**3. Results**

#### *3.1. Mechanical Properties*

The stress-strain curves of a tensile test in Al-9.8%Si-3ppmP cast alloy and Al-10.1%Si-4ppmP-108ppmSr cast alloy are shown in Figure 4. The ultimate tensile strength decreases and elongation to failure increases in both samples with increasing heat treatment time. Both samples show almost a similar stress-strain relationship before and after heat treatment, though tensile strength in Al-10.1%Si-4ppmP-108ppmSr cast alloy is slightly higher than that in Al-9.8%Si-3ppmP cast alloy. In both alloys, heat treatment reduces yield stress and work hardening rate mildly. Elongations to both alloys are almost the same with the same heat treatment time. Note that Vickers hardness in Al-9.8%Si-3ppmP cast alloy and Al-10.1%Si-4ppmP-108ppmSr cast alloy were 58.5 HV and 59.6 HV, respectively. There was no difference in the dendrite secondary arm spacing (DASII)—which was approximately 37 μm—in both of the alloys. Changes in ultimate tensile strength and elongation (average of 5 specimens) are shown in Figure 5. By heat treatment, ultimate tensile strength decreases gradually and elongation increases in both alloys. The changes become particularly remarkable after 1.8 ks of heat treatment. With a short period of heat treatment, no differences are seen in either alloys. However, when applying heat treatment for a longer time, the mechanical properties in Al-10.1%Si-4ppmP-108ppmSr cast alloy become superior to that of Al-9.8%Si-3ppmP cast alloy.

**Figure 4.** Stress-strain curves of tensile test in (**a**) Al-9.8%Si-3ppmP cast alloy and (**b**) Al-10.1%Si-4ppmP-108ppmSr cast alloy.

**Figure 5.** Changes in ultimate tensile strength and elongation during heat treatment at 773 K.

#### *3.2. Morphology Changes of Eutectic Si-Particles*

Three-dimensional volume images of eutectic Si-particles in Al-9.8%Si-3ppmP cast alloy, which are obtained by synchrotron radiation nanotomography, are shown in Figure 6. Interior Si-particles are displayed removing the aluminum matrix in the top part of each figure. It can be confirmed that synchrotron radiation nanotomography is high resolution because the figure indicates the changes in a very small region with a size of 28.4 μm × 56.7 μm × 27.2 μm. In the as-cast state (heat treatment time, t = 0 s) as shown in Figure 6a, most of the eutectic Si-particles are of a straight rod-like shape, and a small plate-like shape is also seen partially. It is observed that the rod and plate-like Si-particles are connecting. Particle growth is confirmed during heat treatment up to 14.4 ks as shown in Figure 6b–g. The number of particles decrease gradually during particle growth. Although particle separation that makes particles segmen<sup>t</sup> into a small size is also observed, most of the particles maintain a high aspect ratio after 14.4 ks annealing.

**Figure 6.** Three-dimensional volume images of eutectic Si-particles in Al-9.8%Si-3ppmP cast alloy. (**a**) As-cast, (**b**) heat-treated at 773 K for 450 s, (**c**) heat-treated at 773 K for 900 s, (**d**) heat-treated at 773 K for 1.8 ks, (**e**) heat-treated at 773 K for 3.6 ks, (**f**) heat-treated at 773 K for 7.2 ks, (**g**) heat-treated at 773 K for 14.4 ks.

Figure 7 shows a three-dimensional volume image of Al-10.1%Si-4ppmP-108ppmSr cast alloy. In the as-cast state (Figure 7a), fine rod-like Si-particles are observed similar to Al-9.8%Si-3ppmP cast alloy. However, the entire morphology of Si-particles in Sr-modified alloy are that of a coral-like shape with multiple branches. Particle size is slightly finer than that in Al-9.8%Si-3ppmP cast alloy. The Si-particles grow gradually with fragmentation during heat treatment, and spheroidize after 14.4 ks. In Al-10.1%Si-4ppmP-108ppmSr cast alloy, formation of Sr precipitations was confirmed in primary α-Al dendrite during heat treatment. Three-dimensional volume images in (a) as-cast and (b) after 7.2 ks heat-treated are shown in Figure 8. New Si-particles, which are not found in the as-cast state, are observed in the outside region of eutectic phase in which Si-particles are gathering. The presence of precipitate Si-particles is also confirmed in the slice image shown in Figure 3b. It can be concluded that the particles are not Sr compounds but Si because the particles have disappeared in the absorption images shown in Figure 3a.

