*3.1. Achieving the Supersaturated Solid-Solution Condition*

SEM images of the as-cast alloys reveal a large number of primary precipitates, which are represented by the areas with bright contrast in Figures 3–5. On the microscale, the precipitates are distributed rather homogeneously in every sample condition. The level of contrast in these images signifies compositional changes in the microstructure, as it depends on the atomic numbers of the elements involved. All samples in the as-cast condition showed particles with a size of 5–10 μm, both within grain interiors and along grain boundaries. EDS analyses were done to reveal the chemistry of the primary precipitates. The analyses were achieved on well-defined points on the sample's surface, as shown in Figure 3, revealing a spectrum of the chemical composition of each point investigated (Figure 4). A total of 10 points were used for the primary precipitates and the surrounding matrix to determine the mean value. The chemistry of these precipitates revealed by EDS is shown in Table 2. Further, phase diagrams from literature were used to estimate the compositions of the precipitates. Those of the primary particles should correspond to Mg2Ca, Zn13Mg12 and Ca2Mg5Zn5 [48]. A more detailed analysis on primary precipitates could have been done by TEM, but that was not the focus of this study. The total volume fraction of primary precipitates was determined to be about ~2% being estimated from the total area of the particles by means of standard image analysis methods.

**Figure 3.** SEM image of Mg5Zn0.3Ca. EDS was carried out at well-defined points (here #6–#10) on the sample's surface. Mainly, the primary precipitates were investigated, but so was the surrounding matrix.

**Figure 4.** Exemplary EDS analysis of point 7 indicated in Figure 3, for a Mg5Zn0.3Ca sample. Intensity peaks (counts/s) for different energies (eV) correspond to EDS signals from Ca, Zn, Mg, Ca, Ca, Zn, and Zn again (in sequence from left to right).

**Figure 5.** SEM image of the as-cast alloy Mg5Zn0.3Ca, (**a**) with about 100× magnification (**b**) with about 500× magnification.


**Table 2.** Chemistry of the precipitates revealed by EDS.

During homogenization, the primary precipitates should have been thermally destroyed. It is not possible, however, to dissolve them completely in the Mg matrix, as its solid solubility is low: Ca has a solubility in Mg of 0.82 at% (1.35 wt%), and Zn has a solubility in Mg of 2.4 at% (6.2 wt%) [49]. Two cooling treatments—quenching and furnace cooling—were performed with all alloys (Figure 6): Quenched samples showed still widely spread precipitates, while the furnace-cooled ones did not, indicating almost a solid-solution state. For the alloys Mg5Zn and Mg5Zn0.3Ca, the volume fraction after quenching could be reduced to ~2% with a precipitation size of less than 5 μm. In case of furnace cooling, the remaining volume fraction of precipitates was less than 1% with a size in the nm-scale. For Mg5Zn0.15Ca and Mg5Zn0.15Ca0.15Zr both cooling treatments could reduce the volume fraction of primary precipitates to below 1%. As a common name for both states, "initial state (IS)" is used in all the following parts of this paper.


**Figure 6.** SEM images of all alloys after best-condition annealing, and quenching or furnace-cooling.

The results of microhardness (HV0.05) measurements of as-cast Mg5Zn0.3Ca, Mg5Zn, Mg0.3Ca, Mg5Zn0.15Ca and Mg5Zn0.15Ca0.15Zr samples, and in their furnace-cooled or quenched states, are listed in Table 3.


**Table 3.** Microhardness results of Mg5Zn0.3Ca, Mg5Zn0.15Ca, Mg5Zn0.15Ca0.15Zr and the two alloys Mg5Zn and Mg0.3Ca in the as-cast, furnace-cooled and quenched states.
