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In materials science, grain growth is the increase in size of grains (crystallites) in a material at high temperature. This occurs when recovery and recrystallisation are complete and further reduction in the internal energy can only be achieved by reducing the total area of grain boundary .
The grain size and shape do not change. [7] The second stage is recrystallization, where new strain-free grains nucleate and grow to replace those deformed by internal stresses. [7] If annealing is allowed to continue once recrystallization has completed, then grain growth (the third stage) occurs. In grain growth, the microstructure starts to ...
Initial grain size affects the critical temperature. Grain boundaries are good sites for nuclei to form. Since an increase in grain size results in fewer boundaries this results in a decrease in the nucleation rate and hence an increase in the recrystallization temperature; Deformation affects the final grain size. Increasing the deformation ...
Recrystallization does not occur until the threshold strain has been reached; The stress-strain curve may have several peaks – there is not a universal equation; Nucleation generally occurs along pre-existing grain boundaries; Recrystallization rates increase as the initial grain size decreases
Recovery is related to the similar processes of recrystallization and grain growth, each of them being stages of annealing. Recovery competes with recrystallization, as both are driven by the stored energy, but is also thought to be a necessary prerequisite for the nucleation of recrystallized grains.
Typically, the inverse Hall-Petch effect will happens at grain size ranging from 10 nm to 30 nm and makes it hard for nanocrystalline materials to achieve a high strength. To push the limit of grain size for strengthening, the hindrance of grain rotation and growth could be achieved by grain boundary stabilization.
These grow larger as the temperature is increased. When cooled very quickly, during a martensite transformation, the austenite grain-size directly affects the martensitic grain-size. Larger grains have large grain-boundaries, which serve as weak spots in the structure. The grain size is usually controlled to reduce the probability of breakage. [14]
This is because when the grain size decreases at nm scale, there is an increase in the density of grain boundary junctions which serves as a source of crack growth or weak bonding. However, it was also observed that at grain size below 3.1 nm, a pseudo Hall–Petch relationship was observed, which results an increase in strength.