Civil Engineering Reference
In-Depth Information
• Two primary theories for electroplasticity are compared by examining the mag-
nitude of energy they impart on a dislocation core. The first theory is based on
Joule heating and the localized heat generated at the dislocation core. The sec-
ond theory analyzed is from direct momentum transfer on the dislocation core
due to the electron wind effect. To perform the comparison, a current density
of 100 A/mm
2
is applied to a pure magnesium metal and the energy transferred
to the core is calculated. The importance of a quantifiable amount of energy
applied to the core is that it can be compared to the activation energy for lattice
diffusion in magnesium. The activation energy to move an ion core is approxi-
mately 1.4 eV/atom, and this equals an activation energy of approximately
98 keV for the entire core. Of note is that the entire dislocation core does not
move all at one time, but regions of the core advance throughout the lattice over
time. Nevertheless, this does not affect the results as they examine the magni-
tude between Joule heating and the electron wind effect. From the analysis, the
Joule heating creating local “hot spots” at defects was shown to provide a sig-
nificant amount of energy (~29 keV) to the core which would have a significant
effect on the dislocations mobility. Also, this amount of energy would signifi-
cantly help to reduce the mechanical stress required to displace the dislocation.
In contrast, the electron wind effect produced a very small amount of additional
energy to the dislocation core (1.29
×
10
−
3
eV).
As a result, it is expected that the electron wind effect will have little effect in
aiding or increasing the mobility of the dislocation. In conclusion, the contribu-
tion toward the observed electrical effects is due to localized areas of increased
atomic vibration from electron scattering (i.e., Joule heating) and not solely due
to direct momentum transfer on the dislocation cores themselves.
• In the case of stationary electrical current application (i.e., no deformation), the
local areas of increased atomic vibration allow for a rapid decrease in the stored
energy of the material by facilitating dislocation motion and annihilation. The local
“hot spots” provide the driving energy to allow the dislocations to move to a sink
such that the overall lattice energy is reduced. In addition, if the material has been
worked (i.e., additional lattice strain present), this increases the driving force for
the movement of the dislocations. Thus, larger effects on the dislocation density
are expected. This theory was supported by the observed mechanical and micro-
structure effects seen by the electrical pre-treating and incremental EAF tests.
• For electrical current applied during deformation, the local “hot spots” created
from greater electron scattering at defects significantly enhance the vibrational
energy in the surrounding area of the dislocation. This greater energy surround-
ing the dislocation allows for enhanced mobility along the slip plane as it can
pass by lattice obstacles with less resistance. Thus, the dislocation has a greater
quantity of energy and can move under a lower required stress (i.e., external
required force for deformation is reduced). Also, for the other defects within
the material (point and interfacial defects), they have an increased vibrational
energy surrounding them as a result of larger amount of electron scattering. As a
result, if dislocations interact or become piled up at these defects, this additional
energy from scattering may allow the dislocation to pass by the obstacle, where
