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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
Subgrains are defined as grains that are oriented at a < 10–15 degree angle at the grain boundary, making it a low-angle grain boundary (LAGB). Due to the relationship between the energy versus the number of dislocations at the grain boundary, there is a driving force for fewer high-angle grain boundaries (HAGB) to form and grow instead of a ...
Using density functional theory computer simulations of grain boundaries have shown that the band gap can be reduced by up to 45%. [18] In the case of metals grain boundaries increase the resistivity as the size of the grains relative to the mean free path of other scatters becomes significant. [19]
There are mainly two types of grain boundary sliding: Rachinger sliding, [2] and Lifshitz sliding. [3] Grain boundary sliding usually occurs as a combination of both types of sliding. Boundary shape often determines the rate and extent of grain boundary sliding. [4] Grain boundary sliding is a motion to prevent intergranular cracks from forming.
The stress required to move a dislocation from one grain to another in order to plastically deform a material depends on the grain size. The average number of dislocations per grain decreases with average grain size (see Figure 3). A lower number of dislocations per grain results in a lower dislocation 'pressure' building up at grain boundaries.
Bulging recrystallization often occurs along boundaries of old grains at triple junctions. At high temperatures, the growing grain has a lower dislocation density than the grain(s) consumed, and the grain boundary sweeps through the neighboring grains to remove dislocations by high-temperature grain-boundary migration crystallization.
Once critical dislocation density is achieved, nucleation occurs on grain boundaries. Grain boundary migration, or the atoms transfer from a large pre-existing grain to a smaller nucleus, allows the growth of the new nuclei at the expense of the pre-existing grains. [3] The nucleation can occur through the bulging of existing grain boundaries.
In general, smaller grain size will make the material harder. When the grain size approach sub-micron sizes, some materials may however become softer. This is simply an effect of another deformation mechanism that becomes easier, i.e. grain boundary sliding. At this point, all dislocation related hardening mechanisms become irrelevant. [1]