After a material yields, the phenomenon where its strength and hardness increase while its plasticity and toughness decrease with increasing deformation is called strain hardening or work hardening.
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Mechanism
As plastic deformation progresses, the dislocation density continuously increases. Therefore, the intersecting of dislocations during movement intensifies, resulting in fixed cleavages, dislocation entanglements, and other obstacles. This increases the resistance to dislocation movement, leading to increased deformation resistance and making further plastic deformation difficult, thus increasing the metal's strength.
Rule: As the degree of deformation increases, the material's strength and hardness increase, while its plasticity and toughness decrease. The dislocation density continuously increases. According to the formula diagram, strength is directly proportional to the first power of the dislocation density ρ. The larger the Burgers vector b of the dislocations, the more significant the strengthening effect.
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Methods
Cold deformation, such as cold pressing, rolling, shot peening, etc.
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Example
Cold-drawn steel wire can increase its strength several times over.
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The Practical Significance of Strain Strengthening (Advantages and Disadvantages)
(1) Advantages:
① Strain strengthening is an effective method for strengthening metals. For materials that cannot be strengthened by heat treatment, strain strengthening can increase their strength by several times.
② It is an important factor in the forming of certain workpieces or semi-finished products, enabling uniform deformation of the metal and making the forming of workpieces or semi-finished products possible, such as cold-drawn steel wire and stamping of parts.
③ Strain strengthening can also improve the safety of parts or components during use. When stress concentration or overload occurs in certain parts of a part, plastic deformation occurs at that location. Due to work hardening, the deformation of the overloaded part stops, thus improving safety.
(2) Disadvantages:
① Strain strengthening also brings trouble to material production and use. Deformation increases strength but decreases plasticity, making further deformation difficult and requiring more power.
② In order for the material to continue deforming, recrystallization annealing is required in between to allow the material to continue deforming without cracking, increasing production costs. Image Image II. Solid Solution Strengthening Image 01 Definition Solid solution strengthening is the phenomenon that the strength and hardness of a solid solution increase while its plasticity and toughness decrease with increasing solute atom content. 02 Mechanism (1) The dissolution of solute atoms distorts the crystal lattice of the solid solution, hindering the movement of dislocations on slip surfaces. (2) Cotillard atmospheres formed by solute atoms segregating on dislocation lines pin dislocations, increasing the resistance to dislocation movement. (3) The segregation of solute atoms in stacking fault regions hinders the movement of extended dislocations. All factors that hinder dislocation movement and increase the resistance to dislocation movement can increase strength. 03 Rules ① Within the solubility range of the solid solution, the greater the mass fraction of the alloying element, the greater the strengthening effect. ② The greater the size difference between solute atoms and solvent atoms, the more significant the strengthening effect. ③ The strengthening effect of solute elements forming interstitial solid solutions is greater than that of elements forming 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.
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Method
Alloying, i.e., adding alloying elements.
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Example
The strength of copper-nickel alloys is greater than that of pure copper and nickel metals.
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III. Grain Refinement Strengthening
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Definition
The phenomenon that the strength, hardness, plasticity, and toughness of a material increase with decreasing grain size is called grain refinement strengthening.
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Mechanism
The principle lies in the hindering effect of grain boundaries on dislocation slip. For polycrystalline materials, dislocation movement must overcome the resistance of grain boundaries. This is because the orientations of dislocations on both sides of a grain boundary are different. Therefore, in a certain grain, a slipping dislocation cannot directly cross the grain boundary into an adjacent grain. Only after a large number of dislocations accumulate at the grain boundary, causing stress concentration, can the movement of existing dislocations in adjacent grains be stimulated to produce slip. Therefore, the finer the grain size, the higher the material strength.
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Rule
Fineer grains result in larger grain boundary areas. According to the Hall-Page formula, the smaller the average grain diameter d, the higher the yield strength σs of the material.
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Methods for Refining Grains
① During crystallization, grains can be refined by increasing supercooling, modification treatment, vibration, and stirring to increase the nucleation rate.
② For cold-deformed metals, grains can be refined by controlling the degree of deformation and annealing temperature.
③ Grains can be refined through normalizing and annealing heat treatments.
④ Alloying elements can be added to steel to form new phases, thereby inhibiting grain growth.
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IV. Second Phase Strengthening
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Definition
The presence of one or more other phases in the metal matrix increases the strength of the metal. Depending on the process of obtaining 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.
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Mechanism
When a dislocation encounters a second phase during its movement, it needs to bypass or cut through the second phase. Thus, the second phase hinders the movement of the dislocation, thereby increasing the strength of the material.
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Example
The presence of cementite in steel increases its strength.
