it can be seen that the deformation
microstructure is significantly finer than that of the
initial microstructure. Most of the parallel boundaries are not
the original high-angle boundaries but the newly developed
dislocation boundaries, indicating the sub-division of existing
grains resulting in grain refinement during ECA pressing.
The features shown in Fig. 3 are very close to those in ECA
pressedFCCmetals [17,18]. Therefore, it is presumed that the
parallel boundaries in the lamellar structure are the geometrically
necessary boundaries (GNBs). Within each lamella,
there are dislocation cell blocks (CBs) divided by incidental
dislocation boundaries (IDBs). The average spacing of
the GNBs and the size of CBs are 0.5 m and 0.4m, respectively.
As shown in the insert of Fig. 3, the SAD taken
from the deformed microstructure indicates that most of the
boundaries are low-angled in this stage because the angular
spread is about 5◦. Unlike the case of pure Ti ECA pressed
in the same die [16], the present Zr702 showed no mechanical
twins, which is probably because of the differences in
the initial grain size and the purity of the specimen chemistry