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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
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.
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.
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 ...
Two main mechanisms for altering grain boundaries have been defined. The first is the process by which quartz softens as temperature increases, providing a means for internal stress reduction by migration of dislocations in the crystal lattice, known as dislocation creep. These dislocations concentrate into walls, forming new grain boundaries.
As recovery proceeds these cell walls will undergo a transition towards a genuine subgrain structure. This occurs through a gradual elimination of extraneous dislocations and the rearrangement of the remaining dislocations into low-angle grain boundaries. Sub-grain formation is followed by subgrain coarsening where the average size increases ...
Dislocations require proper lattice ordering to move through a material. At grain boundaries, there is a lattice mismatch, and every atom that lies on the boundary is uncoordinated. This stops dislocations that encounter the boundary from moving.
Dislocations may be pinned due to stress field interactions with other dislocations and solute particles, creating physical barriers from second phase precipitates forming along grain boundaries. There are five main strengthening mechanisms for metals, each is a method to prevent dislocation motion and propagation, or make it energetically ...