IGCAR : Annual Report - Indira Gandhi Centre for Atomic Research

IGC
Annual Report 2007
immersion transducer were
used to acquire the data and
obtain the C-scan images. The
first backwall echoes obtained
were gated to obtain the C-
scan image corresponding to
their amplitude. Electron back
scattered diffraction (EBSD)
data were collected using a
TSL-OIM (orientation imaging
microscopy) system on FEI
Quanta 200 SEM operating at
20 kV.
From the ultrasonic study, it
has been observed that the
specimens with true strains of
0.2 (Fig.1a) or lower did not
show any variation in the
amplitude of the ultrasonic first
backwall echo. The specimen
with 0.3 strain, on the other
hand, showed visible variation
in the C-scan image. However,
this variation was not
systematic. The specimen
forged to 0.5 strain could
clearly bring out systematic
variations in the amplitude of
the first backwall echo. It
exhibited higher amplitude of
the first backwall echoes along
one of the diagonals and one
of the curved sides (marked as
'A' and 'B' respectively in Fig.
1b).
Fig.2 Image quality (IQ) maps of hot deformed specimens at
0.2 strain. Boundaries with more than 5º misorientation and the twin
boundaries are respectively marked as dark and white lines
Fig.3 (a) Image quality (IQ) map of specimen forged to 0.5 strain.
Graphics conventions are same as in figure 2. (b) Enclosed section of
(a) is brought out (at a higher magnification) as IPF (inverse pole figure) map,
with more than 5º boundaries marked as dark lines.
(c) The 'necklace' grains (fine equiaxed grains around the grain
boundaries) were partitioned from (b). Grains with less than 5 m size
were considered as 'necklace' grains
It has been observed in our
recent study that attenuation of
ultrasonic waves of a particular
frequency in alloy D9 is mainly
governed by grain size. The
specimen with larger grain size
has been found to scatter more
as compared to the finer one.
Accordingly, larger grain size
regions lead to lower amplitude
of the first backwall echoes and
vice versa. This observation
could be employed to analyze
the findings of C-scan images.
Since specimen with 0.2 strain
did not show any perceptible
variation in the amplitude of
the first backwall echo, it is
likely that this specimen
contains almost uniform grain
size distribution. This could be
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