*3.2. Microstructure after ECAP*

Significant grain refinement was observed in specimens processed by an increasing number of ECAP passes. The corresponding EBSD orientation maps of 1P, 4P, and 8P samples are shown in Figure 4a–c, respectively. As is typical for many other Mg alloys, the first pass resulted in the formation of a non-homogeneous microstructure. Grain refinement occurred particularly along the former grain boundaries, and large grains were heavily deformed, as manifested by the variation of colors in individual grains. The large grains of up to 20 μm in diameter were surrounded by small grains with a size of 3 μm. Increasing strain imposed on the material by successive ECAP passes resulted in the higher degree of refinement, and ultimately in the homogenization of the microstructure. The average grain size determined in the 4P and 8P samples was ~2.6 and ~1.5 μm, respectively.

Figure 5 shows the pole figure measured in the 8P sample. The texture exhibits the typical basal slip component, which corresponds to grains with c-axes tilted by 45◦ from the ED. However, a significant volume fraction of grains was oriented with the c-axis perpendicular to the processing direction. As a result, the basal slip texture component reached a texture strength of only ~9. Note that the texture strength of this component was found to be more than 15 in low-alloyed Mg alloys without rare earth elements [27,28].

**Figure 4.** EBSD orientation maps of (**a**) 1P sample, (**b**) 4P sample, and (**c**) 8P sample (cross-section).

**Figure 5.** EBSD pole figure of the 8P sample (cross-section).

TEM investigation of the microstructure of specimens after different numbers of ECAP passes is consistent with the grain refinement observed by EBSD (compare Figures 6 and 7 with Figure 4a–c). In contrast to the as-extruded condition, the distribution of secondary phase particles in the ECAPed specimens was rather homogeneous, that is, they are distributed both along grain boundaries as well as in grain interiors. The size of the particles did not significantly change during the ECAP straining. Interestingly, significant residual strain was observed in the 8P sample (see Figure 7a). Moreover, higher magnification shows that the magnesium matrix contains also a large number of tiny secondary phase precipitates (see Figure 7b). They were identified as ordered Guinier–Preston (GP) zones (hcp, *a* = 0.556 nm, monolayer (0001)<sup>α</sup> disc) and their presence has already been reported elsewhere [7]. It should be noted, however, that these GP zones were not observed in all grains of the 8P sample.

**Figure 6.** TEM images of (**a**) 1P sample and (**b**) 4P sample.

**Figure 7.** TEM images of (**a**) 8P sample and (**b**) detail of ordered Guinier–Preston zones.
