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Grain Boundaries
Crystalline materials can be single crystal or polycrystalline. Polycrystalline materials are composed of lots of small grains (with crystal structure) positioned alongside one another at varying orientations. The boundaries where these grains meet are called grain boundaries. The grain boundaries are a type of interfacial defect.
As the grains are placed at varying orientations (crystallographic misalignment), the atoms in each do not align. The bonds at the grain boundaries are therefore irregular and energy is stored in them. The greater the angle of misorientation of two grains, the more irregular the bonds are and the greater the boundary energy stored will be. The boundary energy makes the grain boundaries more chemically active and reactive and therefore susceptible to defects. The higher energy encourages impurities to propagate. Grain size in the polycrystalline material also affects the amount of boundary energy available and therefore the probability of impurities and defects forming.
Grain boundaries and dislocations
Grain boundaries also interfere with dislocations. Dislocations pass through each grain and have to change orientation when crossing a boundary due to the discontinuity in the slip plane. Sometimes the change in angle is too great for the dislocations to cross the boundary at all, and they pile up instead. The stress produced can lead to new dislocations being formed in the neighbouring grain.
Decreasing the size of each grain increases the number of grain boundaries (increases grain boundary area) and therefore increases possible locations to impede dislocations. The material is stronger with a greater number of smaller grains. The dislocations can lead to different boundary types (tilt boundary, twist boundary), which all also increase boundary energy and the likelihood of defects forming.
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