The successful development of a number of branches of socialist industry, and first of all of metallurgical industry, depends to a large extent on the quality of refractory ceramic products. Modern metallurgy, which uses oxygen blast in blast furnaces and high-speed methods of steelmaking in open-hearth furnaces, needs new, more highly refractory materials than the current ones, whose melting temperature does not exceed 1800- 2000°. The given data (see article) emphasize the great urgency of the problem of new highly refractory materials. The main reason for the difficulties arising in the search for new highly refractory materials is the lack of state diagrams of oxide systems that make up refractory masses. The number of materials from which refractory masses are usually made is rather limited. Therefore, the study of the previously unstudied state diagram of the system MgO - Cr₂O₃ - ZrO₂ (melting point MgO - 2800 °, Cr₂O₃ - 2110 °, ZrO₂ - 2715 °) can have not only theoretical but also practical value in the search for new refractory materials with higher physical and chemical properties. Triple mixtures of magnesium oxide, chromium oxide and zirconium dioxide, whose compositions are located in the region of solid solutions, as not forming eutectics and therefore do not experience softening when heated up to melting temperatures (2200-2600 °), are new, practically important highly refractory materials.

The essence of physicochemical analysis, created and formalized by Nikolai Semenovich Kurnakov into a special department of physical chemistry studying the equilibrium of various systems, consists in the application of physical methods to determine the chemical nature of substances formed in binary and multicomponent systems. The general method of physicochemical analysis consists in the quantitative study of the properties of equilibrium systems formed, depending on their composition, by two or more components. The result of the measured values is a composition‑property diagram consisting of one or more lines, the positions of which determine the state of the system. The scientific works of the Kurnakov school were distinguished by their purposefulness and were mainly aimed at determining the characteristics of a chemical individual formed in binary and multicomponent systems in contrast to an ordinary solution of the same components. In other words, to determine how a substance that we can and should call a chemical individual differs from an ordinary solution of the components that form a chemical individual. The same question was posed to Proust by Berthollet more than a hundred years ago, demanding a precise definition of both concepts. Dalton and Gay‑Lussac, Wald and Ostwald were also concerned with this question.

High refractoriness (2715°), slag resistance, low thermal and electrical conductivity at high temperatures, mechanical strength and thermal stability under sharp temperature fluctuations make zirconium dioxide a very valuable source of highly refractory material, especially after it was discovered that cracking during heating of products made of pure zirconium dioxide, caused by the transformation of one modification into another at a temperature of 1000°, can be eliminated by adding more than 4% magnesium or calcium oxide, which form stable solid solutions with zirconium dioxide. In the Soviet Union there are numerous deposits of zirconium ores, ensuring their industrial use. As for magnesium oxide and calcium oxide, these source materials are available in unlimited quantities. All this shows that the study of the ternary system ZrO₂—MgO—CaO has not only theoretical but also significant practical interest.

Among the refractory materials currently in use, chromite iron ore and magnesite deserve special attention. But, as is known, these refractories in their pure form are not stable enough and are destroyed relatively quickly, which causes frequent repairs of the internal lining of smelting furnaces. Therefore, the study of the dependence of the melting temperature on the composition of mixtures of highly refractory materials, in particular the determination of the areas of formation of solid solutions, is one of the ways to find new highly refractory materials. This prompted us to study the melting diagrams of the systems: Cr₂O₃ — MgO и Cr₂O₃ — ZrO₂.

The first published data date back to 1930, when attempts were made to study the melting diagram of the Cr₂O₃–ZrO₂ system. Having discovered the significant volatility of chromium oxide, the authors abandoned further research. The results of the work are presented in Fig. 1, which shows that the data obtained cannot even serve as approximate. These data exhaust the published information on the Cr₂O₃–ZrO₂ system. Believing that the volatility of Cr₂O₃ in itself cannot serve as an obstacle to studying the system, since the composition of each point can be determined by chemical analysis of the fused portion of the samples, after determining the melting temperature, we decided to study the equilibrium diagram of this system, which is of not only theoretical but also significant practical interest.

The ternary system ZrO₂–MgO–Al₂O₃ as a whole has not yet been studied. Literature data are available only for the binary systems that form the sides of this ternary system. A study of melting temperatures was carried out along three sections in the ternary system ZrO₂–MgO–Al₂O₃, and it was found that in the direction from ZrO₂ to the chemical compound MgO·Al₂O₃, there exists a continuous region of solid solutions whose melting point increases with increasing ZrO₂ content from 2130°C (spinel) to 2715°C (ZrO₂). The absence of a eutectic, which would cause premature softening of the material, makes these mixtures very important as highly refractory materials.

With the very rapid development of the automotive and aviation industries, the question of obtaining materials that are lightweight and at the same time possess sufficient mechanical properties becomes increasingly urgent. The best and, so far, unsurpassed materials in this respect are alloys of the duralumin type, with an approximate composition (see the article). The duralumin alloy, as can be seen, is relatively complex, and therefore experiments have long been conducted in various parts of the globe to obtain simpler alloys that would not be inferior to duralumin in their mechanical qualities. Among such simple alloys, the aluminum-copper alloy enjoys the greatest success, especially in America. The nature of this alloy has not yet been sufficiently clarified. Therefore, for a more detailed investigation of the issue, it seemed appropriate to us to study the change in the solubility of copper in aluminum in the solid state as a function of temperature, since according to available data, such a change in the solubility of various components in alloys is the main reason determining the properties of alloys of the duralumin type, including the property of "aging."