- Email: [email protected]
Heterogeneous distribution of irradiation voids in iron
JOURNAL
OF NUCLEAR
35 (1970)
MATERIALS
HETEROGENEOUS
352-355.
DISTRIBUTION K. FARRELL
Me&
and Ceramics
Division,
Oak Ridge
0 NORTH-HOLLAND
OF IRRADIATION
PUBLISHING
VOIDS
CO., AMSTERDAAI
IN IRON
*
and J. T. HOUSTON
National
Laboratory,
Oak Ridge,
Tennessee
37830,
LCSA
Received 23 January 1970
Microscopic voids, prodwed by fast neutron irradiation at temperatures in the range 0.3 T, to approximately 0.5Tm, (T, is the melting point on the absolute scale) have been found in stainless steel l-4), aluminum 5-g), nickel lo), vanadium 11)) molybdenum 12) and iron 13). From these reports the impression is gained that, apart for denuded zones along grain boundaries, the voids are homogeneously distributed throughout the matrix. Aluminum,
expected to raise the temperature of the specimen to 400 “C; subsequent examination of closely attendant silicon carbide detectors indicated a temperature of 415 & 20 “C. The second specimen was sealed in an argon-filled capsule and heated at 400 “C to simulate the in-reactor heating.
The iron was spectrographic quality obtained from Johnson-Matthey in the form of 5 mm diameter rod. Its chemical composition in weight parts per million was 40 C,2 Hz, 280 N2,
Transmission electron microscopy revealed that partial recrystallization had occurred in both specimens. The extent of the recrystallization was not determined but there was no obvious difference in degree of recrystallization between the two specimens. No voids or dislocation loops were found in the unirradiated specimen. The irradiated specimen contained voids but no readily resolvable loops. The voids were quite clearly dispersed non-uniformly through the specimen. This non-uniformity was over and above that due to the normal void-free regions along grain boundaries. Two types of heterogeneous void distributions
38 02,
were apparent.
exposed to only a moderately high neutron fluence and containing only about 1.3 x 1014 voids/cm3, is an exception 7,s); here the voids are non-randomly dispersed, some of them being in planar arrays, or “walls”. This letter describes some further examples of highly heterogeneous void distributions in iron.
~20 S,6 P,
< 10 Mg,
3
Cr,
~3 Co,20 Ca,10 Mn,
< 1 Cu, and 0.02
B. Transmission
electron microscopy of the as-received rod revealed a microstructure of dislocation cells characteristic of a warm worked material. From this rod we machined two tensile specimens with 2 mm diameter gauge sections. This geometry was necessary to conform to that of other specimens in the irradiation experiment. One specimen of the iron was irradiated in a helium filled can in the High Flux Isotope Reactor to a fast neutron (E> 0.1MeV) fluence of 1.5 x 1021 neutrons/cm2. Gamma heating was * Research sponsored Corporation.
by the U.S.
Atomic
Energy 352
The first of these occurred
in
the recrystallized regions of the specimen only. Here the voids sometimes lay in clusters, were often Rssociated with dislocat,ions, and were frequently arranged in lines (figs. la and b). Many of these lines were very well defined and formed a broken network which, in the whole, would be strikingly like a planar section of a grain boundary network but on a larger scale than that of the subgrain boundaries in the non -recrystallized regions. Stereomicroscopy was difficult because of the magnetic properties of the iron, so we could not determine unCommission
under contract
with the Union
Carbide
IRRADIATION
VOIDS
IN
IRON
363
Fig. 1. Examples of voids in iron irradiated to 1.5X 1021 neutrons/cm2 at approximately 415 ‘C. (a) and (b) Recrystallized regions. (c) Non-recrystallized region. The mottled background in (a) and (b) is caused by a poor surface finish on the thinned foils.
ambiguously whether the apparent lines of voids
were really voids arranged in planes or “walls” like those found in aluminum 7). However, the network pattern suggested that the voids were more likely arranged in walls than in lines. The regions between the walls were r&tively free of voids except for the clusters ; these clusters could represent portions of walls lying almost parallel to the thinned foil. No walls
of voids were found in the non-recrystallized regions of the specimens (fig. lo). The second type of void distribution was evident in the partition of voids between recrystallized and non-~erystallized regions of the specimens. The non-recrystallized regions, composed of recovered subgrain boundaries and some isolated ~slooations, contained fewer and smaller resolvable voids than the recrystallized
354
K.
regions. From photographs equal
areas
of
recovered
BARRELL
AND
that covered almost and
recrystallized
regions of the foils, judged to be about respectively.
1000 A After
T.
Voids are more difficult to resolve in recovered
a
action the
percentage combined
as a
of the total number of voids in the recovered
and recrystallized
regions.
