As discussed in the previous section, the ability of a crystalline
material to plastically deform largely depends on the ability
for dislocation to move within a material. Therefore, impeding
the movement of dislocations will result in the strengthening
of the material. There are a number of ways to impede dislocation
movement, which include:
- controlling the grain size (reducing continuity of atomic
- strain hardening (creating and tangling dislocations)
- alloying (introducing point defects and more grains to pin
of Grain Size
The size of the grains within a material also has an effect on
the strength of the material. The boundary between grains acts
as a barrier to dislocation movement and the resulting slip because
adjacent grains have different orientations. Since the atom alignment
is different and slip planes are discontinuous between grains.
The smaller the grains, the shorter the distance atoms can move
along a particular slip plane. Therefore, smaller grains improve
the strength of a material. The size and number of grains within
a material is controlled by the rate of solidification from the
Strain hardening (also called work-hardening or cold-working)
is the process of making a metal harder and stronger through plastic
deformation. When a metal is plastically deformed, dislocations
move and additional dislocations are generated. The more dislocations
within a material, the more they will interact and become pinned
or tangled. This will result in a decrease in the mobility of
the dislocations and a strengthening of the material. This type
of strengthening is commonly called cold-working. It is called
cold-working because the plastic deformation must occurs at a
temperature low enough that atoms cannot rearrange themselves.
When a metal is worked at higher temperatures (hot-working) the
dislocations can rearrange and little strengthening is achieved.
Strain hardening can be easily demonstrated with piece of wire
or a paper clip. Bend a straight section back and forth several
times. Notice that it is more difficult to bend the metal at the
same place. In the strain hardened area dislocations have formed
and become tangled, increasing the strength of the material. Continued
bending will eventually cause the wire to break at the bend due
to fatigue cracking. (After a large number of bending cycles,
dislocations form structures called Persistent Slip Bands (PSB).
PSBs are basically tiny areas where the dislocations have piled
up and moved the material surface out leave steps in the surface
that act as stress risers or crack initiation points.)
It should be understood, however, that increasing the strength
by cold-working will also result in a reduction in ductility.
The graph to the right shows the yield strength and the percent
elongation as a function of percent cold-work for a few example
materials. Notice that for each material, a small amount of cold-working
results in a significant reduction in ductility.
Effects of Elevated Temperature
on Strain Hardened Materials
When strain hardened materials are exposed to elevated temperatures,
the strengthening that resulted from the plastic deformation can
be lost. This can be a bad thing if the strengthening is needed
to support a load. However, strengthening due to strain hardening
is not always desirable, especially if the material is being heavily
formed since ductility will be lowered.
Heat treatment can be used to remove the effects of strain hardening.
Three things can occur during heat treatment:
- Grain growth
When a stain hardened material is held at an elevated temperature
an increase in atomic diffusion occurs that relieves some of the
internal strain energy. Remember that atoms are not fixed in position
but can move around when they have enough energy to break their
bonds. Diffusion increases rapidly with rising temperature and
this allows atoms in severely strained regions to move to unstrained
positions. In other words, atoms are freer to move around and
recover a normal position in the lattice structure. This is known
as the recovery phase and it results in an adjustment of strain
on a microscopic scale. Internal residual stresses are lowered
due to a reduction in the dislocation density and a movement of
dislocation to lower-energy positions. The tangles of dislocations
condense into sharp two-dimensional boundaries and the dislocation
density within these areas decrease. These areas are called subgrains.
There is no appreciable reduction in the strength and hardness
of the material but corrosion resistance often improves.
At a higher temperature, new, strain-free grains nucleate and
grow inside the old distorted grains and at the grain boundaries.
These new grains grow to replace the deformed grains produced
by the strain hardening. With recrystallization, the mechanical
properties return to their original weaker and more ductile states.
Recrystallization depends on the temperature, the amount of time
at this temperature and also the amount of strain hardening that
the material experienced. The more strain hardening, the lower
the temperature will be at which recrystallization occurs. Also,
a minimum amount (typically 2-20%) of cold work is necessary for
any amount of recrystallization to occur. The size the new grains
is also partially dependant on the amount of strain hardening.
The greater the stain hardening, the more nuclei for the new grains,
and the resulting grain size will be smaller (at least initially).
If a specimen is left at the high temperature beyond the time
needed for complete recrystallization, the grains begin to grow
in size. This occurs because diffusion occurs across the grain
boundaries and larger grains have less grain boundary surface
area per unit of volume. Therefore, the larger grains lose fewer
atoms and grow at the expense of the smaller grains. Larger grains
will reduce the strength and toughness of the material.