(a) 100 µm - Helmholtz-Zentrum Berlin

4.2 Temperature profiles 81
a smaller increment in step time and step temperature. It was found that for this
specimen the decrease in NG,F took place already at a shorter step time of 30 min
to 50 min instead of 40 min to 80 min as for the specimen examined above. In the
experiments both step temperature TS and step time tS were varied in order to de-
termine the course of the annealing temperature limit. Thus, various step time and
step temperature couples from tS = [30, 35, 40, 50 min] and TS = [5, 10, 15, 20, 25 °C]
were chosen. In Fig. 4.25 the difference between NG,F and NG,S, i.e. the additional
average nucleation density, is shown versus the average grain size dG,S at the step
time tS. It can clearly be seen in the figure that upon reaching an average grain
size at tS of about dG,S = 5.5 µm no further nucleation occurs. This behavior is
independent of the step temperature (not shown here). However, when the av-
erage grain size dG,S at tS is below 5.5 µm new nucleation takes place depending
on the step temperature TS. Higher TS are accompanied by an increase of the
nucleation (not shown in Fig. 4.25). The average final grain density does not ex-
ceed NG,F = 3.7 · 10 3 mm −2 in case of dG,S > 5.5 µm. Thus the maximum final
grain size that can be reached is about dG,F = 16.5 µm (dG,F = (1/NG,F ) 1/2 with
NG,F = 3.7 · 10 3 mm −2 ).
This observation can be interpreted as follows. At the step time tS there exists
a zone of dZ = 5.5 µm (at dG,S = 5.5 µm) on each side of the grain where no
new nucleation (dG,F = dG,S + 2dZ) takes place (Fig. 4.26). Once these zones
overlap the temperature can be increased to higher values and still now new grains
nucleate. This is a first experimental proof for the depletion regions responsible
for the suppression of nucleation suggested by Nast et al. [52].
The goal of the heating step experiments was to reduce the process time without
increasing the nucleation density. This goal was achieved. An estimated grain size
limit was found at which the annealing temperature could be increased without
new nucleation. This is a first proof for the depletion regions being responsible for
the nucleation suppression. With increasing the annealing temperature after the
critical grain size is reached the grain growth velocity can be increased strongly
and thus the process time is decreased without further nucleation.