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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
At 0-D there is precipitate and solid solution strengthening with particulates strengthening structure, at 1-D there is work/forest hardening with line dislocations as the hardening mechanism, and at 2-D there is grain boundary strengthening with surface energy of granular interfaces providing strength improvement.
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]
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.
This mechanism is observed in polycrystals or along the surface in a single crystal, which produces a net flow of material and a sliding of the grain boundaries. American materials scientist Robert L. Coble first reported his theory of how materials creep across grain boundaries and at high temperatures in alumina .
The variety of strengthening mechanisms that alter the strength of a material include the mechanism of grain boundary strengthening. Thus, although yield strength is maximized with decreasing grain size, ultimately, very small grain sizes make the material brittle.
Grain boundary sliding (GBS) is a material deformation mechanism where grains slide against each other. This occurs in polycrystalline material under external stress at high homologous temperature (above ~0.4 [ 1 ] ) and low strain rate and is intertwined with creep .