**1. Introduction**

Cast aluminum alloys are used in the automotive and aerospace industries and occupy a special position among structural materials. This is because of the possibility of achieving an optimal combination of basic operational properties (strength, ductility, corrosion resistance, density, etc.) with technological properties, including excellent casting characteristics.

The phase composition, the structural components and the nature of solidification of any cast aluminum alloy are the most important criteria that determine either the operational or technological properties [1].

During the solidification of an alloy, its internal structure is formed, which is one of the determining factors of its operational properties [2]. Solidification is a complex physical and chemical process that can proceed at different rates and under the influence of various external factors. The course of solidification is affected by the physical characteristics of an alloy and the cooling conditions of the casting. Technological factors, such as the temperature of casting the alloy and various types of melt treatment (including modification), significantly affect the solidification process.

**Citation:** Petrov, I.A.; Shlyaptseva, A.D.; Ryakhovsky, A.P.; Medvedeva, E.V.; Tcherdyntsev, V.V. Effect of Rubidium on Solidification Parameters, Structure and Operational Characteristics of Eutectic Al-Si Alloy. *Metals* **2023**, *13*, 1398. https://doi.org/ 10.3390/met13081398

Academic Editor: Wenming Jiang

Received: 29 June 2023 Revised: 27 July 2023 Accepted: 28 July 2023 Published: 4 August 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

In most cases, modification can be considered as the adding of modifying additives into an alloy—elements are added in an amount from 0.001 to 0.3%. Generally, a large number of elements that have a modifying effect on the structure of Al-Si alloys are known.

Modifying elements change the characteristics of the origin, growth and shape of eutectic silicon crystals. Such elements primarily include some alkaline (Na [3–5] and K [6–8]) and alkaline earth metals (Sr [9–11], Ba [12–14] and Ca [12,15,16]). To date, the modifying effect of a number of REMs (Y [12,17], La [18], Ce [19], Sm [20], Eu [21], Ho [22], Er [23,24] and Yb [12,24,25]) on eutectic silicon crystals is known. Such elements are introduced into aluminum alloys with Si content from 6 to 13%, where the eutectic is the main structural component of the alloy. To explain the modification of Al-Si alloys, the theory of supercooling by Edwards and Archer [26] as well as adsorption theories considering the adsorption mechanisms of silicon modification are used. Thus, according to the "Twin plane re-entrant edge" (TPRE) "poisoning" mechanism proposed by Day and Hellawell [27], the modifying element is adsorbed at active growth points in the re-entrant edges of twins, retarding the growth of silicon crystals and changing their growth directions. This deactivates the TPRE mechanism, causing silicon crystals to grow in a more isotropic manner. The basic idea of the "Impurity-induced twinning" (IIT) mechanism proposed by Lu and Hellawell [28] is that the atoms of the modifying element are absorbed on the steps of a growing silicon crystal at the interface between the solid and liquid phases. This leads to the formation of new twins and consequently ensures their growth in other directions. 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.

The interest of researchers in the processes of structure formation during modification of Al-Si alloys has led to the advancement of modification technologies and the production of high-quality castings as well as the development and confirmation of various modification theories.

Of great interest is the use of Rb for modifying Al-Si alloys. There is insufficient literature data on the use of rubidium as a modifier in aluminum–silicon alloys. Rubidium is a chemically active alkali metal with a melting point of 39.3 ◦C [29]. It is close in its physical and chemical properties to sodium; therefore, it is assumed that the modifying effect of rubidium on a eutectic (α + Si) is similar to the action of sodium. Based on the adsorption theory of modification, the most effective modifiers that refine a eutectic (α + Si) are elements with a low surface tension. Rubidium has a surface tension equal to 83 mN/m [30]. It is significantly lower than the surface tension of aluminum (914 mN/m) and silicon (865 mN/m) [30].

Rubidium has not received practical application in the foundry of eutectic Al-Si alloys. This is due to the complexity of adding this metal into the melt, and the lack of information regarding its effect on the properties of Al-Si alloys. Data on its modifying effect on Al-Si alloys are contradictory [6,28,31]. Rubidium in its pure form is difficult to add to the melt; therefore, it was decided to use salt to add it.

The effect of rubidium on the structure, mechanical properties and solidification of a eutectic Al-Si alloy was examined in our study. The duration of the modifying effect of rubidium in the Al-Si alloy melt was determined, and the porosity of an alloy modified with rubidium was evaluated.
