Refractory high entropy alloys (RHEAs) are a new type of alloy composed of a variety of refractory elements. They have excellent high-temperature mechanical properties, high-temperature oxidation resistance, friction and wear properties, corrosion resistance, and radiation resistance. They are expected to be used in aviation, aerospace, nuclear energy, petrochemicals, and medical devices. Limited by traditional alloy melting technology, the high-melting-point refractory high-entropy alloys currently prepared have problems such as small molding size, severe element segregation, and high density, which greatly restrict the development and application of refractory high-entropy alloys. Laser additive manufacturing technology uses a high-energy-density laser beam as a heating source. Through computer-aided design and control, the “discrete-accumulation” forming process of metal materials can be realized, which provides an effective way to break through the research bottleneck of refractory high-entropy alloys. This paper reviews the processing characteristics, microstructure, and performance characteristics of refractory high-entropy alloy coatings (RHEACs) prepared by laser cladding technology in recent years. The effects of alloy composition and processing technology on the phase composition, micromorphology, microhardness, wear resistance, corrosion resistance and oxidation resistance of refractory high entropy alloy coatings were discussed in detail. The research status, shortcomings and challenges of laser cladding refractory high entropy alloys were pointed out, aiming to provide theoretical guidance for subsequent research and to look forward to its future development trend.

Metal materials will undergo obvious structural softening at a service temperature higher than 0.6Tm (Tm is the melting point of the alloy) [1,2]. Traditional high-temperature alloys such as Fe-based, Co-based and Ni-based alloys are prone to premature failure under extreme environments such as high temperature, friction, oxidation and corrosion. Improving the high-temperature wear resistance, corrosion resistance and oxidation resistance of alloys under extreme environments has important practical significance for improving the service life of structural materials. Alloying is the most common and effective method to improve the performance of metal materials. Traditional alloys are usually based on one or two elements, and the service performance of the alloys is improved by adding multiple trace elements. So far, researchers have developed a large number of high-performance traditional alloys using alloying methods. However, the composition of these alloys is usually located at the edge of the phase diagram, which greatly limits the freedom of material design. In 2004, Professor Ye Junwei of National Tsing Hua University in Taiwan [3] prepared a new alloy composed of 10 equimolar elements and found that the alloy not only has a simple phase structure, but also has better performance than traditional single-element alloys. He formally proposed the design concept of “high entropy alloy”.

The high-temperature mechanical properties of alloys are greatly affected by their melting point. When the temperature is  0.6Tm, the dislocation climb mechanism is activated, the deformation mechanism of the alloy changes from non-diffusive deformation to diffusive deformation, and the yield strength will drop sharply. Therefore, adding high-melting-point elements to increase the threshold temperature of diffusive deformation is the simplest and most effective way to improve the high-temperature mechanical properties of alloys. Based on this, in 2011, Professor Senkov of the US Air Force Laboratory [4,5] used refractory elements with high melting points to prepare MoNbTaW and MoNbTaWV high entropy alloys. It was found that the two alloys still maintained ultra-high yield strengths of 405 and 477 MPa at 1600 °C, and proposed the concept of “refractory high entropy alloys (RHEAs)”. Compared with the traditional fcc phase Cantor alloy, RHEAs composed of bcc phase generally have higher high temperature strength and microhardness [6]. In recent years, scholars from various countries have conducted extensive research on the high temperature mechanical properties [7-9], phase stability [10,11], radiation resistance [12,13] and oxidation resistance [14,15] of RHEAs. As a high temperature material, RHEAs show better high temperature mechanical properties than traditional nickel-based high temperature alloys [5] and are expected to become the next generation of high temperature structural materials. However, most RHEAs show obvious brittleness at room temperature and have poor machinability. The common V and VI subgroup components of RHEAs have high melting points. The vacuum arc melting method currently used is limited by the size of the furnace chamber and can only produce button-sized alloy ingots [16]. In addition, the alloy ingots have problems such as coarse structure and severe component segregation. In addition, refractory elements such as Mo, W, Nb, Ta and Hf have high density, and the prepared alloys usually have high density. The above problems greatly limit the application of RHEAs in structural materials, and it is urgent to seek more advanced forming and manufacturing technologies.

Laser cladding (LC) is an advanced technology that has developed rapidly in recent years and is based on digital control to prepare coatings and achieve surface strengthening of metal materials. Compared with traditional surface strengthening technologies, LC technology has the characteristics of high energy density, fast processing speed, less material waste and environmentally friendly process, which helps to form a strong metallurgical bond between the coating and the base alloy, and the coating has a uniform and fine microstructure [17~19]. Studies have shown that LC-formed refractory high entropy alloy coatings (RHEACs) are expected to break through the current research bottleneck of RHEAs. First, high-energy-density laser sources can quickly melt refractory elements such as Mo, Nb, Ta, W, Hf, Zr, Cr and V, and precisely form the molten metal through rapid cooling deposition, which effectively overcomes the shortcomings of RHEAs such as low-temperature brittleness, difficult processing and small forming size. In addition, simple “two-dimensional” coatings can effectively solve the high-density problem of RHEAs as structural materials [20], and enhance the surface properties of the base alloy, realizing low-cost and high-efficiency RHEAs applications.

At present, domestic and foreign researchers have used LC technology to prepare some RHEACs, improving the wear resistance, corrosion resistance and oxidation resistance of base alloys such as M2 steel, 316L steel, IN718 and TC4. This paper mainly reviews the research progress of the processing characteristics of LC technology, the microstructure of RHEACs, and the microhardness, wear resistance, corrosion resistance and oxidation resistance. The problems and challenges of current research are summarized, and the future development trend is prospected, in order to provide valuable reference and reference for the subsequent research of LC-RHEACs.