This is shown in fig. 2. The concentration of voids per unit volume of iron is indicated by fl. There are roughly three times as many resolvable voids in the recrystallized regions as in the recovered regions: the maximum and most frequent void sizes are two and three times larger in the recrystallized material.
of the strong
contrast
the dislocations
with the field of the objective
200
problem.
Sharp focussing becomes a These effects may have hidden some
of the smaller voids but are not likely to have affected the numbers of the larger voids. We are therefore confident that the measured discrepancy between recrystallized and recovered regions is real. This finding may have useful practical consequences. One suggestion for limiting the development of irradiation voids is to incorporate a large number of vacancy
400 VOID
Fig.
2.
Void
distributions
in recrystallized
lens in
microscope.
NON-RECRYSl
0
from
and sub-
grain boundaries. This difficulty is compounded in materials like iron because of magnetic inter-
of voids
size were expressed
because
strain fields around
small correction to equalize the areas the number of a given
HOUSTON
regions
thick, we measured the sizes of all visible voids, 183 and 682 in number,
J.
DIAMETER
600
800
(8)
and non-recrystallized
regions of the iron.
VOIDS
IRRADIATION
sinks such as dislocations, incoherent
particles
would compete
into
subboundaries, the
matrix.
irradiate
E. E. Bloom
for allowing
us to
the iron in one of his experiments.
with void nuclei for the excess
vacancies created by the irradiation, thus retard experimental
355
IRON
We thank
or
These
IN
and would
swelling. Fig. 2 offers support for this thesis.
The non-uniform the recrystallized
distribution
strong
References
1)
of voids within
regions is important
C. Cawthorne
and E. J. Fulton,
The nature of
small defect clusters, M. J. Makin, ed., Harwell (UK)
it casts very serious doubts on the validity of void nucleation theories based on homogeneous nucleation. It is much more compatible with the more recent views 7314915) that void nucleation is heterogeneous and is associated with We suspect that redissolved impurities. crystallization occurred before the voids appeared, and that the walls of voids were developed on impurities lying on the sites of original grain boundaries. It is now generally accepted that dissolved impurities segregate to high angle grain boundaries. It seems reasonable to suppose that these impurities might be left behind when strain-induced recrystallization occurs and may become preferred nucleation sites for voids. Nelson and Mazey 14) have demonstrated that the presence of certain impurities can markedly enhance void formation during heavy ion bombardment. Experiments on neutron irradiated stainless steel 15) and aluminum 9) have shown that preinjected helium has a significant influence on void formation. Kulcinski et al.13) found a uniform distribution of voids in iron. Their iron 16) was from the same source as ours and it received a somewhat higher neutron fluence, but it was annealed at 800 “C for 2 h prior to irradiation. Such a treat’ment presumably destroyed any impurity cells within the new grain structure.
Report
Nature
because
7 3,
J. J. Holmes,
R.
Acta
J. 0.
E.
Robbins, Met.
A. Withop
Design 9 (1969)
J. L. Brimhall
446;
J. L.
16 (1968)
Brimhall 955
J. Nucl.
Mat.
and U. E. Wolff,
Nucl.
265
and B. Mastel,
J. Nucl. Mat. 29
123
R. T. King, K.
p.
173
T. Lauritzen,
(1969)
9
(1966)
Stiegler and E. E. Bloom,
Eng.
5)
5269
575
and B. Mastel, 33 (1969)
4,
AERE-R
216 (1967)
Farrell,
7) J. 0.
E. L. Long, Jr., J. 0. J. Nucl.
Stiegler,
K.
Stiegler and
Mat.
35 (1970)
Farrell,
C. K.
and R. T. King, IAEA
231 H.
Symposium
DuBose
on Radiation
Damage in Reactor Materials, Vienna (June 2-6,
3,
1969) Vol.
2, 215
N. Packan
and D. N. Braski,
(1970)
8)
K. Farrell and R. T. King, Stat.
lo)
J.
Wiffen 1968)
Moteff,
IAEA Vol. )
J. Nucl. Mat. 28
J.
0.
Stiegler,
Rau
Materials,
on
and
F.
D.
Radiation
Metals
Vienna
Kingsbury, Damage
(June
2-6,
in
1969)
B. Mastel and J. L. Brimhall,
Effects
2 (1969)
57
R. S. Nelson and D. J. Mazey, IAEA
E.
J.
2, 269
Radiation
Vienna
Damage
(June 2-6,
E. Bloom
Phys. 13)
C.
G. L. Kulcinski,
on
19
and
R.
Radiation 19
to Phys.
117A
Symposium
Reactor 13
34
115
W.
(Aug.
12)
submitted
B. Ma&e1 and J. L. Brimhall, F.
Mat.
Sol.
(1968)
11)
J. Nucl.
307
Stat.
in
1969) Vol.
and J. 0.
Symposium
Reactor
Materials,
2, 157
Stiegler,
submitted
Sol.
