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."