3.2. Microstructure of the as-extru

3.2. Microstructure of the as-extruded alloyThe OM and EBSD images of the as-extruded alloy are shown in Fig. 2. The microstructure was remarkably refined, indicating that dynamic recrystallization occurred. The LPSO phase in the as-homogenized alloy also inherited to the extruded alloy, oriented along the extrusion direction.A typical bright-field (BF) TEM image of the as-extruded alloy is shown in Fig. 3a. LPSO phase, with size of ~2 μm, was observed. The corresponding SAED pattern of the LPSO phase marked by a white arrow in Fig. 3a is shown in Fig. 3b. This SAED pattern is consistent with that of an 18R LPSO phase.19During the extrusion process at 400 °C, some particles precipitated from the Mg matrix dynamically. As shown in Fig. 4a, a typical particle marked by a white arrow, with a size of ~400 nm, was observed. Fig. 4b shows the corresponding SAED pattern. This particle was indexed consistently to the Mg24(Gd,Y)5 phase (bcc, a=1.13 nm, I 3m) in the alloy. These precipitates were formed adjacent to the grain boundaries, as clearly exhibited in Fig. 4a. Combined with the LPSO phase extending from grain boundaries to grain inside, these particles would exert an important impact on the dynamic recrystallization (DRX) and inhibit grain boundary migration, thereby refining α-Mg grains.