Thread Content
I. Strain strengthening (or deformation strengthening, work hardening) Image 01 Definition: The phenomenon in which, after a material yields, its strength and hardness increase while its plasticity and toughness decrease as the degree of deformation increases is known as strain strengthening or work hardening. 02 Mechanism: As plastic deformation proceeds, the dislocation density continues to increase. As a result, dislocations intersect with each other more frequently during their movement, leading to the formation of obstacles such as fixed stacking faults and dislocation entanglements. These obstacles increase the resistance to dislocation motion, thereby raising the deformation resistance and making further plastic deformation more difficult, which in turn increases the strength of the metal. The rule is that as the degree of deformation increases, the strength and hardness of the material rise, while its plasticity and toughness decrease. The dislocation density keeps increasing. According to the formula, strength is proportional to the square root of the dislocation density ρ; the larger the Burgers vector b of the dislocations, the more significant the strengthening effect. 03 Methods: Cold deformation, such as cold pressing, rolling, shot blasting, etc. 04 Example: Cold-drawing steel wires can double their strength. 05 Practical significance of strain strengthening (advantages and disadvantages) (1) Advantages: ① Strain strengthening is an effective method for strengthening metals. For some materials that cannot be strengthened through heat treatment, strain strengthening can be used to increase their strength, enabling it to double. ②It is an important factor in the processing and shaping of certain workpieces or semi-finished products; it enables uniform deformation of the metal, thus making it possible to shape the workpieces or semi-finished products, such as in the cold drawing of steel wires and the stamping of parts. ③Strain strengthening can also enhance the safety of parts or components during use. When stress concentration or overload occurs in certain areas of a part, plastic deformation takes place in those areas; processing hardening then stops further deformation in those overloaded areas, thereby improving safety. (2) Disadvantages: ① Strain strengthening also causes problems in the production and use of materials; deformation increases strength while reducing ductility, making further deformation difficult and requiring more power. ②To allow the material to continue deforming, recrystallization annealing is required in between, so that the material can keep deforming without cracking, which increases production costs. Image Image II. Solid solution strengthening 01 Definition: The phenomenon in which, as the concentration of solute atoms increases, the strength and hardness of a solid solution rise while its plasticity and toughness decrease is called solid solution strengthening. 02 Mechanism (1) The incorporation of solute atoms causes distortion in the lattice of the solid solution, which hinders the movement of dislocations on the slip planes. (2) The Coherer clusters formed by solute atoms concentrated on the dislocation line act as pinning centers for dislocations, increasing the resistance to their movement. (3) The segregation of solute atoms in the dislocation region hinders the movement of propagating dislocations. All factors that hinder dislocation motion and increase the resistance to dislocation movement can enhance strength. 03 Rules: ① Within the solubility range of the solid solution, the greater the mass fraction of the alloying elements, the greater the strengthening effect. ② The larger the size difference between the solute atoms and the solvent atoms, the more significant the strengthening effect. ③The strengthening effect of solute elements that form interstitial solid solutions is greater than that of elements that form substitutional solid solutions. ④ The greater the difference in the number of valence electrons between solute atoms and solvent atoms, the greater the strengthening effect. 04 Method: Alloying, which involves adding alloying elements. 02 Example: The strength of copper-nickel alloys is greater than that of pure copper and nickel. Image Image III. Fine-grain strengthening Image 01 Definition: The phenomenon in which, as the grain size decreases, the strength and hardness of a material increase while its plasticity and toughness improve is known as fine-grain strengthening. 02 Mechanism Its principle lies in the inhibitory effect of grain boundaries on dislocation slip. In polycrystals, dislocation motion must overcome the resistance at grain boundaries; this is due to the different orientations of dislocations on either side of the grain boundaries. Therefore, within a single grain, dislocations that are sliding cannot pass directly through the grain boundaries into adjacent grains. Sliding can occur only when a large number of dislocations accumulate at the grain boundaries, resulting in stress concentration, which in turn triggers the movement of existing dislocations in the adjacent grains. Therefore, the finer the grain size, the higher the strength of the material. 03 Principle: The finer the grains, the larger the grain boundary area. According to Hall-Petch’s formula, the smaller the average diameter d of the grains, the higher the yield strength σs of the material. 04 Methods for refining grains: ① During the crystallization process, grain refinement can be achieved by increasing the degree of supercooling, applying treatment methods, as well as using vibration and stirring to boost the nucleation rate ； ②For cold-deformed metals, the grain size can be refined by controlling the degree of deformation and annealing temperature ； ③The grain size can be refined through heat treatment methods such as normalizing and annealing ； ④Alloying elements can be added to steel to form new phases, thereby suppressing grain growth. Image Image IV. Phase strengthening in the second stage Image 01 Definition: There are one or several additional phases present in the metal matrix, and the presence of these phases enhances the strength of the metal. Due to the different processes used to obtain the second phase, second-phase strengthening is divided into: ① precipitation strengthening: obtaining the second phase through phase transformation heat treatment; ② dispersion strengthening: obtaining the second phase through powder sintering or internal oxidation. 02 Mechanism: As dislocations move, they encounter the second phase; they must bypass or cut through this phase, and thus the second phase hinders the movement of the dislocations, resulting in an increase in the strength of the material. 03 Example: The presence of cementite in steel enhances its strength.