elements. When forming a lattice, each metal atom will occupy a position in the lattice. However, since the radii between the elements are different, when a fixed lattice is formed, the entire lattice structure will be deformed and distorted, resulting in lattice distortion, as shown in Figure 5. At present, most scholars believe that the average atomic size difference (δ) is the most commonly used parameter to describe the size of the lattice distortion of high-entropy alloys.
See formula (3) in the figure.
Where: rx is the atomic radius; r’ is the average atomic radius; Cx is the atomic percentage.
The distortion calculated by formula (3) is the average dislocation strain, but since the arrangement of atoms in the lattice will have a relaxation process, the actual lattice distortion is often not so large. The average atomic size difference (δ) can clearly determine the degree of distortion of the alloy. The larger the average atomic size difference (δ), the greater the lattice distortion. The appearance of lattice distortion will lead to an increase in the internal energy of the alloy, an increase in microscopic stress, and an obstruction of dislocation slip deformation, which will play a role in solid solution strengthening and further improve the mechanical properties and corrosion resistance of the solid solution.
1.2.3 Slow diffusion effect
Many scholars have found that in high entropy alloys, the diffusion rate of elements is slower than that of other alloys. The main reason is that in the phase transition process, the synergistic diffusion and diffusion effect between the components are the key to achieving phase separation. Because high entropy alloys are composed of multiple main elements, there is no difference in solute or solvent between atoms. All atoms participate in the atomic diffusion process, so there is competition between various atoms near each vacancy. However, due to the different physical properties of elements such as melting point and bond energy, the diffusion speed of elements is different. The slow diffusion effect can effectively inhibit the nucleation and grain growth of phases in the alloy, and increase its recrystallization temperature, thereby improving its thermal stability.
1.2.4 Cocktail effect
Scientist S.Ranganathan first proposed the “cocktail” effect of multi-element high entropy alloys. He believes that high entropy alloys are composed of multiple elements, each with different properties. Through interaction and making full use of their excellent properties, the deficiencies between elements can be compensated, thereby obtaining excellent comprehensive performance, that is, the “cocktail” effect. Simply put, some microscopic properties of the main elements of the alloy will eventually be reflected in the macroscopic properties of the alloy. For high entropy alloys, adding elements with different basic properties will not only affect the overall performance, but also affect the interaction between elements. Therefore, by adjusting the alloy composition ratio and adding a small amount of alloying elements (such as Ti, Si, B, Nb), it is possible to further improve the hardness of the coating. Table 1 summarizes the physical and chemical properties of commonly used elements in high entropy alloy coatings. Figure 6 shows the different core effects produced by different lattice arrangements.
1.3 Relationship between the four major strengthening mechanisms and hardness of laser cladding high entropy alloys
Compared with traditional high entropy alloys, the strengthening mechanism of high entropy alloys prepared by laser cladding has not changed much. Its main strengthening mechanisms can be summarized into the following four types: solid solution strengthening, dispersion strengthening, precipitation strengthening and fine grain strengthening. The type of strengthening mechanism mainly depends on the composition, microstructure and characteristics of the high entropy alloy and the coating. It can be seen that the strengthening mechanism has a direct relationship with the hardness of the coating.
1.3.1 Solid solution strengthening
Adding alloy elements to the high entropy alloy solid solution will produce lattice distortion due to the different atomic radii of the elements in the high entropy alloy. This lattice distortion can enhance the movement resistance of dislocations and prevent slip, thereby improving the mechanical properties of the alloy. In this process, the strength of the metal is enhanced by adding certain solute atoms, which is called solid solution strengthening. The influence of solid solution on high entropy alloys containing more than 4 main elements with different structures is very large. The microhardness of high entropy alloy coatings prepared by laser cladding is mainly improved by solid solution strengthening.
1.3.2 Grain refinement strengthening
Grain refinement can improve the strength of alloy coatings and is a good method to strengthen materials. According to the Hall-Petch formula, grain refinement can significantly improve its mechanical properties.
See formula (4) in the figure.
Where: σ is the yield limit of the material; σ0 is the friction force acting on the dislocation; K is a constant; d is the average size of the grain. The smaller the grain size, the higher the grain yield strength. The strength or hardness of the material tends to increase as the grain size decreases.
1.3.3 Precipitation Strengthening
In addition to the matrix phase, there is also a precipitation phase. When the precipitate phase is dispersed in the matrix in a finely dispersed state, a greater strengthening effect can be obtained. This strengthening phenomenon is called precipitation strengthening. Precipitation strengthening refers to the interaction between the precipitate phase and the dislocation, which prevents the movement of the dislocation, thereby enhancing the deformation resistance of the material and the strength of the alloy material. Carbides and Laves phases can significantly improve the mechanical properties of the material by promoting the precipitation strengthening mechanism.
1.3.4 Dispersion Strengthening
Dispersion strengthening mainly occurs on second phase particles such as carbide particles and hard phases. The strength or hardness of the material is improved by increasing the relative amount of the dispersed phase or refining the relative size of the dispersed phase when the relative amount remains unchanged. Dispersion strengthening is mainly caused by carbide particles and hard phases.
2 Effect of element selection on the hardness of high entropy alloy coatings
2.1 Metal elements
The frequency of using each element in the high entropy alloy composition is shown in Figure 7. The 24th to 29th elements in the fourth period of the periodic table are the main gathering areas of elements in high entropy alloys. So far, Cr, Al, Fe, Co, Ni and Ti are the most commonly used in high entropy alloy compositions. Studies have found that adding metal elements such as Ti and Al can promote phase change. The addition of these metal elements not only changes the chemical composition of the alloy, but also greatly changes the crystal structure and mechanical properties of the material.
