Four processes that make metals stronger

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01 Solid solution strengthening >>>> Definition: The phenomenon in which alloying elements dissolve in the matrix metal, causing a certain degree of lattice distortion and thereby increasing the strength of the alloy. >>>> Principle: The solute atoms dissolved in the solid solution cause lattice distortion, and this lattice distortion increases the resistance to dislocation movement, making slip difficult to occur; as a result, the strength and hardness of the alloy solid solution increase. The phenomenon of strengthening a metal by forming a solid solution through the dissolution of a certain solute element is known as solid solution strengthening. When the concentration of solute atoms is appropriate, the strength and hardness of the material can be increased, whereas its toughness and ductility decrease. >>>> Influencing factors: The higher the atomic fraction of the solute atoms, the greater the strengthening effect; this effect is particularly significant when the atomic fraction is low. The greater the difference in atomic size between the solute atoms and those of the matrix metal, the greater the strengthening effect. Interstitial solute atoms exert a greater solid solution strengthening effect than substitutional atoms; moreover, since the lattice distortion caused by interstitial atoms in body-centered cubic crystals is asymmetric, their strengthening effect is greater than that in face-centered cubic crystals ； However, the solubility of interstitial atoms is very limited, so the actual strengthening effect is also limited. The greater the difference in the number of valence electrons between the solute atoms and the matrix metal, the more pronounced the solid solution strengthening effect; that is, the yield strength of the solid solution increases as the valence electron concentration rises. >>>> The degree of solid solution strengthening mainly depends on the following factors: (1) the size difference between the matrix atoms and the solute atoms. The greater the size difference, the more disturbance the original crystal structure suffers, and the more difficult dislocation slip becomes. (2) 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 reinforcement mechanism, dispersion phase reinforcement. (3) Interstitial solute atoms have a greater solid solution strengthening effect than substitutional atoms. (4) The greater the difference in the number of valence electrons between the solute atoms and the matrix metal, the more significant the solid solution strengthening effect. >>>> Effects: Yield strength, tensile strength, and hardness are all greater than those of pure metals ； In most cases, its ductility is lower than that of pure metals ； Its electrical conductivity is much lower than that of pure metals ； Creep resistance, or strength loss at high temperatures, can be improved through solid solution strengthening. 02 Work hardening >>>> Definition: As the degree of cold deformation increases, the strength and hardness of metal materials rise, but their plasticity and toughness decrease. >>>> Introduction: The phenomenon in which metallic materials experience an increase in strength and hardness, along with a decrease in plasticity and toughness, when subjected to plastic deformation below their recrystallization temperature. Also known as cold work hardening. The reason for this is that during plastic deformation of metal, grain sliding occurs, leading to the entanglement of dislocations; as a result, the grains are elongated, fractured, and fibrousized, and residual stresses are generated within the metal. The degree of work hardening is usually expressed by the ratio of the microhardness of the surface layer after processing to that before processing, as well as by the depth of the hardened layer. >>>> Explanation from the perspective of dislocation theory: (1) Intersections between dislocations create step boundaries that hinder the movement of dislocations ； (2) Reactions occur between dislocations, and the resulting fixed dislocations hinder dislocation motion ； (3) Dislocations proliferate, and the increase in dislocation density further increases the resistance to dislocation motion. >>>> Hazard: Work hardening makes further processing of metal parts difficult. During the cold rolling of steel sheets, they become increasingly hard as rolling continues, to the point where further rolling becomes impossible; therefore, intermediate annealing is necessary during the processing stage to eliminate the work hardening through heating. For example, in cutting processes, it makes the surface layer of the workpiece brittle and hard, thereby accelerating tool wear and increasing cutting forces. >>>> Advantages: It can increase the strength, hardness, and wear resistance of metals, which is particularly important for pure metals and certain alloys that cannot have their strength enhanced through heat treatment methods. Materials such as cold-drawn high-strength steel wires and cold-rolled springs utilize cold working deformation to enhance their strength and elastic limit. Similarly, the tracks of tanks and tractors, the jaws of crushers, and railway switches also utilize work hardening to improve their hardness and wear resistance. >>>> Role in mechanical engineering: Through processes such as cold drawing, rolling, and shot blasting (see surface strengthening), the surface strength of metal materials, parts, and components can be significantly improved ； When a part is under stress, the local stress in certain areas often exceeds