Knowledge

The Basic Process of Metal CrystallizationMainly includes two stages:

Nucleation: When liquid metal is cooled below its melting point (undercooled state), local atoms aggregate through thermal motion to form tiny ordered crystal cores (nuclei).

Homogeneous nucleation: Spontaneous formation of nuclei in a uniform liquid state requires a larger degree of undercooling.

Heterogeneous nucleation: Formation of nuclei attached to existing interfaces such as impurities and mold walls, which requires a smaller degree of undercooling and is more common.

Grain growth: The nuclei gradually absorb the diffusion of surrounding liquid atoms and expand along specific crystal directions, eventually forming grains. When adjacent grains come into contact, grain boundaries are formed.

Conditions for Metal Crystallization

Undercooling (ΔT): Metals must be cooled below their theoretical melting point (equilibrium crystallization temperature) to crystallize. The greater the undercooling, the higher the nucleation rate.

Atomic diffusion ability: The migration ability of atoms in liquid metals affects the crystallization rate. As the temperature decreases, the diffusion ability weakens, which may lead to incomplete crystallization.

Typical structure after crystallization

Polycrystalline structure: Metals are usually composed of a large number of randomly oriented grains, with grain boundaries between them.

Grain size: Affected by cooling rate:

Rapid cooling (such as quenching) → High undercooling → Many nuclei, fine grains;

Slow cooling → Coarse grains.

Crystal defects: Defects such as dislocations and vacancies may form during crystallization, affecting the mechanical properties of metals.

Influencing Factors of Metal Crystallization

Cooling Rate: The faster the cooling, the greater the degree of undercooling, and the finer the grains (such as the formation of fine-grained zones on the surface of castings).

Impurities and Additives: Impurities can act as heterogeneous nucleation sites, refining the grains (for example, adding titanium and boron to aluminum). Some elements (such as sulfur and phosphorus) may inhibit grain growth.

Melt State: The purity of the liquid metal and the degree of overheating treatment (high-temperature melting to reduce impurities) affect the crystallization behavior.

The practical significance of crystallization phenomena

Mechanical property control: Grain refinement can enhance the strength of metals (fine grain strengthening, in accordance with the Hall-Petch formula).

Casting process optimization: Controlling the cooling rate or adding modifiers (grain refiners) to avoid the brittleness caused by coarse grains.

Welding and solidification: The crystalline morphology in the weld zone (such as columnar grains, equiaxed grains) affects the performance of the joint.

Amorphous metals: By rapid cooling (such as copper mold casting), crystallization is suppressed to form an amorphous structure (metallic glass), which possesses high strength, corrosion resistance, and other properties.

Typical Crystal Morphology Examples

Dendrite: A crystal that grows in a tree-like shape, commonly found in the center of slowly cooled castings.

Equiaxed Grain: A grain that grows uniformly in all directions, appearing in rapidly cooled or strongly stirred melts.

Columnar Grain: A long, elongated grain that extends along the heat dissipation direction, commonly seen in directional solidification processes.

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