*2.5. Microstructure Observation*

The microstructure of the specimens was observed by transmission electron microscopy (Fei Tecnai G2 F20, United States FEI Limited Liability Company, Hillsboro, OR, USA) based on Schottky field emission. The acceleration voltage was 200 kV, and the magnification was 25,000–10,300,000×. The specimens were prepared according with the following steps: first, some discs with a thickness of 1 mm and a diameter of 3 mm were prepared; next, the discs were thinned to 50 μm by polishing; finally, the discs were grounded and thinned by electropolishing (MTP-1) using a 30% nitric acid in methanol solution as electrolyte. The temperature was kept between 153 K and 303 K by liquid nitrogen, and the voltage was set to 10–20 V, the current was set to 50–60 mA, and the polishing time was 2 min.

After the HCF tests, the fracture was completely cut off from the specimen and observed by a scanning electron microscope (TESCAN MORA3 LMU, Tescan Company, Brno, Czech Republic). The acceleration voltage was 0.2–30 kV and the magnification was 3.5–1,000,000×. The resolution can reach 1.0 nm in high vacuum mode.

### **3. Results and Discussion**
