## 4 Results and Discussion  
### Microscopic Examinations  
The transformations occurring in the material during FSP treatment are initiated and stimulated by the combined effect of heat and plastic deformation to which the material is subjected. The effect of friction stir processing is the presence of characteristic zones in the modified material, differentiated both in terms of structure and microstructure, and their location in the surface layer of the material. They are the stirring zone (SZ), the thermomechanically affected zone (TMAZ), and the heat-affected zone (HAZ) adjacent to the base material BM. The material located in the stirring zone is subject to the strongest plastic deformation and the highest temperature during friction stir processing. A characteristic feature of the stirring zone is the presence of very fine equiaxed grains, which proves that the microstructure of the material was formed under conditions of dynamic recrystallization. This effect was also noted in the samples examined in this study (Figure 6a). Figure 5: Scheme of FSP hole method (**a**), sample after filling chambers with reinforcing phase (**b**), execution of FSP treatment (**c**), macroscopic effect of FSP treatment (**d**), tool (**e**). It should be noted, however, that the grain size was not the same throughout the stirring zone. This is because the thermal effect generated directly at the surface, i.e., under the tool shoulder, is different than that generated in the lower parts of the pin working zone. Therefore, when assessing the degree of refinement, the average grain size, or even more often, the grain size range is given. In the case of the analyzed samples, the grain size in the stirring zone ranged from 1 to 4 \(\upmu\)m, regardless of the reinforcing phase used. The identical degree of grain refinement in these samples indicates that despite the use of different reinforcing materials, the course and kinetics of dynamic recrystallization were similar in all the composites. Dynamic recrystallization (DRX) is the main mechanism responsible for grain refinement in friction stir processed metals and metal alloys, and similar to static recrystallization (SRX), it may involve the formation and growth of recrystallization nuclei. However, unlike static recrystallization, in the case of DRX, the processes of the nucleation and growth of nuclei occur during plastic deformation of the material, and not after plastic deformation. The type and kinetics of dynamic recrystallization are influenced by many factors, which can be generally divided into factors resulting from the type of material used (e.g., initial grain size, stacking fault energy of the modified material, the presence of second phase particles and their size) and factors resulting from the employed processing parameters (e.g., temperature and strain rate, degree of material deformation, material cooling intensity, pin shape, number of tool passes). As part of this work, all the samples were processed using analogous parameters and cooling conditions; the only difference resulted from the use of different reinforcing materials. The particles of the reinforcing phase may constitute recrystallization nuclei during recrystallization and facilitate recrystallization or lead to slower recrystallization and inhibition of the grain growth of the material. The latter situation occurs when particles are located on the boundaries of newly formed grains, which leads to blockage of the movement of grain boundaries and recrystallization fronts, and thus to the limitation of grain growth. This is the so-called Zener effect. The key parameter determining the actual role of particles and their impact on the kinetics of recrystallization and grain boundary migration is primarily the particle size, which, as should be emphasized, may change during processing with the progressive deformation of the material or the number of repetitions of FSP treatment.