A grain boundary (GB) is the
interface between two grains, or crystallites, in a polycrystalline material. Grain
boundaries are defects in the crystal structure, and tend to decrease the
electrical and thermal conductivity of the material.
A grain boundary is a general
planar defect that separates regions of different crystalline orientation (such
as grains) within a polycrystalline solid. Grain boundaries are usually the
result of uneven growth when the solid is crystallizing. Grain sizes vary from
1 µm to 1 mm.
Most grain boundaries are preferred sites for the onset of corrosion and for the precipitation of new phases from the solid. They are also important to many of the mechanisms of creep. On the other hand, grain boundaries disrupt the motion of dislocations through a material, so reducing crystallite size is a common way to improve the strength of a material.
The juncture between adjacent
grains is called a grain boundary. The grain boundary is a transition region in
which some atoms are not exactly aligned with either grain.
Therefore, the grain boundaries
are:
- Where grains meet in a solid
- Transition regions between the neighboring crystals
- Where there is a disturbance in the atomic packing
Characterization of grain
boundaries is of critical importance in materials studies. The properties of
grain boundaries often determine the grains':
- Formation
- Evolution
- Stabilization (or dissolution)
Grain boundaries have two types,
as per their orientation:
- Low-angle grain boundaries are those with a
misorientation less than about 11 degrees.
- High-angle grain boundaries are whose
misorientation is greater than about 11 degrees.
High-angle boundaries are
considerably more disordered, with large areas of poor fit and a comparatively
open structure. The mobility of low-angle boundaries is much lower than that of
high-angle boundaries. Both low- and high-angle boundaries are retarded by
grain refinement, to minimize or prevent recrystallization or grain growth
during heat treatment.
The grain-boundary atoms are more
easily and rapidly dissolved, or corroded, than the atoms within the grains.
Grain boundaries will oxidize or corrode more rapidly, usually referred to as
grain-boundary penetration or intergranular attack. In under-deposit corrosion
and hydrogen damage, grain boundaries are the site at which the methane
collects that leads to the intergranular cracking characteristic of hydrogen
damage.
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