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 difference in size 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.
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.
The interstitial solute atom has a greater solid solution strengthening effect than the replacement atom.
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.
Yield strength, tensile strength and hardness are stronger than pure metal; in most cases, ductility is lower than pure metal; electrical conductivity is much lower than pure metal; creep resistance, or strength loss at high temperature, can be improved by solid solution strengthening.
With the increase of the degree 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.
The main results are as follows: (1) the intersection between dislocations occurs, and the cut order hinders the movement of dislocations.
(2) the reaction between dislocations occurs, and the fixed dislocations hinder the movement of dislocations.
(3) dislocation multiplication, 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 and increase the cutting force.
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.
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 order to make the surface of the workpiece brittle and hard, increase the cutting force and accelerate the tool wear during re-cutting.
The method of improving the mechanical properties of metal materials by refining grains is called fine grain strengthening. In industry, the strength of materials can be improved by refining grains.
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 grains. 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, which is not conducive to 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. According to τ = ntau 0, the stress concentration is smaller, so the strength of the material is higher.
According to the strengthening law of fine grain strengthening, the more the grain boundary is, the finer the grain is. According to the Hall-Paiqi relation, the smaller the average grain (d) is, the higher the yield strength of the material is.
4. 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.