I. Deformation strengthening (or strain strengthening, work hardening)
- After the material yield, with the increase in the degree of deformation, the material strength, hardness, plasticity, toughness decline in the phenomenon called deformation strengthening or work hardening.
- With the plastic deformation, the dislocation density is increasing, so the dislocations in the movement of the mutual cross-cutting intensified, the result that the production of fixed cut steps, dislocations entanglement and other obstacles to the movement of dislocations to increase the resistance, caused by the increase in the deformation resistance to continue the plastic deformation of the difficulties caused by the deformation of the metal to improve the strength of the law: deformation increases, the material’s strength, hardness, plasticity, toughness decreases, the dislocation density is constantly Increase, according to the formula, it can be seen that the strength and dislocation density ρ one-half of the second power is proportional to the larger the dislocation’s Burr’s vector b, the more significant the strengthening effect.Cold deformation can be used, such as cold pressing, tumbling, and shot peening.
Practical implications of deformation enhancement (pros and cons)
(1) advantage:
①deformation strengthening is an effective way to strengthen the metal, for some materials that can not be strengthened by heat treatment, you can use deformation strengthening methods to improve the strength of the material, which can multiply the strength.
②is an important factor in the processing of certain workpieces or semi-finished products molding, so that the uniform deformation of the metal, so that the workpiece or semi-finished product molding possible, such as cold-drawn steel wire, parts of the stamping molding.
③deformation strengthening can also improve the safety of parts or components in the process of use, some parts of the parts of the stress concentration or overload phenomenon, so that the plastic deformation of the place, due to hardening of the deformation of the overloaded parts of the stop and thus improve safety.
(2) Disadvantages:
①Deformation strengthening also brings trouble to the production and use of materials, deformation increases the strength and reduces the plasticity, so it is difficult to continue to deform, and more power consumption is required.
②In order to allow the material to continue to deform, it is necessary to carry out recrystallization annealing in the middle, so that the material can continue to deform without cracking, increasing production costs.
II. Solid solution strengthening
- With the increase of solute atom content, the solid solution strength, hardness increases, plasticity, toughness decreases the phenomenon called solid solution strengthening.
- The dissolution of solute atoms distorts the lattice of the solid solution and has a hindering effect on the dislocations moving on the slip surface.The Kirchhoff’s air mass formed by the solute atoms polarized on the dislocation line acts as a pinning effect on the dislocations, which increases the resistance to dislocation motion. Polarization of solute atoms in the laminar dislocation region hinders the motion of extended dislocations. All factors that hinder dislocation motion and increase the resistance to dislocation movement can lead to increased strength.
- The greater the mass fraction of alloying elements within the solid solution solubility range, the greater the strengthening effect. The larger the size difference between solute and solvent atoms, the more significant the strengthening effect. The strengthening effect of solute elements forming interstitial solid solutions is greater than that of elements forming replacement solid solutions. The greater the difference in the number of valence electrons between solute and solvent atoms, the greater the strengthening effect.It can be alloyed, i.e. adding alloying elements.
III. Fine grain strengthening
- With the reduction of grain size, the material’s strength, hardness, plasticity, toughness is also improved the phenomenon known as fine grain strengthening.
- The principle lies in the blocking effect of grain boundaries on dislocation slip. For polycrystalline, dislocation movement must overcome the resistance of the grain boundary, which is due to the orientation of the dislocations on both sides of the grain boundary is different, so in a grain, slip dislocations can not be directly across the grain boundary into the neighboring grains, only in the grain boundary at the accumulation of a large number of dislocations caused by the concentration of stress, in order to stimulate the movement of dislocations in the adjacent grains have been generated slip. So the finer the grain, the higher the strength of the material.
- The finer the grain, the larger the grain boundary area, and according to the Hall-Page formula, the smaller the average diameter d of the grain, the higher the material yield strength σs
- We can refine the grain in this way, the crystallization process can be increased by increasing the degree of subcooling, metamorphic treatment, vibration and stirring to increase the rate of nucleation to refine the grain; for cold deformation of the metal can be controlled by the degree of deformation, annealing temperature to refine the grain; can be through the normalizing, annealing heat treatment methods to refine the grain; can be added to the steel alloying elements, the formation of new phases and thus inhibit the growth of the grain.
IV. Second-phase strengthening
- In the metal matrix there exists one or several other phases, the presence of which increases the strength of the metal. Due to the different processes for obtaining the second phase, the second phase strengthening is divided into:
①Precipitation strengthening: obtaining the second phase by phase change heat treatment
②Dispersive strengthening: obtaining the second phase by powder sintering or internal oxidation.
- The dislocations encounter a second phase during their movement, which needs to be bypassed or cut through, thus the second phase hinders the movement of the dislocations and makes the material stronger.The presence of carburizers in steel gives an increase in the strength of the steel.