J. L. Brimhall,
private
correspondence
to
OF NUCLEAR
35 (1970)
MATERIALS
HETEROGENEOUS
352-355.
DISTRIBUTION K. FARRELL
Me&
and Ceramics
Division,
Oak Ridge
0 NORTH-HOLLAND
OF IRRADIATION
PUBLISHING
VOIDS
CO., AMSTERDAAI
IN IRON
*
and J. T. HOUSTON
National
Laboratory,
Oak Ridge,
Tennessee
37830,
LCSA
Received 23 January 1970
Microscopic voids, prodwed by fast neutron irradiation at temperatures in the range 0.3 T, to approximately 0.5Tm, (T, is the melting point on the absolute scale) have been found in stainless steel l-4), aluminum 5-g), nickel lo), vanadium 11)) molybdenum 12) and iron 13). From these reports the impression is gained that, apart for denuded zones along grain boundaries, the voids are homogeneously distributed throughout the matrix. Aluminum,
expected to raise the temperature of the specimen to 400 “C; subsequent examination of closely attendant silicon carbide detectors indicated a temperature of 415 & 20 “C. The second specimen was sealed in an argon-filled capsule and heated at 400 “C to simulate the in-reactor heating.
The iron was spectrographic quality obtained from Johnson-Matthey in the form of 5 mm diameter rod. Its chemical composition in weight parts per million was 40 C,2 Hz, 280 N2,
Transmission electron microscopy revealed that partial recrystallization had occurred in both specimens. The extent of the recrystallization was not determined but there was no obvious difference in degree of recrystallization between the two specimens. No voids or dislocation loops were found in the unirradiated specimen. The irradiated specimen contained voids but no readily resolvable loops. The voids were quite clearly dispersed non-uniformly through the specimen. This non-uniformity was over and above that due to the normal void-free regions along grain boundaries. Two types of heterogeneous void distributions
38 02,
were apparent.
exposed to only a moderately high neutron fluence and containing only about 1.3 x 1014 voids/cm3, is an exception 7,s); here the voids are non-randomly dispersed, some of them being in planar arrays, or “walls”. This letter describes some further examples of highly heterogeneous void distributions in iron.
~20 S,6 P,
< 10 Mg,
3
Cr,
~3 Co,20 Ca,10 Mn,
< 1 Cu, and 0.02
B. Transmission
electron microscopy of the as-received rod revealed a microstructure of dislocation cells characteristic of a warm worked material. From this rod we machined two tensile specimens with 2 mm diameter gauge sections. This geometry was necessary to conform to that of other specimens in the irradiation experiment. One specimen of the iron was irradiated in a helium filled can in the High Flux Isotope Reactor to a fast neutron (E> 0.1MeV) fluence of 1.5 x 1021 neutrons/cm2. Gamma heating was * Research sponsored Corporation.
by the U.S.
Atomic
Energy 352
The first of these occurred
in
the recrystallized regions of the specimen only. Here the voids sometimes lay in clusters, were often Rssociated with dislocat,ions, and were frequently arranged in lines (figs. la and b). Many of these lines were very well defined and formed a broken network which, in the whole, would be strikingly like a planar section of a grain boundary network but on a larger scale than that of the subgrain boundaries in the non -recrystallized regions. Stereomicroscopy was difficult because of the magnetic properties of the iron, so we could not determine unCommission
under contract
with the Union
Carbide
IRRADIATION
VOIDS
IN
IRON
363
Fig. 1. Examples of voids in iron irradiated to 1.5X 1021 neutrons/cm2 at approximately 415 ‘C. (a) and (b) Recrystallized regions. (c) Non-recrystallized region. The mottled background in (a) and (b) is caused by a poor surface finish on the thinned foils.
ambiguously whether the apparent lines of voids
were really voids arranged in planes or “walls” like those found in aluminum 7). However, the network pattern suggested that the voids were more likely arranged in walls than in lines. The regions between the walls were r&tively free of voids except for the clusters ; these clusters could represent portions of walls lying almost parallel to the thinned foil. No walls
of voids were found in the non-recrystallized regions of the specimens (fig. lo). The second type of void distribution was evident in the partition of voids between recrystallized and non-~erystallized regions of the specimens. The non-recrystallized regions, composed of recovered subgrain boundaries and some isolated ~slooations, contained fewer and smaller resolvable voids than the recrystallized
354
K.
regions. From photographs equal
areas
of
recovered
BARRELL
AND
that covered almost and
recrystallized
regions of the foils, judged to be about respectively.
1000 A After
T.
Voids are more difficult to resolve in recovered
a
action the
percentage combined
as a
of the total number of voids in the recovered
and recrystallized
regions.
