Although pure metal has good conductivity, thermal conductivity and good plasticity, and has certain application value in industry, its strength and hardness are not high, and smelting is also very difficult (the purity is also high, and the smelting cost is higher), so its application is limited. In practice, a large number of metal materials used are mostly alloys.

1. Concept of alloy: Alloy refers to two or more metal elements or substances with metal characteristics composed of metal and non-metal. For example, steel and cast iron are alloys of iron and carbon.

Component refers to the most basic and independent unit of alloy. According to the number of components, the alloy can be divided into binary alloy, ternary alloy, etc.

The internal structure of the alloy is composed of phases. The phase in the alloy refers to the uniform part that has the same structure, the same physical properties and chemical properties and has obvious interface separation from the rest of the system.

The phase structure of solid alloy mainly includes solid solution and metal compound.

2. Solid solution

In the solid state, the uniform solution formed by the mutual dissolution of components in the alloy is called solid solution. A component whose lattice remains unchanged in a solid solution is called a solvent. The other components in the solid solution are called solutes.

When the solute atoms occupy the normal nodes of the lattice, the solvent atoms of these nodes are replaced by the solute atoms to form a displacement solid solution; When the solute atoms are embedded in the lattice gap, it is called interstitial solid solution (because the gap size and quantity of the solvent lattice are limited, only the solutes with small atomic radius (carbon, nitrogen, boron and other non-metallic elements) can dissolve into the solvent, which is also called finite solid solution.

3. Metal compounds

Metal compound is a new phase with metal characteristics formed by the interaction of various components in the alloy, which can be generally expressed by chemical formula. Such as Fe3C (cementite) in iron carbon alloy. Metal compounds usually have complex crystal structure and are different from the crystal type of any constituent element.

Metal compounds have high melting point and hard and brittle properties. When they are distributed on the solid solution matrix in a fine and uniform shape in the alloy structure, they can improve the mechanical properties of the alloy. When the amount of metal compounds is large or distributed coarsely and unevenly, the mechanical properties of the alloy will be reduced and the brittleness of the alloy will be increased.