Four Ways to Strengthen Metal Materials

The strengthening of metal materials is a process of improving the strength and hardness of metals through a series of processes and methods. These strengthening mechanisms are based on the microstructure and properties of metals and are achieved by regulating them.

The strengthening of metal materials has the following main uses:

1. Improving mechanical properties: Through strengthening treatment, the strength and hardness of metal materials are improved, which directly enhances the durability and reliability of materials in applications.
2. Optimizing product performance: Different strengthening technologies can optimize the performance of materials in different industrial applications, such as aerospace, automobile manufacturing, and high-performance mechanical equipment.
3. Increasing safety: In parts where stress is concentrated or overloaded, strengthening materials can prevent further deformation, thereby improving safety in use.
4. Promoting industrial development: The development and application of strengthening technology not only promotes the advancement of materials science, but also provides a solid foundation for various industrial applications.
5. Improving economic value: By improving the performance and reliability of materials, strengthening technology helps to reduce maintenance costs and improve production efficiency, thereby bringing economic benefits.

Solid solution strengthening

1. Definition The phenomenon that alloying elements are dissolved in the base metal to cause a certain degree of lattice distortion, thereby improving the strength of the alloy.
2. Principle The solute atoms dissolved in the solid solution cause lattice distortion, which increases the resistance to dislocation movement and makes it difficult to slip, thereby increasing the strength and hardness of the alloy solid solution. This phenomenon of strengthening the metal by dissolving a certain solute element to form a solid solution is called solid solution strengthening. When the concentration of solute atoms is appropriate, the strength and hardness of the material can be improved, while its toughness and plasticity are reduced.
3. Influencing factors The higher the atomic fraction of the 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 in atomic size between the solute atoms and the matrix metal, the greater the strengthening effect. Interstitial solute atoms have a greater solid solution strengthening effect than substitutional atoms, and because the lattice distortion of interstitial atoms in body-centered cubic crystals is asymmetric, their strengthening effect is greater than that of face-centered cubic crystals; but the solid solubility of interstitial atoms is very limited, so the actual strengthening effect is also limited. The greater the difference between the number of valence electrons of the solute atoms and the matrix metal, the more obvious the solid solution strengthening effect, that is, the yield strength of the solid solution increases with the increase of the valence electron concentration.
4. The degree of solid solution strengthening depends mainly on the following factors: The size difference between the matrix atoms and the solute atoms. The greater the size difference, the greater the disturbance to the original crystal structure and the more difficult the dislocation slip. The amount of alloying elements. The more alloying elements are added, the greater the strengthening effect. If too many atoms that are too large or too small are added, the solubility will be exceeded. This involves another strengthening mechanism, dispersed phase strengthening. Interstitial solute atoms have a greater solid solution strengthening effect than substitutional atoms. The greater the difference between the number of valence electrons of the solute atoms and the matrix metal, the more significant the solid solution strengthening effect.
5. Effects Yield strength, tensile strength and hardness are stronger than pure metals; in most cases, ductility is lower than pure metals; conductivity is much lower than pure metals; creep resistance, or strength loss at high temperatures, can be improved by solid solution strengthening.

Work hardening

1. Definition As the degree of cold deformation increases, the strength and hardness of metal materials increase, but the plasticity and toughness decrease.
2. Introduction The phenomenon that the strength and hardness of metal materials increase while the plasticity and toughness decrease when they are plastically deformed below the recrystallization temperature. It is also called cold work hardening. The reason for this is that when metals are plastically deformed, the grains slip, dislocations become entangled, the grains are elongated, broken and fibrous, and residual stress is generated inside the metal. The degree of work hardening is usually expressed by the ratio of the microhardness of the surface layer before and after processing and the depth of the hardened layer.
3. Explanation from the perspective of dislocation theory

(1) Intersection occurs between dislocations, and the steps produced hinder the movement of dislocations;

(2) Reactions occur between dislocations, and the fixed dislocations formed hinder the movement of dislocations;

(3) Dislocations multiply, and the increase in dislocation density further increases the resistance to dislocation movement.