This is shown in fig. 2. The concentration of voids per unit volume of iron is indicated by fl. There are roughly three times as many resolvable voids in the recrystallized regions as in the recovered regions: the maximum and most frequent void sizes are two and three times larger in the recrystallized material.
of the strong
contrast
the dislocations
with the field of the objective
200
problem.
Sharp focussing becomes a These effects may have hidden some
of the smaller voids but are not likely to have affected the numbers of the larger voids. We are therefore confident that the measured discrepancy between recrystallized and recovered regions is real. This finding may have useful practical consequences. One suggestion for limiting the development of irradiation voids is to incorporate a large number of vacancy
400 VOID
Fig.
2.
Void
distributions
in recrystallized
lens in
microscope.
NON-RECRYSl
0
from
and sub-
grain boundaries. This difficulty is compounded in materials like iron because of magnetic inter-
of voids
size were expressed
because
strain fields around
small correction to equalize the areas the number of a given
HOUSTON
regions
thick, we measured the sizes of all visible voids, 183 and 682 in number,
J.
DIAMETER
600
800
(8)
and non-recrystallized
regions of the iron.
VOIDS
IRRADIATION
sinks such as dislocations, incoherent
particles
would compete
into
subboundaries, the
matrix.
irradiate
E. E. Bloom
for allowing
us to
the iron in one of his experiments.
with void nuclei for the excess
vacancies created by the irradiation, thus retard experimental
355
IRON
We thank
or
These
IN
and would
swelling. Fig. 2 offers support for this thesis.
The non-uniform the recrystallized
distribution
strong
References
1)
of voids within
regions is important
C. Cawthorne
and E. J. Fulton,
The nature of
small defect clusters, M. J. Makin, ed., Harwell (UK)
it casts very serious doubts on the validity of void nucleation theories based on homogeneous nucleation. It is much more compatible with the more recent views 7314915) that void nucleation is heterogeneous and is associated with We suspect that redissolved impurities. crystallization occurred before the voids appeared, and that the walls of voids were developed on impurities lying on the sites of original grain boundaries. It is now generally accepted that dissolved impurities segregate to high angle grain boundaries. It seems reasonable to suppose that these impurities might be left behind when strain-induced recrystallization occurs and may become preferred nucleation sites for voids. Nelson and Mazey 14) have demonstrated that the presence of certain impurities can markedly enhance void formation during heavy ion bombardment. Experiments on neutron irradiated stainless steel 15) and aluminum 9) have shown that preinjected helium has a significant influence on void formation. Kulcinski et al.13) found a uniform distribution of voids in iron. Their iron 16) was from the same source as ours and it received a somewhat higher neutron fluence, but it was annealed at 800 “C for 2 h prior to irradiation. Such a treat’ment presumably destroyed any impurity cells within the new grain structure.
Report
Nature
because
7 3,
J. J. Holmes,
R.
Acta
J. 0.
E.
Robbins, Met.
A. Withop
Design 9 (1969)
J. L. Brimhall
446;
J. L.
16 (1968)
Brimhall 955
J. Nucl.
Mat.
and U. E. Wolff,
Nucl.
265
and B. Mastel,
J. Nucl. Mat. 29
123
R. T. King, K.
p.
173
T. Lauritzen,
(1969)
9
(1966)
Stiegler and E. E. Bloom,
Eng.
5)
5269
575
and B. Mastel, 33 (1969)
4,
AERE-R
216 (1967)
Farrell,
7) J. 0.
E. L. Long, Jr., J. 0. J. Nucl.
Stiegler,
K.
Stiegler and
Mat.
35 (1970)
Farrell,
C. K.
and R. T. King, IAEA
231 H.
Symposium
DuBose
on Radiation
Damage in Reactor Materials, Vienna (June 2-6,
3,
1969) Vol.
2, 215
N. Packan
and D. N. Braski,
(1970)
8)
K. Farrell and R. T. King, Stat.
lo)
J.
Wiffen 1968)
Moteff,
IAEA Vol. )
J. Nucl. Mat. 28
J.
0.
Stiegler,
Rau
Materials,
on
and
F.
D.
Radiation
Metals
Vienna
Kingsbury, Damage
(June
2-6,
in
1969)
B. Mastel and J. L. Brimhall,
Effects
2 (1969)
57
R. S. Nelson and D. J. Mazey, IAEA
E.
J.
2, 269
Radiation
Vienna
Damage
(June 2-6,
E. Bloom
Phys. 13)
C.
G. L. Kulcinski,
on
19
and
R.
Radiation 19
to Phys.
117A
Symposium
Reactor 13
34
115
W.
(Aug.
12)
submitted
B. Ma&e1 and J. L. Brimhall, F.
Mat.
Sol.
(1968)
11)
J. Nucl.
307
Stat.
in
1969) Vol.
and J. 0.
Symposium
Reactor
Materials,
2, 157
Stiegler,
submitted
Sol.
J. L. Brimhall,
private
correspondence
to