2.1.1 Ti element
Titanium is a rare metal. It has attracted widespread attention in the development of high entropy alloys because of its large atomic radius and potential to promote the formation of strengthening phases (such as Laves phase). The addition of second phase particles to the alloy matrix can cause dispersion and fine grain strengthening, thereby improving the hardness and strength of the material.
Gao Yulong et al. prepared CoCrNiMnTix (0≤x≤1) high entropy alloy cladding layer on the surface of Q235 steel matrix by laser cladding technology (the following scholars all use this method to prepare high entropy alloys). As can be seen from Figure 8, the hardness of the coating increases with the increase of titanium content. With the increase of Ti content, the coating phase transitions from a single FCC phase to a FCC+Laves phase, and the atomic radius of Ti in this alloy system is the largest. The difference in atomic radius is the main factor for solid solution strengthening of high entropy alloys. The larger the atomic radius, the greater the distance between atoms, the more obvious the lattice distortion, the greater the resistance of dislocation movement in the material, and the hardness and strength of the material will increase accordingly. Therefore, the microhardness of the coating increases with the increase of solid solution strengthening and Laves phase content, and the hardness is the highest when x=1, which can reach 523HV0.1. Zhang et al. prepared AlCoCrFeNiTi(1−x)Zrx (0≤x≤1) cladding layer on 45 steel. The study found that the average microhardness of the coating is much higher than that of 45 steel through solid solution strengthening, fine grain strengthening and carbide dispersion strengthening. The microhardness of the alloy coating increases first and then decreases with the increase of titanium content, and reaches the maximum at x=0.5. Si Wudong prepared CoCrFeMnNiTix (0≤x≤1) high entropy alloy coating on the surface of 40Cr steel. The study showed that the hardness of the alloy coating increases first and then decreases with the increase of titanium content. The lattice distortion and solid solution strengthening caused in the process from x=0 to x=0.75 make the coating hardness higher and higher. However, when the Ti element content continues to increase, the solubility of Ti atoms decreases, resulting in a decrease in hardness. Zuo Runyan et al. prepared CoCrFeNiTix (0≤x≤1) high entropy alloy cladding layer on the surface of 45 steel and found that the hardness of the coating is proportional to the increase of Ti content. In this alloy system, Ti has the largest atomic radius, which will cause lattice distortion and solid solution strengthening, and with the increase of Ti content, a bulk cubic (BCC) phase will be formed. At this time, the grains will gradually refine and be inhibited from growing, thereby improving the hardness of the coating. When x=0.8, the hardness is the highest, reaching 502.39HV0.3. Wang et al. prepared CoCrFeNiTix (0.1≤x≤
0.7) high entropy alloy coating on Q235 steel. The study found that the hardness of the coating is proportional to the increase of Ti content, and both are higher than the hardness of the substrate. Wang Xinyang et al. prepared CoCrFeNiTix (0≤x≤0.7) high entropy alloy coating and found that the hardness of the coating is positively correlated with the element content. When x=0.7, the surface hardness of the coating can reach up to 838.74HV. Huang et al. prepared CoCrCuFeNiTix (0≤x≤1.5) coating on the surface of 45 steel and found that the hardness of the coating gradually increased with the addition of Ti content. Titanium has a large atomic radius, which will cause lattice distortion when dissolved into the solid solution, and the high distortion energy inhibits lattice sliding and deformation, and with the continuous increase of Ti content, it will cause the precipitation of Laves phase and grain refinement, thereby improving the hardness of the coating. When x=1.5, the hardness is the highest, which can reach 556.8HV1.5. Liu Hao et al. prepared CoCrFeMnNiTix (0.25≤x≤1) high entropy alloy coating on the surface of 45 steel and found that the hardness of the coating is directly proportional to the Ti content. The reason is that the large atomic radius of titanium will cause lattice distortion, thereby achieving solid solution strengthening, and some Ti atoms will combine with C atoms to form a hard TiC phase, which will inhibit dislocation slip and increase the hardness of the coating. When x=1, the hardness is the highest, which can reach 364.5HV0.3.
2.1.2 Nb element
Niobium has high plasticity, but its hardness will increase with the increase of impurities. At higher temperatures, its strength and hardness increase. Since niobium is an important rare element, it can be processed into alloys and products for various purposes. When Nb elements with larger atomic radius are added to high entropy alloys, lattice distortion and solid solution strengthening effects will occur, and high-hardness Laves phases may also precipitate, thereby increasing the hardness of the coating.
