*3.3. Microstructural Studies*

The results of the optical microscopy of the structures of the unmodified Al-12wt%Si alloy and those modified with different rubidium content are shown in Figure 2. For alloy 3, the results of the X-ray spectral microanalysis and EDS elementary mapping are presented in Figure 3.

In the Al-12wt%Si alloy under study, the silicon concentration was 11.4%. The microstructure of the unmodified alloy was characterized as coarse eutectic (lSi eut = 14–49.4 μm) with large α-Al dendrites (SDAS 42 μm). Due to the presence of impurities and low cooling rates (casting into a sandy-clay mold), primary silicon crystals could be observed in the structure of the alloy under study. The iron concentration in the alloy was about 0.3%, which can lead to the β-FeSiAl5 phase formation.

**Figure 2.** Microstructure of Al-12wt%Si alloys: (**a**) unmodified; (**b**) alloy 1; (**c**) alloy 2; (**d**) alloy 3; (**e**) alloy 4.

**Figure 3.** *Cont.*

**Figure 3.** Energy-dispersive X-ray spectral microanalysis of a sample of alloy 3 and elementary mapping of the studied area of a sample modified with rubidium. (**a**) Magnification at 1100× (composition according to area); (**b**) magnification at 2150× (composition according to area and mapping according to elements); (**c**) magnification at 11,500× (composition according to area); (**d**) magnification at 4300× (composition according to area and mapping according to elements).

Studies of the microstructure of alloy 1 showed that modification with a minimal amount of rubidium led to a change in morphology, a decrease in the linear dimensions of the eutectic silicon length to 5.42–22.58 μm, and a decrease in α-Al dendrites by 5%. With an increase in the rubidium content in alloys 2 and 3, the respective linear dimensions of the eutectic silicon decreased to 1.33–5.02 and 0.36–2.29 μm (Table 2). As a result, the Si particles acquired a finer fibrous morphology, as shown in Figure 2d,e. An increase in the rubidium content in alloy 4 did not lead to greater refinement of eutectic silicon (Figure 2d). Different contents of rubidium weakly affected the size of the α-Al dendrites, and the SDAS in the alloy decreased by an average of 11–12%. Therefore, in order to obtain a modified eutectic in the alloy under study, it was necessary to ensure the actual rubidium content in the range of 0.007–0.01 wt %. Therefore, further studies were carried out with an estimated rubidium content of 0.5% (corresponding to 0.0075 wt %).

Based on the results of studying the microstructure of the sample of alloy 3, the presence of rubidium in the studied samples was studied using X-ray spectral microanalysis over the area of the image. The structure, X-ray spectrum and composition of the elements are shown in Figure 3a,c.

To confirm the elemental composition, elementary mapping of a sample of alloy 3 modified with rubidium was carried out. The elementary maps of the rubidium-modified sample in the eutectic area (containing aluminum, silicon and the Fe-containing phase) obtained by scanning at a low resolution are shown in Figure 3b, and a higher-resolution map is shown in Figure 3d. The mapping results showed that rubidium atoms were present in the eutectic silicon and had a low concentration.

#### *3.4. Thermal Analysis*

The solidification parameters of the alloys under study (depending on the melt treatment) are presented in Table 3.


**Table 3.** Solidification parameters of the studied alloys.

A DSC curves analysis (Figure 4) showed two-stage solidification of the Al-12wt%Si alloy samples from the liquid state up to complete solidification. In the first stage, the nucleation of aluminum solid solution crystals and their growth occurred, which was recorded in the DSC thermogram as the first thermal effect. The second thermal effect (second stage) was caused by the solidification of the binary eutectic according to the reaction L→α + Si. Since the alloy contained an iron impurity, during solidification of this alloy, not only the formation of binary eutectic (α + Si) was possible but also a more complex iron-containing eutectic L→α + Si +Al5FeSi could form. The solidification temperatures of these eutectics were close [1].

**Figure 4.** Thermograms of experimental alloys.

In the alloys studied, the solidification parameters changed with an increase in the rubidium concentration. Alloy modification with rubidium decreased the solidus temperatures and the temperatures at which the solidification of the eutectic began compared to the unmodified alloy. As a result, the solidification range of the Al-12wt%Si alloys under study expanded (Table 3). According to the data obtained (Figure 4), the most pronounced change in solidification parameters occurred in alloys 3 and 4.

An analysis of the alloy 3 sample's solidification parameters showed that rubidium did not change the liquidus temperature, which remained at the level of the unmodified alloy, while at the same time the solidus temperature decreased by 18.2 ◦C. The solidification range of the alloy with rubidium under study expanded by 18.3 ◦C compared to the unmodified alloy. It should be noted that the temperature of the beginning of the second thermal effect on the alloys was considered as the temperature of the beginning of

solidification of the eutectic (Table 3). Rubidium contributed to a significant decrease in the temperature of the beginning of the solidification of the eutectic (α + Si) compared to the unmodified alloy (equal to 7.6 ◦C).

