Explanation

When a system composed of a large number of atoms or molecules reaches equilibrium under certain external constraints, one or more uniform regions that are different from each other will be formed in the system, which is called phase. When the external constraints (such as temperature, pressure, magnetic field intensity, etc.) change, the number of phase or the properties of phase in the system will change, which is called phase transformation. It is the change of macroscopic state, and the decrease of free energy is the criterion of change direction. There are many types of phase transitions, which can be classified according to certain attribute characteristics: according to thermodynamics criteria, they can be divided into first, second, and multi-level; According to the stability of resisting fluctuations, it can be divided into continuous and discontinuous; According to the control link of new phase growth, it can be divided into diffusion controlled and interface controlled; According to the characteristics of atomic migration during the formation of new phase, it can be divided into diffusion (diffuse mobile type), diffusion free phase transformation (column column mobile type), etc. In addition, there are phase transformation controlled by mass transfer or controlled by heat transfer (such as solidification). Traditionally, it can be divided into reconstruction type and displacement type. The former refers to the formation of a new crystal by disassembling the original chemical bonds and combining them again, while the latter refers to the change of the length and angle of the bonding bonds during the phase transition.

Classification