Wu et al. prepared CoCrFeNiNbx (0≤ x≤0.3) high entropy alloy coatings on 316 stainless steel. The results showed that after adding Nb, the hardness of the alloy coating was improved, and its average hardness is shown in Figure 9. This is because the addition of Nb continuously reduces the average grain size of the high entropy alloy coating, and the extremely fine grains continuously increase the microhardness of the coating. The grain size distribution histogram is shown in Figure 10. In addition, there are differences in the atomic radius of the alloy system. The solid solution strengthening effect continues to increase with the increase of the Nb element content, and the hardness also increases accordingly. Liu Hao et al. prepared CoCrFeNiNbx (0≤x≤1) high entropy alloy coatings. The hardness of the coating first increased and then remained relatively stable with the increase of the Nb element content. Since the difference in the atomic radius of each atom is small, the substitutional solid solution formed is relatively stable, while the larger atomic radius of Nb will cause lattice distortion, thus producing a solid solution strengthening effect. With the increase of Nb elements, a high-hardness Laves phase will precipitate, and the dislocation nucleation will be inhibited and the dislocation slip will be hindered, and the second phase strengthening caused will gradually increase. In addition, with the increase of Nb, a large amount of eutectic structure will be generated. A large number of bonding interfaces will be generated between the eutectic phases, which will hinder the dislocation slip and thus increase the hardness of the coating. However, when the molar mass is 0.5, a complete eutectic structure has been generated. When the molar mass is greater than 0.5, the eutectic structure is reduced but there is still a high-hardness Laves phase precipitated. Therefore, the hardness does not change much when x=0.75 and x=1, and the hardness is slightly higher when x=0.75. Gong Haotian et al. prepared a FeCoNi2CrMnV0.5Nbx (0≤x≤1.2) high entropy alloy cladding layer on the surface of ZG25MnNi cast steel substrate and found that the hardness of each coating is positively proportional to the Nb element content. The addition of Nb elements with a larger atomic radius will produce a lattice distortion effect, thereby achieving a solid solution strengthening effect [79]. In addition, with the addition of Nb elements, high-strength Laves phase is gradually generated, and grain refinement is also generated. The number of grain boundaries increases, thereby hindering dislocation slip, so the hardness will gradually increase. When x=1.2, the hardness is the highest, reaching 631.1HV. Sun Zeyang clad the surface of 304 stainless steel with a high-entropy alloy coating of FeCoNiCuAlNbx (0.25≤x≤1). The results show that the microhardness of the alloy coating increases first and then decreases with the increase of Nb content. When x=0.5, the hardness of the alloy coating is the highest, reaching 835.8HV. This is because when x<0.5, the laves phase gradually increases, resulting in the gradual strengthening of the second phase, and from x=0.75, excessive Nb leads to the formation of coarse grains, which slightly reduces the hardness of the coating. Zhang et al. [81] prepared FeCoNi2CrMnV0.5Nbx (0≤x≤1.2) high entropy alloy coating on ZG25MnNi cast steel and found that the microhardness of the coating was proportional to the Nb content. Since the high-hardness Laves phase was formed in the coating [76], the grains gradually refined, the number of grain boundaries increased, and the slip of dislocations was hindered, the hardness of the coating was improved. When x=1.2, the hardness was the highest, reaching 631.1HV. Cengiz et al. prepared CoCrFeNiNb (x 0.2≤x≤1) high entropy alloy coatings with different Nb contents. It was found that the addition of Nb reduced the volume fraction of the FCC phase in the coating, while the Laves gradually increased and transformed into a dual-phase (FCC+ Laves phase) structure, and finally transformed into a completely eutectic structure, thereby effectively improving the hardness of the alloy.
2.1.3 Al element
Aluminum is a metal element with a low carbon content and is ductile. Its hardness is second only to diamond. It has a high melting point and is resistant to acid and alkali. It is often used to make some workpieces such as bearings. In a high entropy alloy system, as the Al content increases, a BCC structure will be formed, and the alloy will become stronger, but at the cost of reduced ductility, which is the main factor of hardening.
Han et al. [86] prepared AlxCoCrFeNiSi (x=0.3, 1.0) high entropy alloy coating on the surface of AISI 304H stainless steel. The study found that the higher the aluminum content, the better the hardness of the coating. The microhardness of the alloy coating is shown in Figure 11. This is mainly because the addition of Al atoms with a larger atomic radius (1.43 Å) causes lattice distortion, resulting in an increase in lattice energy, hindering the diffusion of elements, inhibiting grain growth, and refining the quality grains. The grain size distribution of AlxCoCrFeNiSi (x=0.3, 1.0) high entropy alloy coating is shown in Figure 12. According to the Hall-Petch theory, grain refinement is beneficial to improving the strength and hardness of the coating. Di Yingnan [88] prepared AlxCoCrFeMoVTi (0≤x≤1.5) high entropy alloy coating on the TC4 surface and found that the hardness of the HEA coating showed a trend of gradually increasing and then slowly decreasing with the increase of aluminum content. When x<1, the hardness of the material continued to increase with the increase of aluminum content. However, when x=1.5, when the aluminum content was too high, a large number of holes would be formed inside the coating, resulting in defect formation, which would reduce the hardness of the coating. However, due to the different atomic radii of each element, lattice distortion will occur, which will in turn produce a solid solution strengthening effect. The intense heating and cooling of the high-energy laser beam will refine the grains, so the hardness of each group of coatings has been greatly improved. Zheng Biju et al. prepared AlxCrFeCoCuNi (0 ≤x≤4) coatings on AISI1045 steel substrates and found that the hardness of the coatings is proportional to the Al content. This is because with the increase of Al content, the FCC phase structure gradually transforms into a BCC phase structure with higher hardness. In addition, the laser cladding method itself has the characteristics of rapid cooling, which will form finer grains, thereby improving the hardness of the coating. Li et al. prepared
AlxCrFeCoNiCu (0≤x≤2) coatings and found that their hardness increases with the increase of Al content. For coatings with low aluminum concentrations, aluminum is dissolved in the FCC crystal structure. Since Al has a larger atomic radius, the resistance to dislocation movement increases with the increase of aluminum content, and the microhardness of the coating is also higher. For coatings with medium aluminum concentration, the increase of aluminum increases the volume fraction of the BCC phase, which has fewer sliding bands and high accumulation failure energy, so it is difficult to deform under load, resulting in increased hardness. For coatings with high aluminum concentration, as the aluminum content increases, the content of the BCC phase will also increase, resulting in an upward trend in the hardness of the coating. Nguyen et al. prepared AlxFeMnNiCrCu0.5 (0≤x≤1) coatings on AISI 1045 steel substrates. By continuously adding aluminum elements, the BCC phase fraction gradually increased, thereby improving the hardness of the material. In addition, the lattice distortion caused by the continued addition of Al elements with larger atomic radius is more severe, and a solid solution strengthening effect is produced, which further increases its microhardness to 541HV0.2. Bao Yayun et al. prepared FeCrNiCoCuAlx (0≤x≤4) high entropy alloy cladding layer on the surface of Q345 steel substrate. The results showed that the phase structure of the alloy coating changed significantly with the increase of Al content, gradually changing from a two-phase mixed solid solution structure of BCC and FCC to a full BCC structure. The microhardness of the high entropy alloy layer was gradually improved under the influence of the phase structure. Xu et al. prepared AlxCoCrFeNiTi(1−x) (x=0.5, 0.8) high entropy alloy coating on AISI1045 steel and found that the average microhardness of the hard alloy coating reached the maximum value of 743HV0.2 at x=0.5, which was about 6 times that of the substrate.
