# Crystallization Kinetics of Sr-Modified Precipitation Hardening Al-Si-Cu Alloys
## 3 Discussion of the Research Results
### Thermal-Derivative Analysis
For the reference material, the undercooling is less than 1 \({}^{\circ}\)C, and, for AlSi6 Cu4 Sr0.2 and AlSi6 Cu4 Sr1 alloys, it is 1.6 \({}^{\circ}\)C and 1.7 \({}^{\circ}\)C.  
Figure 2: Cooling, crystallization and base curve—AlSi6 Cu4 alloy. Figure 4: Cooling, crystallization and base curve—ASI6 Cu4 Sr1 alloy. Figure 3: Cooling, crystallization and base curve—ASI6 Cu4 Sr0.2 alloy. Temperature T\({}_{(Al\text{-Si-Cu})}\), at which the nucleation of phases with copper with the simultaneous crystallization of \(\alpha+\beta\) eutectic under strontium increased by \(\sim\)6 \({}^{\circ}\)C, which corresponds to other research results [18]. The following successive nucleation of the fine eutectic form of phases with copper and phases with magnesium also occurs at a slightly higher temperature T\({}_{(Al\text{-Si-Cu-Mg})}\). The end of crystallization of the studied alloys occurs at point XIII, and the T\({}_{\text{SOL}}\) temperature for the AlSi6 Cu4 Sr0.2 alloy reaches higher values by 1.2 \({}^{\circ}\)C. Strontium increased the solidification time of the AlSi6 Cu4 Sr0.2 alloy by almost 34 s, while, for the AlSi6 Cu4 Sr1 alloy, this time is 121 s longer. At 0.2% Sr content, there is no apparent inflection in the derivative curve of the solidifying AlSiSr phase. The addition of higher amounts of strontium leads to a visible overcooling of the alloy presented in Figure 5. The reason for this is the extended formation of intermetallic phases with strontium, which have a lower solidification temperature compared to the basic aluminum matric and Al-\(\alpha\) phase. Figure 5 shows a summary of the solid fraction contribution curves during solidification. The addition of Sr caused a change in the shape of the solid fraction curve as a result of the crystallization of the Al2 Si2 Sr phases before nucleation of the \(\alpha\) phase dendrites. The latent heat of crystallization is proportional to the share of individual phases in the alloy, and the thermal-derivative analysis allows us to calculate the proportions of phases crystallized in the alloy. The calculation is based on characteristic points determined on the differential curve. These points reflect the thermal effects occurring in the alloy during crystallization and depend on the chemical composition of the alloy, the cooling rate, the rate of crystallization heat generation and the temperature of the liquid metal, i.e., the parameters affecting the final structure of the alloy. These parameters also characterize the crystallization kinetics of the alloy. Tables 3-5 show the determined values of specific heat and the latent heat of crystallization. The value of specific heat, both in the liquid and solid state, decreased with the addition of strontium. The least latent heat of crystallization was given off during solidification of the alloy with 1% Sr. The amount of this heat is \(\sim\)10% less (Table 5) than the heat given off during crystallization of the reference material (Table 3). \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Point/Temperature**} & \multicolumn{3}{c}{**Temperature, \({}^{\circ}\)C**} \\ \cline{2-5}  & **AlSi6 Cu4** & **AlSi6 Cu4 Sr0.2** & **AlSi6 Cu4 Sr1** & **Reaction** \\ \hline IA/TL & - & - & 683.2 & Liq \(\rightarrow\) AlSiSr \\ \hline I/TDN & 608.5 & 610.2 & 616.8 & \\ \hline II/Tdmin & 603.2 & 604.3 & 611.3 & \\ \hline III/TDKP & 603.5 & 605.0 & 611.9 & Liq \(\rightarrow\alpha\) \\ \hline IV/TG & 603.6 & 605.9 & 612.3 & \\ \hline V/T(Al-Mn-Fe-Si) & 603.5 & 605.2 & - & Liq \(\rightarrow\alpha\) + AlMnFeSi \\ \hline VI/TE(\(\alpha+\beta\))N & 565.6 & 555.8 & 553.0 & \\ \hline VII/TE(\(\alpha+\beta\))min & 561.8 & 551.4 & 549.1 & Liq \(\rightarrow\alpha+\beta\) \\ \hline VIII/TE(\(\alpha+\beta\))G & 562.6 & 553.0 & 550.8 & \\ \hline IX/T(Al-Si-Cu) & 502.9 & 509.0 & 509.1 & Liq \(\rightarrow\alpha\) + AlCu + \(\beta\) \\ \hline X/T(Al-Si-Cu-Mg) & 499.6 & 502.4 & 502.9 & Liq \(\rightarrow\alpha\) + AlCu + MgSi + \(\beta\) \\ \hline XI/TSOL & 480.5 & 481.7 & 480.9 & Solid \\ \hline \hline \end{tabular}
\end{table}