#### *3.5. Study of the Effect of Rubidium on the Porosity of the Alloy*

A qualitative assessment of the effect of rubidium on the formation of gas porosity in the experimental alloys at different concentrations of rubidium was carried out (Figure 5).

**Figure 5.** Porosity distribution in Al-12wt%Si alloy castings when modified with rubidium: (**a**) unmodified; (**b**) alloy 1; (**c**) alloy 2; (**d**) alloy 3; (**e**) alloy 4.

The studies showed that the samples of the Al-12wt%Si alloys treated with rubidium had a decrease in gas porosity compared to an unmodified alloy, the porosity of which corresponded to four levels on the standard ISO 10049:2019(E) porosity scale. In alloy 1, the porosity corresponds to level 3, and with an increase in the rubidium content in alloy 4, the porosity decreased to level 1 according to the porosity scale (Figure 5).

Studies were also conducted on the effect of modification of the Al-12wt%Si alloy with rubidium on the manifestation of volumetric shrinkage of the experimental alloys. The formation of shrinkage defects in a massive element, which was the thermal center of the casting, was studied. Since the unmodified Al-12wt%Si alloy had a narrow crystallization interval of 24.2 degrees, it crystallized frontally (successively) (Figure 6a). In this case, a shrinkage cavity was formed on the upper surface of the sample, and the shrinkage porosity was formed on the opposite side. Alloy 3 modified with rubidium had a larger crystallization interval that equaled 42.5 degrees. Therefore, it was more prone to bulk crystallization. At the same time, a shrinkage cavity was formed at the top, and distributed porosity was formed below it (Figure 6b).

**Figure 6.** The manifestation of volumetric shrinkage in the Al-12wt%Si alloy: (**a**) unmodified; (**b**) modified with Rb.

## *3.6. Evaluation of the Duration of the Modifying Effect of Rubidium in the Melt*

Based on the studies shown above, alloy 3 was selected to determine the duration of the modifying effect. Treatment of the melt with rubidium led to an increase in the duration of the effect of modifying the Al-Si alloy (Figure 7a,b) compared to sodium modifiers [7]. During the first 30 min after the melt treatment, the σ value of the Al-12wt%Si alloy modified with rubidium was 176 MPa. However, with a longer holding time of up to 60 and 120 min, the ultimate strength of the rubidium-modified alloy decreased slightly to 173 and 170 MPa, respectively. With further holding of the melt up to 240 min, the ultimate strength of the rubidium-modified alloy decreased to 155 MPa.

**Figure 7.** Effect of melt holding time after rubidium modification on: (**a**) ultimate strength; (**b**) relative elongation; (**c**) the average length of eutectic silicon particles.

The values of the relative elongation of the Al-12wt%Si alloy modified with Rb remained at approximately the same level (from 9.1 to 8.46%) during the first 45 min. However, with a longer holding time from 60 to 180 min, the relative elongation of the Al-12wt%Si alloy modified with rubidium decreased from 7.5% to 6.32%.

When holding the melt up to 240 min, the relative elongation of the rubidium-modified alloy decreased significantly to 4.48%.

When modifying the Al-12wt%Si alloy with rubidium, the refined eutectic silicon phase endured for 45 min. A thin fibrous morphology of eutectic silicon was observed (Figure 8a–d). With the increase in the holding time from 60 to 120 min, a smooth size enlargement and coarsening of the eutectic (α + Si) was observed in the microstructure of the alloy (Figure 8e,f). With further holding of the melt up to 180 min, changes to the thin-plate shape in the morphology of the eutectic silicon were observed (Figure 8g). With the proceeding increase in the holding time to 240 min, an abrupt enlargement in size and coarsening of the eutectic (α + Si) was observed in the microstructure of the alloy. The morphology of eutectic silicon changed to needle-like (Figure 8h). However, even with such a long melt holding time, the resulting structure could be characterized as partially modified.

Quantitative analysis of the microstructure of the Al-12wt%Si alloy modified with rubidium showed that an increase in the melt holding time led to an increase in the linear size of the eutectic silicon (Figure 7c). After holding the melt with rubidium modification for 60 min, the average length of eutectic silicon particles increased from 1.3 to 2 microns, after 120 min—to 2.7 microns, and after 180 min—to 3.7 microns. With further exposure of the melt for another 60 min, a sharp increase in the linear sizes of the silicon particles to 16.2 microns occurred.

**Figure 8.** Effect of the melt holding time after modification of the microstructure of alloy 3. (**a**) 0 min.; (**b**) 15 min.; (**c**) 30 min.; (**d**) 45 min.; (**e**) 60 min.; (**f**) 120 min.; (**g**) 180 min.; (**h**) 240 min.

#### **4. Discussion**

To determine the possibility of modifying the Al-12wt%Si alloy under study with RbNO3 salt, a thermodynamic analysis of the probable chemical Reactions (2) and (3) was carried out. The formation of rubidium and nitrogen in the melt was thermodynamically possible and confirmed experimentally. During the experimental melting, the release of gaseous compounds during the adding of the salt into the melt was noted.