1. Composition design of refractory high-entropy alloys by laser cladding
Multi-principal element is the most important feature of high-entropy alloys that distinguishes it from traditional alloys. It provides a broad composition design space for the development of high-performance materials. However, its composition is complex and its organization is changeable, which also brings certain challenges to the element and composition screening of alloys. In the early stage of research, people tend to design alloys with equimolar composition in order to obtain the maximum mixing entropy value, so as to improve the formability and stability of single-phase solid solution in the alloy. With the deepening of research, people found that mixing entropy does not have enough driving force to inhibit the formation of intermetallic compounds in most alloy systems, and the change of mixing enthalpy brought about by composition change determines the formability and service performance of solid solution alloys to a certain extent. Therefore, the purpose of laser cladding refractory high entropy alloy composition design is similar to that of traditional refractory high entropy alloys, which is mainly to optimize the phase composition and microstructure to achieve the expected performance of the alloy. The difference is that the coating composition will be diluted by the elements in the base alloy during laser cladding, and the focus of coating composition design is to study the influence of the type and content of the main element on the solid solution forming and alloy properties. In addition, the laser cladding process involves rapid heating and cooling. When designing the composition, it is necessary to select alloy elements with suitable thermal expansion coefficients, thermal conductivity and melting points to avoid large thermal stresses inside the cladding layer and between the cladding layer and the substrate during rapid heating and cooling, which may cause cracking or shedding of the coating. Laser cladding needs to ensure that the cladding layer has good metallurgical bonding with the substrate. When designing the composition, it is necessary to consider the compatibility between the elements to ensure good interface bonding between the two and avoid the formation of brittle phases or interface defects. HEACs usually contain expensive metal elements such as Ta, Zr and Hf. The traditional “cooking-style” composition design not only has a long experimental cycle, but also high research costs. Based on this, researchers have developed a variety of theoretical models and simulation calculation methods for the design of RHEAs.

1.1 Empirical parameter method
The empirical parameter method belongs to the theoretical paradigm of research through models or induction. The characteristic of the empirical parameter method is that it can quickly and easily predict the phase structure of the alloy through physical and chemical parameters. Zhang Yong et al. [21] found through the analysis of a large amount of experimental data that the solid solution forming ability of the alloy is related to the mixing enthalpy value ΔH mix and the atomic size difference δ in the system. The absolute value of the mixing enthalpy represents the resistance to the formation of a disordered solid solution structure between elements. A more positive mixing enthalpy indicates that the repulsive force between elements is greater, and the alloy tends to undergo spinodal decomposition or form an ordered structure; a more negative mixing enthalpy indicates that the elements attract each other, and the alloy tends to form an intermetallic compound; and the closer the mixing enthalpy is to zero, the easier it is for the alloy to form a disordered solid solution. In addition, the smaller the atomic size difference is, the easier it is for the elements to replace each other to form an infinite solid solution. Therefore, Zhang Yong et al. [21] proposed that the mixing enthalpy in the alloy system is −15 kJ/mol ≤ΔH mix ≤5 kJ/mol and the atomic size difference δ ≤ 6.5% is the condition for the alloy to form a simple solid solution structure. With the deepening of research, Yang and Zhang [22] believed that the simple ΔH mix-δ criterion condition ignored the influence of mixing entropy on the stability of the solid solution, and proposed a new criterion parameter Ω to represent the solid solution forming ability of high entropy alloys during solidification: see formula (1) in the figure. In the formula, Tm is the melting point of the alloy, and ΔSmix is ​​the mixing entropy of the alloy. When Ω < 1, the mixing enthalpy value dominates the solid solution forming ability of the alloy during solidification, and the alloy is easy to form intermetallic compounds and undergo spinodal decomposition. When Ω > 1, the mixing entropy value determines the solid solution forming ability of the alloy, and the alloy preferentially forms a simple disordered solid solution structure. Based on this, Yang and Zhang[22] proposed that when the alloy system satisfies the conditions of Ω≥1.1 and δ≤6.6%, the structure of the alloy tends to form a solid solution during solidification. So far, most research works have successfully achieved the prediction of the solid solution structure of high entropy alloys using the above empirical model. However, although the empirical parameter method is simple and fast, its application accuracy for certain alloy systems needs further verification[11].

1.2 Computational simulation method
Computational simulation method belongs to the computational paradigm of using computers for simulation research. The computational simulation methods currently used in laser cladding composition design are mainly divided into first-principles and phase diagram calculation methods. The first-principles calculation method is based on the interaction between atoms and electrons. Through approximate assumptions such as density functional theory, the Schrödinger equation is solved to obtain the physical, chemical and mechanical properties of the material[23]. The first-principles calculation first constructs a crystal model based on the alloy lattice. On this basis, the crystal model is structurally optimized to obtain the most stable structure of the alloy system in the computational environment. The structure mainly contains the wave function of the alloy system. Finally, the alloy-related properties are obtained by extracting the wave function information.
At present, the research field of laser cladding coating mainly uses first-principles calculation to screen the elements and optimize the composition of the cladding layer. Mu Yongkun [24] used the first-principles method to calculate the influence of Cr and Al elements on the thermophysical parameters such as binding energy, formation enthalpy, elastic modulus and bulk modulus of TiVMoNb RHEAs. The results show that TiVMoNb-X (X = Cr, Al, CrAl) RHEAs are all single bcc structures, and the Cr element makes the formation enthalpy of the alloy closer to zero, while the Al element makes the formation enthalpy of the alloy more negative, indicating that the Al element will stabilize the bcc phase of the alloy, and the Cr element will cause the bcc phase to become unstable. In addition, the elastic modulus of the alloy decreases when Al and Cr elements are added separately, while the elastic modulus of the alloy increases when Al and Cr elements are added simultaneously, indicating that alloying affects the stiffness of the material to a certain extent.
Although first-principles calculations can observe and obtain the internal crystal structure information of alloy systems at the atomic scale, and analyze and understand the nature of matter and related mechanisms from the perspective of electronic structure, their crystal structure modeling methods and density functional theory use a large number of approximate assumptions, resulting in certain errors in the simulation results [25]. In addition, the unit cell is the basis for first-principles calculations. When studying the influence of composition on the physical and chemical properties of alloys, it is necessary to establish a huge database of crystal structure models. First-principles calculations are usually inefficient when dealing with multi-atom supercell structure optimization, and it is difficult to meet the high-throughput requirements in the field of material design [25]. Phase diagram calculation is a method for parameterizing phase equilibrium and thermodynamic information. The essence of this method is to optimize the low-order thermodynamic parameters of the target system through iterative fitting of experimental and theoretical calculation data, and establish a thermodynamic description of the high-order system through extrapolation. This thermodynamic description can quickly establish the relationship between alloy composition, phase structure and temperature [26]. The accurate alloy composition design by phase diagram calculation requires sufficient low-order thermodynamic information and a large amount of key experimental information verification. Therefore, the phase diagram calculation method is used to iteratively fit the thermodynamic information and experimental data of the first-principles calculation to build a complete thermodynamic and kinetic database of high-entropy alloys. This is a feasible solution for the rapid optimization of the composition and material development of laser cladding high-entropy alloy coatings.

