**1. Introduction**

The addition of neodymium into the magnesium–zinc–zirconium system leads to a change in the phase composition of magnesium-based alloys. In this case, simultaneously with the known strengthening phases of magnesium with neodymium (for example, Mg12Nd), phases of a more complex stoichiometric composition are formed, containing magnesium, neodymium, and zinc, for example Nd15Mg65Zn20, Nd16Mg37Zn47, Nd6Mg41Zn53 [1–7]. Due to the presence of these phases in the system, a combination of highly plastic and structural characteristics of magnesium alloys, as well as their high-temperature strength, is ensured, since the thermal expansion coefficient of long period stacking ordered (LPSO) phases is much lower than that of the basic α-solid solution [8–11].

It is known that the production of magnesium and aluminum alloys is carried out by dissolving in the melt double and ternary master alloys, which are produced by fusing the components, using metallothermic reduction of alloying components from their compounds (for example, oxides or fluorides) or electrolysis [12–15]. In recent years, there has been a tendency to produce more complex master alloys, for example, ternary or quadruple [16–18]. It was found that ternary master alloys can be effectively used in the production of alloys based on light metals [19,20].

In this regard, it seems relevant to develop new solutions for the production of complex magnesium master alloys including rare earth metals (REEs). It should be noted that for the domestic magnesium industry the task of obtaining magnesium alloys is of grea<sup>t</sup> importance in connection with the approval of the Strategy for the development of the metallurgical industry in Russia for the period up to 2030. In accordance with the sustainable development plan, the task of increasing the production of metallurgical products with high added value was approved, which should lead to a reduction in imports of such products [21]. In addition, the states priority is to improve the quality of production of Russian metallurgical companies in order to increase the efficiency of processing mineral raw materials [22–29].

The aim of the work is to develop technological solutions that ensure high extraction of neodymium in the ternary Mg–Zn–Nd master alloy during metallothermic reduction of neodymium fluoride, which can be used in the production of high-strength and heat-resistant magnesium and aluminum alloys.

#### **2. Materials and Methods**

Differential thermal analysis (DTA) was carried out using a (NETZSCH, Selb, Germany) Simultaneous thermal analyzer (STA 429 CD) in a high purity argon flow at a gas flow rate of 50 mL per minute [30]. For the analysis, a salt mixture 35KCl–35NaCl–30CaCl2–NdF3, granular zinc and magnesium were weighed into an alundum melting pot. Two heating–cooling cycles were carried out at a rate of 10 ◦C per minute. In the first cycle, the melting pot with the weighed portion was heated to a temperature of 800 ◦C, then cooled to 200 ◦C; in the second successive cycle, the melting pot was heated from 200 to 800 ◦C and cooled again.

Experimental studies were carried out on a laboratory setup, which includes the following items: the shaft electric furnace with silicon carbide heating elements, the thermostat, the furnace control panel, the thermocouple and the stirring device. The synthesis of ternary master alloy Mg–Zn–Nd was carried out according to the following procedure. A mixture of salts consisting of Potassium chloride, (35 wt.%), Sodium chloride (35 wt.%), Calcium chloride (30 wt.%) was previously prepared. To this mixture was added neodymium fluoride (NdF3) and the mixture stirred continuously. Then the resulting technological salt mixture was added to magnesium and zinc and placed in an alundum melting pot in the furnace. In the first series of experiments, in order to determine the optimal temperature condition for the synthesis of the master alloy, the melting pot was kept in a furnace with temperatures variations from 550 to 700 ◦C, and the residence time from 15 to 30 min, with a constant ratio of chlorides to neodymium fluoride. In the second series of experiments, in order to identify the most optimal technological condition, the synthesis temperature was varied from 650 to 700 ◦C, the residence time from 20 to 30 min, the ratio of chlorides to neodymium fluoride from 1:4 to 1:6. After the exposure time, in all experiments, the melt was settled for 5 min. The surface part of the molten salt was poured out, and the resulting master alloy was poured into molds. The studies were carried out with a constant mass ratio of Mg: Zn 1:2 and components of the salt mixture 35KCl–35NaCl–30CaCl2, the qualifications of the initial salts of the mixture are: KCl, NaCl, CaCl2,—chemically pure, NdF3—pure. The average values of the degree extraction for neodymium obtained from the results of three parallel experiments are presented in the discussion of the results.

Elemental analysis of master alloy samples, which were obtained after DTA and experiments on a laboratory setup, was carried out on the sequential Wavelength Dispersive X-Ray Fluorescence Spectrometer XRF-1800 (Shimadzu, Kyoto, Japan). The metallographic study for the samples of the obtained master alloy was carried out on the Axiovert 40 MAT optical microscope (Carl Zeiss, Oberkochen, Germany) and the VEGA electron microscope (TESCAN, Brno, Czech Republic) with the INCAx-act energy dispersive spectrometer (Oxford Instruments, Abingdon, UK). The research was carried out with the involvement of the laboratory facilities of the Common Use Center of the Saint-Petersburg Mining University.
