1. Deformation hardening (or strain hardening, work hardening)
After yielding, with the increase of deformation degree, the strength and hardness of the material increase, and the plasticity and toughness decrease.
This phenomenon is called deformation hardening or work hardening.
With the progress of plastic deformation, the dislocation density increases continuously, so the mutual delivery of dislocations during movement intensifies, resulting in the generation of fixed cutting steps, dislocation entanglement and other obstacles, which increase the resistance of dislocation movement and cause the increase of deformation resistance, which makes it difficult to continue plastic deformation, thus improving the strength law of metal: the degree of deformation increases, the strength and hardness of the material increases, the plasticity and toughness decrease, and the dislocation density increases.
According to the formula,
It can be seen that the strength is directly proportional to the half power of the dislocation density ρ, and the larger the Burt vector B of the dislocation, the more obvious the strengthening effect is.
Cold deformation, such as cold pressing, rolling, shot peening, etc.
Cold drawn steel wire can double its strength.
5. Practical significance of deformation strengthening (advantages and disadvantages)
① Deformation strengthening is an effective method to strengthen metals.
For some materials that cannot be strengthened by heat treatment, the strength of the materials can be improved by deformation strengthening, which can increase the strength by several times.
② It is an important factor in the processing and forming of some workpieces or semi-finished products.
It can make the metal deform uniformly and make the forming of workpieces or semi-finished products possible, such as cold drawn steel wires and stamping of parts.
③ Deformation strengthening can also improve the safety of parts or components in the process of use.
When stress concentration or overload occurs in some parts of parts, plastic deformation will occur.
The deformation of overload parts will be stopped due to work hardening, thus improving the safety.
① Deformation strengthening also brings trouble to the production and use of materials. Deformation increases the strength and reduces the plasticity, and it is difficult to continue to deform, which requires more power.
② In order to enable the material to continue to deform, recrystallization annealing is required in the middle, so that the material can continue to deform without cracking, which increases the production cost.
2. Solid solution strengthening
With the increase of solute atom content, the strength and hardness of solid solution increase, and the plasticity and toughness decrease, which is called solid solution strengthening.
(1) The lattice of the solid solution is distorted by the solute atoms, which hinders the dislocations moving on the slip plane.
(2) The Kirschner gas mass formed by the solute atoms on the dislocation line has a pinning effect on the dislocation and increases the resistance of dislocation movement.
(3) The segregation of solute atoms in the stacking fault region hinders the movement of spreading dislocations.
All factors that hinder the movement of dislocations and increase the resistance of dislocation movement can improve the strength.
① In the range of solid solution solubility, the larger the mass fraction of alloying elements, the greater the strengthening effect
② The larger the size difference between solute atoms and solvent atoms, the more remarkable the strengthening effect;
③ The strengthening effect of solute elements forming interstitial solid solution is greater than that of elements forming displaced solid solution;
④ The larger the difference between the valence electron numbers of solute atoms and solvent atoms, the greater the strengthening effect.
Alloying is the addition of alloying elements.
The strength of the copper nickel alloy is greater than that of the pure metals of copper and nickel.
3. Fine grain strengthening
With the decrease of grain size, the strength and hardness of the material increase, and the plasticity and toughness are also improved, which is called fine grain strengthening.
The principle lies in the retardation effect of grain boundary on dislocation slip.
For polycrystals, the movement of dislocations must overcome the resistance of grain boundaries. This is because the orientations of dislocations on both sides of the grain boundaries are different. Therefore, in a certain grain, the sliding dislocations cannot directly cross the grain boundaries and enter the adjacent grains.
Only when a large number of dislocations are accumulated at the grain boundaries and stress concentration is caused, can the movement of existing dislocations in the adjacent grains be excited to generate slippage.
Therefore, the finer the grain, the higher the strength of the material.
The finer the grain, the larger the grain boundary area.
According to hall page formula,
the smaller the average diameter d of the grain, the higher the yield strength σs of the material.
4. Method of grain refinement
① During the crystallization process, the grain can be refined by increasing the undercooling degree, modifying treatment, vibration and stirring to increase the nucleation rate;
② For the cold deformed metal, the grain can be refined by controlling the deformation degree and annealing temperature;
③ Grain can be refined by normalizing and annealing heat treatment;
④ Alloy elements can be added to the steel to form a new phase to inhibit grain growth.
4. Second phase strengthening
There are also one or several other phases in the metal matrix, and the presence of these phases improves the strength of the metal.
Due to different processes for obtaining the second phase, the second phase strengthening is divided into:
① Precipitation strengthening: the second phase is obtained by phase transformation heat treatment;
② Dispersion strengthening: the second phase is obtained by powder sintering or internal oxidation.
The dislocation encounters the second phase in the process of movement and needs to bypass or cut through the second phase, so the second phase hinders the movement of the dislocation and improves the strength of the material.
The strength of steel is improved by the presence of cementite in steel.