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 point, linear, or planar imperfections.
There are two types of point imperfection.
- Vacancies are points 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 are points at which there is an atom that shouldn't be there. This usually occurs in alloys where the atomic size of one element is significantly smaller than the other.
Linear imperfections are all 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.