2 Processing characteristics of laser cladding refractory high-entropy alloys
2.1 Preparation method and powder feeding system of LC-RHEACs raw materials
LC is essentially a processing technology belonging to the category of powder metallurgy. The raw materials for forming are usually regular spherical metal powders. In order to prepare coatings with good morphology and quality, the powder must meet the requirements of low oxygen content, high sphericity and high purity. Figure 1 is a schematic diagram of the principle and powder morphology of the commonly used method for preparing alloy powders [27,28]. Figure 1a [27] is a basic principle diagram of the vacuum plasma atomization method. The molten metal is broken into fine droplets under the high-pressure gas in the atomization chamber, and is continuously cooled and spheroidized into metal powder. As shown in Figure 1b[29], the metal powder particles prepared by vacuum plasma atomization are regular spherical and have good fluidity, which is suitable for coaxial powder feeding laser additive manufacturing. It is the type of metal powder commonly used in the preparation of RHEACs. Figure 1c[30] is a basic principle diagram of the spray granulation method. Polyvinyl alcohol or distilled water is used to prepare irregular metal raw materials into prefabricated alloy slurry. The slurry is dried and agglomerated under the action of spray hot air to finally obtain spherical alloy powder. As shown in Figure 1d[31], the spray granulation method can prepare irregular pure metal powder into prefabricated alloy powder with good sphericity, in which the high melting point element particles have a smaller size and can be fully melted under the irradiation of the laser beam. This method is suitable for additive manufacturing of alloy powders with large differences in element melting points. Studies have shown that the coating structure of high-melting-point RHEACs formed by spray granulation powder is more uniform than that prepared by plasma atomization alloy powder, and the high-melting-point W and Mo element particles in the coating are completely melted. However, due to the poor density of the alloy powder prepared by spray granulation and the contamination of impurity carbon elements in polyvinyl alcohol, the prepared coating has obvious pores and impurity ceramic phase defects [31].

In the LC processing process, alloy powder is usually sent to the laser focus in the form of pre-laying or coaxial powder feeding for melting and deposition. Among them, LC in the form of pre-laying powder is only suitable for processing base alloys with simple planar shapes, while coaxial powder feeding LC can achieve coating cladding on the surfaces of various complex structure alloys such as crankshafts and planes by locally melting the powder and depositing it to form a molten pool. In addition, coaxial powder feeding LC technology has the advantages of high efficiency and fast deposition rate, and is more suitable for application in the field of industrial production [32]. Within a reasonable range of processing parameters, coaxial powder feeding can gradually change the composition of the mixed powder in the powder feeder to prepare composition gradient alloys with high throughput, thereby realizing rapid screening of high-performance alloys [33,34].

2.2 Common defects of LC-RHEACs
During laser processing, the molten pool has a high heating and cooling rate, and the unavoidable thermal internal stress leads to a certain amount of macroscopic and microscopic defects in the coating. The melting points of the constituent elements of RHEAs are generally greater than 1650 ℃, and the melting points of Ta and W elements even reach about 3000 ℃, which requires a higher energy density to be input during laser processing to ensure that the elements are fully melted. Since the melting points of some elements are even higher than the boiling points of other elements, low-melting-point elements will evaporate or high-melting-point elements will not fully melt during the cladding process. When the overflow rate of the supersaturated gas generated by the evaporation of low-melting-point elements is less than the solidification rate, bubbles will be retained in the coating to form defects such as holes [35,36]. In addition, the inadequate melting of high-melting-point elements will reduce the fluidity of the melt in the molten pool, making it difficult for bubbles to “escape” from the molten pool, and even produce defects such as cracks around the unmelted powder under the action of internal stress [35,36]. Studies have shown [35,37] that the external auxiliary field during the cladding process can effectively improve the forming quality of the coating, and the coating structure after solidification is more uniform, and defects such as pores and cracks disappear. Figure 2 [35] shows the SEM morphology of MoNbTaW RHEACs prepared by LC and magnetic field-LC. As shown in Figure 2a [35], the coating structure prepared without adding an auxiliary magnetic field is uneven, and there are obvious pores and cracks (Figure 2b1~d1 [35]), while the coating structure prepared with an auxiliary magnetic field becomes uniform, and the pores and cracks disappear (Figure 2a2~d2 [35]). This is mainly due to the fact that the external auxiliary field increases the flow rate of the Marangoni fluid in the molten pool, and the supersaturated gas can quickly reach the melt surface and overflow the molten pool. In addition to the external auxiliary field, reasonable optimization of processing parameters can also effectively improve defects such as unmelted metal powder and pores in the coating. The defects in the coating gradually disappear with the increase of laser power [38]. This is mainly attributed to the fact that increasing the laser power increases the melting temperature in the molten pool, and the high-melting-point element particles are fully melted. In addition, the increase in the molten pool temperature effectively prolongs the solidification time of the melt, and the bubbles generated by the melt have sufficient time to overflow the molten pool. In addition to optimizing the laser power, speeding up the laser scanning speed can reduce the powder delivery ratio, which will also reduce the proportion of unmelted powder after coating formation and reduce defects in the coating [38].
In addition to pores and unmelted particles, RHEAs are prone to microcracks during the forming process due to their intrinsic brittleness and the internal stress generated by continuous heating and cooling during laser processing [39,40]. Figure 3 [41] is a SEM morphology of laser-clad MoFexCrTiWAlNby RHEACs. As shown in Figure 3a and b [41], no obvious cracks are generated in the Fe1Nb1 and Fe1.5Nb1 coatings shown in Figures 3a and b [41]. When the Fe and Nb contents are high (Figure 3c~i[41]), a large amount of Laves_Fe2Nb hard phase is generated in the coating, which increases the crack sensitivity of the coating and generates a large number of microcracks. However, after adding Cu, the microcracks in the coating disappear significantly[42]. This is mainly attributed to the fact that Cu improves the coating’s resistance to plastic deformation and effectively inhibits the initiation and propagation of cracks. In addition, Cu has a lower melting point, which can prolong the solidification time of the coating, allowing the residual liquid to effectively fill the gaps in the solidified structure, thereby further inhibiting the formation of cracks. Similarly, reducing the content of elements such as Mo and W can also effectively reduce the brittle tendency of the coating and improve the crack resistance of the coating. Sun et al.[31] laser clad 3 layers of MoNbTaW RHEACs on a Mo matrix alloy. The results show that the cracks in the coating gradually disappear with the increase of coating thickness. Analysis of the reasons shows that the reduction in cracks is mainly due to the fact that the dilution rate of the coating decreases with increasing thickness, and the reduction of the Mo element content inhibits the generation of microcracks. In summary, the process parameters of laser processing and the alloy composition jointly determine the forming quality of RHEACs. Establishing a perfect forming control mechanism is of great significance to improving the machinability of RHEACs and achieving good preparation of the coating.

