Crystals are entities of atoms, ions or even polymer strings in which the subunits (i.e. atoms, ions or polymers) are arranged in a certain pattern. Crystal growth is the addition of new subunits in this arrangement, with inherently a volume rise of the crystal. In metals growth is often occompanied by a relative decrease in surface area (with high energy) of the crystal. The bulk energies are lower and therefore subunits prefer a larger crystal. In ion or polymer crystals the subunit-subunit bonding itself might be sufficient cause for crystal growth.

Growth starts with the nucleation on a small inclusion in the material or boundaries between other crystals. The newly formed crystal then grows while the old crystals shrink when their atoms are taken by the new chrystal. Some important features during growth are the arrangement, the origin of growth, the interface form (important for the driving force) and the final size. Arrangement in metals is often a body centered cubic, face centered cubic or hexagonal close packed. When origin of growth is only in one direction for all the crystals, this can have as result that the material becomes very anisotropic (different properties in different directions). The interface form determines the additional free energy for each volume of crystal growth. The final size is important for mechanical properties of materials (in metals it is widely ackknowledge that large crystals can stretch further due to the longer deformation path and thus lower internal stresses).

Sorry I didn't refer, but I only passed this missing part during graduation and wanted to add somthing