Fig. 1. True stress-strain curves of (a) as-extruded pure Mg and Mg-0.3Ce samples, and (b) E280 samples annealed at 300 °C for 0 h, 3 h, 6 h, and 12 h.

Fig. 2. EBSD images of as-extruded alloys, including (a, d) pure Mg extruded at 180 °C, (b, e) Mg-0.3Ce extruded at 280 °C, and (c, f) Mg-0.3Ce extruded at 350 °C samples. (a), (b), (c) display the IPF maps and (d), (e), (f) display the IPF images. The mean DRXed grain size profile is also included as inset image.

Fig. 3. TKD images over cross-sectional of the as-extruded E280 sample, including (a, b) typical region A with lower degree of DRX, and (c, d) typical region B with higher degree of DRX. Figs. b, d display the IGMA results for the region A and B, respectively. The LAGBs are marked by the red lines, and the HAGBs are displayed as black lines.

Fig. 4. Typical STEM-DF images and 3D-APT result of the E280 sample, (a, b) showing non-DRXed region involving LAGBs and residual dislocations, (c) DRXed region with string like sub-/DRXed grains. Compared with STEM-DF image in (d), the Z-contrast image in (e) of the same region shows the non-DRXed region with no obvious segregation, and the DRXed region with partial segregations. Also, the Z-contrast images in (f, g) show the existences of nano-particles. 3D-APT result confirms the Ce segregation along HAGB in (h).

Fig. 5. Typical EBSD results of pure Mg-180 sample, including (a) ～17.5 mm, (b) ～12.5 mm, (c) ～7.5 mm and (d) ～2.5 mm away from the die-exit. (e) Misorientation angle profile and (f) inverse pole figures at different positions are also included. The characterized position is elucidated as inset in (d).

Fig. 6. Optical images of the interrupted Mg-0.3Ce extrusion sample at different positions below extrusion die exit, showing (a) montaged large area near the die exit and (b) the enlarged images at different distances from the die exit.

Fig. 7. EBSD results of microstructure evolution during extrusion process of Mg-0.3Ce-280 sample, including (a-e) IPF maps of 22.5-2.5 mm positions below extrusion die exit, (f) point-to-origin line profiles of misorientation angle along paths indicated by AB and CD in (e), (g) the enlarged IPF map from the marked region in (d), and (h) the corresponding orientation map based on quaternion disorientaion coloring mode. The IGMA images for positions at 7.5 mm and 2.5 mm and the misorientation angle profile for 22.5 mm, 17.5 mm and 12.5 mm positions are displayed in (i) and (j), respectively.

Fig. 8. Typical TEM images at the position of 17.5 mm below extrusion die exit of Mg-0.3Ce billet, (a, b) showing the existance of <c+a> dislocations under WBDF condition and (c, d) particles/second phases under STEM condition.

Fig. 9. Typical TEM images at the position of 12.5 mm below extrusion die exit of Mg-0.3Ce billet, showing <c+a> dislocation and <a> dislocation identified by WBDF conditions of (a, c) g=0002 and (b, d) g=$ 11\bar{2}0$, and (e) the dislocation tangles and (f) chain-like distributed particle.

Fig. 10. Typical TEM images at the position of 7.5 mm below extrusion die exit of Mg-0.3Ce billet (a, b, c) showing the initial formation stage of LAGBs which are perpendicular to basal plane via <c+a> dislocations accumulation, under the two-beam conditions of (a, b) g=0002 and (c) g=$10\bar{1}0 $, and (d) the corresponding schematical image.

Fig. 11. Typical TEM images at the position of 2.5 mm below extrusion die exit of Mg-0.3Ce billet, (a) showing the formation of polygonized sub-grains, (b) LAGBs being perpendicular and parallel to basal plane, and (c) LAGBs being askew to basal plane. Typical TEM images under conditions of (d) bright-field, (e) dark-field with g=0002, (f) dark-field with g=$ 11\bar{2}0 $, (g) dark-field with g=$\bar{1}\bar{1}22$, (h) dark-field with g=$11\bar{2}2$, and (i) the corresponding schematical image for dislocations and LAGBs.

Fig. 12. Typical EBSD images for E280 sample annealed at 300 °C for 3 h, (a) showing the IPF map and corresponding pole figure. And (b, c) the enlarged inverse pole figure maps in rectangular regions marked as “b” and “c” in (a), which include misorientation profiles along the direction indicated by arrows of AB and CD in region “b”, and sub regions with grain size less and above 3 μm in region “c”, respectively.

Fig. 13. Typical EBSD results of E280 sample annealed at 300 °C, showing (a) the inverse pole figure map of 6 h-annealed sample and (b) inverse pole figure, and (c, e) the corresponding sub region maps for DRXed grains less or larger than 5 μm, and (d, f) the pole figure and IPF, and (g) the inverse pole figure map of 12 h-annealed sample with (h) whole and DRXed region IPF, and (i) RE-textured grains map with the threshold of 15° and (j) the distribution of 〈$ 10\bar{1}0 $〉 and $\langle 44\bar{8}3\rangle$ orientated grains.

Fig. 14. Schematic illustration showing (a) the loading condition of the billet during indirect extrusion, and (b) the corresponding stress state of the billet as well as (c-g) the deformation mechanisms at different position below extrusion die exit including (c, d) twinning dominated region, (d) <c+a> dislocation activated region, (e, f) formation of LAGBs and (g) the finally formation of strings of sub-grains and/or HAGBs.

Fig. 15. Schmidt factor maps and statistical Schmidt factor values of (a-c) basal slip, (d-f) prismatic slip, (g-i) pyramidal I slip and (j-l) pyramidal II slip systems at the position of (a, d, g, j) 7.5 mm and (b, e, h, k) 2.5 mm. Average Schmidt factor values were calculated and indicate in (c, f, i, l). The stress state is shown in top-right corner exhibiting the geometrical condition of c-axis under compression, in (b).