3 Structural characteristics of laser cladding refractory high entropy alloy coatings
The microstructure of the alloy, such as phase composition, grain size, grain structure and dislocation density, determines the expected performance and modification method of the material. Therefore, it is crucial to fully understand the microstructural evolution behavior during the laser cladding coating forming process. The microstructure of RHEACs is mainly determined by the alloy composition and processing technology. This section will review the research progress of the microstructure of LC-RHEACs and analyze the influence mechanism of alloy composition and processing technology on the phase composition and microstructure of RHEACs.

3.1 Phase composition
The content of high entropy alloy elements is usually equimolar or approximately equimolar. According to the classic Boltzmann formula [3], the more components a high entropy alloy system has, the higher the mixing entropy. High mixing entropy is conducive to reducing the free energy of the system and promoting the formation of solid solution phase [43,44]. RHEAs are mainly composed of refractory elements of groups IV, V and VI. Group V and VI elements are all bcc structures, while the reference state of Ti, Zr and Hf elements of group IV is hcp phase, which will undergo allotropic transformation at high temperature and thus convert into bcc structure. According to statistics, most of the more than 400 RHEAs reported so far are single bcc phase structures[45], and sometimes secondary phases such as hcp phase, B2 phase and Laves intermetallic compound are precipitated. Unlike bulk alloys, the composition of RHEACs will also form phases such as carbides and nitrides due to dilution of matrix elements. Table 1 lists the coating types and phase structures of LC-RHEAs in recent years as well as the types of matrix alloys.

As can be seen from Table 1, the phase structure of the alloy coating composed of refractory elements is a single bcc solid solution. When elements such as Al, Cr and Fe with relatively negative mixing enthalpy with refractory elements are added, intermetallic compounds such as Laves are easily generated in the coating. In addition, when the metal coating is prepared by LC technology, the composition of the coating is diluted by the matrix alloy elements to form new phases. Guo et al. [47] and Kuang et al. [51] used LC to prepare CrTiMoWNbFeAl and MoFeCrTiWAlNb3 RHEAs on M2 high-speed steel, respectively. X-ray diffraction (XRD) test results show that Nb and Ti elements have a strong bonding ability with the C element in the matrix alloy to form a (Nb, Ti)C phase [64]. Similarly, Liu et al. [42] found that when laser cladding AlNbMoTaCux RHEAs on the surface of TC4 alloy, a large amount of Ti in the TC4 matrix was diluted into the coating, forming AlCu2Ti and Ti(Al, Cu)2 compounds with Cu and Al in the coating, and forming TiN hard phase with N in the atmosphere. Therefore, in order to improve the accuracy of coating composition design, it is crucial to optimize the laser processing technology to reduce its dilution rate.

