*2.2. Microstructure Characterization*

The microstructure of Mg-9Li dual phase alloy after ECAP and rolling was analyzed by Optical Microscopy (OM, Olympus BX51M, Tokyo, Japan). The samples were cut into 10 × 10 mm small squares, then grounded using sandpaper increasing the grit size after each grinding cycle. The samples were then polished using 0.5 μm Al2O3 suspension solution until a mirror-like specimen surface was observed. The polished samples were etched with a solution mixture of 4.3 mL picric acid, 95 mL ethanol, and 0.7 mL phosphoric acid.

Transmission electron microscopy (TEM, FEI Tecnai G2, Hillsboro, OR, USA) was applied to observe the microstructure and grain size of the Mg-9Li alloy after ECAP and post-rolling. The samples were cut into a 10 <sup>×</sup> 10 <sup>×</sup> 0.6 mm3 pieces. The cut piece was grounded using a 1000 and 2000 grit sandpaper to about 100 μm. The sample was then punched into circular disks of 3 mm diameter and a special auxiliary equipment was used to polish the disc to 20–30 μm on the 1500 grit sandpaper. The TEM foil was finally perforated by ion milling (Gatan 695C, Pleasanton, CA, USA).

The phase composition of Mg-9Li dual phase alloy in different states was qualitatively characterized by X-ray diffractometer with a Bruker D8 (XRD, Karlsruhe, Germany) with test conditions: Cu target Ka-ray, tube current 40 mA, scanning range 10–90◦, scanning speed 5◦/min. The texture of the samples in different states were observed by XRD. The samples were polished using a 1000 grit sandpaper to ensure a smooth surface and etched with 3% nitric acid to remove its surface stress with the direction of extrusion and rolling indicated on the sample surface.
