**5. Conclusions**

In this study, a series of three-dimensional morphology changes of fine eutectic Si-particles during heat treatment have been investigated in Self-modified and Sr-modified Al-10%Si cast alloys by means of synchrotron radiation nanotomography using a Fresnel zone plate and a Zernike phase plate. The morphology of eutectic Si-particles was rod-like in shape in the self-modified sample of Al-9.8%Si-3ppmP cast alloy. In the Sr-modified sample of Al-10.1%Si-4ppmP-108ppmSr cast alloy, Si-particle was a coral-like shape in the as-cast. The coral-like shape particles observed in Al-10.1%Si-4ppmP-108ppmSr cast alloy fragmented at branch and neck during heat treatment at 773 K. The fragmentation occurred up to 900 s. After that, the fragmented particles grew and spheroidized by Ostwald ripening. The rate of Ostwald ripening in Al-10.1%Si-4ppmP-108ppmSr cast alloy was faster than that in Al-9.8%Si-3ppmP cast alloy. On the other hand, rod-like shaped eutectic Si-particles observed in Al-9.8%Si-3ppmP cast alloy were larger in size compared to the particle size in Al-10.1%Si-4ppmP-108ppmSr cast alloy. In Al-9.8%Si-3ppmP cast alloy, separation of eutectic Si-particles occurred up to approximately 900 s, which was similar tendency to that in Al-10.1%Si-4ppmP-108ppmSr cast alloy. The frequency of separation was low due not to the coral-like shape but the rod-like shape. Three-dimensional morphology changes of fine eutectic Si-particles in both cast alloys, specifically fragmentation and spheroidizing, can be connected to

changes in mechanical properties. In the rod-like shape of Si-particles obtained in a self-modified sample of Al-9.8%Si-3ppmP cast alloy, however, it was found that the morphology change behavior was very complex. By non-destructive continuous observation using nanotomography, it was revealed that relatively long rod-shape particles grew slowly without separation. It is also observed that a protuberance was absorbed into a small plate-shape part. Moreover, very complex behavior was observed where a rod-shape particle separated at the neck, spheroidized and was then absorbed by a neighboring larger particle.

**Author Contributions:** Methodology, K.U and A.T.; Validation, T.A and H.M; Investigation, S.F and M.K.; Resources, S.F, T.A and H.M; Data Curation, K.U and A.T.; Writing—Original Draft Preparation, M.K.; Writing—Review & Editing, T.A and H.M; Visualization, S.F.; Project Administration, M.K.

**Funding:** The authors appreciate the financial assistance of the Light Metal Educational Foundation.

**Acknowledgments:** The synchrotron experiment in this study was performed in subject No. 2016B1106 in SPring-8. This research was also partially supported by JST under Industry-Academia Collaborative R&D Program "Heterogeneous Structure control: Toward Innovative Development of Metallic Structural Materials".

**Conflicts of Interest:** The authors declare no conflict of interest.
