Iron-based amorphous alloys have great application prospects in industry due to their excellent mechanical, chemical and physical properties and low cost. However, the lack of toughness and plasticity of iron-based amorphous alloys has always restricted its development in the field of coating applications. To solve this problem, researchers have conducted a lot of research on laser cladding preparation technology and element formula, and achieved fruitful results. The coexistence of amorphous phase and crystalline phase can effectively improve the toughness or plasticity of iron-based amorphous alloy composite coatings. The grain size, structure and distribution of the crystalline phase have a great influence on the toughness or plasticity of iron-based amorphous alloy composite coatings, and the mechanism of dual-phase plasticization and toughening is explored. The research progress in the plasticization and toughening of laser cladding iron-based amorphous alloy composite coatings in recent years in terms of laser process parameters, element formula and auxiliary methods is summarized and summarized, the main problems currently existing are analyzed, and the future development direction is prospected. (Fe-based amorphous; laser cladding; alloy; composite coating; toughness; plasticity)

1 Introduction
Amorphous alloys have excellent corrosion resistance and mechanical properties due to their unique atomic structure: short-range order, long-range disorder, and the absence of crystal defects such as grain boundaries, dislocations, and slip [1,2]. Therefore, the application of amorphous alloys in water conservancy and hydropower, marine equipment, aerospace, semiconductors and other fields is very promising [3-8]. Among them, Fe-based amorphous alloys have attracted the attention of researchers because of their abundant resources, low prices, and high hardness, strength, wear resistance and corrosion resistance. Their application prospects are also very broad. However, the preparation process of Fe-based amorphous alloys requires a high critical cooling rate and is difficult to prepare. At the same time, Fe-based amorphous alloys also have the disadvantage of room temperature brittleness, which seriously limits their wide application [9,10].

In order to solve the above difficulties and improve the amorphous forming ability of Fe-based amorphous alloy coatings, researchers have focused on the feasibility of preparing Fe-based amorphous alloy coatings by supersonic flame thermal spraying and laser cladding technology [11-14]. Both technologies have a high cooling rate and are very suitable for the substrate conditions of amorphous formation. Among them, laser cladding technology has more prospects in engineering applications because the prepared coating is metallurgically bonded to the substrate and the bonding strength is much higher than that of the coating prepared by supersonic flame spraying. In order to solve the room temperature brittleness problem of Fe-based amorphous coatings, domestic and foreign researchers have conducted a lot of research on laser cladding process parameters and material formulations. The study found that the coexistence and interaction of amorphous phases and crystalline phases can effectively improve the toughness or plasticity of Fe-based amorphous alloy composite coatings, which is an effective way to solve the room temperature brittleness problem of Fe-based amorphous coatings. This paper focuses on summarizing and analyzing the research on laser cladding process parameters, material formulation and auxiliary methods in the plasticization and toughening of Fe-based amorphous alloy composites, and looks forward to the future development direction.