the material’s yield limit, leading to plastic deformation. Since work hardening limits the further progression of this plastic deformation, it is possible to improve the safety of the parts and components ； When metal parts or components are stamped, strengthening occurs at the sites of plastic deformation, causing the deformation to be transferred to the surrounding unstrained portions. Through such repeated alternating actions, cold-stamped parts with uniform cross-sectional deformation can be obtained ； It can improve the cutting performance of low-carbon steel, making it easier to separate the chips. However, work hardening also makes further processing of metal parts difficult. In the case of cold-drawn steel wires, work hardening results in high energy consumption for further drawing, and the wires may even break; therefore, intermediate annealing is necessary to eliminate work hardening before proceeding with drawing. For example, in cutting processes, to make the surface layer of the workpiece brittle and hard, the cutting force is increased during further cutting, which accelerates tool wear and so on. 03 Grain refinement strengthening >>>> Definition: The method of improving the mechanical properties of metal materials by refining their grains is known as grain refinement strengthening; in industry, grain refinement is used to enhance the strength of materials. >>>> Principle: Typically, metals are polycrystals composed of many grains, and the size of these grains can be expressed by the number of grains per unit volume; the higher this number, the finer the grains. Experiments show that fine-grained metals at room temperature possess higher strength, hardness, ductility, and toughness than coarse-grained metals. This is because when fine grains undergo plastic deformation under external forces, this deformation can occur across a larger number of grains, resulting in more uniform plastic deformation and less stress concentration ； Furthermore, the finer the grains, the larger the grain boundary area, and the more tortuous the grain boundaries, which makes it less favorable for crack propagation. Therefore, in industry, the method of improving material strength by refining grains is called grain refinement strengthening. >>>> Effect: The finer the grains, the smaller the number of dislocations (n) in the dislocation clusters, the less stress concentration there is, and the higher the strength of the material ； The strengthening mechanism of fine-grain strengthening is such that the more grain boundaries there are and the finer the grains, the smaller the average grain size (d) according to Hall-Petch’s equation, and thus the higher the yield strength of the material. >>>> Methods to refine grains: Increase supercooling ； Modification treatment ； Vibration and stirring ； For cold-deformed metals, grain refinement can be achieved by controlling the degree of deformation and annealing temperature. 04 Second-phase strengthening >>>> Definition: Compared to single-phase alloys, multiphase alloys contain a second phase in addition to the matrix phase. A significant strengthening effect is produced when the second phase is uniformly distributed as fine, dispersed particles within the matrix phase. This strengthening effect is known as second-phase strengthening. >>>> Classification: Regarding the movement of dislocations, the second phases present in the alloy fall into two categories: (1) the strengthening effect of ind deformable particles (bypass mechanism). (2) Strengthening effect of deformable particles (shear mechanism). Both dispersion strengthening and precipitation strengthening are special cases of second-phase strengthening. >>>> Effect The main reason for the strengthening in the second phase is their interaction with dislocations, which hinders dislocation movement and increases the deformation resistance of the alloy. In summary, the most important factors affecting strength are the composition of the material itself, its microstructure, and its surface condition ； Next is the loading condition; factors such as the speed at which force is applied and the manner of loading—whether it is simple stretching or repeated stress—will all result in different strengths ； In addition, the geometry and dimensions of the specimen, as well as the test medium, also have a significant impact; sometimes they are even decisive. For instance, the tensile strength of ultra-high-strength steel can be reduced severalfold in a hydrogen atmosphere. There are essentially two ways to strengthen metallic materials: one is to enhance the interatomic bonding force in the alloy, thereby increasing its theoretical strength and producing defect-free, perfect crystals, such as whiskers. It is known that the strength of iron whiskers is close to the theoretical value; this can be attributed to the absence of dislocations in the whiskers, or to the presence of only a small number of dislocations that cannot multiply during deformation. Unfortunately, the strength drops sharply when the diameter of the whiskers is large. Another way to strengthen materials is to introduce a large number of crystal defects into the crystal lattice, such as dislocations, point defects, foreign atoms, grain boundaries, highly dispersed particles, or inhomogeneities (such as segregation). These defects impede the movement of dislocations and can significantly increase the strength of metals. It has proven to be the most effective way to improve metal strength. For engineering materials, better overall performance is generally achieved through comprehensive strengthening effects.