Study of Dislocation Densities Through the Thickness of 7050 Aluminum
Cory Parker, David Field
WSU REU Program
This work was supported by the National Science
Foundation’s REU program under
grant number DMR-1062898.
Methods
Introduction
7050 Aluminum is a lightweight, yet strong, alloy
primarily used in the aerospace industry. The alloy
has a complicated chemistry that has made the
study of its microstructure rather difficult due to the
presence of a wide range of particles in the
substrate. 7050 goes through a rolling process at
high temperatures in order to allow for easy
deformation of large plates. This process greatly
deforms the crystal lattice in the alloy by introducing
dislocations. Because the process induces so much
change in the matrix these dislocations occur
frequently and begin to stack up as they have more
and more difficulty moving through the lattice. Soon
the density of these dislocations is so massive that
grains begin to have noticeable variation in their
orientation. Maintaining this structure is not
optimal and so requires a larger amount of energy to
maintain compared to a smooth lattice.
Electron Backscatter
Diffraction was used in
this study. The sample
must be held at a 70
rotation and a
stationary electron
beam is fired from an
SEM. This creates
refraction bands
created by the lattice
structure that are then
reflected onto a
phosphor screen
attached to a TV
camera. These bands
are read and used to
determine orientation
data.
Motivation
This concentration of high energy can cause
problems during aging of the plate. Typically after
being rolled the plates are solutionized at a high
temperature and aged at a moderate temperature.
During this phase the high concentration of energy
from dislocations causes new grains to form through
the substrate. These newly recrystalized grains
cause inhomogeneity in the lattice of the alloy.
These inhomogeneities are prime locations for cracks
to begin to form during normal use as a wing spar. If
the rolling process can be controlled to account for
these dislocation densities, or at least the areas with
the highest densities determined, then these alloys
can be made safer and more dependable.
Left: Figure showing
individual grains from a
scan taken
approximately 0.4 cm
from the surface of the
plate. Grains are small
and the rolling direction
is evident.
Right: Figure showing
individual grains from a
scan taken
approximately 4.8 cm
from the surface of the
plate. Grains are
substantially larger.
5 samples were prepared in total. 4 samples
prepared from the Plan View and 1 sample from the
Long Transverse of the plate. These samples were
all prepared using standard metallographic practices
which include cutting, sanding, and polishing the
samples. The final step of polishing was performed
by vibropolish for approximately 75 minutes for each
sample. Plan View samples were polished
automatically while the LT Sample was polished by
hand due to its large size. Samples were then
examined using Electron Backscatter Diffraction
using a step size of 2 microns for every scan.
Data
GND calculations from
the Long Transverse
sample showing a
downward trend from
surface to t/2.
Above: Key for
Orientation Maps.
Left: Orientation Maps
from various positions
through the thickness
of the 5-inch plate.
Distance from the
surface of the plate
increases moving down
the figures. Black areas
represent areas that
have been expunged
from data due to low
confidence.
Right: GND mapping of
the Orientation Map
opposite. Scale runs
from .01x10^11/m^2 to
2000 x10^11/m^2 with
blue representing the
lower bound and red
representing the upper
bound. Black lines
represent grain
boundaries, which are
not considered.
The conversion from the data on the left side to those
on the right side is done by add-on code written for
OIM Analysis 5.1. This code analyzes the orientation
at every point in relation to the points around it and
determines the Geometrically Necessary Dislocations
in order to achieve the change in orientation that
occurred. This comes from using the equations:
ij = eikl (ejl,k + gjl,k)
ij =  k (bik zjk)
to solve for the dislocation density tensor, .
GND calculations from
the Plan View samples
showing the same
downward trend.
Percent of scans
showing grains that are
already recrystalized,
defined as having a
Grain Orientation
Spread less than 1
degree, upon arrival. A
somewhat parabolic
correlation can be seen
from surface to t/2.
Results
•Moving from the surface to t/2 positions GND
decreased and grain size increased.
•The general trend for recrystalization already
present through the thickness of the plate is that
recrystalization is greatest near the surface and t/2
positions with a vaguely parabolic behavior between
these two areas.
•The plate is much stronger near the surface due to
high strain resulting from the rolling processing
reducing the size of grains, however this area is most
likely to encounter recrystalization.
•A connection between percent recrystalization and
GND does not seem to exist. Because recrystalized
grains have low dislocation content they should
decrease overall density but this doesn’t occur.