2.1.4 Co element
Cobalt is a special-purpose metal material with hard and brittle texture. It can be preserved for a long time and maintain good performance. It is often used to make generators, transformers, motors, and electric transmission parts of bearings on some large machines. The Co element is crucial in the high entropy alloy system. On the one hand, Co can be used as a constituent element of the matrix phase. On the other hand, it is also conducive to the formation of some precipitates.
Wang Guiping et al. prepared the AlCoxCrFeNiCu (0≤x≤2) high entropy alloy cladding layer by laser cladding process and found that the hardness of the AlCoxCrFeNiCu (0≤x≤2) high entropy alloy coating is much greater than the hardness of the matrix. The hardness of the coating also shows a trend of gradually increasing and then slowly decreasing with the increase of Co element. As shown in Figure 13, when x≤1, the diffraction peak intensity of the BCC phase is significantly increased, while the intensity of the FCC diffraction peak decreases, which improves the hardness of the coating. When x≥1.5, Cu in the alloy precipitates in the dendrites, making the FCC phase diffraction peak on the alloy surface more obvious, while the hardness of the coating decreases. Qiu Xingwu et al. prepared Al2CrFeCoxCuNiTi (0.5≤x≤2) high entropy alloy coating and found that the hardness of the coating was significantly improved after laser cladding treatment. This is because the solid solution strengthening effect of the alloy and the precipitation of the second phase can achieve the effect of dispersion strengthening. The hardness of the coating is inversely proportional to the Co content. As the Co content gradually increases, the content of the FCC structure with lower hardness in the alloy increases, so the hardness decreases. As shown in Figure 14, when x=0.5, the maximum hardness is 1 013HV.
2.1.5 Cu element
Copper (Cuprum) is a ductile element with lower strength than other elements and can form a variety of alloys. Copper is an important metal element. At the same time, it is also a major element in high entropy alloys, which can play a role in self-lubrication and improve its high-temperature wear resistance. Cu promotes the formation of FCC phase, improves plasticity, and the presence of Cu can improve the mechanical properties and hardness of high entropy alloys.
Meng et al. prepared CoCrFeNiCux (0≤x≤1.5) high entropy alloy cladding layer on the surface of Q10 steel substrate. The results show that with the increase of Cu, the hardness of the coating tends to decrease gradually, as shown in Figure 15. Due to the influence of temperature gradient during solidification, the microstructure at the top of the coating is finer than that at the bottom, so the hardness value at the top is higher. However, with the increase of Cu content, the increase of Cu-rich phase improves the ductility of the coating, thereby reducing the hardness of the coating. Wang Gen prepared a new CoCuxFeNiTi-M (0≤x≤2) high entropy alloy coating on the surface of the pick (40Cr substrate surface) and found that the hardness of the coating was inversely proportional to the Cu content. As the copper content gradually increased, the hardness of the coating showed a gradual decrease with the reduction of the intermetallic compound phase. In addition, from the “cocktail effect”, it can be seen that each element added to the alloy system can show their basic characteristics. Therefore, when copper elements with lower hardness are added, the plasticity of the alloy can be improved, but the hardness of the alloy will be reduced. However, with the increase of Cu content, the hardness of the coating is also significantly higher than that of the substrate material. Their mechanical properties such as hardness and strength will change due to different Cu content, but they can all have a good protective effect on the substrate. Yang Chaomin et al. prepared FeCoCrNiCu (x 0≤x≤1) high entropy alloy coating on SUS304. The results showed that when the copper content increased, the hardness of the coating first decreased and then slowly increased. When x=0.5, the microhardness of the coating surface is the lowest. Due to the low Cu content, the toughening effect of FeCoCrNiCu0.5 coating is not as good as that of the equal molar ratio, and the addition of soft Cu is an important reason for the decrease in hardness. When x=1, the coating has the maximum hardness. Although the doping of Cu element will reduce its hardness, when Cu element is doped at the same molar ratio, due to the presence of Cu element, its lattice undergoes severe distortion, which causes solid solution strengthening, thereby increasing its hardness. Liu et al. prepared AlNbMoTaCux (0≤x≤0.8) coating and found that the microhardness of the coating decreases with the increase of Cu element, but it is improved compared with the substrate. The microhardness of the coating without copper is the highest. This is because the coating is a single BCC structure, and the solid solution strengthening effect improves the hardness of the coating. Liu Liang et al. prepared AlFeCrNiTiCux (0≤x≤2) high entropy alloy coating and found that the hardness of the coating is inversely proportional to the Cu content. This is mainly because the enrichment of copper on the grain boundary promotes the formation of FCC phase, thereby reducing the hardness of the coating.
2.1.6 W element
Tungsten (Wolfram) is a rare element with the highest melting point among all metal elements. It has high elastic modulus, high density, high thermal conductivity, and excellent mechanical properties at high temperatures. Tungsten is required as a reinforcing material in many industrial fields to improve strength, hardness and toughness.
