Civil Engineering Reference
In-Depth Information
The micrographs for the deformation test (5.08 mm) with a single pulse of
1000 A for 1 s are given in Figs. C.30 and C.31 for orientation 1 and orientation 2,
respectively. The average grain size of Sample 10 in orientation 1 is 5.66 µm with
a standard deviation of 1.63 µm, whereas orientation 2 has an average grain size of
5.51 µm with a standard deviation of 1.89 µm. As seen, the results are very simi-
lar to the deformed test (Sample 9) with no electrical current application (grain
size and amount of twinning). This is also statistically verified by the two-sample
t test where the grain size means are equivalent for both orientations. However,
the variances are only equivalent for orientation 2. Thus, it is suggestive that the
reduced yield point is a result of the interaction of heat generation/electroplasticity
within the material lattice and not an alteration of the material grain size/shape.
This interaction with the lattice is suggested to be a modification of the dislocation
density within the material.
8.3.5 Microstructure Analysis Conclusions
The main conclusions drawn from the analysis in this section are as follows:
• The as-received material had an average grain size that is in agreement with
literature. Also, this material was free from twin boundaries as a result of the
material being warm-rolled.
• The room temperature forming microstructure showed a deformed structure
(non-equiaxed grains) and the presence of a large number of twin bounda-
ries which were a result of the limited number of slip systems active at room
temperature.
• The stationary electrical square wave (electrical treatment) showed no indica-
tion of a direct electrical effect at the grain level. Consequently, the observed
variation in the mechanical response is suggested to be a result of the applied
current altering the dislocation density of the material.
• The square wave electrical tests all showed similar microstructures to the room
temperature forming test, but the amount of twinning appeared to be reduced.
This could be a consequence of the electrical current allowing for pinned dislo-
cations to be freed, thus reducing the necessity of twin boundary formation for
continued deformation. Also, there was a microstructure gradient present due
to the diffuse necking of the specimen (non-uniform strain). This non-uniform
strain is a result of the thermal gradient in the specimen due to forming setup
geometry. Overall for these tests, the grain size was affected by the amount of
deformation imposed on the material and the current magnitude and pulse dura-
tion did not appear to modify the grain size to any significant level. Thus, this
suggests that the microstructure is not affected by pulsed current, but the acting
mechanism (thermal/electroplastic) for force reduction is occurring only at the
atomic level (in the material's lattice).
• The continuous EAF tests also displayed a microstructure gradient along the
specimen length due to non-uniform strain. Also, dynamic recrystallization was
