**1. Introduction**

Since the building of large synchrotron radiation facilities throughout the world in the latter half of the twentieth century, the performance of synchrotron radiation tomography has gradually improved up to the present day [1–3]. Currently, in the Japanese synchrotron radiation facilities, SPring-8, three-dimensional non-distractive observation with a spatial resolution of 50–160 nm is available constantly in the imaging beamline by using an X-ray focusing device of a Fresnel zone plate [4–6]. Phase-contrast imaging techniques have been also developed for those samples for which visualizations are difficult by X-ray absorption contrast (i.e., these densities are very close) [7]. Furthermore, improvement of the performance of X-ray 2D detector system has rapidly shortened scanning time. Therefore, synchrotron radiation tomography can be used for various studies in various fields.

The advantages of X-ray tomography are that three-dimensional morphologies are obtained, and that the observation is non-destructive. In studies of structural materials, material behaviors changing over time can be visualized, for instance, damage and fracture mechanisms [7–14], fatigue and crack propagation phenomena [15–18] and so on. We can deeply understand various phenomena

affected by microstructures from a series of observed images. In Al-Si cast alloys, it is well known that eutectic Si-particle strongly affects mechanical properties. Many studies with regard to the morphology and distribution of eutectic Si-particles have been conducted to date [19–21], because the spheroidizing of Si-particles brings, in particular, ductility improvement by heat treatment. However, most of the research had been performed on the basis of 2D observation by polishing of a heat-treated sample after cross section cutting. Although three-dimensional evaluation also exists using Focus Ion Beam tomography [22], unfortunately this method is destructive.

In the application of hypoeutectic Al–Si alloys for automobile parts which require sufficient toughness, Si-particle refinement is applied by adding trace Sr to improve the mechanical properties [23–25]. The addition of trace Sr prevents aluminum phosphide, AlP which become the nuclei of coarse Si particles [26], and then changes the solidification process of hypoeutectic Al-Si alloys [27]. Note that the origin of phosphorus is the impurity of Si. Si-particles modified by trace Sr addition become very fine at less than 1 μm. A Sr-modified hypoeutectic Al-Si alloy demonstrates excellent mechanical properties. Furthermore, with applying heat treatment to the alloy, its strength and ductility can be controlled. The changes of Si-particles morphology during a heat treatment are considered as follows; firstly, Si-particles with necking divide into parts by Plateau–Rayleigh instability [28,29]. This separation is a change which decreases system energy quickly. Next, the fragmented Si-particles grow into spherical shapes by diffusion-controlled Ostwald ripening to reduce their surface energy.

By contrast, self-modification (self-refinement) of eutectic Si-particles is also possible by killing an impurity element of P, which is contained in Si and forms AlP as the solidification nuclei of Si. This P-free solidification process has been reproduced by phase-field model simulation by Eiken [27]. The morphologies of eutectic Si-particles which are formed in the different solidification processes—self-modification and Sr-modification—are different. Synchrotron radiation nanotomography has revealed that the morphologies of Si-particles in self-modification and Sr-modification are of a rod-like shape and coral-like shape, respectively, by casting self-modified and Sr-modified samples and investigating practically [30]. Therefore, in this study, to clarify the behavior of morphological changes during heat treatment and the effect of them on mechanical properties, hypoeutectic Al-10%Si alloys were cast using two different solidification processes (Self-modification and Sr-modification) that produce different morphology of eutectic Si-particles (rod-like and coral-like). The changes of mechanical properties were investigated in the prepared samples. The three-dimensional morphology changes of eutectic Si-particles during the heat treatment process were observed in both alloys by using nanotomography with a Fresnel zone plate and a Zernike phase plate.

#### **2. Materials and Methods**

Al-10%Si alloy was selected as the sample of this study. Two kinds of Al-10%Si alloy, self-modified and a Sr-modified sample, were prepared by gravity casting. It is known that three-dimensional morphology of eutectic Si-particles is different between the two alloys [30]. High purity Al (99.99%) and high purity Si (99.9999%) were melted in a graphite crucible at 993 K in air atmosphere using an electrical resistance furnace (Hamamatsu heat-tech, Hamatsu, Shizuoka, Japan). The molten metal was degassed by hexachloroethane. After the degassing treatment, molten metal was cast into a boat-shaped iron-mold heated at 473 K with a cavity size of 150 mm × 25 mm × 25 mm as a self-modified sample. For the Sr-modified sample, the degassed molten metal was cast into the mold soon after the 100 ppm Sr addition. The chemical compositions of cast alloy samples detected by spark emission spectrometer (OBLF QSN750-II, Witten, Germany) are listed in Table 1. Hereafter, two prepared cast alloys are named as Al-9.8%Si-3ppmP and Al-10.1%Si-4ppmP-108ppmSr on the basis of the result of composition analysis. Photos of microstructures in Al-9.8%Si-3ppmP cast alloy and Al-10.1%Si-4ppmP-108ppmSr cast alloy are shown in Figure 1. The microstructures of both alloys are almost the same in the two-dimensional image. It is difficult to distinguish them.


