JAEA-Review-2010-065.pdf:15.99MB - 日本原子力研究開発機構

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Vacancy Generation around an SCC Crack Tip
in Stainless Steels Probed by a Positron Microbeam
In the 1970’s, the stress corrosion cracking (SCC)
occurred frequently in reactor materials of Type 304
austenitic stainless steels. It has been recognized that the
degradation of the corrosion resistance is caused by the
reduction of chromium concentration near the grain
boundary due to chromium carbide formation during
welding. This chromium carbide formation phenomenon is
called sensitization, and nuclear-reactor materials were
replaced by low-carbon stainless steels to avoid sensitization.
Nevertheless, the SCC still occurs in the high-temperature
pure-water reactor environment. This suggests that the
suppression of the corrosion resistance degradation can not
prevent the SCC failure. Recently, the SCC propagation
model that the vacancy-type defects that accumulate in the
1, 2)
crack tip play a role as a crack nucleus was proposed .
However, there is no information available on the generation
or accumulation mechanism of such vacancies around the
SCC crack tips. In this study, lattice defect spatial
distributions around the SCC crack tip in an austenitic
stainless steel have been probed by a positron microbeam.
A Type 304 stainless steel foil with 5 × 10 mm in size
and 30 μm in thickness was annealed for 24 hours at 650 o C
in the vacuum. The foil was attached to the compact
tensile test specimen holder and tensile stress was applied.
The holder was exposed into boiling MgCl2 solution, so the
SCC crack was introduced. The SCC crack specimen was
3)
loaded into the positron microbeam apparatus . The SCC
crack specimen was irradiated with a 20-keV positron
microbeam. When vacancy-type defects are present in a
(a) Optical Image
(b) S-parameter Image
1 mm
A. Yabuuchi, M. Maekawa and A. Kawasuso
Advanced Science Research Center, JAEA
0.98 0.99 1.00 1.01 1.02
Normalized S parameter
0.98 0.99 1.00 1.01 1.02
Fig. 1 (a) Optical image of the SCC crack in the
sensitized Type 304 stainless steel and (b) S-parameter
image around the SCC crack. The black pixel in the
S-parameter image corresponds to the pre-notch and
the SCC crack gap.
JAEA-Review 2010-065
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material, annihilation gamma-ray peak intensity is increased.
We have evaluated the change in peak intensity using the S
parameter which increases with increasing peak intensity.
Figure 1 shows the measured S-parameter distribution
map obtained from an SCC crack specimen. From this
result, the increase of S parameter was observed around the
SCC crack. To investigate what type of defect has caused
the increase of this S parameter, the gamma-ray spectrum
obtained from the surrounding of the SCC crack was
compared with the spectrum obtained from the tensile test
specimen. The tensile specimen was prepared from a
50-μm thick Type 316L stainless steel foil. The test piece
was solution-annealed and tensile-deformed with a nominal
strain of 22%. The spectra obtained from both specimen
are plotted in Fig. 2 with the ratio change from the standard
spectra. These two ratio curves agree well, which means
that the same type of defect is included in the two specimens.
Furthermore, the calculated spectrum of mono-vacancy in
bcc-Fe replicates the experimental spectra. On the contrary,
corrosion test revealed corrosion-induced defects were
introduced only to 100 nm from the surface. These results
indicate that the increase of S parameter near the SCC crack
was caused by plastic-deformation-induced vacancies.
References
1) R.W. Staehle, "Proc. Int. Conf. Water Chem. of Nucl.
React. Sys.", Jeju Is., Korea (2006).
2) K. Arioka et al., INSS Journal 13 (2006) 168.
3) M. Maekawa and A. Kawasuso, Appl. Surf. Sci. 255
(2008) 39.
Ratio to bulk bulk bulk
1.2
1.0
0.8
0.6
512 514 516 518
-ray -ray -ray Energy, E E E / keV
V 1
Fig. 2 The ratio curves for SCC crack specimen (●)
and tensile test specimen (○). The solid line
represents the calculation result with annihilating at
mono-vacancy.