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An alloy is a metallic solid that results from the mixing and melting of one metal with another or several metals or non-metals, followed by cooling and solidification. The change from metal to alloy is generally not as significant as in other chemical reactions, with relatively small heat effects. Alloys generally possess certain metallic properties. Based on the number of constituent elements, alloys can be classified into binary alloys, ternary alloys, and multicomponent alloys. According to the phase diagram of alloys, alloy phases can generally be divided into three categories: metallic solid solutions, intermetallic compounds, and interstitial compounds. When the electronegativity, chemical properties, and atomic size of two metal elements are similar, solid solutions tend to form; when there is a large difference in electronegativity and atomic radius between the elements, intermetallic compounds are likely to be formed. Transition metal elements easily form interstitial compounds with non-metal elements such as H, B, C, and N, which have very small atomic radii – these compounds are created as atoms with small radii fill the interstices between metal atoms. Metallic solid solutions. It is a system formed when the atoms of one metal randomly replace some of the atoms in the lattice of another metal; the structural pattern of metal solid solutions is generally the same as that of pure metals. A complete solid solution system can be formed only when the radius difference between the two metal atoms is less than 15%, and when the two metals have the same crystal structure and similar chemical properties. For example, nickel and copper can form a complete solid solution series. Sodium and potassium do not form solid solutions; zinc and copper only form incomplete solid solution series. Metal intermetallics. Alloys are those in which the various components that make up the alloy form compounds with each other. Many metal intermetallics have structures similar to those of β, γ, and ε bronzes; their properties are determined by the ratio of electrons to atoms, which is why they are also known as electron compounds. Metal interstitial compounds. Compounds formed between transition metals and the second-period elements B, C, N, and O are often intermetallic compounds. In such compounds, small atoms such as B, C, N, and O are inserted into the gaps within the structure formed by transition metal atoms. Interstitial compounds formed by transition metals from Group ⅢB to Group ⅥB mostly have an MX stoichiometry and a NaCl structure. Interstitial compounds generally possess good electrical and thermal conductivity, with the electrical conductivity decreasing as temperature rises; they have a metallic luster; they have very high melting points and high hardness but are brittle; regardless of the structural pattern of the pure metal itself, most of them adopt a NaCl structure. Alloys possess superior mechanical, physical, and chemical properties compared to pure metals, and most of the metal materials used in industry are alloys. The most common non-metallic alloying element is carbon, which accounts for 1.7% in steel. Other metals added to steel, such as chromium and molybdenum, can enhance the steel’s hardness and corrosion resistance. Alloys often exhibit significant differences in their properties depending on the components they contain. Based on whether they contain iron, alloys can be divided into two main categories: ferrous alloys and non-ferrous alloys. Flux and Wood’s alloy are representative of fusible alloys that melt at lower temperatures. A high-melting-point alloy that melts at over 1,650°C, and is a metal solution of tungsten, cobalt, nickel, and molybdenum. An amalgam is an alloy of mercury with other metals, which can be prepared in a liquid state. Alloys have wide-ranging applications in various fields, and research on alloys holds great practical significance.