3.2 Micromorphology characteristics
In the process of laser cladding refractory high entropy alloys, the laser beam melts the metal powder in a very short time, and the coating has a heating and cooling rate of up to 106 K/s, which is much higher than the cooling rate of traditional vacuum arc melting alloys (10~102 K/s) [65]. The high cooling rate can effectively increase the solidification nucleation rate of the alloy, which is conducive to the formation of ultrafine nano-scale grains in the alloy [66,67]. Li et al. [68] used electron backscatter diffraction (EBSD) to compare the microstructures of laser melting deposition and vacuum arc melting MoNbTa alloys. The results are shown in Figure 4 [68]. As can be seen from the figure, the alloy prepared by laser deposition in Figure 4a [68] is mainly composed of columnar crystals with a grain size of about 13.18 μm, which is significantly smaller than the equiaxed grains (112.52 μm) prepared by vacuum arc melting in Figure 4b [68]. Guan et al. [50] prepared TiNbZr RHEACs by laser cladding technology. The coating is composed of irregular equiaxed crystals with an average grain size of 24.5 μm, while the grain size of the alloy prepared by conventional vacuum arc melting is about 100 μm [69]. It can be seen that laser processing technology has great potential in the preparation of high-performance ultrafine-grained coatings.
The laser processing process involves complex heat, mass and momentum transfer in the molten pool, so the coating has a rich microstructure in different spatial distributions. According to the classical metal solidification theory, the temperature gradient is the main driving force for the solidification of the melt, and the temperature gradient and the solidification rate jointly determine the morphology of the solidified structure. During the laser cladding process, the high-energy laser directly acts on the surface of the molten pool. The temperature in the molten pool gradually decreases from the surface to the bottom, and the temperature gradient gradually increases. Figure 5a[60] is a schematic diagram of the structure of the laser cladding molten pool. The molten pool boundary consists of the solidification front and the melting front. The solidification rate R of the melt depends on the laser scanning rate vd and the solidification angle θs: R = vd cosθs (2). The solidification angle gradually increases from the surface to the bottom of the molten pool. Under the condition that the laser scanning rate vd is constant, the solidification rate will gradually decrease. Figure 5b is a schematic diagram of the relationship between the temperature gradient G, the solidification rate R, and the grain size and morphology of the solidified structure during the metal solidification process. It is well known that the growth direction of the grain is affected by the temperature gradient, and the solidification size of the grain is determined by the solidification rate. It can be seen from the constructed solidification diagram that G·R determines the grain size of the solidification structure, and G/R determines the grain shape of the solidification structure. When the temperature gradient at the bottom of the molten pool is large enough, it is difficult to form a composition supercooling zone at the front of the solid-liquid interface, and the solidification structure will grow in the form of a planar crystal cell with a smooth interface and no solute segregation. With the growth of the planar crystal, the latent heat is released at the solidification interface, G in the molten pool gradually decreases, while R gradually increases, forming a constitutive supercooling at the solidification front, and the solidification structure grows epitaxially into columnar grains along the heat flow direction. With the decrease of the temperature gradient and the continuous increase of the solidification rate, G/R is not enough to maintain the normal growth of the columnar crystal, and a columnar dendrite structure with slight branches will be formed in the local area. Finally, the solidification structure forms cellular and equiaxed crystal grains driven by the solidification rate.

The microstructure of the metal coating prepared by laser cladding is mainly determined by the processing parameters. Zhou et al. [60] studied the effects of LC, high-speed LC (HLC) and ultra-high-speed LC (EHLC) on the temperature field of TiNbTaZr RHEACs. The results show that with the increase of scanning rate, the solidification rate of the molten pool is significantly accelerated. Figure 6 [60] shows the microstructure morphology of the coating prepared by LC at different scanning rates. As can be seen from the figure, the top (Figure 6a~c[60]), middle (Figure 6d~f[60]) and bottom (Figure 6g~i[60]) of the coating are composed of cellular crystals, dendrites and columnar crystals, respectively. During the cooling process of the molten pool, the grains nucleate and grow by absorbing the surrounding molten metal liquid. When the solidification rate is faster, the grain growth time becomes shorter, forming dense and fine grains. Therefore, with the increase of scanning rate, the cellular crystals at the top of the coating have a significant refinement effect, the grain boundaries become narrower, and the secondary dendrite arms of the middle dendrites become shorter and denser.

4 Performance characteristics of laser cladding refractory high entropy alloy coatings
The alloys and coatings prepared by laser additive technology usually have fine structures, uniform structures, and excellent comprehensive properties. In addition, excellent high-temperature mechanical properties are the main advantages of RHEAs compared with other alloys. Therefore, RHEACs prepared by LC technology have broad application prospects in the field of surface protection of hot end components. At present, the research on RHEACs at home and abroad is still in the preliminary stage of process and composition optimization. This section will analyze and summarize the microhardness, wear resistance, corrosion resistance and oxidation resistance of reported RHEACs.

4.1 Microhardness of laser cladding refractory high entropy alloy coatings
The high mixing entropy of high entropy alloys causes its lattice lattice to have obvious distortion, giving the alloy a large solid solution strengthening effect. Therefore, high microhardness is the most representative performance characteristic of high entropy alloys. RHEAs, as an important branch in the high entropy alloy system, is mainly composed of bcc phase. Compared with the fcc phase, the bcc lattice is denser, has fewer movable slip systems, and the alloy has a more obvious work hardening effect. According to literature reports [66-69], due to the rapid cooling and heating characteristics of laser processing, the microstructure of the alloy is significantly refined and enriched. Therefore, the hardness of RHEAs prepared by laser cladding is 2-3 times that of traditional melting technology. Table 2 summarizes the microhardness of RHEACs currently prepared by LC technology.

The strengthening mechanism of LC-RHEACs includes fine grain strengthening, solid solution strengthening, precipitation strengthening and dispersion strengthening. Among them, fine grain strengthening and solid solution strengthening are the most common strengthening mechanisms in laser-prepared alloys. Chen et al. [54] compared the microhardness of laser-clad AlTiVMoNb RHEACs and arc-melted cast alloys. The results showed that the microhardness of RHEACs was 888.5 HV0.2, which was significantly higher than 536.6 HV0.2 of the cast alloy. The analysis results show that RHEACs have a smaller grain size, and the fine grain strengthening effect improves the microhardness of the coating. At the same time, the tensile stress generated by frequent heating and cooling during the laser cladding process causes serious distortion of the coating lattice, generates a certain amount of elastic stress field, and hinders the movement of dislocations through pinning, which plays a significant role in solid solution strengthening. In addition to the internal stress strengthening effect generated during the processing, the addition of large-sized elements such as Zr and Ta will also enhance the solid solution strengthening effect of the coating [53,63,70], significantly improving the microhardness of the coating.

