Civil Engineering Reference
In-Depth Information
• Impurities can be intentionally added to a material for strengthening purposes
(i.e., alloying in aluminum or stainless steel alloys). These impurities can be
intended to increase the strain hardening of the metal in order to make it stronger
for a particular application. As the number of impurities in a metal increases, the
magnitude of strain hardening increases.
• Grain boundaries are the locations where the orientation and direction of the
lattice change, and different grains are spread throughout the microstructure of
the metal. Each time a dislocation comes to a grain boundary; it cannot progress
through the boundary. The dislocation creates a stress at this boundary such
that a new dislocation is created from the stress at the grain boundary inside the
new grain, and then, this dislocation can begin moving through the new grain's
different oriented slip systems. For the same bulk dimensions, a metal with a
smaller grain size would have more grain boundaries and a greater grain bound-
ary surface area for the dislocations to travel through so they would need excess
energy each time they reached a grain boundary in order to change direction/
orientation, or to create stress to produce a new dislocation. Therefore, a metal
with a smaller grain size is typically stronger than a metal with a larger grain
size. In addition, the number of slip systems also plays a role in the strain hard-
ening because a metal with fewer slip systems will have a less likely chance of
the slip planes lining up well at the grain boundaries, and the dislocations will
have a higher probability to need to change directions to a larger degree.
• The dislocations are also a defect, but they are not stationary, like the impuri-
ties and the grain boundaries. If a moving dislocation comes into contact with
another dislocation, they could either cancel each other out or they could repel
each other. If the first dislocation cannot move, because it is being held up
(repelled) by another dislocation, then the pileup will continue until enough
energy is now supplied such to provide the dislocation at the front of the pileup
with enough energy to break the bonds in front of it and travel through the lattice.
Aside from alloying, potential lattice defects can also be intentionally created
(strengthening mechanisms) or removed (weakening mechanisms). For strengthening
mechanisms, heat treating the metal to produce a small grain size could strengthen the
metal so there are more grain boundaries for the dislocation to pass through. Also, the
metal could be pre-worked so that the dislocation density within the metal is higher
than in the annealed state and these dislocations dispersed through the metal are all
additional defects. Dislocation density is presented in terms of the total of all the dislo-
cation lengths within a unit volume [mm/mm
3
], or it could be presented as the number
of dislocations intersecting a 2-D unit area of a material [#/mm
2
]. For a polycrystal-
line metal, the dislocation density at an annealed state can be about 10
7
mm/mm
3
and
the dislocation density in a worked state can be about 10
12
mm/mm
3
, which means
that the overall dislocation density between annealed and worked conditions could be
about 100,000 times different [
23
]. Heat treating could also be performed to weaken
the metal, by way of allowing for recrystallization and grain growth to occur. The
larger grains would equate to a fewer number of grain boundaries that a dislocation
must pass through and change its direction, hence less external force would be needed.
