Third, fine crystal strengthening
01. Definitions
With the decrease of grain size, the strength and hardness of the steel material increase, and the plasticity and toughness are also improved, which is called fine crystal strengthening.
02. Mechanism
The principle lies in the retarding effect of grain boundary on dislocation slip. For polycrystals, dislocation movement must overcome the resistance of grain boundaries, which is due to the different orientation of dislocations on both sides of the grain boundary, so in a grain, the slip dislocation can not directly cross the grain boundary into the adjacent grain, only after a large number of dislocations are packed at the grain boundary to cause stress concentration, in order to stimulate the movement of the adjacent grain has a fault to produce slip. So the finer the grain, the higher the strength of the material.
03. Rules
The finer the grain size, the larger the grain boundary area, according to the Hall-Page formula
The smaller the average diameter d of the grain, the higher the yield strength σs of the material
04. Refining grain method
(1) During the crystallization process, the grains can be refined by increasing the degree of supercooling, metamorphic treatment, vibration and agitation to increase the nucleation rate;
② For cold deformed metals, grains can be refined by controlling deformation degree and annealing temperature;
③ The grains can be refined by normalizing and annealing heat treatment methods;
④ Alloying elements can be added to the steel to form a new phase to inhibit grain growth.
Fourth, the second phase of strengthening
01. Definitions
There is also one or more other phases in the metal matrix, the presence of these phases to improve the strength of the metal. Due to the different processes for obtaining the second phase, the second phase strengthening is divided into: ① precipitation strengthening: the second phase is obtained by phase change heat treatment; ② dispersion strengthening: the second phase is obtained by powder sintering or internal oxidation.
02. Mechanism
The dislocation encounters the second phase in the process of movement and needs to bypass or cut through the second phase, which hinders the movement of the dislocation and makes the strength of the material increase.
03. Examples
The strength of steel is improved by the presence of cementite in steel.
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