2 Effect of laser process parameters on plasticization and toughening of Fe-based amorphous alloy coatings
Laser cladding is a very complex integrated metallurgical process, which includes a series of changes such as light absorption, heat transfer, mass transfer, melting and solidification. The laser cladding layer has a large temperature gradient, which makes the cladding layer have unique structural characteristics, that is, the cladding layer has a stratified structure. From the surface of the cladding layer to the interface between the coating and the substrate, some equiaxed crystals, cellular crystals, dendrites and other structural characteristics may appear in sequence [15]. At present, it is still difficult to completely block the epitaxial growth of crystals in the preparation of Fe amorphous alloy coatings by laser cladding. The first condition for amorphization is to block the epitaxial growth of crystals. Therefore, the amorphous coating prepared by laser cladding is often a composite coating composed of amorphous, nanocrystals and metal compound crystals [16]. However, this feature has a beneficial effect on improving the plasticity or toughness of Fe-based amorphous alloy composite coatings. Especially in application scenarios where the coating needs to cope with high-intensity and high-frequency impact forces, such as turbine impellers, large ship blades and other flow-through parts, laser cladding of Fe-based amorphous alloy composite coatings is more advantageous. To this end, domestic and foreign scholars have conducted a lot of research on the plasticization and toughening of Fe-based amorphous composite coatings in terms of laser process parameters and have achieved many results. Basu et al. [17] tried to laser clad Fe48Cr15Mo14Y2C15B6 amorphous alloy coatings on AISI 4140 substrates. By changing the laser power and laser scanning speed, a large number of process tests were carried out to study the influence of process parameters on the microstructure and coating performance of the coating. By optimizing the process parameters, the ratio of amorphous phase to crystalline phase can be changed to improve the wear resistance and toughness of the coating. Zhu et al. [18, 19] used laser cladding to prepare Fe-Ni-B-Si-V amorphous-nanocrystalline composite coatings with different process parameters. The study found that the coating microstructure changes along the depth direction. Different microstructures have different strengths and toughness. Most of the amorphous phase is concentrated in the middle of the coating, accompanied by a small amount of nanocrystals. This area has the highest strength and toughness. Li Zhuguo’s research group at Shanghai Jiaotong University [20-23] developed Fe-(Ni,Co)-B-Si-Nb laser cladding powder and studied the effects of laser process parameters such as laser cladding scanning rate, power and laser remelting on the formation of amorphous and crystalline phases and the mechanical properties of the coating. Their research results show that: ① Dilution rate and scanning speed are the decisive factors for the preparation of amorphous coatings in the laser cladding process. Low dilution ratio and higher scanning speed can increase the possibility of amorphous phase formation. By changing the crystallinity of the coating, the toughness and plasticity of the coating can be regulated; ② The solidification process after laser remelting is conducive to inhibiting the growth of grains, reducing the crystallinity of the coating, reducing the grain size, and effectively regulating the crystal phase ratio and grain size of the coating. From the fracture morphology of the coating, it can be seen that the laser remelting process can improve the brittle fracture performance of the coating; ③ Through the nanoindentation test method, it was found that the amorphous nanocrystalline structure of the Fe-Ni-B-Si-Nb amorphous nanocrystalline composite coating after laser remelting can effectively absorb the deformation work of the coating, reduce the generation of coating cracks, and have high hardness and toughness. The typical coating morphology they studied is shown in Figure 1, which shows the macroscopic morphology of the cladding layer and the changes in the microstructure along the depth direction. The microstructure is divided into three layers, namely Layer I (columnar dendrites), Layer II (equiaxed dendrites and some white particles) and Layer III (gray matrix and many white particles); the XRD test results of different areas of the coating are shown in Figure 2. It can be seen from Figure 2 that the gray matrix is ​​an amorphous structure and the white particles are NbC. Wang Yanfang’s research group at China University of Petroleum [24,25,26] systematically studied the phase composition, microstructure, and formation mechanism of Fe-based amorphous alloy cladding layers, established physical and mathematical models of laser cladding process parameters, obtained the temperature gradient and cooling rate variation of the laser molten pool along the depth direction, and revealed the growth mechanism and refinement mechanism of the crystal phase in the Fe-based amorphous composite coating; revealed the regulation of laser process parameters such as laser scanning speed on crystal epitaxial growth and refinement, and the beneficial effect of crystal phase grain refinement in Fe-based amorphous alloys on coating toughness and plasticity. Mojaver et al. [27] studied the possibility of complete amorphization of Fe49Cr18Mo7B16C4Nb3 coating and found that laser power and scanning speed had a very important influence on the amorphous content of Fe49Cr18Mo7B16C4Nb3 coating. The coating composition change and crack growth were affected by the laser heat input and melting ratio change. They also pointed out that there were multiple regions of the coating that were mixed with amorphous phase and ultrafine grain phase. The crack growth in these regions was significantly suppressed and the fracture toughness was significantly improved. In summary, when laser cladding was used to prepare Fe-based amorphous alloy composite coatings, it was difficult to completely amorphize the coating, and a dual-phase structure coating with coexistence of amorphous phase and crystalline phase was obtained with a high probability. The process parameters of laser cladding (laser power, scanning speed, laser remelting, etc.) have a great influence on the characteristics of the coating structure and composition, such as the ratio of amorphous phase and crystalline phase, the grain size of the crystalline phase, the interface bonding mode between the crystalline phase and the amorphous phase, the fracture failure effect of the dual-phase structure amorphous coating, etc.

The change of the coating structure is directly related to the plasticity and toughness of the coating. For example: ① The increase of the crystalline phase ratio within a suitable range will improve the plasticity or toughness of the coating; ② The appropriate interface bonding mode and distribution characteristics of the crystalline phase and the amorphous phase can effectively hinder the crack propagation when the coating is subjected to stress, thereby effectively improving the toughness and plasticity of the coating; ③ The smaller the grain size, the more effectively the strength and toughness of the Fe-based amorphous alloy composite coating; ④ The plasticity and toughness of the Fe-based amorphous composite coating with amorphous-nanocrystalline structure are significantly improved.

