01. Solid solution strengthening
The phenomenon that the solid solution of alloy elements in the matrix metal causes a certain degree of lattice distortion so as to improve the strength of the alloy.
The solute atoms dissolved in the solid solution cause lattice distortion, which increases the resistance of dislocation movement and makes it difficult to slip, thus increasing the strength and hardness of the alloy solid solution.
This phenomenon of metal strengthening by dissolving into a certain solute element to form a solid solution is called solid solution strengthening.
When the concentration of solute atom is appropriate, the strength and hardness of the material can be improved, but its toughness and plasticity decrease.
The higher the atomic fraction of solute atoms, the greater the strengthening effect, especially when the atomic fraction is very low, the strengthening effect is more significant.
The greater the difference of the atomic size between the solute atom and the matrix metal, the greater the strengthening effect.
The interstitial solute atom has a greater solid solution strengthening effect than the replacement atom, and because the lattice distortion of the interstitial atom in the body-centered cubic crystal is asymmetric, its strengthening effect is greater than that of the face-centered cubic crystal, but the solid solubility of the interstitial atom is very limited, so the actual strengthening effect is also limited.
The greater the difference of the number of valence electrons between the solute atom and the matrix metal, the more obvious the solid solution strengthening effect is, that is, the yield strength of the solid solution increases with the increase of valence electron concentration.
The degree of solid solution strengthening mainly depends on the following factors:
(1) The size difference between matrix atoms and solute atoms.
The larger the size difference is, the greater the interference of the original crystal structure is, and the more difficult the dislocation slip is.
(2) The amount of alloy elements.
The more alloy elements are added, the greater the strengthening effect is.
If you add too many atoms that are too large or too small, it will exceed the solubility.
This involves another strengthening mechanism, dispersion phase strengthening.
(3) The solid solution strengthening effect of interstitial solute atoms is greater than that of replacement atoms.
(4) The greater the difference in the number of valence electrons between the solute atom and the matrix metal, the more obvious the solid solution strengthening effect.
The yield strength, tensile strength and hardness are stronger than those of pure metal.
In most cases, the ductility is lower than that of pure metal.
The electrical conductivity is much lower than that of pure metal.
Creep resistance, or strength loss at high temperature, can be improved by solid solution strengthening.
02. Work hardening
With the increase of cold deformation, the strength and hardness of metal materials increase, but the plasticity and toughness decrease.
The phenomenon that the strength and hardness of metal materials increase while the plasticity and toughness decrease during plastic deformation below the recrystallization temperature.
Also known as cold work hardening.
The reason is that during the plastic deformation of the metal, the grain slips, the entanglement of dislocation occurs, which makes the grain elongate, break and fibrosis, and the residual stress occurs in the metal.
The degree of work hardening is usually expressed by the ratio of microhardness of the surface layer after processing to that before processing and the depth of the hardened layer.
From the perspective of dislocation theory:
(1) The dislocations intersect, and the resulting cut order hinders the movement of dislocations;
(2) The fixed dislocations formed by the reaction between dislocations hinder the movement of dislocations;
(3) Dislocation proliferation occurs, and the increase of dislocation density further increases the resistance of dislocation movement.
Work hardening brings difficulties to the further processing of metal parts.
For example, in the process of cold rolling, the steel plate will be rolled harder and harder so that it can not be rolled, so it is necessary to arrange intermediate annealing in the processing process to eliminate its work hardening by heating.
For example, in the cutting process, the surface of the workpiece is brittle and hard, so as to accelerate the tool wear, increase the cutting force and so on.
It can improve the strength, hardness and wear resistance of metals, especially for those pure metals and some alloys which can not improve the strength by heat treatment.
Such as cold-drawn high-strength steel wire and cold-coiled spring, cold working deformation is used to improve its strength and elastic limit.
For example, the crawler of tank and tractor, the jaw plate of crusher and the turnout of railway also use work hardening to improve its hardness and wear resistance.
Role in mechanical engineering
The surface strength of metal materials, parts and components can be significantly improved by cold drawing, rolling and shot peening (see surface strengthening).
