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Figure 1: Hall–Petch strengthening is limited by the size of dislocations. Once the grain size reaches about 10 nanometres (3.9 × 10 −7 in), grain boundaries start to slide. In materials science, grain-boundary strengthening (or Hall–Petch strengthening) is a method of strengthening materials by changing their average crystallite (grain
To push the limit of grain size for strengthening, the hindrance of grain rotation and growth could be achieved by grain boundary stabilization. The construction of nanolaminated structure with low-angle grain boundaries is one method to obtain ultrafine grained materials with ultra-strength.
The Hall–Petch method, or grain boundary strengthening, is to obtain small grains. Smaller grains increases the likelihood of dislocations running into grain boundaries after shorter distances, which are very strong dislocation barriers. In general, smaller grain size will make the material harder.
Micrograph of a polycrystalline metal; grain boundaries evidenced by acid etching. Differently-oriented crystallites in a polycrystalline material. In materials science, a grain boundary is the interface between two grains, or crystallites, in a polycrystalline material.
In metallurgy, materials science and structural geology, subgrain rotation recrystallization is recognized as an important mechanism for dynamic recrystallisation.It involves the rotation of initially low-angle sub-grain boundaries until the mismatch between the crystal lattices across the boundary is sufficient for them to be regarded as grain boundaries.
As grain size decreases, the surface area to volume ratio of the grain increases, allowing more buildup of dislocations at the grain edge. Since it requires much energy to move dislocations to another grain, these dislocations build up along the boundary, and increase the yield stress of the material.
The reduction in the non-recrystallization region induces the formation of deformation bands and activated grain boundaries, which are alternative ferrite nucleation sites other than grain boundaries. [12] Other alloying elements are mainly for solid solution strengthening including silicon, manganese, chromium, copper, and nickel. [14]
The exceptional yield strength of nanocrystalline metals is due to grain boundary strengthening, as grain boundaries are extremely effective at blocking the motion of dislocations. Yielding occurs when the stress due to dislocation pileup at a grain boundary becomes sufficient to activate slip of dislocations in the adjacent grain.