## 3 Results
### Analysis of the Transition Zone in the SiC/Zn1.5 Mg Joint  
An undulated boundary with small depressions was formed at the SiC/solder boundary, resulting from the assistance of ultrasound power. A detail of this boundary is shown in Figure 14.  
Figure 14: EDX point analysis of SiC/Zn1.5 Mg joint. Figure 13: Microstructure of the boundary in the SiC/Zn1.5 Mg/Cu joint from the SEM observation in BSE mode. The chemical composition and identification of specific phases in the soldered joint were determined using the application of EDX analysis. The measurement was carried out at six points, namely Spectrum 1 to 6-Table 6.  
In the measurement points Spectrum 1, 3, 4, and 5, spot local analyses from the contact zone of Zn1.5 Mg solder/SiC substrate were performed. The results were affected by the position of the measurement point in the boundary. The closer the measurement point to the SiC substrate, the higher the Si content observed. A narrow zone formed in the boundary, where an interaction occurred between Zn-Mg solder and the SiC substrate. Spectrum 5 slightly deviates in Zn and Si content compared to the three remaining point analyses. The average zinc content in this zone (Spectrums 1, 3, 4) is 64.4 at. % Zn, 29.2 at. % Si, 4.2 at. % Mg, and 2.2 at. % Cu. The presence of Cu in this zone may be due to conventional and diffusion processes of copper occurring in the solder under the effect of ultrasound, where the second joint was formed by the combination of Cu/Zn1.5 Mg solder. The phase diagram Mg-Si (Figure 15) is of the eutectic type. One eutectic is formed by the reaction from the melt at 637.6 \({}^{\circ}\)C, resulting in the creation of a eutectic mixture of Mg\({}_{2}\)Si + Mg, whereas the second reaction takes place at 945.6 \({}^{\circ}\)C with the creation of a eutectic Mg\({}_{2}\)Si + Si mixture. Both Mg and Si elements are mutually practically insoluble. Spectrum 2 and 6 measurement points are the zones with a high zinc content (about 96 at. %, about 3.3 at. % Cu and 1 at. % Mg). The solubility of Mg in zinc at normal temperatures is negligible (<1 at. %), similar to the copper solubility in zinc. In the peritectic reaction at the temperature of 425 \({}^{\circ}\)C in the Cu-Zn system, the copper content in the solid solution of Zn is 2.75 wt. % Cu. The Cu solubility in (Zn) gradually decreases with decreasing temperature. In this case, the solid solution of zinc with a low content of Cu and Mg is concerned. \begin{table}
\begin{tabular}{c c c c c} \hline
**Spectrum** & **Zn [at. \%]** & **Mg [at. \%]** & **Si [at. \%]** & **Cu [at. \%]** \\ \hline Spectrum 1 & 65.0 & 4.4 & 28.6 & 2.0 \\ Spectrum 2 & 96.8 & - & - & 3.2 \\ Spectrum 3 & 65.4 & 1.0 & 30.4 & 3.2 \\ Spectrum 4 & 63.0 & 7.3 & 28.2 & 1.5 \\ Spectrum 5 & 49.1 & 2.7 & 46.2 & 2.0 \\ Spectrum 6 & 95.1 & 1.4 & - & 3.5 \\ \hline \end{tabular}
\end{table} 
Table 6: Results of point EDX analysis of SiC/Zn1.5 Mg joint. Figure 15: Equilibrium binary diagram of Mg-Si [23]. The interaction between the solder and ceramic is well illustrated by the planar distribution of magnesium (Figure 16b) in the boundary of the SiC/Zn1.5 Mg joint. In this joint boundary, the newly formed intermetallic phases of magnesium were segregated, which enhanced the bond formation. The line analyses (Figure 17) illustrate the concentration profile along the SiC/solder boundary, covering a total length of 9 \(\upmu\)m. The interaction between the solder and the SiC substrate took place predominantly at the boundary, resulting in the formation of an extremely thin layer, approximately 1 to 2 \(\upmu\)m thick. There is also an apparent zinc and silicon interaction, which overlaps 2 \(\upmu\)m in length. Figure 16: The planar distribution of elements (**b**) Mg, (**c**) Si, and (**d**) Zn in the joint boundary (a) SiC/Zn1.5 Mg.  
Figure 17: The line EDX analysis of SiC/Zn1.5 Mg joint, (**a**) transition zone with a marked line, (**b**) concentration profile of Mg, (**c**) Zn, and (**d**) Si elements.