Crystalline materials (mainly metals and alloys, but also stoichiometric salts and other materials) are made up of solid regions of ordered matter (atoms placed in one of a number of ordered formations called Bravais Lattices). These regions are known as crystals. A perfect crystal is one that contains no imperfections (obviously...). Crystal imperfections can be point, linear or planar.
Point imperfections are:
- Vacancies: a point at which there should be an atom but there is none - the vacancy concentration tends to increase with increaing temperature as the atomic vibrations increase,
- Intersticials: A point at which there should be no atom but one does exist - this usually occurs in alloys where the atomic size of one element is significantly smaller than the other.
Linear imperfections are termed dislocations and can be simply explained as an extra half-plane of atoms wedged in the lattice. Dislocations are very important to the mechanical properties of a metal as their concentration affects its strength, ductility and other mechanical properties. The higher the dislocation density, the stronger and less ductile it is.
Planar imperfections are the boundaries between crystals in a material. These are also known as grain boundaries. The size of grains in a metal also affects its mechanical properties - the smaller the grain size, the stronger and less ductile it is.
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