Solid solutions are phases in which one of the components (the solvent) retains its crystal lattice, and atoms of other (dissolved) components are located in its lattice, distorting it.
Throughout the history of the metallurgical industry, studying solid solutions has been one of the key aspects of understanding and improving properties of metal alloys. Solid solutions are the basis for creating diverse materials, from bronzes, brasses, stainless and structural steels to high-strength alloys, including magnesium and aluminum. These materials play a key role in creating strong, light structures, finding application in various fields including aviation and automotive.
Properties of solid solutions are actively regulated by their composition and production technology, especially processes of heat or thermomechanical treatment. This allows engineers and metallurgists to create materials with certain mechanical and chemical properties.
When transitioning from one solid solution to another, steels and alloys can acquire new, sometimes unique properties. For example, as happens when quenching steels. At the beginning of heating the material the crystal lattice expands and atoms of alloying elements dissolve in the main metal matrix, forming a solid solution. When the steel cools the crystal lattice contracts. As a result martensite forms, which has a different crystal structure compared with the original austenite. This change in the material’s crystal lattice affects its mechanical properties, in particular increasing hardness and strength.
Precipitation hardening (aging) is also accompanied by changes in the material’s crystal structure: decomposition of a supersaturated solid solution occurs and new phases precipitate that strengthen the material. This process is characteristic, for example, of aluminum alloys.
In materials three main types of solid solutions are distinguished: substitution, interstitial, and subtraction. These varieties have certain features of crystal structure that determine material properties.
Substitution solid solutions are formed by replacing part of the solvent atoms in its crystal lattice with atoms of the dissolved component. As a rule this is disordered placement, because atoms of the dissolved element do not occupy special places in the crystal lattice but only replace solvent atoms at random sites.
It can also be the opposite, when atoms of solvent and dissolved substance elements are located in different crystallographic planes. Such solid solutions are called ordered; they are characterized by higher hardness and brittleness.
The possibility of ordering the crystal lattice of solid solutions was discovered by Nikolai Semenovich Kurnakov in 1914. The Russian chemist discovered the ordering phenomenon when studying electrical resistance of copper and gold alloys. X-ray analysis confirmed that change in material properties is linked to redistribution of atoms inside the crystal lattice. Under certain conditions atoms of the dissolved element can transition from chaotic distribution to ordered. In industry regulating ordering processes of solid solutions plays a key role in producing precision alloys based on iron and cobalt.
Interstitial solid solutions arise when atoms of the dissolved component embed into voids of the crystal lattice.
Subtraction solid solutions form due to appearance of vacant sites in the crystal lattice when one of the components of a chemical compound dissolves.
Researching solid solutions and ways of acting on their structure has fundamental significance for modern metallurgy. This knowledge makes it possible to create materials with certain physical, mechanical, and chemical characteristics that will maximally meet requirements of modern technologies and innovative solutions.
