## 3 Results and Discussion
The thermal diffusivity gap between the Al-1.1 Mg\({}_{2}\)Si and Al-3.5 Mg\({}_{2}\)Si samples was measured as approximately 15 mm\({}^{2}\)/s at 25 \({}^{\circ}\)C. This difference of thermal diffusivity between the two samples was attributed to two reasons: the content of the alloying element in alloys and the concentration of dissolved solute in the Al matrix after quenching. The additive atoms have a different size than the Al atoms. In general, the addition of alloying elements led to a decrease in thermal diffusivity. Further, if impurity atoms had dissolved in the Al matrix, those impure atoms caused a partial lattice distortion in the matrix; this distortion interrupted the movements of the heat carriers. Thus, thermal diffusivity decreased as the concentration of impurity atoms increased [16; 20; 21]. Figure 4: X-ray diffraction patterns of Al-1.9 Mg2 Si specimens: (**a**) as-quenched sample (supersaturated solid solution state) and (**b**) DSC analyzed sample. Figure 3: The heat flow curves of solid solution-treated samples according to temperature increases after quenching: (**a**) Al-1.1 Mg2 Si, (**b**) Al-1.9 Mg2 Si, and (**c**) Al-3.5 Mg2 Si. In general, thermal diffusivity linearly decreased with the increase of temperature above room temperature. This was because the heat vibrations of the matrix atoms interrupted the movements of free electrons in the lattice [10]. However, in our study, the thermal diffusivity tendency of each specimen changed with temperature and content. In Figure 5a, the slope of the thermal diffusivity curve had a negative value at zone 1; however, the Al-3.5 Mg\({}_{2}\)Si specimen (Figure 5c) had a small positive slope in the same zone. Zone 1 was defined as the range in which thermal diffusivity was continuously and linearly changed corresponding to the increase of temperature. [PERSON] et al. [25] demonstrated that the co-cluster and the Guinier-Preston (GP) zone combined and precipitated at below 200 \({}^{\circ}\)C. This zone included the precipitation temperature of the GP zone. It appears that the precipitation of the solute affected the increase in thermal diffusivity. The thermal diffusivities of the samples rapidly increased at a specific temperature range, as shown in Figure 5; this temperature range was called zone 2. The temperature of zone 2 matched the precipitation temperatures of the \(\beta^{\prime\prime}\) and \(\beta^{\prime}\) phases (A and B peaks) in Figure 3. As the GP zone developed to the \(\beta^{\prime\prime}\) and \(\beta^{\prime}\) phases, the precipitations of the \(\beta^{\prime\prime}\) and \(\beta^{\prime}\) phases reduced the concentration of solute atoms in the Al matrix [10; 26]. As a result, heat transfer was facilitated, and the thermal diffusivity increased [27]. The thermal diffusivity of the Al-3.5 Mg\({}_{2}\)Si specimen increased in zones 1 and 2, as shown in Figure 5c. Al-3.5 Mg\({}_{2}\)Si had the largest amount of dissolved solute in the matrix, and the precipitation of the dissolved solute contributed to the increased thermal diffusivity. The thermal diffusivities of all of the samples decreased at temperatures above zone 2, as shown in Figure 5, due to the thermal vibration of the atoms in the matrix, which interrupted the heat transfer [28]. The heat energy excited the metal atoms and increased the temperature of the metal. The thermal vibration of the excited atoms interrupted the free electron transfer through the matrix [29]. Therefore, the thermal diffusivities of the specimens decreased with the increase of temperature in zone 3. The as-quenched samples were aged at temperatures ranging from 180 to 240 \({}^{\circ}\)C for 5 h. The Al-1.9 Mg\({}_{2}\)Si specimens were analyzed with OM, SEM, and TEM, and the results of these analyses are shown in Figure 1.0 The precipitate cannot be observed using SEM, and as shown in Figure 6a, only the AlFeSi phase appeared.