Plastic deformation and recrystallization of metals
The plasticity of metal is an important characteristic, and this characteristic has very important significance for the application of metal materials, for example.
After the metal undergoes plastic deformation, its shape and size have changed from a macro point of view, but this is not the content of our research. What we want to study is what changes have occurred in the internal structure and properties of the metal after deformation, and what changes have occurred in these structures and properties when heated, and finally how to use this knowledge to improve the properties of the material.
Section 1 Plastic deformation of metal
The main content of this section is from the atomic point of view, how does the plastic deformation of metal occur?
When an external force acts on the metal, such as tension, the distance between atoms in the metal becomes larger. If this change is within the elastic range, when the external force is removed, the atoms can return to their original state; if the external force is large, this This kind of change has reached the plastic stage. When the external force is removed, some of the changes cannot be restored, and the metal undergoes plastic deformation. As a limit, when the external force reaches a certain level, the bonding force between atoms is broken, and the metal is broken.
The main ways of metal plastic deformation are slip and twinning.
1. Plastic deformation of single crystal
This is a method adopted for the convenience of learning, because most of the actual metals are polycrystalline.
1. The definition and phenomenon of slip; (using wall charts and slides to explain), there are five questions here: slip line, slip belt, slip distance, slip surface and slip direction;
2. Slip system: a slip surface and a slip direction constitute a slip system. The number of slip systems determines the plasticity of the metal. The more the number, the better the plasticity;
Body-centered cubic lattice: 6×2=12 slip systems; low plasticity and good; such as iron, chromium, etc.;
Face-centered cubic lattice: 4×3=12 slip systems; the best plasticity; such as gold, silver, copper, aluminum, etc.;
Close-packed hexagonal lattice: 1×3=3 slip systems; plasticity is the worst; such as zinc, cadmium, etc.;
3. The mechanism of slip: non-rigid sliding, but realized by the movement of dislocations (proposed in 1934).
1) Only a few atoms near the dislocation line move;
2) The distance the atom moves is less than one atom distance;
Therefore, when slipping is realized by dislocation, the required force is small; for example. (flip chart)
Summary: The plastic deformation of metal is carried out by slipping, and slipping is realized by the movement of dislocations. Therefore, as long as the movement of dislocations is hindered, the progress of slip can be hindered, thereby increasing the resistance to plastic deformation and increasing the strength. The five strengthening methods commonly used in metal materials (solid solution strengthening, work hardening, grain refinement, dispersion strengthening, and quenching strengthening) are all realized through this mechanism.
2. Plastic deformation of polycrystals
The deformation of a single crystal is the same as that of a single crystal. The difference is that the polycrystal is composed of multiple crystals, and the orientation of each crystal is different, so the plastic deformation of the polycrystal does not occur at the same time, but proceeds step by step. . The resistance to plastic deformation is higher than that of single crystals.
The second issue to pay attention to is the influence of grain boundaries. Grain boundaries are places where atoms are arranged irregularly, which hinder the movement of dislocations. In order for dislocations to pass through the grain boundaries, the outside must exert greater force on it, so the strength at the grain boundaries is higher than that in the crystal.
The finer the crystal grains, the more grain boundary area per unit volume, the greater the hindering effect on dislocations, and the higher the strength of the metal. There is an empirical formula for the relationship between grain boundaries and strength (Hall-Petch formula):
Materials with fine grains not only have high strength, but also have high plasticity and toughness, which cannot be achieved by other strengthening methods.
We generally call the method of improving the properties of the material by making the structure of the material finer as grain refinement.
3. Changes in metal properties and structure after cold deformation
1. Changes in mechanical properties
As the amount of deformation increases, the strength and hardness of the metal after cold plastic deformation increase, but the plasticity and toughness decrease. This phenomenon is called work hardening.
Work hardening is very important in the use of metal materials. It is an important strengthening method for metal materials, especially for materials that cannot be heat treated, such as pure metals, high manganese steels and austenitic stainless steels. At the same time, work hardening enables the subordinate materials to be uniformly deformed during cold processing; in addition, when the material is accidentally overloaded, work hardening prevents the material from undergoing a large amount of plastic deformation and fracture, which increases safety.
Work hardening also has disadvantages in the use of materials. For example, it reduces the plasticity and must be carried out several times when deforming with a large amount of deformation, which increases consumption and reduces labor productivity.
