The development of new materials is not only closely related to the study of the composition, structure, and properties of materials, but also directly related to their preparation methods.
Different materials require different processes for preparation and synthesis:
Metal materials: Solidification
Ceramic materials: Sintering
Polymers: Reaction synthesis
The preparation process of materials has a significant impact on their physical, chemical, and mechanical properties. Understanding the basic process of material preparation, mastering the basic theories, techniques, and methods of material preparation, is of great significance for material selection and further improvement of its performance.
Solidification: The process of a substance changing from a liquid to a solid state.
Crystallization: The process of a substance changing from a liquid to a crystal (solid) state.
Crystallization is a form of solidification.
Characteristics of the Structure of Liquid Metals#
- Short-range order and long-range disorder in atomic arrangement
Gas (long-range disorder) $ \Rightarrow $ Liquid (short-range order, long-range disorder) $ \Rightarrow $ Crystal/Solid (long-range order)
- Existence of structural fluctuations (phase fluctuations)
Structural fluctuations: Ordered atomic clusters in liquid metals appear and disappear instantaneously, fluctuating and changing unpredictably. This is the structural condition for metal crystallization.
Crystallization Process of Pure Metals#
Macroscopic Phenomena of Metal Crystallization#
The macroscopic phenomena of crystallization mainly manifest as the occurrence of supercooling and the generation of latent heat of crystallization.
Supercooling is the energy condition for metal crystallization (thermodynamic condition).
Microscopic Process of Metal Crystallization#
The crystallization of metals consists of two basic processes: nucleation and growth of nuclei.
After the growth of nuclei, they become grains.
Nucleation - Formation of Nuclei#
Two conditions:
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Structural condition - structural fluctuations
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Energy condition - energy fluctuations
Two ways of nucleation:
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Homogeneous nucleation (spontaneous nucleation) - Nuclei are spontaneously formed from the interior of the liquid. Requires a large amount of energy, so a larger degree of supercooling is needed for nucleation to occur.
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Heterogeneous nucleation (non-spontaneous nucleation) - Nuclei are formed by attaching to impurities in the liquid. Requires less energy and often plays a dominant role in the crystallization of practical metals.
Growth - Growth of Nuclei#
The essence of the growth of nuclei is the transfer of atoms from the liquid to the solid surface.
Modes of growth for pure metals and alloys:
- Dendritic growth - Similar to the growth of a tree, the nuclei first grow into trunks, and then branches grow on the trunks. Subsequently, smaller branches can grow on the branches (cooling is faster and temperature is lower at the corners). Grains that grow in a dendritic manner are called dendritic crystals or dendrites.
Control of Grain Size#
Influence of Grain Size on Metal Properties#
At room temperature, the smaller the grain size of a metal, the better its comprehensive mechanical properties. That is, refining the grain size of a metal can not only improve its strength and hardness, but also its plasticity and toughness.
Evaluation of Grain Size#
Grain Size#
An indicator for evaluating grain size. Generally expressed by the average diameter or average area of grains.
In industry, grain size is often represented by grain size grades. Standard grain size is divided into eight grades, with Grade 1 being the coarsest and Grade 8 being the finest.
Factors Affecting Grain Size#
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Nucleation rate N - The number of nuclei formed per unit volume of liquid metal per unit time.
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Growth rate G - The length of crystal growth per unit time.
The larger the ratio $ N/G $, the smaller the grain size obtained after crystallization, and vice versa.
Methods for Controlling Grain Size#
Control of Supercooling#
Measure: Lowering the pouring temperature.
Principle: Increase the cooling rate. A larger degree of supercooling leads to a larger ratio $ N/G $.
Modification Treatment#
Measure: Adding modifiers to the liquid metal.
Principle: Modifiers act as the core of heterogeneous nucleation, increasing the nucleation rate N.
Vibration Stirring#
Measure: Applying mechanical vibration, ultrasonic vibration, or electromagnetic stirring to the molten metal.
Principle: Through vibration stirring, dendrites in the growing crystals are broken, increasing the nucleation rate N, and inputting energy to promote spontaneous nucleation.
Polymorphous Transformation#
Polymorphous Phenomenon and Polymorphs#
Some metals exist in two or more crystal lattice types in the solid state, which is called polymorphism or polymorphic behavior.
The same metal existing in different crystal structures is called polymorphs of that metal.
Polymorphous Transformation#
Under certain conditions, polymorphs of the same metal can transform into each other. Polymorphous transformation is the phenomenon of one crystal lattice transforming into another crystal lattice with a change in temperature in the solid state.
Polymorphous transformation of pure iron:
$ \delta-Fe \Leftarrow^{1394^{\circ}C}\Rightarrow \gamma-Fe \Leftarrow^{1394^{\circ}C}\Rightarrow \alpha-Fe $
Characteristics of Polymorphous Transformation#
The polymorphous transformation of metals is similar to the crystallization process of liquid metals, so it is called secondary crystallization or recrystallization.
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Polymorphous transformation has a specific transformation temperature and requires supercooling to occur, releasing latent heat.
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Polymorphous transformation occurs in the solid state, so it requires a large degree of supercooling.
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The formation of polymorphs goes through the processes of nucleation and growth. New phase nuclei often form on the grain boundaries or specific crystal planes of the old phase.
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Polymorphs of the same metal usually have different lattice densities, so their molar volumes are different. The transformation often leads to changes in volume, resulting in the generation of internal stresses.
Molar volume: The volume occupied by a unit weight of a substance.
Crystallization of Metal Ingots#
1: Fine-grained zone
2: Columnar zone
3: Equiaxed zone
This article is also updated on xLog by Mix Space.
The original link is https://nishikori.tech/posts/tech/Basic-process-of-material-preparation