# Crystallization Kinetics of Sr-Modified Precipitation Hardening Al-Si-Cu Alloys
## 3 Discussion of the Research Results
### Microstructure and Chemical Composition Analysis  
The microstructure was observed in planes perpendicular to the vertical axis of the samples. The AlSi6 Cu4 alloy is characterized by the presence of phases with silicon in the form of long lamellae, which are non-uniformly distributed in the interdendritic spaces. Under the influence of a strontium addition, the silicon crystals were fragmented, and the lamellar silicon eutectic was transformed into a eutectic with a fibrous morphology of eutectic silicon. As a result of the addition of Sr, fine silicon crystals occupy a larger area of the study area (Figure 6b).  
Strontium also increased the share of large phases with copper, the morphology of which took the form of lumps (Figure 6c,d). In aluminum-silicon alloys containing copper, two forms of phases are present, i.e., Al\({}_{2}\)Cu--large and eutectic, differing in morphology and size, and the reason for the increased ratio of large phases with copper to those of a eutectic nature may be the addition of strontium [29; 30]. The presence of strontium in the added amount of 0.2% caused the formation of phases with strontium, probably Al\({}_{2}\)Si\({}_{2}\)Sr (Figure 6c,d) [8]. In the case of an aluminum alloy with 1% Sr added, Al\({}_{2}\)Si\({}_{2}\)Sr phases taking various polygonal shapes or elongated forms are present (Table 6, Figure 7). According to the equilibrium diagram [31], it is difficult to precipitate the Al\({}_{2}\)Si\({}_{2}\)Sr phase in the alloy with Sr content \(\leq\) 2000 ppm by weight. The Sr content around the primary Si solidification front is sufficient to ensure nucleation and the growth phase of Al\({}_{2}\)Si\({}_{2}\)Sr. Thus, the Al\({}_{2}\)Si\({}_{2}\)Sr phase is susceptible to nucleation at the solidification front and increases in size with the primary Si, and some of the solidification area in the alloy with a higher cooling rate causes local segregation, which favors the precipitation of the Al\({}_{2}\)Si\({}_{2}\)Sr phase [32]. The reasons for the  
Figure 5: Summary of solid fraction curves during solidification.  
formation of pores in aluminum alloys with the Sr addition were explained by [PERSON]. [33; 34]. Also the work performed by [PERSON] and [PERSON] offers valuable insights and contextualizes our finding. The authors also observed a similar fibrous morphology transformation with the addition of 0.03 wt% of Sr to Al-Si-Cu. Based on this paper, first-principal calculation results showed that Sr has a strong affinity to vacancy and interfaces, prohibiting the nucleation of the eutectic Si and porosity. This might be able to further support the thermal analysis results [20; 35].  
\begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**Point/Area**} & \multicolumn{4}{c}{**Mass Concentration of Elements, \%**} \\ \cline{2-5}  & **Al** & **Si** & **Cu** & **Sr** \\ \hline
1 & 27.7 & 31.5 & - & 40.8 \\ \hline
2 & 27.8 & 29.9 & - & 42.3 \\ \hline
3 & 28.0 & 30.9 & - & 41.1 \\ \hline
4 & 55.3 & 43.1 & 1.6 & - \\ \hline \end{tabular}
\end{table}
Table 6: Mass and atomic concentration of elements in Figure 7.  
Figure 6: The microstructure of alloys after solidification: (**a**) AlSi6 Cu4; (**b**,_c_) AlSi6 Cu4 Sr0.2; and (**d**) AlSi6 Cu4 Sr1; # 1—\(\alpha\), # 2—\(\beta\), # 3—Al2 Cu, # 4—Al15(Fe,Mn)3 Si2, and # 5—Al2 Si2 Sr.