4. Hazards Work hardening makes it difficult to further process metal parts. For example, in the process of cold rolling steel sheets, the steel sheets will become harder and harder until they cannot be rolled. Therefore, it is necessary to arrange intermediate annealing during the processing to eliminate the work hardening by heating. Another example is that in the cutting process, the surface of the workpiece is made brittle and hard, thereby accelerating the wear of the tool and increasing the cutting force.
5. Benefits It can improve the strength, hardness and wear resistance of metals, especially for pure metals and certain alloys that cannot be improved by heat treatment. For example, cold-drawn high-strength steel wire and cold-rolled springs use cold working deformation to improve their strength and elastic limit. For example, the tracks of tanks and tractors, the jaw plates of crushers and railway switches also use work hardening to improve their hardness and wear resistance.
6. Role in Mechanical Engineering Through processes such as cold drawing, rolling and shot peening (see surface strengthening), the surface strength of metal materials, parts and components can be significantly improved; after the parts are subjected to force, the local stress in some parts often exceeds the yield limit of the material, causing plastic deformation. Since work hardening limits the continued development of plastic deformation, the safety of parts and components can be improved; when metal parts or components are stamped, their plastic deformation is accompanied by strengthening, so that the deformation is transferred to the surrounding unhardened parts. After such repeated alternating actions, cold stamping parts with uniform cross-sectional deformation can be obtained; the cutting performance of low-carbon steel can be improved, making it easy to separate chips. However, work hardening also brings difficulties to the further processing of metal parts. For example, cold-drawn steel wire, due to work hardening, further drawing consumes a lot of energy and may even be broken, so it must be annealed in the middle to eliminate work hardening before drawing. Another example is in cutting processing, in order to make the surface of the workpiece brittle and hard, the cutting force is increased during cutting, and the tool wear is accelerated.

Grain refinement strengthening

1. Definition The method of improving the mechanical properties of metal materials by refining the grains is called grain refinement strengthening. In industry, grain refinement is used to improve the strength of materials.
2. Principle Metals are usually polycrystalline composed of many grains. The size of the grains can be expressed by the number of grains per unit volume. The more the number, the finer the grains. Experiments show that fine-grained metals at room temperature have higher strength, hardness, plasticity and toughness than coarse-grained metals. This is because the plastic deformation of fine grains under external forces can be dispersed in more grains, the plastic deformation is more uniform, and the stress concentration is smaller; in addition, the finer the grains, the larger the grain boundary area, the more tortuous the grain boundary, and the less conducive to the expansion of cracks. Therefore, in industry, the method of improving the strength of materials by refining the grains is called grain refinement strengthening.
3. Effect The finer the grains, the smaller the number of dislocations (n) in the dislocation cluster. According to τ=nτ0, the stress concentration is smaller, so the strength of the material is higher. According to the strengthening law of fine grain strengthening, the more grain boundaries there are, the finer the grains are. According to the Hall-Petch relationship, the smaller the average value of the grains (d), the higher the yield strength of the material.
4. Methods for grain refinement Increase the degree of supercooling; Modification; Vibration and stirring; For cold-deformed metals, the grains can be refined by controlling the degree of deformation and annealing temperature.

Second phase strengthening

1. Definition Compared with single-phase alloys, multiphase alloys have a second phase in addition to the matrix phase. When the second phase is uniformly distributed in the matrix phase as fine dispersed particles, it will produce a significant strengthening effect. This strengthening effect is called second phase strengthening.
2. Classification For the movement of dislocations, the second phase contained in the alloy has the following two situations: (1) Strengthening effect of non-deformable particles (bypass mechanism).(2) Strengthening effect of deformable particles (cut-through mechanism). Dispersion strengthening and precipitation strengthening are both special cases of second phase strengthening.

3. Effect 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. Summary

The most important factors affecting strength are the composition, structure and surface state of the material itself; the second is the stress state, such as the speed of force application, the loading method, whether it is simple stretching or repeated force, which will show different strengths; in addition, the geometry and size of the specimen and the test medium also have a great influence, sometimes even decisive, such as the tensile strength of ultra-high strength steel in a hydrogen atmosphere may decrease exponentially.

There are only two ways to strengthen metal materials. One is to improve the interatomic bonding force of the alloy, improve its theoretical strength, and produce defect-free complete crystals, such as whiskers. It is known that the strength of iron whiskers is close to the theoretical value. It can be considered that this is 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 whiskers is large, the strength will drop sharply.

Another strengthening method 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 inhomogeneities (such as segregation). These defects hinder the movement of dislocations and can also significantly increase the strength of the metal. Facts have proved that this is the most effective way to increase the strength of metals. For engineering materials, generally, better comprehensive performance is achieved through comprehensive strengthening effects.