Deformation of Metals - Electrically Assisted Forming - page 14

amount of excess space within a unit cell of a metal. The higher the APF, the less
excess space there is for any other atomic level particles to fit through. The num-
ber of slip systems that a metal has is important for plastic deformation, because
these are the only methods by which dislocations can travel in order to facilitate
deformation (ignoring nano-deformation mechanisms).
1.4.3.1 Face-Centered Cubic (FCC)
The coordination number of a FCC metal is 12, which is the number of atoms a
single atom touches within a unit cell. In addition, the APF (i.e., the volume of
the atoms in a unit cell compared to the total volume of the unit cell) is 0.74. FCC
metals have 12 slip systems. As stated above, these are like “roadways” for the
dislocations to travel on in order to allow for plastic deformation. Examples of
some FCC metals are copper, aluminum, and some stainless steels [ 21 , 22 ].
1.4.3.2 Body-Centered Cubic
The number of neighboring atoms contacting each atom in a Body-Centered Cubic
(BCC) metal is 8 (the coordination number). Additionally, the packing factor for
BCC metals is 0.68, which is lower than the FCC metal. The BCC metals can have
12 or 24 slip systems, depending on the different slip plane/direction combina-
tions. Iron is an example of a BCC metal [ 21 , 22 ].
1.4.3.3 Hexagonal Close-Packed
The coordination number and the APF for Hexagonal Close-Packed (HCP) metals
are the same as for FCC metals, which are 12 and 0.74, respectively. The HCP met-
als have the lowest number of slip systems, with three; therefore plastic deformation
by way of dislocation motion can be difficult and makes this class of metals act brit-
tle. These metals often twin to facilitate deformation, due to the lack of active slip
systems. Twinning is when a mirror representation of a particular lattice arrange-
ment is created across a given plane (i.e., a twin plane). As the shear force within the
metal increases, it will generate a mirror representation of a particular lattice struc-
ture. This newly generated lattice structure will provide reoriented slip systems that
will offer the dislocations potential pathways for which to travel on and continue
deformation. Examples of HCP metals are titanium and magnesium [ 21 , 22 ].
1.4.4 Lattice Defects
A characteristic of a crystalline material is a repetitive atomic structure or pattern
across its lattice. However, there can be several different types of defects through-