IGCAR : Annual Report - Indira Gandhi Centre for Atomic Research

IGC
Annual Report 2007
was determined by step height
measurements. Figure 1 shows
the void swelling in the two
alloys as a function of the
irradiation temperature. It is
found that the peak swelling
temperatures and the
magnitude of swelling for the
two alloys are different.
Whereas the alloy with 0.15%
Ti displayed a swelling of
~15% at the peak swelling
temperature of 923 K, the alloy
with 0.25% Ti has a swelling
maximum of ~ 4% at 823 K.
With similar thermomechanical
treatment effected
on both the alloys the
difference in void swelling
behavior is solely due to the
effect of chemical composition.
In order understand the
drastically different behaviour
of the two alloys with regard to
void swelling, the TiC
precipitate formation in these
two alloys was studied by
positron lifetime measurements.
The un-irradiated alloys were
subjected to isochronal
annealing and the positron life
time was measured after each
annealing. These alloys show
different TiC precipitate
formation behavior (Fig.2). The
observed variation of lifetime τ
displays distinct stages viz., a
monotonic decrease in τ from
the initial cold worked state
upto ~900 K in alloy B and ~
800 K in alloy A. This is
followed by a stage where there
is an increase in lifetime to
saturation, followed by a
decrease in lifetime. The first
stage corresponds to point
defect recovery arising out of
the migration of vacancies to
sinks such as dislocations .The
subsequent stage where there is
an increase in lifetime τ is the
result of positron trapping by
the TiC precipitate which forms
during the heat treatment. The
increase in average lifetime of
positrons in this stage for alloy
A in comparison to alloy B is
due to the higher number
density of TiC precipitates in the
former. The observed lower
void swelling in alloy A can be
attributed to the higher number
density of TiC precipitates .It is
further seen (Fig.2) that there is
a difference in the onset of TiC
precipitation by 100K between
the two alloys. The Shift in the
peaking swelling temperature
can be attributed to the
difference in the onset
temperature for TiC
precipitation for the two alloys.
Thus, positron lifetime
measurements helps to
rationalize the different swelling
behavior in alloys with different
Ti concentrations.
Fig.1 Temperature dependence of void swelling measured
by surface profilometry for the D9alloys with different
titanium concentration
Fig.2 Variation of positron life time with annealing
temperature for the cold -worked D9 alloys with
different titanium concentration, alloy A (Ti / C = 6) and
alloy B (Ti /C = 4)
46 R&D FOR FBRs