Duan et al. prepared FeCrCoNiMo 0.5Wx (0≤x≤1.0) high entropy alloy coating on Q10 steel substrate. As can be seen from Figure 16, the hardness of the coating is much higher than that of the Q10 steel substrate, and the hardness of the coating increases with the addition of tungsten. Among them, the W1.0 coating has the highest hardness, up to 720HV0.5, which is 4.09 times the hardness of the substrate (170HV0.5). First, when the W element with a large atomic radius enters the lattice to form a substitutional solid solution, it causes a lattice distortion effect, and the solid solution strengthening effect is enhanced, which increases the strength and hardness of the alloy material. At the same time, the addition of tungsten is conducive to promoting grain refinement, thereby significantly improving its hardness. Finally, the increase in W content promotes the formation of μ-phase intermetallic compounds. This hard phase can effectively inhibit dislocation movement and increase resistance to dislocation slip. Dislocations are entangled near the μ phase during deformation. With the increase of μ phase in the coating, the blocking ability of dislocation movement is also enhanced, so the enhancement of the second phase is the key to improving the hardness of the alloy. Ma Shizhong et al. prepared CoCrFeNiWx (0≤x≤0.8) high entropy alloy coating on the surface of 45 steel. The hardness of the coating is positively proportional to the W content. Starting from x=0.6, the hardness of the cladding layer is significantly improved. First, because tungsten has a large atomic radius, when substitution occurs to form a substitutional solid solution, a large lattice distortion will be produced, thereby achieving solid solution strengthening; secondly, the W element can promote the formation of μ phase, increase the dislocation slip resistance and hinder the grain growth to a certain extent, the grains are refined, and the microstructure of the coating is effectively improved. At the same time, the second phase (μ phase strengthening) and fine grain strengthening are obtained, thereby improving the hardness of the coating. Liu et al. prepared CoCrFeNiWx (0≤x≤1.0) cladding layer on 45 steel. The study found that the hardness of the coating is positively proportional to the W content. When x≤0.5, because the atomic radius of W is large, lattice distortion will occur after addition, which plays a role in solid solution strengthening, improves the anti-slip ability of dislocations, and W atoms tend to dissolve into the FCC phase, so the microhardness increases. When x≥0.75, the microhardness of the coating increases more obviously due to the second phase strengthening of the μ phase. In addition, Fe reacts with W to produce a rich (FeCoCr)7W6 intermetallic compound phase. When the W concentration is large enough, the presence of fine lamellar eutectic structure will result in a large number of grain boundaries. Dislocations will be blocked by grain boundaries, making them unable to move, and the hardness of the coating will increase accordingly.
2.1.7 Mo element
Molybdenum is a transition metal element that is hard and tough. Molybdenum alloys are mainly strengthened by solid solution strengthening, precipitation strengthening, and work hardening, and these strengthening effects are closely related to dislocation movement. In a high temperature environment, crystals or grains are formed, thereby improving the uniformity and stability of the structure. In addition, other elements such as chromium can be added to enhance its hardness and wear resistance, so it is widely used in chemical machinery, construction and other fields.
Chen et al. prepared a CrFeNbTiMox (0≤x≤1) high entropy alloy coating on the surface of a 40Cr substrate. As can be seen from Figure 17, the addition of molybdenum is beneficial to improving the hardness of the coating. The hardness of the Mo1 coating is the highest, at 841HV, which is about 7.40 times that of the 40Cr substrate. Combined with XRD, it can be seen that the BCC phase content in the coating gradually increases with the increase of the molybdenum content, and the change in phase structure improves the hardness of the coating. In addition, the addition of the Mo element also refines the microstructure of the coating, and the hardness of the refined coating will also be improved. Bao Yefeng et al. prepared a high entropy alloy FeCoCrNiB0.2Mo (x 0≤x≤1) coating on Q235, and the hardness of the coating is directly proportional to the Mo content. Since the atomic radius of each element is different, the Mo element has a larger atomic radius, which is easy to achieve solid solution strengthening effect, and ultimately the hardness of the high entropy alloy coating continues to increase. Liu Hao et al. prepared CoCrFeMnNiMox (0≤x≤1) high entropy alloy cladding layer on the surface of 45 steel. The results showed that the hardness of the coating gradually increased with the increase of the molybdenum content in the coating. Because the Mo atom substitution solid solution causes lattice distortion, it hinders the slip of dislocations, thereby increasing the hardness of the coating. When x=1, the coating has the highest microhardness, and the microhardness of the coating is increased by 3.5 times compared with that when x=0. Zhou et al. prepared FeCoNiCrMox (0≤x≤1.2) high entropy alloy coating on 40Cr substrate. The hardness performance of the coating is positively proportional to the Mo content. First, because the atomic radius of the molybdenum element is relatively large, as the Mo element increases, it will continuously squeeze the surrounding atoms, aggravate the lattice distortion, hinder the movement of dislocations, increase the difficulty of sliding, achieve the solid solution strengthening effect, and thus improve the hardness of the coating. In addition, since the laser cladding method will cause the high entropy alloy coating to cool rapidly, resulting in the refinement of the coating grains, the number of grain boundaries will be increased, and the hardness of the coating will be improved. In addition, the increase in Mo content will not only enhance the effect of fine grain strengthening, but also cause the σ phase to be continuously generated as a hard phase, thereby improving the hardness of the coating. Wu et al. prepared Al2CrFeNiMox (0≤x≤2) high entropy alloy coating and found that the hardness of the coating is positively proportional to the Mo content. The atomic radius of molybdenum is relatively large. The increase in molybdenum atoms will occupy lattice sites and produce lattice distortion, which plays a role in solid solution enhancement, thereby improving the hardness of the coating. Gu et al. prepared Ni1.5CrFeTi2B0.5Mox (0≤x≤1) on 904L stainless steel and found that the hardness of the coating can be significantly improved, with a maximum hardness of 673HV.