Compounds such as hcp, Laves and MC are common second phases in RHEACs, which will produce significant precipitation strengthening in the coating. When the coating contains elements such as Ti, Zr and Hf, the hcp phase is likely to exist. Studies have shown that the microhardness of the alloy coating gradually increases with the addition of hcp structural elements such as Zr [53]. The main reason is that the increase in Zr content improves the stability of the hcp phase [71,72], and needle-shaped hcp strengthening phases gradually precipitate in the bcc phase of the coating matrix. In addition, according to the binary phase diagram of refractory elements [73], the atomic radius of Cr is small, and it is easy to form Laves compounds with large atomic radius elements such as Zr, Ta and W. Laves compounds (the crystal structure is divided into MgZn2-type C15 phase structure and MgCu2-type C14 phase structure) are a typical topological close-packed phase with high microhardness, corrosion resistance and oxidation resistance [74]. Reasonable optimization of alloy composition and processing technology and regulation of the phase fraction of Laves phase can achieve the preparation of high microhardness RHEACs. Wang et al. [48] studied the effect of Nb content on the microstructure and microhardness of CrTiMoWNbxFe1.5Al RHEACs. The results show that with the addition of Nb element, the in-situ generated Laves_Fe2Nb compounds in the coating gradually increase, and the microhardness gradually increases from 810 HV0.2 to 910 HV0.2. After annealing at 650 ℃, the coating gradually precipitates Laves_Fe2W compounds, and the microhardness reaches 954 HV0.2.

4.2 Wear resistance of laser cladding refractory high entropy alloy coating
The wear resistance of metal materials is evaluated by the comprehensive properties of the materials. Good wear resistance first requires that the material has a high microhardness to resist the volume cutting wear caused by the high hardness material pressing into the metal material. In addition, materials with both strength and toughness have excellent deformation resistance, which can effectively reduce the fatigue peeling caused by elastic failure of the material. Metal materials will produce obvious structural softening phenomenon at high temperature, and their strength will decrease parabolically. Therefore, the wear resistance of metal structural materials is usually also closely related to temperature. As a typical bcc structure high temperature alloy, RHEAs usually have high microhardness and excellent resistance to high temperature softening. The coatings prepared from them show excellent wear resistance at both room temperature and high temperature. Table 3 summarizes the wear resistance of LC-RHEACs reported.

Generally, the wear mechanism of metal materials is divided into abrasive wear, adhesive wear, oxidative wear and fatigue wear. Among them, abrasive wear and adhesive wear mechanisms are more common. When the small and sharp protrusions on the friction pair surface contact the coating surface, the coating surface will be micro-cut to cause abrasive wear. In addition, with the repeated friction between the friction pair and the coating surface, the temperature of the contact point gradually increases to form a “welding” reaction[75]. Under the shear force generated by the repeated movement of the friction pair and the sample, the welding points on the coating surface with lower strength will be torn, forming local adhesive wear. It can be seen that the higher the microhardness of the coating, the stronger its ability to resist micro-cutting of the friction pair surface, and theoretically has better wear resistance[76]. Wang et al. [48] used a reciprocating wear tester to study the room temperature wear resistance of CrTiMoWNbxFe1.5Al RHEACs. The results showed that with the increase of Nb element content (x = 1.5~3.0), the friction coefficient of the coating decreased from 0.72 to 0.52, and the wear volume decreased from 0.078 mm3 to 0.045 mm3, which was significantly smaller than the friction coefficient and wear volume of the M2 steel substrate (0.87 and 0.4314 mm3), indicating that CrTiMoWNbxFe1.5Al RHEACs has excellent wear resistance. This is mainly because with the addition of Nb content, the in-situ generated Laves_Fe2Nb intermetallic compounds in the coating gradually increase, which improves the microhardness of the coating and effectively enhances the coating’s resistance to friction microcutting. Figure 7[48] is a SEM morphology of the coating surface wear. The wear morphology of the Nb1.5 coating consists of tearing deformation caused by elastic failure and narrow furrows caused by abrasive wear. Area A is the deformation area and area B is the wear area, as shown in Figure 7a. With the addition of Nb content (Figure 7b~d), the furrows on the coating surface become thinner and shallower, showing a wear mechanism dominated by abrasive wear. The M2 steel substrate with lower hardness is pierced by the high-hardness friction pair, showing the characteristics of plastic failure accompanied by adhesive wear mechanism. The surface wear morphology consists of a large number of wide and deep furrows and large-area peeling, as shown in Figure 7e. In addition to Laves intermetallic compounds, the presence of high-hardness phases such as hcp solid solution and TiN compound in RHEACs will also significantly reduce the abrasive wear of the friction pair on the coating and improve the wear resistance of the coating.

The wear resistance of the coating is not only related to its microhardness, but also closely related to its strength-plasticity matching. Studies have shown that RHEAs usually exhibit a more obvious brittle tendency at room temperature. While the microhardness of the coating increases with the increase of the content of hcp phase and Sigma compound, the plasticity also decreases rapidly, which makes the coating prone to microcrack initiation and expansion under load. Under the action of the additional horizontal shear stress of the friction pair, the crack zone gradually peels off. Zhao et al. [53] studied the effect of Zr element on the wear resistance of AlNbTaZrx RHEACs and found that with the addition of Zr element, hcp strengthening phase gradually precipitated in the coating, resulting in its microhardness gradually increasing and increasing its resistance to microcutting. The wear morphology of the coating surface obtained by SEM analysis is shown in Figure 8. With the increase of Zr content, the furrows (area D) on the coating surface caused by wear gradually decreased (Figure 8a~c), and a large amount of oxide glaze layer (area E) was produced. When the Zr content continues to increase to 1.0, the precipitation of a large amount of hcp phase in the coating causes its toughness to drop sharply, the ability to resist brittle debonding during wear becomes weaker, and a “fish scale-like” peeling area appears on the local surface (Figure 8d). Therefore, adding a small amount of toughness elements to the coating can effectively reduce the cracking sensitivity of the coating under high stress friction and improve the wear resistance of the coating. Liu et al. [42] studied the effect of Cu element on the wear resistance of AlNbMoTaCux refractory high entropy alloy coating. The results showed that the wear volume of the coating with a small amount of Cu element added (x = 0.4) was reduced from 5.54 mm3 to 0.41 mm3, which significantly improved the wear resistance of the coating. The analysis shows that the AlNbMoTa coating has a greater tendency to be brittle. With the addition of Cu, although the microhardness of the coating decreases from 836 HV0.2 to 568 HV0.2, the fcc phase is generated around the bcc phase and hcp phase, which effectively reduces the brittleness of the coating, inhibits the initiation and expansion of cracks and the formation of the spalling layer, and significantly improves the wear resistance of the coating.