3 Effect of material formula on plasticization and toughening of iron-based amorphous alloy coatings
Iron-based amorphous alloy coatings are representative of brittle amorphous alloy coatings.
Macroscopically, they hardly show room temperature deformation. Generally, once the elastic limit is exceeded, they will break. Their plastic strain is usually less than 0.2%[28]. In order to give full play to the advantages of iron-based amorphous alloy coatings and make iron-based amorphous alloy materials widely used, researchers have conducted a lot of research on the effect of iron-based amorphous alloy powder formula on the toughness and plasticity of coatings, and have achieved some results.
Introducing a crystalline phase into the amorphous phase, using the crystalline phase to hinder the expansion of the shear band in the amorphous coating, so that the composite coating can accommodate more plastic deformation and achieve the effect of plasticization and toughening.
This method was first discovered in the study of Zr-based amorphous coatings. In 2000, Hays et al. [29] discovered a ductile crystalline β-phase Ti-Zr-Nb with a BCC structure in a Zr-based amorphous coating. Under an external mechanical load, the failure plastic strain and impact toughness of the composite coating increased significantly. Later in 2006, Shen et al. [30] introduced Cu into FeCoBSiNb amorphous alloy, causing α-(Fe, Co) plastic phase to precipitate in the FeCoBSiNb amorphous alloy, significantly increasing the yield strength of the alloy. At the same time, the plastic strain capacity of the alloy was also improved by 0.6%. This idea provides an important reference for scholars’ future research. Makino et al. [31] studied the effect of Cu on the plasticity of Fe-Si-B-P amorphous alloy. The results showed that the plastic deformation of Fe-Si-B-P amorphous alloy was increased to 3.1%, and multiple high-density shear bands were observed at the fracture interface. The main reason was the effect of in-situ synthesized nanocrystalline α-Fe (grain size less than 10 nm) embedded in the amorphous phase. Guo et al. [32] developed an in-situ formed tough α-Fe dendrite reinforced iron-based amorphous composite based on composition design. The composite showed a compressive plastic strain of 37.5% and a high fracture strength of 3.0 GPa. The study showed that the significant enhancement of plasticity was due to the strong interaction between the shear band and the α-Fe dendrite, which blocked the rapid propagation of the main shear band and promoted the generation of multiple shear bands. As shown by the arrows in Figure 3, the interaction between the dendrite phase and the shear band hindered their rapid propagation on the one hand, and induced the bifurcation and proliferation of the shear band on the other hand, thereby improving the overall plasticity. Poon et al. [33] added hard ceramic particles to the surface of amorphous alloy steel by using a suitable cladding temperature (the cladding temperature is higher than the melting point of Fe and lower than the melting point of ceramic particles such as TiC and NbC) and studied the changes in its mechanical properties. The results showed that the addition of hard particles significantly enhanced the stiffness, shear modulus and Poisson’s ratio of the composite material. In summary, from the perspective of material formulation, the principle of enhancing the toughness and plasticity of iron-based amorphous alloy materials is mainly to suppress the expansion of the shear band of the amorphous alloy by synthesizing the crystal phase or adding the crystal phase, thereby solving the room temperature brittleness problem of iron-based amorphous alloy materials. There are still many aspects worthy of further study, such as the chemical reaction, mechanical action, interface bonding mechanism, etc. between the crystal phase and the amorphous phase of different elements.

4 Auxiliary methods for plasticization and toughening of Fe-based amorphous composite coatings
In addition to improving the toughness and plasticity of Fe-based amorphous composite coatings and improving the room temperature brittleness of Fe-based amorphous composite coatings by adjusting laser process parameters and material formulations, some scholars have also studied the use of auxiliary process methods to improve the toughness and plasticity of coatings during the preparation of Fe-based amorphous coatings.

