# Structural Particularities, Prediction, and Synthesis Methods in High-Entropy Alloys
## 3 Synthesis Methods and Their Influence on the Structure and Properties of HEA Alloys
### Influence of the Synthesis Method on the Structure and Properties of HEA Alloys
#### 3.2.1 Influence on the Microstructure of Different HEA Alloys  
Figure 7 illustrates the impact of different production techniques on the microstructure of High-Entropy Alloys (HEAs). It highlights the advantages and disadvantages of arc melting, mechanical alloying, casting, and additive manufacturing in terms of achieving homogeneity, grain refinement, and controlling microstructural characteristics. These techniques are assessed for their ability to produce fine-grained and uniform structures, manage segregation, and handle contamination. Understanding these influences is crucial for optimizing the microstructural properties of HEAs for various engineering applications. In 2015, a group of researchers from Australia [74] published a study demonstrating the influence of synthesis methods on the structure of HEA alloys by comparing the microstructures of samples fabricated through direct laser synthesis (using an elemental powder mix, single solidification) and those produced via arc melting (undergoing five remelting cycles). They tested three types of Al\({}_{x}\)CoCrFeNi alloys: Al\({}_{0.3}\)CoCrFeNi with an FCC structure, Al\({}_{0.6}\)CoCrFeNi with a dual FCC + BCC structure, and Al\({}_{0.85}\)CoCrFeNi with a BCC structure. Their results revealed that both the Al\({}_{0.3}\) FCC and Al\({}_{0.85}\) BCC alloys exhibited similar microstructures and textures regardless of the processing method used, showing a pronounced \(\langle 001\rangle\) fiber texture aligned with the solidification direction. In the case of the FCC-BCC alloy (Al\({}_{0.6}\)CoCrFeNi), arc melting resulted in a dendritic structure, whereas direct laser synthesis produced a Widmanstatten structure. This variation in microstructure was attributed to the faster solidification conditions during direct laser synthesis compared to arc melting. In 2021, a group of South African researchers published a review article [5] highlighting several studies investigating the influence of synthesis methods through mechanical alloying and consolidation via SPS on the microstructure of various types of HEA alloys. Their primary conclusion was that during the mechanical alloying (MA) process, the energy generated as heat influences the formation of the phases. Severe plastic deformation during mechanical alloying (MA) results in a higher density of dislocations and induces strain within the lattice of the system. Key parameters, including grinding duration and speed, as well as the ball-to-powder ratio, are essential in determining phase evolution within  
Figure 7: The influence of synthesis methods on the structure of HEA alloys. powders. Furthermore, the initial structure of binary alloy pairs, the order of element addition, and the mixing sequence also impact phase development. During the consolidation process via spark plasma sintering (SPS), lattice distortion and the slow diffusion of elements further affect phase evolution in HEA alloys. Consequently, it can be concluded that the process parameters employed during MA and SPS have a significant influence on the microstructure and phases of HEA alloys. According to the same review article from 2021 [5], phase changes occur during solidification (cooling) in the melting and casting processes of HEA alloys. The formation of phases during solidification is affected by how constituent elements move or are distributed within the alloy. However, the cooling rate, local atomic arrangement variations, and the different diffusion rates of elements can significantly influence the solid phases and the overall microstructure of the alloys. Alloys produced through melting and casting often display dendritic microstructures with segregations between the dendrites. Therefore, melting and casting techniques with higher cooling rates tend to favor the presence of a single dominant phase and limit the formation of secondary phases. Additionally, the formation of HEA phases during melting and casting relies more on the interactions between binary alloy pairs than on the individual constituent elements.