*3.7. Electron Microscopy Analysis of Precipitate Structure Evolution*

TEM analyses were done in order to understand how the mechanical properties of Mg samples are related to microstructural features, in particular, to different shapes and densities of precipitates that form during heat treatments. The samples were chosen based on the significance of microhardness results shown in Figure 10, for the alloy Mg5Zn0.3Ca, in the following three states:


The three precipitate states reveal differences in morphology, size and density of precipitates in the range of 3–100 nm. Primary precipitates, as scattered residuals after annealing, did not show any substantial evolution during the processing history; see Figure 16—first row, SEM 200×.

**Figure 16.** SEM (upper two rows) and STEM (bottom row) images of Mg5Zn0.3Ca samples (**a**) after HPT-processing; (**b**) after HPT and additional heat treatment at 100 ◦C for 24 h; and (**c**) after HPT and additional heat treatment at 125 ◦C for 24 h. HPT-processing was carried out by 0.5 rotations under 4 GPa pressure at RT.

Precipitates' structures have been analyzed in all three states (Figure 16—second row, SEM about 10,000×), namely: polyhedral precipitates (P1), elongated precipitates (P2) and thinner phases. The thinner phases are at the boundaries of grains, or follow their contours. P1 and P2 do not change at all during processing, and can be observed in all states (Figure 17). P1 and P2 contain Zn and Ca, as seen in exemplary EDS maps of P1 precipitates (Figure 18).

**Figure 17.** STEM image of Mg5Zn0.3Ca, HPT-processed at 0.5 rotations, showing P1, P2 and thinner precipitates (blue arrows). The TEM image in the top left corner is a close-up of the P1 precipitate. Thinner precipitates are located at the grain boundaries, following their contours.

**Figure 18.** EDS maps of a P1 precipitate in a Mg5Zn0.3Ca sample. These precipitates contain Zn (green) and Ca (blue); Mg (red) was mainly observed in the surrounding matrix. O (pink) appeared in all sample areas; it most likely came from the atmospheric oxidation of prepared TEM foils.

Besides P1, P2 and thinner phases, further precipitates were observed in the heat-treated samples (Figure 16—third row, STEM, about 200,000×). P3-precipitates could be observed in both the heat -treated samples (Figure 19). In the sample heat treated at 100 ◦C, the P3-precipitates were located at the boundaries between grains and had a size of 20-50 nm. In the sample heat treated at 125 ◦C, those phases grew in size up to 100 nm. Additionally, ultrafine P4 precipitates (Figure 19) with a size less than 10 nm and up to 50 nm appeared. They were oriented normal to <0001> direction, but further verifications are required. These oriented platelets were not observed in the other two conditions.

**Figure 19.** TEM images of the HPT-processed (0.5 rot) and heat-treated Mg5Zn0.3Ca sample (**a**) at 100 ◦C for 24 h and (**b**) HPT processed and heat treated at 125 ◦C for 24 h showing P3 and P4 precipitates.
