## 3 Results and Discussion
In Figure 6b, a small precipitate was present after the aging treatment at 180 \({}^{\circ}\)C for 5 h. The size of this small Mg\({}_{2}\)Si phase (\(\beta^{\prime\prime}\)) was 30 nm. The particles were too small to be observed in the SEM image of Figure 6a. Precipitate was detected by the optical microscope after aging at 240 \({}^{\circ}\)C. The precipitate seems to be similar to the black dot in Figure 6c. The average size of the Mg\({}_{2}\)Si was less than 1 \(\upmu\)m with an aging temperature of 240 \({}^{\circ}\)C. In Figure 6d, a lot of Mg\({}_{2}\)Si was randomly distributed on the Al matrix of 4-6 \(\upmu\)m.  
Figure 1.1 Thermal diffusivity changes of as-quenched samples according to increasing temperature: (**a**) Al-1.1 Mg\({}_{2}\)Si, (**b**) Al-1.9 Mg\({}_{2}\)Si, and (**c**) Al-3.5 Mg\({}_{2}\)Si. Aged Al-1.1 Mg\({}_{2}\)Si was also analyzed in terms of thermal diffusivity using LFA and the results are shown in Figure 1.2 Thermal diffusivity at 25 \({}^{\circ}\)C increased from 77 mm\({}^{2}\)/s to 84 mm\({}^{2}\)/s with the increase of the aging temperature from 180 to 240 \({}^{\circ}\)C. The thermal diffusivity of the Al-1.1 Mg\({}_{2}\)Si samples decreased with the increase of temperature in zone 1 (below 200 \({}^{\circ}\)C). In zone 2, the thermal diffusivity of the 180 \({}^{\circ}\)C aged sample increased with the increase of temperature. The increment of thermal diffusivity in this zone was approximately 2 mm\({}^{2}\)/s. However, the thermal diffusivities of both samples decreased with the increase of temperature when samples were aged at 220 \({}^{\circ}\)C and 240 \({}^{\circ}\)C. The thermal diffusivities of all of the samples reached similar values, then reduced with the increase of temperature in zone 3. The aged Al-1.1 Mg\({}_{2}\)Si samples were investigated in terms of calorimetric analysis and shown in Figure 1.3  
Figure 1.4 The precipitation of Mg\({}_{2}\)Si phase in Al-1.9 Mg\({}_{2}\)Si alloy: (**a**) scanning electron microscope (SEM) photograph after aging treatment at 180 \({}^{\circ}\)C for 5 h; (**b**) transmission electron microscope (TEM) photograph after aging treatment at 180 \({}^{\circ}\)C for 5 h; (**c**) optical microscope (OM) photograph after aging treatment at 240 \({}^{\circ}\)C for 5 h; and (**d**) OM photograph after aging treatment at 340 \({}^{\circ}\)C for 5 h.  
Figure 1.5 The thermal diffusivity changes of Al-1.1 Mg\({}_{2}\)Si samples according to the temperature increase with various contents and aging treatment conditions. The exothermic peaks of A, B, and C were generated due to the precipitation reactions of \(\beta^{\prime\prime}\), \(\beta^{\prime}\), and \(\beta\), respectively, as mentioned earlier in Figure 1.6  
The exothermic reactions A, B, and C of the heat flow curve decreased with the increasing aging temperature. The area of the exothermic curve (enthalpy) indicates the number of precipitates generated during the calorimetric analysis [8]. Thus, the number of residual solutes decreased with the increasing aging temperature. The Al-1.1 Mg\({}_{2}\)Si specimen aged at 180 \({}^{\circ}\)C had the largest number of residual solutes, and the thermal diffusivity of this specimen was the lowest at 25 \({}^{\circ}\)C, as shown in Figure 1.7 However, in the case of the 240 \({}^{\circ}\)C aged sample, the precipitations of the \(\beta^{\prime\prime}\) and \(\beta^{\prime}\) phases were nearly completed after the aging treatment. The precipitation reaction could not occur during the thermal diffusivity measurement in the 240 \({}^{\circ}\)C aged sample. The thermal diffusivity of this sample only decreased in zone 2, as shown in Figure 1.8  
The thermal diffusivity and the heat flow curves for the Al-3.5 Mg\({}_{2}\)Si aged specimens are shown in Figures 9 and 10. In Figure 9, the 180 \({}^{\circ}\)C aged sample had a nearly regular thermal diffusivity value of 62 mm\({}^{2}\)/s in zone 1. The thermal diffusivity values at 25 \({}^{\circ}\)C increased with the increasing aging temperature.