Ma Yuliang [34] studied the effect of transition layer on the performance of laser cladding Fe-based amorphous coatings. By first electroplating a Ni-P transition layer on the Q235 steel substrate as a substrate, and then laser cladding Fe-based amorphous coatings, the research results showed that the increase in the P content in the Ni-P transition layer increased the amorphous phase content in the coating, slightly reduced the hardness, and significantly improved the room temperature brittleness problem. Some scholars have also added electromagnetic fields in the process of laser cladding Fe-based amorphous coatings. Bu Liming [35] studied the effects of electromagnetic field and Ni-P transition layer on laser cladding Fe-based amorphous coating. The results showed that the electromagnetic field made the cladding layer more uniform and weakened the flow of the melt during laser cladding. The Ni-P cladding layer melt blocked the mixing of amorphous powder and matrix melt, which improved the amorphous formation ability of the laser cladding layer. The two worked together to effectively improve the impact toughness of the cladding layer. At the same time, the solidification characteristics, organizational structure and corresponding mechanism of the cladding layer under the action of the electromagnetic field were characterized, which provided a theoretical basis for the later industrial application of electromagnetic field in laser cladding Fe-based amorphous coating. Yuan Wuyan et al. [36] proposed a method for preparing Fe-based amorphous coatings by ultra-high-speed laser cladding assisted by alternating magnetic fields. The research results showed that the alternating magnetic field can make it difficult for dendrites to grow, or be broken or crushed during the solidification of the cladding layer, thereby increasing the amorphous phase content of the cladding layer, refining the grains, inhibiting defects such as cracks and pores in the cladding layer, and effectively improving the room temperature brittleness of the cladding layer. Jiang Fengchun et al. [37] proposed a method for preparing Fe-based amorphous coatings by ultra-high-speed laser cladding assisted by ultrasonic impact micro-forging technology. Ultrasonic impact micro-forging technology can control and improve the solidification process of amorphous coatings. The ultrasonic energy field can produce cavitation in the molten pool, reduce the instantaneous temperature of the micro-area melt, increase the solidification rate, and increase the proportion of amorphous phase. At the same time, it optimizes the stress state of the cladding layer, eliminates defects, and significantly improves the brittle fracture problem of Fe amorphous coatings. Wang Tiancong[38] studied the effect of different contents of nickel-plated carbon nanotubes on the microstructure and mechanical properties of laser-clad Fe-based amorphous coatings. The results showed that with the increase of the mass fraction of nickel-plated carbon nanotubes (0-1%), the hardness of the amorphous region of the coating decreased by 9% and the fracture toughness increased by 33.4%. Therefore, suitable auxiliary process methods can effectively improve the toughness and plasticity of laser-clad Fe-based amorphous composite coatings, thereby helping to solve the room temperature brittleness problem of the coatings. 5 Conclusion With the continuous development of marine, aerospace, petroleum, national defense and other industries, the requirements for material properties are becoming more and more stringent. Material properties are developing towards stronger, tougher, more corrosion-resistant and wear-resistant. Laser cladding preparation of Fe-based amorphous alloy coatings has many research results and applications in the above fields due to its metallurgical combination with the substrate, easy intelligent integration, high construction efficiency, and green environmental protection, but there are still some shortcomings. At present, one of the main research directions for optimizing the mechanical properties of Fe-based amorphous materials is to plasticize and toughen Fe-based amorphous materials to solve the room temperature brittleness problem. To this end, domestic and foreign scholars have conducted a lot of research and achieved some results, but the following aspects need further research. (1) Fine-tuning of various structures of laser cladding Fe-based amorphous composite coatings. Through the precise control of the crystallization degree of the material by the component composition and process, and the development of the crystal phase structure in the direction of nano- or sub-micronization, the coating structure with ultra-fine reinforcement phase and toughness phase uniformly distributed is obtained. (2) Further strengthen the research on the second phase based on dislocation strengthening theory such as in-situ generation of toughness phase, addition of nanoparticle toughness phase, and addition of metal ceramic particles, and further explore the mechanism of dual-phase plasticization and toughness. Under the guidance of theory, a uniform Fe-based amorphous structure with high-density dislocation entanglement nano-precipitation phase is prepared. (3) The process of laser cladding preparation of Fe-based amorphous alloy coatings involves many complex processes such as phase transformation thermodynamics, kinetics, and phase interface theory. For the complex process of multi-field coupling, the data of the multi-field coupling process can be further monitored through advanced information and digital technology. Through data analysis, the dual-phase intrinsic formation and action mechanism in the preparation process of laser cladding Fe-based amorphous alloy coating can be studied, and the performance prediction of the plasticity and toughness of Fe-based amorphous alloy materials can be carried out to carry out in-depth and systematic research.