**Figure 7.** Three-dimensional volume images of eutectic Si-particles in Al-10.1%Si-4ppmP-108ppmSr cast alloy. (**a**) As-cast, (**b**) heat-treated at 773 K for 450 s, (**c**) heat-treated at 773 K for 900 s, (**d**) heat-treated at 773 K for 1.8 ks, (**e**) heat-treated at 773 K for 3.6 ks, (**f**) heat-treated at 773 K for 7.2 ks, (**g**) heat-treated at 773 K for 14.4 ks.

**Figure 8.** Three-dimensional volume images in Al-10.1%Si-4ppmP-108ppmSr cast alloy; (**a**) as-cast and (**b**) after 7.2 ks heat-treated. New Si-particle precipitate during heat treatment.

Figure 9 indicates changes in total Si-particle volume, number of Si-particles and average Si-particle size (sphere-equivalent diameter) during heat treatment. These statistics were obtained from the microstructures shown in Figures 6 and 7 by three-dimensional image processing analysis. The total Si-particle volume in Al-10.1%Si-4ppmP-108ppmSr cast alloy is larger than that in Al-9.8%Si-3ppmP cast alloy. Volume fraction of Si phase in Al-10%Si alloy should be approximately 11.4%. However, the volume fractions were slightly small in the volumes analyzed and were 7.8% and 8.7% in Al-9.8%Si-3ppmP cast alloy and Al-10.1%Si-4ppmP-108ppmSr cast alloy, respectively. This is due to inhomogeneities of microstructure and the small field of view size in nanotomography. The amount of Si content does not differ in both alloys. The total Si-particle volume in Al-9.8%Si-3ppmP cast alloy looks to slightly decrease during heat treatment. This change is due to particles on the edge of view. Total Si-particle volume is almost constant during heat treatment in both alloys. In the as-cast, the number of Si-particles in Al-10.1%Si-4ppmP-108ppmSr cast alloy is larger than that in Al-9.8%Si-3ppmP cast alloy. This is because total Si-particle volume is large in Al-10.1%Si-4ppmP-108ppmSr cast alloy, and the particle size is small as observed in Figure 7. The number of particles decreases in both alloys during heat treatment. In Al-9.8%Si-3ppmP cast alloy, fragmentation of Si-particles, which is observed in the early stage of heat treatment, causes an increase of number of particles temporarily. As shown in Figure 8, Si-particles precipitate into α-Al dendrites during heat treatment. However, the precipitation has no effect on the number of Si-particles, because the number increase is small compared with the number decrease by particle growth. In Al-9.8%Si-3ppmP cast alloy, average Si-particle size decreases, and then increases. This decrease at the early stage is also due to fragmentation of Si-particles. In case of Al-10.1%Si-4ppmP-108ppmSr cast alloy, a little increase of Si-particle size is found in the early stage of heat treatment, and then the size increases rapidly in the later period of heat treatment.

**Figure 9.** Changes in total Si-particle volume, number of Si-particle and average Si-particle size during heat treatment. Initial total Si-particle volume was 3416.645 μm<sup>3</sup> and 5217.278 μm<sup>3</sup> in Al-9.8%Si-3ppmP cast alloy and Al-10.1%Si-4ppmP-108ppmSr cast alloy, respectively. (**a**) total Si-particle volume, (**b**) number of Si-particles and (**c**) average Si-particle size.