In addition to abrasive wear and adhesive wear, the wear mechanism of alloy coatings also includes oxidation wear that is prone to occur at high temperatures. Mo, Nb and Ta, which are common elements in RHEAs, are difficult to form a dense oxide layer at high temperatures and usually show poor oxidation resistance. However, the oxide film formed by the coating prepared by it at high temperature will be compacted under high stress [77], forming a dense oxide film glaze layer, which further reduces the wear of the coating by resisting the cutting of the friction pair. Wang et al. [61] studied the high-temperature wear resistance of MoNbTaW RHEACs composited with Y2O3 ceramic particles. The results showed that the wear resistance of the coating increased significantly with increasing temperature. The analysis results showed that as the temperature increased, the oxide glaze layer on the surface of the alloy coating could play a better lubricating role in the friction and wear process, reducing the wear of the coating. Zhao et al. [36] studied the high-temperature wear resistance of MoNbTaW RHEACs and found that the wear resistance of the coating decreased with increasing temperature. This is mainly because the coating composition is diluted by the In718 matrix alloy, and there are a large number of low-strength fcc_(Ni, M) phases in the coating. As the temperature increases, the fcc_(Ni, M) phase softens significantly and cannot provide structural support for the oxide layer, resulting in large-scale peeling of the oxide layer. It can be seen that RHEACs have excellent room temperature and high temperature wear resistance. Whether the microhardness and strength and plasticity match is the key to determining its room temperature wear resistance, while the high-temperature wear resistance of RHEACs is mainly determined by its resistance to high-temperature softening.

4.3 Corrosion resistance and oxidation resistance of laser cladding refractory high entropy alloy coatings
Corrosion is one of the most common failure forms of metals. Alloy corrosion can be divided into uniform corrosion, local corrosion and galvanic corrosion. Among them, local corrosion is the most important corrosion failure form of metal structural materials in industrial applications. The main reason for local corrosion is the obvious potential difference between different phases or different composition regions in the alloy, which forms an electrolytic cell in the local area, causing rapid corrosion of the material. RHEAs are usually composed of simple bcc solid solutions, which have better corrosion resistance than fcc solid solutions in Cl- and acidic solutions. Secondly, laser rapid solidification can effectively eliminate the intergranular segregation of elements, which is conducive to obtaining a more uniform microstructure. Therefore, RHEACs usually exhibit excellent corrosion resistance. Dynamic potential polarization curve is a standard electrochemical technique for characterizing the corrosion resistance of materials. Table 4 summarizes the corrosion resistance evaluation results of LC-RHEACs by electrochemical technology.

In the dynamic polarization curve of the alloy measured by the electrochemical method, the larger the corrosion potential, the smaller the corrosion current density, indicating that the material has lower corrosion sensitivity and fewer corrosion sites, and better corrosion resistance [79]. As can be seen from Table 4, RHEACs have a larger corrosion potential and a smaller corrosion current than TC4, SS 316 and other base alloys, showing better corrosion resistance. X-ray photoelectron spectroscopy (XPS) analysis of the passive film composition shows that the Ti, Cr, Mo and W elements in RHEACs will form stable high-valent TiO2, Cr2O3, MoO3 and WO3 passivation films during the corrosion process of NaCl solution [63], preventing the continuous corrosion of the metal and slowing down the corrosion rate of the coating. Ta and Nb elements form unstable multivalent passivation films, and Zr and V elements are almost not passivated. Therefore, adding elements that are easy to form stable passivation films is an effective way to improve the corrosion resistance of the coating.

In addition to adding passivating elements, improving the phase composition of the coating through component design can effectively reduce the galvanic corrosion of the coating and improve its corrosion resistance. Liao et al. [56] used the CALPHAD method to study the effect of phase composition on the corrosion resistance of AlCoCrMoVx refractory high entropy alloy coatings. The dynamic polarization curve and impedance spectrum show that when the V element content is 0.8, the coating has the lowest corrosion current density and the largest Nyquist radius, indicating that the coating has the best corrosion resistance. Analysis shows that with the increase of V element content, the microstructure of the AlCoCrMoVx coating gradually transforms into a single bcc phase, which effectively reduces the galvanic corrosion rate and improves the corrosion resistance of the coating. When the V element content increases to 1.0, the corrosion current density of the coating increases slightly. XPS analysis results show that this is because the content of passivating elements such as Mo, Cr and Al in the coating decreases with the addition of V element, resulting in a decrease in the content of compounds such as MoO3, Cr2O3 and Al2O3 in the passivation film, which increases the corrosion rate. In general, LC-RHEACs show excellent corrosion resistance compared with traditional nickel-based, iron-based and titanium-based alloys, but there are relatively few reports on the corrosion behavior of LC-RHEACs in the literature. In order to accelerate the application of coatings in extreme corrosive environments, exploring the corrosion mechanism of RHEACs under various corrosive environments under laser processing technology is the focus of future research.

Good oxidation resistance is an important technical indicator for achieving high-temperature applications of RHEACs. Common components such as Mo, Nb, Ta, W and V in RHEAs are easily oxidized elements, and the oxidation products are relatively loose, which are easy to debond and peel off during the oxidation process. Therefore, the oxidation resistance of RHEAs is not ideal. However, the addition of antioxidant elements such as Cr, Al, Ti and Si can form a protective Cr2O3, Al2O3, TiO2 and SiO2 dense oxide layer on the surface of the alloy, effectively improving the oxidation resistance of the alloy. In addition, due to the complex composition of high entropy alloys, the synergistic effect between elements will form composite oxides[80,81], which significantly improves the oxidation resistance of the alloy[82].

