1、 Deformation hardening (or strain hardening, work hardening)

After yielding, with the increase of deformation degree, the strength and hardness of the material increase, while the plasticity and toughness decrease, which is called deformation strengthening or work hardening.

mechanism

With the progress of plastic deformation, the dislocation density increases continuously, so the mutual intersection of dislocations during movement is intensified, resulting in fixed cutting steps, dislocation entanglement and other obstacles, which increase the resistance of dislocation movement, cause the deformation resistance to increase, and cause difficulties in continuing plastic deformation, thus improving the strength of metal

Rule: the deformation degree increases, the strength and hardness of the material increase, the plasticity and toughness decrease, and the dislocation density continues to increase. According to the formula picture, we can know the strength and dislocation density ρ The bigger the Bernoulli vector b of the dislocation is, the more significant the strengthening effect is.

Method: Cold deformation, such as cold pressing, rolling, shot peening, etc. Example: Cold drawn steel wire can double its strength.

Practical significance of deformation strengthening (advantages and disadvantages)

(1) Benefits:

① Deformation strengthening is an effective method to strengthen metals. For some materials that cannot be strengthened by heat treatment, the strength of materials can be improved by deformation strengthening, which can double the strength.

② It is an important factor in the processing and forming of some workpieces or semi-finished products, which makes the metal deform evenly and makes the forming of workpieces or semi-finished products possible, such as the stamping of cold drawn steel wires and parts.

③ Deformation strengthening can also improve the safety of parts or components in the use process. When stress concentration or overload occurs in some parts of the parts, plastic deformation will occur there, and the deformation of overload parts will stop due to work hardening, thus improving the safety.

(2) Disadvantages:

① Deformation strengthening also brings troubles to the production and use of materials. Deformation increases the strength and reduces the plasticity. It is difficult to continue deformation and requires more power consumption.

② In order to allow the material to continue to deform, recrystallization annealing is required in the middle, so that the material can continue to deform without cracking, increasing the production cost.

2、 Solution strengthening

With the increase of solute atom content, the strength and hardness of solid solution increase, and the plasticity and toughness decrease.

mechanism

(1) The dissolution of solute atoms will distort the lattice of solid solution and hinder the dislocations moving on the slip surface.

(2) The Coriolis air mass formed by the solute atoms on the dislocation line plays a role of pinning the dislocation, increasing the resistance of dislocation movement.

(3) The segregation of solute atoms in the stacking fault region hinders the motion of extended dislocations. All factors that hinder dislocation movement and increase the resistance to dislocation movement can increase the strength.

law

① In the range of solid solution solubility, the greater the mass fraction of alloy elements, the greater the strengthening effect

② The larger the size difference between solute atoms and solvent atoms, the more significant the strengthening effect.

③ The strengthening effect of solute elements forming interstitial solid solution is greater than that of elements forming displacement solid solution

④ The greater the difference between the valence electron numbers of solute atoms and solvent atoms, the greater the strengthening effect.

Methods: Alloying, that is, adding alloying elements.

Example: The strength of copper nickel alloys is greater than that of pure copper and nickel metals.

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.

mechanism

Its principle lies in the retardation effect of grain boundary on dislocation slip. For polycrystals, the dislocation movement must overcome the resistance of the grain boundary. This is because the orientation of the dislocations on both sides of the grain boundary is different. Therefore, in a certain grain, the slipping dislocations cannot directly cross the grain boundary into the adjacent grains. Only when a large number of dislocations are piled up at the grain boundary, the stress concentration is caused, can the movement of the existing dislocations in the adjacent grains be stimulated to produce slip. Therefore, the finer the grains, the higher the strength of the material.

law

The finer the grain, the larger the grain boundary area. According to the picture of Hall Page formula, the smaller the average diameter d of the grain, the yield strength of the material σ The higher s

Methods for refining grains

① During crystallization, the grain can be refined by increasing undercooling, modification, vibration and stirring to increase nucleation rate;

② For the cold deformed metal, the grain can be refined by controlling the deformation degree and annealing temperature;

③ The grain can be refined by normalizing and annealing;

④ Alloying elements can be added to steel to form new phases to inhibit grain growth.

4、 Second phase strengthening

There are also one or several other phases in the metal matrix, which can improve the strength of the metal. Due to different processes for obtaining the second phase, the second phase strengthening can be divided into: ① precipitation strengthening: obtaining the second phase through phase change heat treatment; ② dispersion strengthening: obtaining the second phase through powder sintering or internal oxidation.

mechanism

Dislocation meets 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 dislocation and improves the strength of materials.

Example: The existence of cementite in steel improves the strength of steel.

Source: Material base

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