**2. Materials and Methods**

The experimental magnesium alloy ZN11 (Mg–1 wt.% Zn–1 wt.% Nd) was cast in Helmholtz-Zentrum Geesthacht, Germany, and extruded at 400 ◦C, with an extrusion ratio of 30 and a crosshead speed of 1 mm/s. Bars with the cross-section 10 <sup>×</sup> 10 mm2 were machined from the extruded rod. Samples with a length of 10 cm were processed by ECAP, with the die having the inner angle of 90◦, following route BC [15]. The processing temperatures and pressing speeds for ZN11 after extrusion are shown in Table 1. Each pass of the ECAP process was performed at the lowest possible temperature, because deformation temperature has a very strong effect on the resulting microstructure and mechanical properties, as was shown in the AX41 magnesium alloy by Kraj ˇnák et al. in [16] and for other materials [17–22]. Note that further decrease of the processing temperature would lead to occurrence of cracking and billet segmentation. Four conditions were investigated in this study: as-extruded (EX) condition, and conditions after one pass through ECAP (1P), four passes (4P), and eight passes (8P).

**Table 1.** ECAP (Equal-Channel Angular Pressing) procedure parameters of the investigated alloy.


The microstructure of all samples was investigated by scanning and transmission electron microscopy (SEM, TEM). An SEM Zeiss Auriga Compact (Zeiss, Oberkochen, Germany), equipped with an EDAX electron backscattered diffraction (EBSD) camera (EDAX Inc., Mahwah, NJ, USA), was used for the investigation of the grain structure. EBSD analysis was performed with a scan size of 400 <sup>×</sup> 400 <sup>μ</sup>m<sup>2</sup> and a step size of 0.4 <sup>μ</sup>m in the case of the extruded sample, and a scan size of 100 × 100 μm and step size of 0.1 μm in the case of the samples processed by ECAP. The raw data were partially cleaned in OIM TSL software (EDAX Inc., Mahwah, NJ, USA) using one step of confidence index (CI) standardization and one step of grain dilatation. Note that only points with CI > 0.1 were used for calculations. Analysis regarding occurrence of twinning in the pre-deformed sample was performed by in-built feature in OIM TSL software. Only one type of twin, tension twin 1012 1011 (the misorientation angle between the matrix and the twin–86.4◦ around *a*-axis), was observed. TEM JEOL 2200FS (JEOL USA Inc., Peabody, MA, USA), operating at 200 kV, was used for secondary phase analysis. Samples for EBSD measurements and thin TEM discs were mechanically prepared and subsequently ion-polished using a Leica EM RES102 (Leica Microsystems GmbH, Wien, Austria). The texture of the initial as-extruded condition was investigated by an X-ray PANalytical XPert MRD diffractometer (XRD) (Malvern Panalytical Ltd, Grovewood Road Malvern, UK). CuK radiation and polycapillaries in the primary beam were employed during the measurements. The full pole figures were calculated using MTEX software (TU Chemnitz, Chemnitz, Germany) [23].

Mechanical properties were studied using the Vickers microhardness measurement and compression and tensile deformation tests. Microhardness measurements were performed by a Qness Q10 device (Qness GmbH, Colling, Austria) with a load of 0.5 kg (HV 0.5) for 10 s. Microhardness was measured on a cross-section plane. More than 100 experimental points (indents) were evaluated for each specimen. Compression and tensile deformation tests were performed using a universal testing device (Instron 5882, Instron, Coronation Road, Buckinghamshire, UK). Compression tests were performed on cuboid specimens with the aspect ratio of 2:3 (4 <sup>×</sup> 4 <sup>×</sup> 6 mm3). The samples were machined with the deformation axis parallel to the extrusion direction (ED), normal direction (ND) and transverse direction (TD), see Figure 1. Note that compression tests along ND and TD were performed only for the 8P sample. Tensile deformation tests were performed only for the 8P sample along ED. Bone-shaped flat samples with a rectangular cross-section of 4 <sup>×</sup> 1.4 mm2 and a gauge length of 10 mm were machined. At least three samples for each combination of deformation axis, deformation direction and materials condition were tested. Finally, the true stress–strain curves were calculated from the engineering stress–strain curves (see [24], for example).

**Figure 1.** Mutual orientation of the equal-channel angular pressing (ECAP) die and the billet.

### **3. Results**

#### *3.1. As-Extruded Condition*

The initial, as-extruded, microstructure of the investigated alloy was studied by EBSD, TEM, and XRD. Figure 2a shows the EBSD orientation map revealing the grain structure of the material. It is clear that extrusion resulted in a significant grain refinement, and the formation of a homogeneous, fully recrystallized microstructure. The grain size—determined as the average value weighted by the area fraction—was ~11 μm.

The presence of small secondary phase particles is clearly seen in the TEM micrograph in Figure 2b. Many grain boundaries are decorated by small particles in a necklace-like formation. Local chemical analysis and selected area electron diffraction confirmed that the particles are stable γ-phase Mg3 (Nd, Zn), face-centered cubic (FCC) structure, *Fm*3*m*, with *a* = 0.70 nm [7]. The size of the γ-phase particles varied from ~100 nm to 1 μm. In addition, Figure 2b discloses that the grain interiors are almost strain-free.

XRD measurement was performed to acquire information about the texture formation during the extrusion. Extrusion of magnesium alloys usually leads to the formation of the typical - 1010 fiber texture, which is often strong (explained theoretically by Mayama et al. [25]). However, Figure 3 shows that the texture observed in the investigated ZN11 alloy was completely different. This kind of texture has already been observed in Mg–RE alloys after extrusion, and is referred to as "rare earth texture" [26]. In the investigated sample, this texture component is weak, and is represented by grains which have their c-axis tilted from ED by approximately 45◦.

**Figure 2.** (**a**) Electron backscattered diffraction (EBSD) orientation map and (**b**) transmission electron microscopy (TEM) micrograph of the as-extruded (EX) sample (cross-section).

**Figure 3.** X-ray diffractometer (XRD) pole figures of the EX sample (cross-section).
