Metallic materials may be shaped by applying external forces to them without reducing their structural cohesion. This property is known as the formability of metal. Deformation or flow occurs when the rows of atoms within the individual crystalline grains are able, when stressed beyond a certain limit, to slide against one another and cohesion between the rows of atoms takes place at the following atomic lattice. This sliding occurs along planes and directions determined by the crystalline structure and is only made possible by, for example, dislocations (faults in the arrangement of the atomic lattice). Other flow mechanisms such as twin crystal formation, in which a permanent deformation is caused by a rotation of the lattice from one position to another, play only a minor role in metal forming technology.
