Based on the observation that the average grain diameter was
only ~50 nm after 96 h at 800 C, it appears that grain growth is
inhibited in the multilayered samples. Since the initial structure
has long columnar grains and the final structure has only fine
equiaxed grains, the structural evolution is characterized by
dramatic changes in grain shape, and the before and after grain
sizes cannot be directly compared. However, average grain volumes
were estimated by assuming an initial columnar grain
length of 500 nm (which is reasonable based on DF-TEM images
such as Fig. 1b and d). This yields initial average grain volumes of
3 105 and 7 105 nm3 for Sample A and B, respectively. By
assuming spherical grains in the heat-treated samples, the
average grain volumes are approximately 7 104 nm3 for both
HT-Sample A and B. Note that these grain sizes are volumetrically
smaller by an order of magnitude after heat treatment; not only
does the thermal exposure not seem to cause grain growth by,
e.g., lateral growth of the columnar grains, it is associated with
substantial structural refinement to an equiaxed structure. For
the monolithic Sample C, the grains changed from an average
column width of 57 nm to an average grain diameter of 196 nm
(Fig. 3c). For a 500 nm long columnar grain, this translates to an
i