2.1.8 Mn Element
Manganese is a hard and brittle metal that can be alloyed with other alloys such as nickel and cobalt. It is often used in the industry to manufacture alloys and alloy structural steels, and is also used to cast aluminum alloy parts or welding materials. Mn is cheap and abundant, making it an economical candidate element for high entropy alloys. Li et al. prepared AlCrFeNiMnx (0 ≤x≤1) high entropy alloy coatings on pure copper substrates. As can be seen from Figure 18, the proper addition of manganese can increase the hardness of the coating, but the hardness begins to decrease after excessive addition. First, the addition of Mn promotes the formation of BCC phase, and the hardness of BCC phase is higher than that of FCC phase. Secondly, the atomic radius of Mn and other elements is different. After adding manganese, lattice distortion will occur between alloys, and the resistance of dislocation movement will increase, thus achieving the effect of solid solution strengthening. In addition, laser cladding on the surface of the coating layer, followed by rapid cooling, the melting and cooling speeds are very fast, so a high-density and low-oxidation coating can be produced, and the grains in the coating are fine and uniform, which is conducive to improving the hardness of the coating. However, as the Mn content continues to increase, its dilution ratio also increases, and the Cu of the pure copper substrate will diffuse more easily into the cladding, thereby reducing the hardness of the coating. When x=0.5, the hardness of the Mn0.5 coating is the highest. Liu et al. prepared AlxCoCrFeNiMn(1−x) (0 ≤x≤1) high entropy alloy coating on AISI1045 steel substrate. The results showed that under the condition of 0≤x≤0.6, the average hardness of the coating remained basically unchanged. Combined with the XRD spectrum, it can be seen that the coating is mainly composed of FCC phase, with only a small amount of BCC phase, so the hardness of the coating will not be significantly improved. With the increase of Al element, the Mn element decreases. When 0.8≤x≤1.0, combined with XRD, it can be seen that the BCC in the coating is relatively large, and the change in phase structure significantly improves the microhardness of the coating.
2.1.9 V element
Vanadium is a rare element with a relatively high melting point. Pure vanadium is hard, non-magnetic, ductile, elastic and tough, has a certain hardness, but is not wear-resistant. It is an important metallurgical material and a raw material for making alloys. Vanadium is an important component of many steels, such as alloy steel and stainless steel. Adding a certain amount of V element to high entropy alloys can refine the grains and improve the strength of the coating.
Liao et al. prepared AlCoCrMoVx (0≤x≤1) high entropy alloy coatings on 904L stainless steel and found that the hardness of the coating first decreased and then increased with the increase of V content. As shown in Figure 19, the microhardness decreases slightly when x=0.2 because a small amount of V may be burned during the laser cladding process. However, there are more second-phase particles in the AlCoCrMoVx high-entropy alloy coating, which can hinder the movement of dislocations and thus increase the hardness. According to the Orowan mechanism, if the second-phase particles are smaller and more evenly distributed, the lattice dislocations will encounter greater resistance when moving around them, and the strength and hardness of the material will be easier to improve.
2.2 Non-metallic elements
In addition to metal elements such as Ti, Nb, Al, and Co, according to metallurgical theory, non-metallic elements also play a key role in the organization and properties of high-entropy alloys. High-entropy composite materials with the addition of non-metallic alloy elements (such as C, B, and Si) may have new functions and applicability.
2.2.1 C element
Carbon is a black solid with stable chemical properties and is insoluble in acids, alkalis, and other substances. It can be used as an alloy additive. In many industries, carbon needs to be added as the main alloying element for strengthening treatment to improve its performance. For example: high-strength steel, low-alloy steel, etc. Carbon has been proven to be one of the effective interstitial alloying elements in steel, which can significantly enhance mechanical and wear properties through solid solution strengthening and precipitation of carbides. With the increase of carbon content, the number of carbon atoms increases, the lattice distortion gradually becomes stronger, and the mechanical properties and wear resistance of the alloy are significantly improved with the increase of carbides in the matrix.
Liu Jingzhou et al. prepared CoCrFeMnNiCx (0≤x≤0.15) high entropy alloy coating on 45 steel matrix. The results showed that with the addition of C, the microhardness of the coating showed a trend of gradually increasing and then slowly decreasing. First, when C is added to the alloy, C and the metal elements in the coating generate intermetallic compounds, which significantly improves the hardness of the coating. Secondly, as the C content continues to increase, the intermetallic compounds decrease, the precipitation strengthening effect weakens, and the hardness of the laser cladding layer gradually decreases.
2.2.2 Si element
Silicon is the second most abundant element. Since silicon usually forms alloys with metals such as iron and cobalt and produces a series of excellent physical and chemical properties, it has good application prospects in casting. Adding silicon high entropy alloys in laser cladding manufacturing can not only produce coatings with good macroscopic quality, but also strengthen the coating and improve the hardness and wear resistance of the coating. In addition, Si is a promoter of the BCC phase. After adding Si, the FCC phase structure will be converted into the BCC phase structure, thereby improving the hardness of the coating.