**Table 1.** Chemical composition of prepared cast alloys (wt.%).

Specimens for nanotomography were cut from the cast ingot. Very small stick-shaped specimens with a section size of 50 μm × 50 μm and length of about 8 mm were manufactured by hand polishing. Five tensile specimens with 19.75 mm<sup>2</sup> section × 30 mm length in a gauge part, which is a half size of JIS No.13 B (JIS Z 2241), were prepared from a position of 2 mm above from the bottom of the cast ingot, then the mechanical properties of the cast alloys were examined by a tensile testing machine (SHIMADZU AG-100 kNX, Kyoto, Japan).

**Figure 1.** Optical micrographs; (**a**) Al-9.8%Si-3ppmP cast alloy and (**b**) Al-10.1%Si-4ppmP-108ppmSr cast alloy.

Synchrotron radiation nanotomography was used for observation of three-dimensional morphology change in eutectic Si-particles during heat treatment. The synchrotron radiation experiment was performed at the undulator beam line of BL47XU in the Japanese synchrotron radiation facility, SPring-8 (Hyogo, Japan). A schematic illustration of the nanotomography set-up ¯ in the experimental hutch is shown in Figure 2. X-ray energy of 8 keV, which was adjusted by a silicon (111) double-crystal monochromator (SPring-8 Standard Monochromator, sKohzu Precision Co.,Ltd, Kawasaki, Kanagawa, Japan), was selected for this observation. A Fresnel zone plate with an outermost zone width of 50 nm was installed as an X-ray objective of an imaging X-ray microscope (NTT-AT, Kawasaki, Kanagawa, Japan). A Zernike phase plate made from tantalum with a thickness of 0.96 μm was also installed at the back focal plane of the Fresnel zone plate. A two-dimensional image detector system consisting of a Gd2O2S:Tb scintillator, an optical relay lens and a complementary metal oxide semiconductor camera (Hamamatsu Photonics K.K., C11440-22C, Hamamatsu, Shizuoka, Japan) was used. Since the difference in atomic number between them is only one, there is little X-ray absorption contrast in the Al phase and Si phase as shown in Figure 3a. Therefore, Si-particles in the inside of the aluminum matrix were visualized by phase contrast using a Zernike phase plate as shown in Figure 3b. Exposure time of 250 ms was used and 1800 projections were captured during a 180◦ rotation, for tomography. Voxel size of (37.8 nm)<sup>3</sup> was achieved in the reconstructed volume image in the set-up of this study.

**Figure 2.** The schematic illustration of set-up of nanotomography in the experimental hutch.

**Figure 3.** Slice images of nanotomography in Al-10.1%Si-4ppmP-108ppmSr cast alloy heat-treated at 773 K for 7.2 ks. The same slices are shown by (**a**) absorption contrast and (**b**) phase contrast. Field of view and analyze-volume position are indicated by white dashed-line circle and box. Eutectic Si-particles and precipitate Si-particles can be recognized as white objects in (**b**), though specimen surface only can be seen in (**a**).

Experimental procedure for the synchrotron radiation nanotomography observation was simple. The initial state of the sample, i.e., as-cast sample, was scanned. After the first tomography scan, the sample was heat-treated by taking it in and out of a compact air atmosphere furnace maintained at 773 K, and then was tomography scanned repeatedly at the same position at 450 s, 900 s, 1.8 ks, 3.6 ks, 7.2 ks and 14.4 ks. X-ray scanned data were reconstructed into a three-dimensional volume image by a conventional filtered convolution back-projection algorithm. A three-dimensional median filter (3 × 3 × 3) was applied to three-dimensional volume images reconstructed in order to reduce artifacts and image noise. Si-particles observed in the volume images were binarized and segmented with the thresholds value that were decided by comparing the obtained volume images to one another. The result of Si-particles segmentation was checked by visual inspection. Then if wrong connections existed among particles, such connections were carefully corrected one by one. Volume rendering software (VG studio Max 2.0, Volume Graphics, Heidelberg, Germany; and Amira 4.0, Thermo Fisher Scientific, Waltham, MA, USA) was used to visualize the three-dimensional morphology of Si-particles. The analyzed region for the morphology changes of eutectic Si-particles was extracted from a lower effect region of artifacts inside the sample. The analyzed regions were 56.7 μm × 28.4 μm × 27.2 μm and 37.8 μm × 37.8 μm × 41.8 μm in Al-9.8%Si-3ppmP sample and Al-10.1%Si-4ppmP-108ppmSr, respectively. Si-particles within the analyzed regions were segmented and labeled after binarization. Then, volume and surface area were measured for each of the labeled Si-particles.