After the parts are subjected to stress, the local stress in some parts often exceeds the yield limit of the material, resulting in plastic deformation. because work hardening limits the continued development of plastic deformation, the safety of parts and components can be improved.
When a metal part or component is stamped, its plastic deformation is accompanied by strengthening, so that the deformation is transferred to the unworked hardened part around it.
After such repeated alternating action, the cold stamping parts with uniform cross section deformation can be obtained.
The cutting performance of low carbon steel can be improved and the chips can be easily separated.
However, work hardening also brings difficulties to the further processing of metal parts.
Such as cold-drawn steel wire, due to work-hardening, the further drawing consumes a lot of energy, and even is broken, so it must be annealed in the middle to eliminate work-hardening and then drawn.
For example, in the cutting process, in order to make the surface of the workpiece brittle and hard, increase the cutting force and accelerate the tool wear during re-cutting.
03. Fine grain strengthening
The method of improving the mechanical properties of metal materials by refining grains is called fine grain strengthening.
In industry, grain refinement is used to improve the strength of materials.
Usually, metals are polycrystals composed of many grains, and the size of grains can be expressed by the number of grains per unit volume.
The more the number, the finer the grain.
The experimental results show that fine-grained metals have higher strength, hardness, plasticity and toughness than coarse-grained metals at room temperature.
This is because the plastic deformation of fine grains caused by external force can be dispersed in more grains, the plastic deformation is more uniform, and the stress concentration is smaller.
In addition, the finer the grain is, the larger the grain boundary area is and the more tortuous the grain boundary is, the more disadvantageous to the crack propagation.
Therefore, in industry, the method of improving material strength by refining grains is called fine grain strengthening.
The finer the grain is, the smaller the number of dislocations (n) in the dislocation cluster is, the smaller the stress concentration is, and the higher the strength of the material is.
According to the strengthening law of fine grain strengthening, the more grain boundaries are, the finer the grains are.
According to the Hall-Paige relation, the smaller the average value of grains (d), the higher the yield strength of the material.
The method of grain refinement:
Vibration and stirring.
For cold deformed metals, the grains can be refined by controlling the degree of deformation and annealing temperature.
04. Second phase strengthening
In addition to the matrix phase, there is a second phase in multiphase alloys compared with single-phase alloys.
When the second phase is uniformly distributed in the matrix phase with fine dispersed particles, it will have a significant strengthening effect.
This strengthening is called second phase strengthening.
For the movement of dislocations, the second phase contained in the alloy has the following two cases:
(1) Strengthening effect of non deformable particles (bypass mechanism).
(2) Strengthening effect of deformable particles (cutting mechanism).
Both dispersion strengthening and precipitation strengthening belong to the special case of second phase strengthening.
The main reason for the strengthening of the second phase is the interaction between them and dislocations, which hinders the movement of dislocations and improves the deformation resistance of the alloy.
The most important factors affecting the strength are the composition, microstructure and surface state of the material itself.
The second is the stress state, such as the speed of adding force, the mode of loading, whether it is simple tension or repeated force, will show different strength.
In addition, the geometric shape and size of the sample and the test medium also have a great influence, sometimes even decisive, for example, the tensile strength of ultra-high strength steel in hydrogen atmosphere may decrease exponentially.
There are only two ways to strengthen metal materials, one is to improve the interatomic binding force of the alloy, to improve its theoretical strength, and to prepare defect-free complete crystals, such as whiskers.
It is known that the strength of iron whiskers is close to the theoretical value, which can be considered to be because there are no dislocations in the whiskers, or only a small number of dislocations that cannot be multiplied during deformation.
Unfortunately, when the diameter of the whisker is larger, the strength will decrease sharply.
Another strengthening way is to introduce a large number of crystal defects into the crystal, such as dislocations, point defects, heterogeneous atoms, grain boundaries, highly dispersed particles or inhomogeneity (such as segregation).
These defects hinder the movement of dislocations and significantly improve the strength of the metal.
Facts have proved that this is the most effective way to improve the strength of metals.
For engineering materials, it is generally through the comprehensive strengthening effect to achieve better comprehensive properties.