## 2 Materials and Methods
### Casting of Mg Alloys  
Two Mg-based alloys (with 2 wt.% and 3 wt.% of Zn, and 0.6 wt.% Si) were produced using the directional solidification technique with raw materials of commercial purity. The mechanical properties of these alloys have been discussed in previous work by [PERSON] et al. [42]. The Mg used in this research was obtained from Rima Industrial (Distrito Industrial, Brazil), while Al, Ca, Fe, Cu, Zn and Si were obtained from Ted Importacao, Comercio e Representacao de Me-tais Ltd. (Sao Caetano do Sul, Brazil). In the directional solidification method, a cylindrical ingot mold made of 316 L stainless steel, and a base made of 1020 steel, were utilized. The inner wall of the cylindrical region of the ingot mold was coated with a graphite-based layer to facilitate demolding of the ingot. Additionally, on this cylindrical side, eight K-type thermocouples were arranged longitudinally to enable temperature measurements at various distances from the base. The metallic alloys of interest were produced, cast, and solidified in ingot molds using a Vacuum Induction Melting (VIM) system from GCA Vacuum Industries, in an atmosphere rich in Argon. The ingot molds, filled with the solidified alloys of interest, were then attached to the directional solidification system. A layer of salt, composed of 50% magnesium chloride and 50% potassium chloride, was deposited on the solidified material. Within the directional solidification system, the ingot molds were heated until complete melting of the alloys was achieved. During this heating process, the salt layer melted, thereby generating a layer of protection for the Mg alloys from the surrounding atmosphere. Once the metallic alloy had melted, it was cooled until it reached a temperature equal to about 1% of the alloy's liquidus temperature at the thermocouple closest to the base of the ingot mold. Cooling was started when the temperature recorded by the first thermocouple was 5% above the liquidus temperature. At this point, a flow of water was activated against the outer surface of the ingot mold, forcing upward solidification (bottom up). During the solidification process, temperature readings were captured for each of the eight thermocouples at a frequency of 5 Hz. These measurements were utilized to calculate the thermal parameters of solidification at different positions within the ingot. This approach facilitated the creation of gradients of solidification parameters (cooling rate) for each alloy of interest. Figure 1 illustrates the experimental process utilized in the fabrication of directionally solidified Mg-0.6 Si-2 Zn (0.6 wt.% Si and 2 wt.% Zn) and Mg-0.6 Si-3 Zn (0.6 wt.% Si and 3 wt.% Zn) ingots. Table 1 shows a summary of the samples used in the current study and presents details of the corresponding cooling rates for each sample. An amount of 0.6 wt.% of Si was chosen based on the optimum concentration for ideal mechanical properties of the alloy, as well as due to its use in previous studies on Mg-Si-Zn alloys in as-cast conditions [16; 26; 28]. Further, as previously mentioned, the 2-3 wt.% Zn range chosen represents the optimal limits for Zn concentration in Mg-Zn alloys [25; 26; 27; 28; 29]. And finally, the cooling rate selected allowed the growth of the Zn-rich intermetallic particles (hereafter referred to as MgZn particles) such that these could be observed using the lab-scale X-ray Computed Tomography (XCT) technique--thereby enabling the assessment of the morphological evolution of the phase at the initial stages of its formation. In tandem with the solidification experiments, thermodynamic calculations of the solidification paths for the alloys were conducted with the assistance of the Pandat software v2021a, from CompuTherm (CompuTherm LLC, Middleton, WI, USA). The Mg-Si-Zn-Ca database was used to provide insights into the potential phases formed. After solidification, the samples were extracted from directionally solidified cylindrical ingots.