## 4 Results and Discussion
### Microscopic Examinations
The reinforcing phases used in this work were characterized by a predominant share of small-sized particles, the presence of which in the material favored the formation of a fine-grained microstructure. It is worth noting here that high grain refinement is one of the main goals of FSP treatment, and the obtained degree of grain refinement is the effect of heat acting on the material or the amount of plastic deformation of the material. As indicated by the research results of [PERSON] et al. [49; 50] regarding the production of composites via deformation-driven metallurgy, due to the strong plastic deformation of the material and dynamic recrystallization, not only can strong refinement of the matrix grains be achieved, but also uniform dispersion of the reinforcing phase in the composite. Figure 6: Microstructure of stirring zone (**a**) and thermomechanically affected zone (**b**). Etched, light microscopy. Multiple FSP passes also allow greater grain refinement and, at the same time, improve the uniformity of the distribution of the reinforcing phase. Each subsequent pass of the tool leads to the re-occurrence of dynamic recrystallization of the material and a more thorough mixing of the material, which results in an increase in the degree of grain refinement and higher homogeneity of the composite microstructure. An additional improvement in the uniformity of the distribution of the reinforcing phase can also be achieved by changing the direction of mixing the material with each subsequent pass of the tool [51]. The hole solution with the shifted working zone of the pin used in this work is a two-stage solution, thanks to which it is possible to obtain better microstructural effects than in the case of single-stage processing. Multiple processing is particularly recommended when producing particle-reinforced composites that tend to form clusters and agglomerates. It is also worth adding that the degree of grain refinement may also be influenced by additional cooling during processing. This is currently an important trend in FSP and a very promising direction of research [52]. Taking into account the main goal of the work, special attention was paid to the assessment of the composite microstructure in the context of the reinforcing phase introduced into the matrix. Examples of microstructures of the obtained composites are shown in Figure 7. The microscopic examinations revealed that as a result of FSP treatment, a composite layer with a thickness of approximately 4 mm to 5 mm was created. The thickness of the composite layer usually corresponds to the length of the pin of the tool employed in the processing, although the dimensions of this layer are also influenced by other processing parameters. The microscopic examinations also disclosed that the particles of the reinforcing phase were located primarily in the stirring zone. In the zone adjacent to the stirring zone, i.e., in the thermomechanically affected zone, there were also particles of the reinforcing phase, but their number was much smaller. A characteristic feature of the TMAZ zone was the presence of grains with an elongated shape reflecting the direction of flow of the plasticized material. It is worth explaining here that in the thermomechanically affected zone, the material also undergoes plastic deformation, but dynamic recrystallization does not occur or its scope is limited, which results in the formation of a microstructure that is at most partially recrystallized. In none of the analyzed samples were particle clusters or agglomerates found in the stirring zone, which proves the correct selection of the FSP parameters. The uniformity of particle distribution in the matrix is an extremely important criterion for assessing the microstructure of the composite, determining its properties and behavior during operation. Nonetheless, when comparatively assessing the microstructures presented in Figure 7, it can be seen that the most uniform distribution of the reinforcing phase in the matrix was obtained in the case of SiC powder.