The essence of metal plastic deformation is mainly realized by sliding and other movements. Metal plastic deformation refers to the irreversible permanent deformation of metal parts under the action of external force. The reason of plastic deformation is the sliding dislocation of atoms.

The plastic deformation of single crystal is caused by the sliding dislocation of atoms. In the plastic deformation of polycrystals (most of the metals actually used are polycrystals), besides the deformation inside each grain (intragranular deformation), there is also the deformation between each grain (called intergranular deformation). The plastic deformation of polycrystals is the sum of intragranular deformation and intergranular deformation.

It has been a long time for human beings to process metal materials by plastic deformation, but the theory of plastic deformation was established more than 100 years ago. From 1864 to 1868, H.Tresca, a Frenchman, put forward the maximum shear stress condition of plastic deformation in a series of papers.

The research on the plasticity of metal crystals began with the manufacture and development of metal single crystals. Application of x-ray diffraction. The early research results are included in the works of Elam in Britain, Schmidt in Germany and Barrett in the United States. This paper mainly studies the main forms of plastic deformation in metal crystals & slip and twin deformation.

According to the different research purposes and methods, the theory of metal plastic deformation can be divided into two categories:

1. Based on the macroscopic mechanical parameters and the assumption of uniform and continuous medium, the stress and strain in plastic deformation body are studied to solve the strength design and variable problems in plastic processing of materials. This theory is often called plastic mechanics or plastic theory (see the mechanical principle of plastic deformation).

2. The relationship between plastic deformation and crystal structure of metal crystals and the mechanism of plastic deformation are studied. This theory is often called the crystal paradigm.