Liu et al. prepared AlCoCrFeNiSix (0≤x≤0.5) high entropy alloy coating on AISI 304 substrate. As can be seen from Figure 20, the microhardness of the high entropy alloy coating gradually increases with the increase of silicon content. When x=0.5, the average hardness of the coating is the highest, which is 8.19 GPa. Combined with the microstructure and XRD spectrum of the AlCoCrFeNiSix high entropy alloy coating, it can be seen that the increase in microhardness is because the addition of Si element causes the radius of the alloy system to differ, resulting in lattice distortion, thereby achieving solid solution strengthening, and dislocation strengthening and fine grain strengthening further improve the hardness and performance of the coating. Liu Hao et al. prepared AlCoCrFeNiSix (0≤x≤0.5) cladding layer on 304 stainless steel and found that the hardness of the coating gradually increased with the increase of Si content. First, the rapid solidification effect of laser cladding will indirectly increase the shrinkage stress during the solidification process, which is easy to lead to the formation of high-density dislocations in the alloy coating. The greater the dislocation density, the higher the hardness of the high-entropy alloy coating. Secondly, after adding Si elements, the pinning effect induced by grain refinement and AlNi phase precipitation hinders the slip of dislocations, further leading to an increase in dislocation density and hardness. Finally, as the content of C elements continues to increase, a small amount of Cr23C6 carbides precipitate along the grain boundaries. These high-hardness carbides will play a role in second-phase hardening, thereby increasing the hardness of the coating, and the maximum hardness can reach 848.1HV0.3. Zhang et al. and Hao Wenjun et al. prepared CoCrFeNiSix (0≤x ≤2) high-entropy alloy coatings on 45 steel substrates and found that the microhardness of the coating is proportional to the Si content. First, the atomic radius of each element in the CoCrFeNiSi system is different, which will produce lattice distortion, which can achieve solid solution strengthening effect, thereby improving hardness. When x=2.0, the hardness is the highest, which can reach 556.5HV0.5. Zhang Guozhong et al. [137] prepared AlCoCrFeNiSix (0≤x≤0.5) cladding layer on 304 stainless steel and found that the hardness of the coating is proportional to the Si content. First, as the Si content increases, the difference in atomic radius of each element causes lattice distortion, thereby achieving solid solution strengthening effect. In addition, dislocation strengthening also has a great influence on the improvement of coating hardness. When x=0.5, the average hardness is the highest, which can reach 829.4HV0.3.
2.2.3 B element
Boron, with relatively high hardness. During the laser cladding process, when the B element encounters certain elements, it will promote the formation of borides, thereby improving the strength and hardness of the coating. In addition, the atomic radius of the B element is small, and it is easy to diffuse after being added, which will produce fine crystal strengthening and solid solution strengthening, thereby improving the mechanical properties of the coating.
Lin Jianquan et al. prepared CoCrFeMnNix (0≤x≤0.8) high entropy alloy coating on the surface of H13 steel. The results showed that the hardness of the coating gradually improved with the addition of boron content. When x=0.8, the hardness of the coating was the highest, up to 739.8HV0.2. Zhang Chong et al. prepared FeCrNiCoMnBx (0≤x≤1.25) high entropy alloy coating on the surface of 45 steel. The hardness of the coating gradually increased with the increase of boron content, as shown in Figure 21. When x≤0.75, the rising trend of the coating hardness is the same as the formation trend of borides, indicating that the increase in the content of hard phase borides in the coating is an important reason for the increase in hardness. However, when x=1, the average hardness of the coating reaches 6690 MPa, which shows a leap-forward growth. Zhao Longzhi et al. prepared FeCoCrNiSiBx (0≤x≤0.08) high entropy alloy cladding layer. The results show that when x≤0.06, as the content of B element gradually increases, the precipitation of boride gradually increases, which improves the hardness of the coating. When x≥0.08, the boride increases, but the FCC phase structure in the coating gradually increases, which reduces the microhardness of the coating. Ding et al. prepared CoCrFeNiTiNbBx (0≤x≤1.25) cladding layer on 45 steel substrate. The study found that the addition of B element promotes the formation of BCC phase, and the hardness of the alloy coating gradually improves with the increase of BCC phase content. In addition, the high entropy effect and rapid solidification of laser cladding promote the dissolution of alloy elements into the HEA matrix and produce a solid solution strengthening effect, thereby further improving the hardness of the coating. Lin et al. [143] prepared FeCoCrNiAlBx (0≤x≤0.75) high entropy alloy coating on Q235 steel. The results showed that the microhardness of FeCoCrNiAlBx (0≤x≤0.75) high entropy alloy coating gradually improved with the increase of B content. When x=0.75, the hardness reached 726HV.
2.3 Multi-element synergistic change
The elemental composition of HEA directly determines its properties. In recent years, adding a single element to HEA to improve its performance has attracted widespread attention. However, there are relatively few studies on the influence of multi-component synergistic effects on the comprehensive performance of materials. Synergistic changes can give full play to their respective advantages, help to better optimize the alloy performance, better understand the contribution of different elements to the alloy performance, and flexibly adjust the ratio of the two elements in the alloy to achieve the required performance requirements.
Ma et al. prepared AlCoCrFeNiTi(1–x)Zrx coating on 45 steel. As shown in Figure 22, the microhardness of the alloy coating increases first and then decreases with the increase of Zr content. It reaches the highest when x=0.5. This is because the dissolution of Ti and Zr elements with larger atomic radius in the BCC matrix will cause lattice distortion to a certain extent, enhance the solid solution strengthening effect, increase the resistance to dislocation movement, make it difficult to slide, and thus improve the hardness of the HEA coating. As x continues to increase, the content of Ti in the solid solution decreases with the increase of x, while Zr mainly participates in the formation of carbides and oxides, and has weak solid solubility with other elements. Therefore, the microhardness of the coating is low when x=0.75 and 1.0. Bu Shanfei et al. prepared FeCoNiCrNbxTiy coating on 42CrMo steel. The study found that with the increase of Nb and Ti content, the hardness of the cladding layer gradually increased. Among them, the average microhardness of FeCoNiCrNb0.5Ti0.9 was the highest, which was 493.58HV1.0, about 1.79 times that of the substrate. The main reason is that with the increase of Nb and Ti content, their large atomic radius will lead to strong lattice distortion, resulting in strengthening effects such as solid solution strengthening and fine grain strengthening, thereby improving the hardness of the coating. In addition, the increase of Nb element leads to the generation of laves hard phase. With the increase of Ti element, the BCC phase and laves phase generated in the coating will greatly increase its hardness. Li et al. prepared CoCrFeNiAlxMo(2–x) cladding layer on Q235 steel. The study found that with the increase of x, the hardness of the coating first increased and then decreased. When x=1, the hardness was the highest, which was 1 008.4HV0.2. Liu et al. prepared AlxCoCrFeNiMn(1–x) cladding layer on 45 steel. The study found that with the increase of x, the hardness of the coating first remained relatively stable and then began to increase gradually. This is mainly because when x was just added, the coating consisted of a two-phase structure of FCC phase and BCC phase, which had no obvious effect on improving the hardness. As x continued to increase, the Al element promoted the phase transition from FCC to BCC, allowing the skeleton of the harder and heavier BCC phase to support the softer FCC phase. Therefore, the microhardness increased significantly.