The oxidation resistance of the coating is related to the stability of the oxidation products. The stability of the oxidation products can be evaluated by the PBR value (Pilling-Bedworth ratio)[83]. The PBR value represents the volume change ratio of the metal atom before and after oxidation. When the PBR value is 1~2, the metal oxide film is relatively dense and has a good atmospheric isolation effect. A higher PBR value indicates that the volume expansion of the metal atom is large when it is oxidized, the oxide film is easy to break, and the oxidation resistance is poor. Zhao et al.[53] found that ZrO2 has a smaller PBR value when studying the oxidation performance of AlNbTaZrx RHEACs oxidized at 1000℃ for 50 h. With the increase of Zr content, the oxidation weight gain of the coating gradually decreases. Figure 9[53] shows the morphology of the oxide layer on the coating surface. It can be seen from the figure that some fine cracks can be clearly observed in the oxide layer formed on the Zr0.2 coating, and obvious peeling occurred in some areas (Figure 9a). The oxide layer formed on the Zr1.0 coating is relatively flat (Figure 9b). It can be seen that selecting suitable components to form a dense oxide layer is of great significance to improving the oxidation resistance of refractory high entropy alloy coatings. In addition, how to make full use of the synergistic effect between different elements through composition design is the key to further improving the oxidation resistance of refractory high entropy alloy coatings.

5 Summary and Outlook
Laser cladding, as an advanced additive manufacturing technology, provides an effective way to solve the bottleneck problems of difficult preparation, high cost and high density of refractory high entropy alloys. At present, researchers have used laser cladding technology to prepare refractory high entropy alloy coatings with uniform composition, fine structure, strong interface bonding and good forming quality, and the coatings show excellent comprehensive properties. First, refractory high-entropy alloy coatings show excellent high-temperature stability and oxidation resistance in high-temperature applications, and are expected to be widely used in the protection of high-temperature components such as turbine blades and combustion chambers of aircraft engines and gas turbines. Secondly, in the energy field, refractory high-entropy alloy coatings containing elements such as Zr and W are considered to be ideal materials for nuclear power plant reactor cladding due to their excellent radiation resistance, low neutron absorption cross-section and low sputtering rate. In the field of petrochemicals, refractory high-entropy alloy coatings can be used on the inner walls of chemical reactors, pipelines and storage tanks, significantly improving their corrosion resistance and wear resistance. In addition, refractory high-entropy alloy coatings containing Ti elements can be used for the protection of medical implants and surgical instrument surfaces due to their good biocompatibility and antibacterial properties, thereby extending the service life of implants. Overall, laser cladding refractory high-entropy alloy coatings have considerable application prospects in extreme application fields. However, the current research on laser cladding refractory high entropy alloy coatings is still in the laboratory research stage of process optimization and composition regulation. There is still a certain distance from the actual application of engineering. It is urgent to further improve the basic theoretical research work, which is mainly reflected in the following aspects:
(1) There are more than 400 refractory high entropy alloys that have been developed so far, but the theoretical alloy systems are far more than 31,000. Therefore, the role of the high-throughput material integrated computing database of the Materials Genome Project should be brought into play to achieve rapid screening of alloy materials with expected performance. In addition, it is necessary to combine the advantages of coaxial powder feeding in laser cladding and prepare composition gradient coatings by in-situ alloying based on high-throughput design. By quickly characterizing the microstructure and service performance of the coating, efficient screening of alloy components can be achieved, shortening the research and development cycle of new materials and reducing research and development costs.
(2) The alloy system of the refractory high entropy alloy coating currently developed is mainly composed of refractory elements and strengthening elements. Studies have shown that non-metallic elements such as O, N and C and ceramic particles such as CeO2, Al2O3 and WC often make the cast alloy show excellent strength-toughness synergy. Therefore, in the future, the research on non-metallic elements and ceramic particles to strengthen refractory high entropy alloy coatings can be focused. In addition, the high solidification rate during laser forming can effectively inhibit the nucleation and growth of crystals, which is conducive to the formation of amorphous phase structure in the alloy. The amorphous phase structure has excellent mechanical properties, especially wear resistance and corrosion resistance, but no research has been carried out in related fields. Therefore, the research on laser surface amorphization technology in the field of preparing refractory high entropy alloy coatings should be accelerated.
(3) The refractory high entropy alloy coatings currently prepared generally have the problem of high dilution rate (greater than 30%). Too high dilution rate will deteriorate the microhardness, corrosion resistance and oxidation resistance of the coating, and even affect the formability of the solid solution in the refractory high entropy alloy coating. Too low dilution rate will reduce the metallurgical bonding between the coating and the substrate. In addition, due to the different phase compositions of the substrate alloy and the coating, this metallurgical bonding often makes the bonding area present a complex microstructure. Under the influence of thermal stress, phase change stress and constraint stress during laser processing, microcracks are easily formed in the bonding area, resulting in crack propagation and eventual failure of the coating during use. Therefore, in order to obtain refractory high entropy alloy coatings with good metallurgical bonding and excellent surface properties, controlling the dilution rate and deeply studying the influence of the micromorphology and phase composition of the bonding zone on the bonding strength of the coating are the main problems that need to be overcome in future research.
(4) As a new generation of high-temperature surface protection materials, refractory high entropy alloy coatings are targeted at aerospace, nuclear energy, chemical industry, military equipment and other fields with high requirements for high-temperature service performance. However, the current research on its high-temperature service performance is not in-depth and comprehensive enough, and it is necessary to further understand the high-temperature service behavior and failure mechanism of laser additively manufactured refractory high entropy alloy coatings, such as high-temperature creep and fatigue resistance. In addition, there is no systematic study on the radiation resistance and biocompatibility of refractory high entropy alloy coatings. The corresponding theoretical system research should be formed and improved as soon as possible to expand the application scope of refractory high entropy alloy coatings.