Civil Engineering Reference
In-Depth Information
they can affect dislocations that are traveling through the different slip systems
within the lattice. Some examples of lattice defects are voids, impurities or alloy-
ing components, grain boundaries, phase boundaries, and other dislocations.
• A vacancy is an empty location in the lattice structure where an atom would
normally appear. At a vacancy site, the neighboring atoms either cannot fully
bond or must bond with atoms further away, both of which will increase the
energy state of the bonding.
• An impurity can be any element in the atomic structure that is not normally
included in the lattice. Each element has atoms which have a specific atomic
radius. To this end, if all the atoms in the lattice were the same size (i.e., a pure
material), the bond spacing between the atoms would be equal. However, if an
original atom is replaced with an atom of a different size atomic radius, the bond
spacing at and near this impurity will now be affected. There can be two types of
impurities. A substitutional impurity is when the atom of a base metal is replaced
with an atom of another material. The difference in atomic size of the new atom
will provide for either a state of compression or tension on the surrounding
bonds. The second type of impurity is the interstitial impurity, where an atom
from a new element is aligned in the free space between two bonded atoms of
the base metal. As is the case with the substitutional impurities, the interstitial
impurities also distort the bonds in the local region.
• Grain boundaries mark the end of one lattice in a crystal structure and the begin-
ning of another. The lattice structure consists of slip planes oriented in particular
directions with respect to the local grain. Once into another grain boundary, the
slip planes on either side of the grain boundary will be oriented with respect to
the particular grain they are in, which will be different compared with the previ-
ous or next grain. In dual-phase materials, the different phases act the same as
the grain boundaries, in that they break up the consistency of the lattice pattern.
• Dislocations can also be considered lattice defects because they represent por-
tions of un-bonded atoms within a lattice.
Regardless of the type of lattice defect, any flaw in the lattice of the material will
have an effect on the dislocations that are moving through the metal. In the case
of a perfect lattice, the dislocations will move by way of breaking and reforming
bonds due to external forces that are exerted, as was described in the previous sec-
tions. However, when there is something in the structure of the lattice that is not
normally there (i.e., a defect), the dislocations now have a harder time advancing
past this point, unless extra energy is provided to do so. The halting or hinder-
ing of dislocation motion leads to dislocation pileups. If the dislocations cannot
move through the metal, the metal cannot continue deforming (since a collection
of many dislocation motions result in deformation at the bulk level). Continuing
this example, if the metal was being plastically deformed and dislocation pileups
were beginning to occur, the force required to continue deforming the metal would
increase, and this additional force is required to provide the dislocations with the
extra force they need to surpass the defects within the lattice. In general, this is the
basis for strain hardening. Different types of lattice defects can cause dislocation
pileups in different ways.
