Strengthening of materials, deformation strengthening, solid solution strengthening, fine grain strengthening, second phase strengthening

I. Deformation strengthening (or strain strengthening, work hardening)
Definition
After the material has yielded, with the increase in the degree of deformation, the strength and hardness of the material increases and the plasticity and toughness decrease is called deformation strengthening or work hardening.

Mechanism
As plastic deformation proceeds, the density of dislocations increases, so that the dislocations in the movement of the mutual cut intensifies, the result is that the fixed cut step, dislocation entanglement and other obstacles, so that the resistance to dislocation movement increases, causing deformation resistance to increase, to continue plastic deformation caused by difficulties, thus increasing the strength of metal
Law: the degree of deformation increases, the strength of the material, hardness, plasticity, toughness, dislocation density increases, according to the formula picture, it can be seen that the strength and dislocation density ρ one-half of the square is proportional to the larger the dislocation of the Burns vector b, the more significant the strengthening effect.

Methods
Cold deformation, e.g. cold pressing, rolling, shot peening, etc.

Example
Cold drawing of steel wire increases its strength exponentially.

Practical implications of deformation strengthening (pros and cons)

(1) Advantages.
① Deformation strengthening is an effective method of strengthening metals. For some materials that cannot be strengthened by heat treatment, deformation strengthening can be used to improve the strength of the material and can increase the strength exponentially.
② is an important factor in the processing and forming of certain workpieces or semi-finished products, so that the metal is uniformly deformed, making it possible to shape workpieces or semi-finished products, such as cold-drawn steel wire, stamping and forming of parts.
③ deformation strengthening can also improve the safety of parts or components in the process of use, certain parts of the parts of the stress concentration or overload phenomenon, so that the plastic deformation of the place, due to work hardening so that the deformation of the overload parts stop thus improving the safety.

(2) Disadvantages.
The deformation increases strength and decreases plasticity, making further deformation difficult and requiring more power to be consumed.
(2) In order to allow the material to continue to deform, intermediate recrystallisation annealing is required so that the material can continue to deform without cracking, which increases production costs.
Picture
Picture


II. Solid solution strengthening


Definition
With the increase of solute atomic content, the solid solution strength and hardness increase, plasticity and toughness decrease is called solid solution strengthening.

Mechanism
(1) The dissolution of solute atoms distorts the lattice of the solid solution and prevents dislocations from moving on the slip surface.
(2) The formation of kerogen clusters of solute atoms on the dislocation line, which act as pins for the dislocations, increases the resistance to dislocation movement.
(3) The polarisation of solute atoms in the dislocation zone impedes the motion of extended dislocations. All factors that impede dislocation movement and increase the resistance to dislocation movement can lead to increased strength.

Laws
① The greater the mass fraction of alloying elements in the solid solution solubility range, the greater the strengthening effect
②The larger the difference in size between the solute and solvent atoms, the more significant the strengthening effect.
③ the strengthening effect of solute elements forming interstitial solid solutions is greater than that of elements forming replacement solid solutions
④The greater the difference in the number of valence electrons between the solute atom and the solvent atom, the greater the strengthening effect.

Method
Alloying, i.e. the addition of alloying elements.

Example
The strength of a copper-nickel alloy is greater than that of copper and nickel as pure metals.

III. Fine grain strengthening
Definition
The reduction in grain size increases the strength and hardness of a material and improves its plasticity and toughness, a phenomenon known as fine grain strengthening.

Mechanism
The principle lies in the hindering effect of grain boundaries on the slip of dislocations. For polycrystals, dislocation movement must overcome the resistance of the grain boundary, which is due to the different orientation of dislocations on both sides of the grain boundary, so in a grain, the slippage of dislocations can not directly across the grain boundary into the adjacent grain, only at the grain boundary after the accumulation of a large number of dislocations caused by stress concentration, in order to stimulate the movement of dislocations in the adjacent grain to produce slip. So the finer the grain, the higher the strength of the material.

The law
The finer the grain, the larger the grain boundary area, according to the Hall-Page formula picture, the smaller the average diameter of the grain d, the higher the yield strength of the material σs

Methods for refining grains
(i) The grain can be refined during crystallisation by increasing the subcooling, densification, vibration and stirring to increase the nucleation rate.
②For cold deformed metals the grain can be refined by controlling the degree of deformation and annealing temperature.
③ can refine the grain by normalising and annealing heat treatment methods.
④ Alloying elements can be added to the steel to form new phases thus inhibiting grain growth.

Fourth, the second phase of strengthening

Definition
The presence of one or several other phases in a metal matrix, the presence of which increases the strength of the metal. Depending on the process used to obtain the second phase, second phase strengthening is divided into: ① precipitation strengthening: the second phase is obtained by phase change heat treatment ② dispersion strengthening: the second phase is obtained by powder sintering or internal oxidation.

Mechanism
The dislocation encounters the second phase during its movement and needs to bypass or cut through the second phase, thus the second phase impedes the movement of the dislocation, resulting in an increase in the strength of the material.

Example
The presence of carburite in steel gives the steel its increased strength.
Disclaimers.

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