# Crystallization Kinetics of Sr-Modified Precipitation Hardening Al-Si-Cu Alloys
## 3 Discussion of the Research Results
### Structural X-Ray  
Phase analysis using the X-ray diffraction technique confirmed the presence of alloying elements such as aluminum and silicon, as well as some intermetallic phases. In the studied AlSi6 Cu4 alloy subjected to supersaturation and aging, the detected intermetallic phases are those with copper (Al\({}_{2}\)Cu) and manganese- and iron-rich phases--most likely to be the Al\({}_{15}\)(Fe,Mn)\({}_{3}\)Si\({}_{2}\) alloy (Figure 8). Both in the alloy with the 0.2 wt.% strontium addition and in the AlSi6 Cu4 Sr1 alloy, Al\({}_{2}\)Si\({}_{2}\)Sr phases were detected.  
The interplanar distance for the AlSi6 Cu4 alloy after precipitation hardening is 2.350 A, while this parameter for the reference material, the AlSi6 Cu4 alloy in the post-TDA state, reaches 2.336 A [35]. Thus, the precipitation hardening process slightly increased the interplanar distance due to phase separation in the matrix region. The addition of strontium also increased it by more than 0.1 A (Table 7). Alloying elements form intermetallic phases during solidification, i.e., they separate from the solution without causing deformation of the matrix network. The \(\theta\) phases dissolved during supersaturation precipitate during aging, which is the reason for the slight difference in the interplanar distance.  
After the addition of 1% strontium, there is still present porosity (Table 8) in the microstructure of this alloy. As is revealed in other studies' processes, innovation is urgently needed. To fundamentally improve the microstructure and reduce defects, we must control the distribution of composition and microstructure morphology in melt solidification and decrease the formation temperature [36]. One possible approach could be the use of semisolidicasting, which is a progressive and active near net forming technique that can be used to solve the above problems and to prepare high-quality castings.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Alloy** & **Interplanar Distance, Å** \\ \hline AlSi6 Cu4 & 2.350 \\ \hline AlSi6 Cu4 Sr0.2 & 2.471 \\ \hline AlSi6 Cu4 Sr1 & 2.481 \\ \hline \hline \end{tabular}
\end{table}
Table 7: Interplanar distance for the tested alloys after precipitation hardening.  
Figure 7: Morphology of the strontium phases in the AlSi6 Cu4 Sr1 alloy.  
Figure 8: X-ray diffractograms for alloys after supersaturation and aging: (**a**) AlSi6 Cu4, (**b**) AlSi6 Cu4 Sr0.2, and (**c**) AlSi6 Cu4 Sr1.  
The addition of Sr to the Al-6%Si alloy increases its hardness and wear resistance. On the other hand, the addition of 1%Sr tends to decrease the mechanical properties compared the alloy when there was no addition of strontium to the alloy. However, the effect appears to decline as soon as the addition exceeds the 0.5% level, which shows negative effects in response to the mechanical properties because of the presence of some compounds like, for example, the brittle Al3 SrSi3 phase, which coarsens the eutectic silicon and reduces the properties of the alloy, causing them to revert to values more typical of unmodified material.  
Analysis of microscopic images was used to analyze the porosity of the alloys. The ImageJ program was used for this purpose. ImageJ is a freely available multi-platform image processing program commonly used by scientists. The pores in the microscope images were extracted using the 'Threshold' filter and then counted using the 'analyze particles' function. The following porosity values were obtained in the alloy samples: AlSi6 Cu4--0.28%; AlSi6 Cu4 Sr0.2--0.35%; and AlSi6 Cu4 Sr1--0.05%. In Figures 9 and 10