2. Organizational changes
1) Form cold deformed fibrous structure;
2) The substructure is refined and the dislocation density increases, which is generally considered to be the cause of work hardening;
3) Form a deformed texture;
3. Cold deformation internal stress
Most of the work done during plastic deformation of the material is converted into heat, but a small part remains inside the material, which is the residual internal stress. Residual internal stress can lead to material deformation. If it is superimposed with the working stress of the material, it will accelerate the failure of the material, so it should be paid attention to.
1) Macroscopic internal stress: caused by uneven deformation in the macroscopic range. Sometimes this internal stress can be used to offset each other with the working stress and prolong the life of the parts. For example, the shot peening or rolling of a leaf spring will form a residual compressive stress on the surface of the spring, and the working stress on the surface —– -Tensile stress cancels each other out.
2) Microscopic internal stress: caused by uneven deformation between grains;
3) Internal stress of lattice distortion: the range of action is inside a crystal grain.
Section 2 The transformation of cold deformed metal when heated
First review the changes in metal structure and properties after cold deformation (question), three points:
1. Work hardening;
2. Cold deformed fibrous structure;
3. Deformation residual internal stress;
It is derived that cold deformed metal is in an unstable state and will change significantly when heated. This process is divided into three stages. That is, recovery, recrystallization and grain growth.
It occurs when the heating temperature is low. At this stage, because the temperature is not high, the atomic activity is small, and it cannot be diffused in a large range. Only the number of point defects such as vacancies and interstitial atoms is reduced, and the dislocations are rearranged. The organization and performance have not changed much. The main reason is that the internal stress is greatly reduced, the resistance is reduced, and the corrosion resistance is improved.
The recovery is mainly used to eliminate the residual internal stress in the cold deformed metal, to hide the size, reduce the resistance, and improve the corrosion resistance. Such as the shaping of cold coil springs, the stress relief of brass shells, etc.
Occurs at higher temperatures, about 450°C or higher for steel. At this time, the mobility of atoms is greatly enhanced, crystal defects are eliminated, recrystallization occurs, work hardening is eliminated, and the cold-deformed fiber structure becomes polygonal again until the axis grains, that is, the structure and performance of the material are restored to those before cold deformation. status.
The recrystallization process is also a process of nucleation and growth, but the crystal structure of the material has not changed, so it is not a phase transition process.
1. Recrystallization temperature
The minimum temperature at which metal recrystallization occurs is called recrystallization temperature, which is related to the degree of deformation and composition of the metal. The greater the degree of deformation, the lower the recrystallization temperature. Elements such as W, V, Ti, and Cr can increase the recrystallization temperature. Generally, the recrystallization temperature of the material is determined according to an empirical formula:
T then = 0.4×T0 (melting point of metal)
According to this formula, the recrystallization temperature of iron can be calculated to be about 450℃. The recrystallization temperature of other metals can be consulted in related manuals.
2. Application of recrystallization
Recrystallization is generally used to eliminate work hardening and restore plasticity. In production, this treatment method is called recrystallization annealing.
3. Grain growth
No, students will learn by themselves.
Section 3 Thermal Deformation Processing
Cold deformation processing: cold drawing, cold drawing, cold rolling, cold stamping, etc.;
Hot deformation processing: hot rolling, forging, etc.;
1. The difference between cold and hot deformation processing
From the perspective of metal science, cold and hot deformation processing is limited by the recrystallization temperature. The deformation below the recrystallization temperature is called cold deformation; the deformation above the recrystallization temperature is called hot deformation. Metals such as tin, lead and other metals have a low recrystallization temperature, and their deformation at room temperature is considered to be hot deformation processing; while the recrystallization temperature of tungsten is as high as 1200℃, then the deformation at 1000℃ is still cold. Deformation processing.
Another issue to note is that during thermal deformation processing, the change in material properties is bidirectional, because at the same time as work hardening, recrystallization also occurs, that is, hardening due to deformation and softening due to recrystallization. At the same time. Which aspect dominates depends on the specific conditions of deformation and heating temperature.
2. Changes in metal structure and properties during thermal deformation
1. It can eliminate some organizational defects and make the organization more dense;
2. Streamline: Some impurities and inclusions in the metal are formed by extending along the deformation direction. If the streamline is distributed along the contour of the part, the performance of the material can be improved. Therefore, some important parts are formed by forging.
3. Banded structure: Ferrite or cementite in steel nucleates with elongated impurities as the core to form a banded structure. Banded organization can cause material anisotropy, which should be paid attention to.