*3.1. Alloy Microstructure and Constitution before Oxidation*

The Al71Co29 and Al76Co24 alloys were prepared by arc-melting. The microstructure of the as-solidified Al71Co29 alloy is presented in Figure 2. In this alloy, three different microstructure constituents have been found. The image was acquired in backscatter electron mode to provide element resolution. The chemical composition of the constituents, measured by EDS point analysis, is provided in Table 1. The dendritic constituents have a significantly higher Co concentration (44.8 at.%) compared to the remainder of the alloy. The other two constituents have 28.1 and 25.4 at.% Co, respectively. The black areas, found in the microstructure, are pores.

**Figure 2.** Microstructure of the as-solidified Al71Co29 alloy.

**Table 1.** Chemical compositions of microstructure constituents observed in the as-solidified Al71Co29 and Al76Co24 alloys.


The microstructure of the as-solidified Al76Co24 alloy is presented in Figure 3 in BSE imaging mode. In this alloy, three different microstructure constituents have been found (light grey, medium grey and dark grey). The light grey constituent in the Al76Co24 alloy has 74.4 at.% Al and 25.6 at.% Co (Table 1). As such, its chemical composition is comparable to chemical compositions of the dark grey constituent observed in the as-solidified Al71Co29 alloy (Figure 2). The medium grey constituent contains 75.2 at.%

Al and 24.8 at.% Co. The dark grey constituent of the as-solidified Al76Co24 alloy has 81.5 at.% Al and 18.5 at.% Co. The black areas located within the dark grey constituent are pores (Figure 3).

**Figure 3.** Microstructure of the as-solidified Al76Co24 alloy.

A phase assignment of the alloy's microstructure constituents is presented in Table 1. The assignment has been made based on the experimental chemical composition of the constituents obtained by EDS and crystal structure of the phases identified by XRD in our previous work [57,58]. In the studied alloys, altogether five different phases have been identified: β-AlCo, Al5Co2, Z-Al3Co, m-Al13Co4 and Al9Co2 (Table 1).

The presence of different phases in the alloys indicates that non-equilibrium processes had been taking place during rapid solidification. The dendritic shape of β-AlCo in the as-solidified Al71Co29 alloy suggests that it solidified first, directly from the melt. The dendritic β-AlCo is located inside the Al5Co2 phase. Therefore, Al5Co2 was probably formed by partial transformation of β. The Al5Co2 phase is located next to Z-Al3Co (Figure 2). As such, Z-Al3Co was probably formed by peritectic reaction of Al5Co2 with the surrounding melt.

The as-solidified Al76Co24 alloy was found to consist of Z-Al3Co, m-Al13Co4 and Al9Co2, respectively (Figure 3). The m-Al13Co4 phase is located next to Z-Al3Co. As such, it was probably formed by peritectic reaction of Z-Al3Co with the melt. The Z-Al3Co phase has a lower Al concentration compared to m-Al13Co4 (Table 1). Al9Co2, observed in the as-solidified Al76Co24 alloy, is located next to m-Al13Co4 (Figure 3). As such, Al9Co2 was probably formed by peritectic reaction of m-Al13Co4 with the remaining melt. It was observed to be porous (Figure 3). The pores are usually formed by vacancy migration, with sub-grains and natural surfaces serving as sinks [62,63]. In the present case, the pores were found in the interior of Al9Co2. The preferential pore formation indicates that the pores could be a result of rapid transformation of the liquid Al9Co2 into solid in the final step of solidification.
