**2. Materials and Methods**

Three alloys were investigated in this study: a ZK60 Mg-based alloy with the alloying element concentrations of 6 wt% Zn and 0.5 wt%, Zr, and two other materials in which 2 wt% Y or 2 wt% Ce-rich RE elements were added to the ZK60 alloy (the relative fractions of the constituents in wt% were 50.6% Ce, 23.2% La, 20.3% Nd, and 5.9% Pr). Similar to our previous work [18], all three materials were prepared by melting them in an induction furnace at 740 ◦C. After casting and homogenization at 440 ◦C for 12 h, extrusion was performed on the samples at 180 ◦C at the extrusion ratio of 18:1.

The phase composition of the alloys was analyzed by X-ray diffraction (XRD) using a Smartlab diffractometer (manufacturer: Rigaku, Tokyo, Japan). In the XRD experiments, a divergent beam was applied in Bragg–Brenatno (*θ*–*θ*) diffraction geometry using CuKα radiation (wavelength: 0.15418 nm).

The microstructure was studied by scanning electron microscopy (SEM) using a ZEISS SIGMA VP microscope (manufacturer: Carl Zeiss AG, Jena, Germany). During the surface preparation, the samples were polished with an alumina suspension (particle size: 0.5 μm) and then etched in a picral solution at room temperature. The same instrument was used for energy-dispersive X-ray spectroscopy (EDS) in order to determine the chemical composition of the secondary phases.

The crystallite size and the dislocation density were investigated by XLPA [22]. This analysis was carried out on diffractograms measured by a high-resolution rotating anode diffractometer (type: RA-MultiMax9, manufacturer: Rigaku, Tokyo, Japan) using CuKα<sup>1</sup> radiation (wavelength: 0.15406 nm). The measured XRD patterns were evaluated by the convolutional multiple whole profile (CMWP) fitting method which yielded the crystallite size and the dislocation density in the studied samples. In this procedure, all peaks in the measured pattern are fitted simultaneously using theoretical profile functions calculated on the basis of the kinematical theory of XRD [23]. These calculated functions contained unknown parameters of the microstructure, such as the median and the variance of the crystallite size distribution and the dislocation density, which were then changed during the CMWP procedure in order to obtain the best fit between the measured and the calculated patterns. It should be noted that the calculated XRD pattern contains not only the diffraction peaks but also the background under the peaks which was approximated by a spline. The strongest 16 reflections of the main Mg phase in the diffraction angle (2*θ*) range between 30 and 125◦ were used in the evaluation. An example for CMWP fitting will be shown in Section 3.1. It should be noted that the recent development in XLPA enables the determination of the microstructural parameters with the help of artificial intelligence [24]. This novel machine learning-based method is much easier to use and faster than the conventional pattern-fitting procedure. However, the present Mg alloys cannot be evaluated by this new method since it is available only for face-centered cubic (fcc) structures as yet.

The EBSD technique was used to investigate the crystallographic texture and to characterize the grain boundaries in the samples. First, the surface of the specimens was mechanically polished with a diamond paste down to 1 μm, and then refined by ion beam polishing using a Leica EM RES102 system (manufacturer: Leica Mikrosysteme, Wetzlar, Germany). The EBSD was carried out with Zeiss Cross Beam Auriga SEM (manufacturer: Carl Zeiss AG, Jena, Germany) with a step size of 0.8 μm. The misorientation distribution was determined by the EDAX OIM software (manufacturer: EDAX Inc., Mahwah, NJ, USA). In addition, the OIM software was also used for the determination of the area-weighted mean grain size from the EBSD images. In this evaluation, the regions containing at least 4 pixels and bounded by high-angle grain boundaries (HAGBs) with misorientation angles higher than 15◦ were considered grains.

An electrodischarge machine (EDM) was used to prepare tensile specimens parallel to the extrusion direction (ED) according to the ASTM E8-04 standard. The tensile tests were carried out on at least three samples for each condition using a SANTAM tensile machine with a capacity of 2 tons and at an initial strain rate of 10–3 s–1. The samples containing Y and the specimens with Ce-rich rare earth elements are denoted as ZK60–2Y and ZK60–2RE, respectively.

## **3. Results and Discussion**
