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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 ...
As the grain is bent further, more and more dislocations must be introduced to accommodate the deformation resulting in a growing wall of dislocations – a low-angle boundary. The grain can now be considered to have split into two sub-grains of related crystallography but notably different orientations.
The result is that the dislocation must bend (which requires greater energy, or a greater stress to be applied) around the precipitates, which inevitably leaves residual dislocation loops encircling the second phase material and shortens the original dislocation. This schematic shows how a dislocation interacts with solid phase precipitates.
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 pile-up of dislocations at grain boundaries and Orowan loops around strong precipitates are two main sources of these back stresses. When the strain direction is reversed, dislocations of the opposite sign can be produced from the same source that produced the slip-causing dislocations in the initial direction.
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 materials that were deformed under very high temperatures, lobate grain boundaries may be taken as evidence for diffusion creep. [7] Diffusion creep is a mechanism by which the volume of the crystals can increase. Larger grain sizes can be a sign that diffusion creep was more effective in a crystalline material.