**2. Materials and Methods**

Ingots of Mg-RE alloy were prepared by induction melting the mixture of industrial AZ31, Mg-10%La and Mg-20%Ce (wt%) master alloys in an induction furnace at 993 K for 30 min under the protection of high-purity argon. The chemical compositions of the ingots were measured by X-ray fluorescence spectrometry, and the results are listed in Table 1. Figure 1 is a schematic diagram of the manufacturing process for magnesium alloy sheets. The Mg-based sheets were prepared by VTRC. The roll, which had a diameter of 300 mm and a width of 100 mm, was made from a copper alloy. Twin roll casting experiments were carried out under casting conditions with a casting speed of 10 m/min and a pouring temperature 973 K. Because the casting produces separation force during the casting process, the metal block was set at the side of the moving roller to form a supporting force in order to minimize the gap between the rollers as much as possible during the casting process. Afterward, the melt of magnesium alloy flowed through a nozzle to a position between running rolls. The initial roll gap was set to 0 mm. An oil tank was set under the roll and the Mg-RE alloy sheet was completely submerged in the tank when the casting process was completed to prevent further grain growth. The final thickness of the sheets between 0.5 mm and 1.1 mm and the width of the strip rang is 25 mm to 50 mm.

From the Mg-RE master ingots and the as-extruded Mg-RE sheets, samples with a dimension of 10 × 10 × 1 mm<sup>3</sup> were firstly grounded with SiC papers to1200 grid, and then by diamond pastes down to #0.25 μm grade. The microstructures of the polished surfaces were observed using a field emission scanning electron microscope (FE-SEM, JMS-6301, Tokyo, Japan) and the elements distribution maps were observed by electron probe micro-analysis (EPMA, JXA-8530F, Tokyo, Japan). The alloy phases

were obtained by an X-ray diffractometer (XRD, D/Max 2500 PC, Tokyo, Japan). For TEM (HF-3300, Tokyo, Japan) analysis, the focused ion beam (FIB, JIB-4500, Tokyo, Japan) was used for preparation. During the original work of sample preparation, the alloy sheet was cut into 10 mm<sup>2</sup> square shapes, and the thinned section thickness was about 60 μm. Square metal sheets with an area of 1.5 mm<sup>2</sup> were cut off from the as-cast sheet and stuck together on a Mo grid with some resin glue, and then the sample was further cut with FIB to obtain the ultimate sample thickness of 0.1 μm.

**Table 1.** Composition and the atomic radius of the Mg-RE alloy (atomic radius difference between Mg (Al) and other elements is symbolized by ARDMg (ARDAl).


**Figure 1.** Schematic diagram of the manufacturing process for magnesium alloy sheets.

The corrosion behaviors of the alloys were studied by potentiodynamic polarization (HZ700, Tokyo, Japan) and electrochemical impedance spectroscopy (Modulab XM, Tokyo, Japan), using a three-electrode cell comprising an auxiliary electrode of platinum counter, a reference electrode of Ag/AgCl electrode, and a working of the samples. The exposure area was 1 cm2. After immersion for 0.5 h, electrochemical impedance tests were operated at the open circuit potential with signal amplitude of 10 mV over a frequency varying between 100,000 Hz and 0.01 Hz. After immersion for 1 h, potentiodynamic polarization was performed at a scanning rate of 1 mV/s. All electrochemical tests were conducted in PBS (phosphate buffer saline) solution at 310 K. A triplicate electrochemical test was carried out to ensure the reproducibility of the results. For immersion tests, for each sample, only one side of 1 cm<sup>2</sup> was exposed. Then, the samples were immersed in PBS solution at 310 K for 30 days. After immersion, the section corrosion morphologies were observed by SEM. The concentration of metal ions in the solutions were analyzed by Inductively Coupled Plasma (ICP) (ICPS-7000 Sequential Plasma Spectrometer, Tokyo, Japan).

All animal experiments were approved by the University of Saitama Institute of Technology Animal Care and Use Committee (Grant NO. 2019-5). The project recognition date is 20 May 2019. 5 white rats (12 weeks of age, 0.36 ± 0.02 kg) from Tokyo University Institute of medicine were used in this study. Rats were anesthetized with isofluane and the right thighs were shaving and disinfected. The skin and muscle of thighs were carefully retracted to expose the femurs. Single screw fixation was used in this implantation experiment. Sheets were 22 × 4.8 mm<sup>2</sup> with a thickness of 1 mm. Screws were 5 mm in length, with an outer shaft diameter of 1.75 mm and shaft inner diameter of 1 mm. Animals were monitored daily for general behavior, movement, and food and water intake. High resolution microCT (R.mCT2, Rigaku, Tokyo, Japan) was used to assess Mg-RE sheets degradation and new bone formation.
