*2.2. Microstructure Characterization*

The cast ZK60 samples (10 <sup>×</sup> <sup>10</sup> <sup>×</sup> 10 mm3) were sealed with epoxy resin (working area <sup>=</sup> 1.0 cm2), ground with 2000 grit SiC paper, and polished with 3 μm diamond paste. Then, they were etched with a picric acid solution for metallographic analysis. The metallographic structure of the cast ZK60 sample was observed using a 3D optical microscope (VHX-1000, KEYENCE, Osaka, Japan), SEM (Quanta 200, FEI, Eindhoven, The Netherlands) equipped with EDX (EDAX-Genesis), and transmission electron microscopy (TEM). The TEM sample was firstly mechanically ground to a thickness of about 20 μm, and then, it was ion milled at 4 keV and 4◦, cooled by liquid nitrogen. TEM observations were carried out with an FEI Talos F200X transmission electron microscope (FEI, Portland, OR, USA) operated at 200 kV. The element content and elemental distribution of the cast ZK60 sample were characterized by EDX and an electron probe micro-analyzer (EPMA-8050G, SHIMADZU, Kyoto, Japan), respectively. Phase structure analysis was performed by XRD (X'Pert PRO, PANalytical B.V., Almelo, The Netherlands) using Cu Kα radiation. The scan range of 2θ was from 20◦ to 90◦ with a scan step of 0.02◦. The XRD pattern was analyzed with the X'Pert HighScore Plus software (2.0, PANalytical B.V., Almelo, The Netherlands). SKPFM (SPM-9700, SHIMADZU, Kyoto, Japan) was used to measure the relative Volta potential differences among different microstructural constituents to show their relative nobility. Meanwhile, the corresponding topography maps of the same area were also obtained.