Based on the obtained data from the atomic emission spectral analysis and energydispersive X-ray spectral microanalysis of the samples modified with Rb, it can be concluded that rubidium passed into the melt when added with its nitrate.

The assimilated amount of the modifying element in the range of 0.007–0.01% was sufficient to modify the structure and increase the mechanical properties of the alloy under study.

When modified with rubidium, a thin fibrous eutectic structure was obtained, which is typical of a structure with standard Al-Si alloy modifiers such as sodium and strontium [4,5,36].

The modification mechanism of eutectic silicon by rubidium can be explained using the theory of supercooling and adsorption theory; in particular, the adsorption mechanism of the modification of eutectic silicon.

Firstly, the basis of the theory of supercooling is the data of thermal analysis. According to the theory of supercooling [26], modification of Al-Si alloys lowers the temperature of the beginning of the solidification of the eutectic. An analysis of the DSC thermograms confirmed that rubidium contributed to a significant decrease in the temperature of the beginning of the solidification of the eutectic (α + Si), which led to supercooling of the melt contributed to the appearance of more embryos, and consequently led to the modification of the eutectic.

Secondly, since rubidium is close in its physical and chemical properties to sodium and is a surface-active element, it can be assumed that it will work via a "poisoning" (TPRE) mechanism [27].

On the other hand, according to the IIT mechanism, only elements showing the "ideal" ratio of atomic radii *ri*/*r* ~ 1.646 (where *ri* is the atomic radius of the element and *r* is the radius of silicon) are modifiers.

However, rubidium has a ratio of its atomic radii of *ri*/*r* = 2.08 [28] and does not meet this criterion, as do some other modifying elements (for example, Sr, Ba and Na). This indicates that the ratio of the atomic radii of a potential modifying element by itself is not capable of predicting the spheroidization of eutectic silicon.

Thus, besides the atomic radius, other factors that are important for the modification of eutectic silicon (the vapor pressure, the formation of secondary compounds or oxides as well as phase equilibria with aluminum and silicon phases, and the cooling rate) probably exist.

The concept of the "poisoning" mechanism (TPRE) and IIT suggests the formation of a high density of twinning in modified silicon crystals [26,28,37,38]. In order for the impurity rubidium atoms to cause such a high twinning density, it is necessary that they are relatively evenly distributed (at least at the nucleation stage) throughout the silicon crystal, as was observed in the results of the energy-dispersive X-ray spectral microanalysis and elementary mapping in our studies. Thus, these results confirmed the operation of the "poisoning" mechanism (TPRE) and IIT in the rubidium-modified 12wt%Si alloy.

It should be noted that the mechanism of rubidium modification has not yet been fully determined and requires further research.

The observed decrease in gas porosity when modifying the studied alloys with rubidium can be caused by the degassing effect of nitrogen formed during the decomposition of the RbNO3 salt according to Reactions (2) and (3). The hydrogen dissolved in the melt diffused into the floating nitrogen bubbles and was removed from the liquid metal. An increase in the amount of salt added to the melt for its modification led to an increase in the degree of degassing of the 12wt%Si alloy.

The formation of shrinkage defects in a casting is affected by the nature of the solidification of the alloy. Under the same cooling conditions, the main effect on the nature of the solidification of alloys is exerted by the temperature range of solidification. Narrow-interval alloys, which include the 12wt%Si alloy, are characterized by sequential or frontal solidification with slight supercooling and the formation of a concentrated shrink shell [39–41].

With an increase in the solidification range of an alloy, the tendency the alloy to lose porosity increases. It is known that when modifying Al-Si alloys with surfactants, which include rubidium, the degree of supercooling of such alloys increases [40,41]. When modifying the 12wt%Si alloy with rubidium, the liquidus temperature of the alloy practically did not change compared to the unmodified alloy (Table 3). At the same time, as a result of the adsorption of rubidium atoms onto the solidification centers of the eutectic (Figure 3), their deactivation occurred. The solidus temperature of the experimental alloy decreased, and its degree of hypothermia increased. According to the authors of [42,43], due to the neutralization of solidification, eutectic colonies grow in the form of spheroids, forming isolated pores when closing. As a result, volumetric solidification and the formation of shrinkage porosity occurred in the thermal center of the casting (Figure 6b).

The results of the studies conducted to determine the duration of the modifying effect showed that when treated with rubidium, the modifying effect in the melt lasted up to 180 min. This was indirectly confirmed by the mechanical properties and quantitative analysis of the microstructure at different melt holding times. Apparently, the duration of the modifying effect depended on the kinetics of oxidation of rubidium in the melt, which requires additional research. Compared to commonly used eutectic silicon modifiers [44,45], the retention time of the modifying effect of rubidium is higher than that of Na but lower than that of Sr.