2.4 Selection and design of alloying elements
When designing the elements required for high entropy alloys, the phase composition and microstructure of the alloy can be changed according to the principles and methods of preparing high entropy alloys. By drawing on previous research results, metal elements with excellent performance and good stability (such as titanium (Ti), aluminum (Al), nickel (Ni), cobalt (Co) and iron (Fe)) can be selected to give full play to the “cocktail” effect of high entropy alloys. The chemical interactions between these elements and their molar ratios have an important influence on the stability, strength, ductility and corrosion resistance of high entropy alloys. In summary, it can be concluded that some metal and non-metal elements have an impact on the organization and performance of high entropy alloy coatings.
The hardness change of high entropy alloys with different compositions and ratios prepared by laser cladding will be affected by many factors. During the laser cladding preparation process, the material will undergo a rapid cooling and solidification process, during which poor microstructural uniformity, nanocrystalline, amorphous and other structures may be formed, which is the reason for the change in the hardness of high entropy alloys. 45 steel is a high-quality carbon structural steel with excellent performance, low cost and reasonable chemical composition. It can meet most of the needs through post-processing and is widely used in machining, automobile manufacturing and shipbuilding industries. The hardness values of different high entropy alloy coatings prepared on 45 steel substrate and 45 steel hardness values are shown in Figure 22. It can be found that the hardness values of different high entropy alloy coatings are higher than that of 45 steel substrate. In descending order, Al4CrFeCoCuNi and AlFeMnNiCrCu0.5 have very high hardness, exceeding 700HV. This is because the hardness is significantly improved by adding strong forming elements such as aluminum and titanium; CoCuFeMnNiC0.9 and CoCuFeMnNiMo contain more high entropy alloys such as copper and manganese, so they are relatively soft and have a hardness of only about 230HV. In short, the preparation of different high entropy alloys by laser cladding needs to be evaluated and optimized according to the specific conditions such as the selected material composition and structure. With the development of materials science research, more elements will be introduced into high entropy alloy design to meet the needs of different scenarios.
3 Conclusion and Prospect
High entropy alloy is a new type of material, also known as multi-element alloy or composite alloy, with a unique structure. Due to its excellent mechanical properties, it can be used as a substitute for traditional alloys. The high hardness of high entropy alloy coatings is mostly due to the effects of solid solution strengthening and second-stage strengthening. When preparing high entropy alloy coatings, since the radius of elements such as Al, Ti, Mo, and V is larger than that of elements such as Co, Cr, Fe, and Ni, the coating will be lattice distorted, and dislocation movement will be hindered, thereby forming solid solution strengthening, which will promote the formation of BCC phase. With the change of the main element content in the high entropy alloy system, the FCC phase may be reduced and the BCC phase structure may increase. The hardness of the BCC phase in the coating is higher than that of the FCC phase. Some alloys contain hard Laves phases, thereby achieving the effect of strengthening the coating performance by the second phase.
High entropy alloy coatings prepared by laser cladding will obtain excellent high hardness performance under certain conditions, making it have a very broad application prospect in the field of high hardness repair and protection of parts and components. How to maximize the hardness of the workpiece under the optimal working conditions is a hot topic at present. In order to obtain high-entropy alloy coatings with excellent comprehensive performance, the following work needs to be carried out:
1) Adding elements with special properties to the high-entropy alloy system can usually improve the performance of the coating. However, as the element content continues to increase, the hardness of the alloy coating sometimes tends to decrease. The addition of elements has a great influence on the phase structure composition of the alloy coating, but the optimal element addition amount needs to be determined through continuous experiments or simulation regulation. Usually, researchers use some software (such as Materials Studioa, Lmmps, VASP, JMatPro, etc.) to simulate the effects of different element contents on the phase composition and microstructure of high-entropy alloy coatings, so as to further predict the effects of element changes on coating performance.
2) High-entropy alloys have superior mechanical properties, such as good strength, hardness, and wear resistance, but no practical applications have been found to replace traditional alloys (such as titanium alloys and nickel alloys). Therefore, in the subsequent research on high-entropy alloys, the focus should be shifted to the design of application-oriented high-entropy alloys. In addition, high-entropy alloys should be consciously developed for functional properties, which may lead to the innovation of new processes and products, thereby responding to the needs of the new era.
3) While solving the application problem of high entropy alloys, its design cost is another obstacle. Therefore, they can only replace existing materials if they are sustainably developed and reasonably priced (processing cost, component cost and handling cost). Therefore, the design of high entropy alloys must take into account the cost to enhance the hardness and tribological properties.
4) Existing studies have focused on the effect of a single component on its hardness properties, and the mechanism of the hardness properties of the coating under the synergistic effect of multiple components needs to be further studied.
5) At present, compared with conventional alloys, the hardness of high entropy alloy coatings has been greatly improved, but its plasticity and toughness have decreased. Ensuring its plasticity and toughness while improving the hardness of the coating is a future research direction.
