*3.1. Thermodynamic Analysis of the Interaction of Rubidium Nitrate with Melt*

The study of an alloy modification with rubidium using RbNO3 salt is impossible without a preliminary thermodynamic analysis. In this regard, the thermodynamic probability of the salt-decomposition process and its interaction with the melt up to a temperature of 1200 K (927 ◦C) was determined. Calculations of the free reaction energy were carried out while taking into account phase transformations based on the literature data [34,35].

It is known [34] that rubidium nitrate RbNO3 is a low-melting salt (melting point t = 313 ◦C) and that when heated above the melting point, it decomposes to form a nitrite and oxygen according to Reaction (1):

$$\text{RbNO}\_3 = \text{RbNO}\_2 + 0.5\text{O}\_2 \tag{1}$$

The resulting RbNO2 reacts with aluminum and is reduced to rubidium according to Reaction (2):

$$\text{RbNO}\_2 + \frac{4}{3}\text{Al} = \frac{2}{3}\text{Al}\_2\text{O}\_3 + \frac{1}{2}\text{N}\_2 + \text{Rb} \tag{2}$$

At the same time, direct interaction of the nitrate with aluminum according to Reaction (3) with the formation of rubidium is also possible:

$$\text{RbNO}\_3 + 2\text{Al} = \text{Al}\_2\text{O}\_3 + \frac{1}{2}\text{N}\_2 + \text{Rb} \tag{3}$$

According to the calculations obtained, the free reaction energy for Reactions (2) and (3) at the temperatures of 600–1200 K was negative (Figure 1). Consequently, at the salt input temperature of 1023 K (750 ◦C), the formation of rubidium in the aluminum melt was possible according to Reactions (2) and (3), for which the energy of the system reached values of −432 and −367 kJ/mol, respectively.

**Figure 1.** Temperature dependences of the free reaction energy of Reactions (2) and (3) on temperature.

#### *3.2. Mechanical Properties*

The results of mechanical tests of the Al-12wt%Si alloy after modification with rubidium are presented in Table 2. It was experimentally established that adding rubidium caused an increase in the mechanical properties (ultimate strength σ<sup>B</sup> and relative elongation δ).


Note: the numerator shows the average size of the structural components and the denominator shows the minimum and maximum size.

The largest increase in the mechanical properties of the alloy 3 is explained by the modified microstructure of the alloy. An increase in the Rb content in alloy 4 did not lead to a further increase in mechanical properties, which remained at the level of alloy 3.
