## 3 Microstructure of the Filling Materials  
### Filling Material Type AlMgZr  
Three types of AlMgZr alloys with different weight percentages of zirconium were cast for the study. Analysis of the chemical composition together with the X-ray spectrum of the cast filling materials showed that the amount of this element was, respectively, 0.35 wt.% (Figure 4a), 0.67 wt.% (Figure 4b) and 0.93 wt.%. (Figure 4c). The magnesium content varied within 5 wt.% in all castings made.  
The observation and metallographic analysis of each of the aforementioned alloys was then carried out. Figure 4 shows example images of the microstructure of AlMg5 Zr type castings with different zirconium contents. As can be seen in Figure 4a, the addition of  
Figure 3: Dimensions of the custom sample used for the tensile strength test of the resulting castings.  
Figure 2: Macroscopic view of one of the received castings.  
In addition, iron precipitates in the characteristic \"Chinese script\" form were observed in the AlMg5 Zr0.67 casting, as well as the formation of a triple eutectic \(\alpha\) + Mg2 Si + Si with significant intensification for AlMg5 Zr0.93 (Figure 6).  
Agglomerates of \"rod-shaped\" (Al,Si)\({}_{3}\)Zr precipitates with a size of 30-50 \(\upmu\)m and chemical composition as in Figure 7 were visible in all castings.  
In the AlMgZr castings, small spherical Al\({}_{3}\)Zr precipitates ranging from 1 \(\upmu\)m to 3 \(\upmu\)m in size could also be observed, unevenly distributed throughout the casting at grain boundaries and in small amounts in the interdendritic spaces (Figure 8).  
Figure 8: Al\({}_{3}\)Zr precipitates in an AlMgZrO.67 casting.  
Figure 6: X-ray microanalysis of the chemical composition of the precipitate phases of the AlMg5 Zr0.93 casting.  
Figure 7: Cluster of club-shaped precipitates in an AlMgZrO.84 casting.  
It is known that in binary alloys, zirconium remains in the form of a supersaturated solution upon rapid cooling of the casting. Furthermore, the decomposition of the presaturated solid solution manifests itself through the formation of a metastable L12 Al\({}_{3}\)Zr phase characterised by negligible compatibility with the aluminium matrix. Further annealing at temperatures in the range 400-500 \({}^{\circ}\)C leads to transformation of the equilibrium DO23 Al\({}_{3}\)Zr phase [52]. In multicomponent alloys, the strong Al-Zr bonding of the solid solution is replaced by weaker Mg-Zr reducing the solubility of zirconium. This effect is amplified when silicon is present in the alloy [53]. This may explain the agglomeration of rod-shaped (Al,Si)\({}_{3}\)Zr precipitates as a product of undissolved zirconium in the aluminium matrix. On the other hand, the location of the spherical Al\({}_{3}\)Zr precipitates in the spaces of the primary dendritic grains indicates that the zirconium content of the crystallising liquid was sufficient for their nucleation. The number, type of precipitates and their size were probably related to the temperature and annealing time of the homogenising casting. To achieve maximum precipitation hardening by forming high-density Al\({}_{3}\)Zr phase dispersoids, a two-step annealing would have to be carried out [54]. Given the scientific use of the alloy as a possible additional AlMgZr material, which involves its re-melting, two-step annealing was not carried out.