*3.2. Microstructural Evolution*

To understand the microstructure and phases of the Al-Mg-Si-*x*La alloys the backscattered SEM images are presented in Figure 4. Overall, the microstructures include gray α-Al dendrite arms and intermetallic particles of various shapes and colors in the surroundings. The Mg2Si phase, appeared in black color and was quite visible in all the images. Other phases mostly contained La and some traces of Fe impurity, elements with a high atomic number, which were presented as bright inclusions. The composition of di fferent phase-referred areas, determined by EMPA, are summarized in Table 3. As is shown, there was no appearance of Al3Mg2 in La-rich alloys due to dissolution of Mg in α-Al.

**Figure 4.** Backscattered SEM micrographs showing as-cast microstructural evolution of Al—4 wt.% Mg—0.5 wt.% Si alloy with increasing in La content: (**a**) L0 alloy; (**b**) L1 alloy; (**c**) L2 alloy; (**d**) L3 alloy; (**e**) L4 alloy; (**f**) L5 alloy.


**Table 3.** Chemical composition of the microstructural components in experimental alloys according to electron microprobe analysis (EMPA) 1.

1 See referenced alloys microstructure images and referenced specters in Figure 4.

Following the base L0 alloy as a reference (Figure 4a), a relatively high volume of lamellar and needle-like Mg2Si phase, and bright small inclusions of Fe-containing phase (likely, Al13Fe4 phase, according to calculation) were recorded. The latter was very fine and of particular low proportion compared to the Mg-rich phase. Apart from the formation of the Fe-rich phase, it is remarkable how precisely the microstructure agrees with Scheil–Gulliver prediction.

Observations of the La-containing alloys show significant changes in intermetallic shapes. The addition of 0.1 wt.% La resulted in thinning of the Mg2Si phase branches, though they retained lamellar morphology. Meanwhile, some traces of La-rich phase resembled the AlLaSi compound due to the prominent presence of Si and some amount of Mg possibly captured during spectral emission (Table 3). Besides, having exhibited the Al13Fe4 phase, it was complicated to distinguish two binary intermetallics (Fe- and La-rich phases). However, the AlLaSi phase was far more differentiated, while the Al13Fe4 phase was mostly of a small blocky shape. The appearance of the AlLaSi phase becomes more evident with an increase in La due to thicker branches of intermetallics and less capturing of Mg-rich α-Al phase coupled by more evident Si content. Strikingly, the addition of 0.25 wt.% La almost eliminated the presence of the Mg2Si phase. As is seen from the microstructure of the L2 alloy (Figure 4c), the AlLaSi phase is located in the vicinity of a very fine eutectic Mg2Si phase. This partially confirms the absorbing effect of the La-rich phase. This effect may have suppressed the anisotropic growth of the Mg2Si phase inhibiting its shape factor. On the other hand, since the amount of the Mg-rich phase is visibly low, the thermodynamically predicted relationship between the volume of AlLaSi and Mg2Si is confirmed by the presence of a higher amount of a AlLaSi phase and a lower amount of the Mg2Si phase. On further investigation of different alloys, the incredible growth of the AlLaSi phase and evolution of the Mg2Si phase down to submicron in thickness, accompanied by an increase in Mg solubility in α-Al (e.g., from 1.83 wt.% in the L0 alloy vs. 5.36 wt.% in the L4 alloy) was noted. Under the further increase in La content, the shape of the AlLaSi phase progresses dramatically from

acicular in the L3 alloy (Figure 4d) up to needle-like the L4 alloy (Figure 4e) and to coarse star-like shape in L5 alloy intrinsic for Al13Fe4 phase in Fe-rich aluminum alloys. This advancement is accompanied by a prominent change in morphology of the Mg2Si phase, which became both slender and vermicular in the L5 alloy microstructure.

In addition, the microstructural analysis revealed that almost all the microstructures included Fe-rich intermetallics (Al13Fe4). Generally, they must have a commonly adverse needle-like morphology in most alloys, even at a low concentration of Fe. It is of particular note that in the experimental alloys, Al13Fe4 shows a short flake-like shape at a relatively high Fe content (up to 0.12 wt.%). Indeed, it was reported that La may hamper the orientation growth of the Al13Fe4 phase in a similar manner as of Mg2Si phase [5]. However, La content of more than 0.25 wt.% brings degradation and formation of very coarse acicular intermetallics related to the AlLaSi phase, which may act as stress raisers, thus being detrimental to mechanical properties.

It should be taken into account that quantitative EPMA analysis produces a misleading backlighting effect due to the capturing elemental emissions from nearby areas. However, an EMPA mapping of the representative L4 alloy (Figure 5) is strongly consistent with previous thermodynamic predictions and microstructural investigations. The existence of three types of microstructural components was noted, and the corresponding elemental correlations provide a reliable qualitative explanation of their chemical composition. It was found that Mg (red-color in the maps) is distributed mostly in the region related to the α-Al phase, advocating its dissolution. Its presence in regions adjacent to La-bearing phases (turquoise-color in the maps) may also be observed, which is probably related to Mg micro-segregation enriching to the edges of the α-Al phase dendrite cells because of non-equilibrium solidification conditions [23]. Besides, Mg along with Si (deep-green-color in the maps) are incorporated into the Mg2Si phase of worm-like shape. Ultimately, while Fe is bonded into unevenly distributed fine particles of less than 3 μm in size, the most dominating microstructural component is a very coarse AlLaSi phase revealed by correlation among Al, Si, and La enriched regions.

**Figure 5.** EMPA mapping (Al, Mg, Si, Fe, La) of the phases presented in the microstructure of the L4 alloy.
