Materials Transactions, Vol. 49, No. 10 (2008) pp. 2259 to 2264
#2008 The Japan Institute of Metals
Precipitation of Solid Transmutation Elements in Irradiated Tungsten Alloys
Takashi Tanno1; *1 , Akira Hasegawa1 , Mitsuhiro Fujiwara1 , Jian-Chao He1; *1 ,
Shuhei Nogami1 , Manabu Satou1 , Toetsu Shishido2 and Katsunori Abe1; *2
1
Department of Quantum Science and Energy Engineering, Graduate School of Engineering,
Tohoku University, Sendai 980-8579, Japan
2
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
Tungsten-based model alloys were fabricated to simulate compositional changes by neutron irradiation, performed in the JOYO fast test
reactor. The irradiation damage range was 0.17–1.54 dpa and irradiation temperatures were 400, 500 and 750 C. After irradiation,
microstructural observations and electrical resistivity measurements were carried out. A number of precipitates were observed after 1.54 dpa
irradiation. Rhenium and osmium were precipitated by irradiation, which suppressed the formation of dislocation loops and voids. Structures
induced by irradiation were not observed so much after 0.17 dpa irradiation. Electrical resistivity measurements showed that the effects of
osmium on the electrical resistivity, related to impurity solution content, were larger than that of rhenium. Measurements of electrical resistivity
of ternary alloys showed that the precipitation behavior was similar to that in binary alloys. [doi:10.2320/matertrans.MAW200821]
(Received April 23, 2008; Accepted August 8, 2008; Published September 18, 2008)
Keywords: tungsten, solid transmutation element, precipitation, neutron irradiation, composition change, electrical resistivity
1.
Introduction
Tungsten (W) is one of the candidate materials for fusion
reactors because of its high melting point, high sputtering
resistivity and high temperature strength. During fusion
reactor operation, it is anticipated that due to 14 MeV neutron
irradiation, in addition to irradiation damage, solid transmutation elements, such as rhenium (Re) and osmium (Os),
will be induced in pure tungsten. Consequently, with
increasing neutron fluence, the solid transmutation elements
will accumulate and the original pure tungsten will change
to W-xRe or W-xRe-yOs alloys. For example, calculations
predict that pure W will change to a W-18Re-3Os alloy after
50 dpa irradiation, corresponding to a neutron fluence of
10 MWy/m2 .1)
It is well known that tungsten shows brittle behavior at
room temperature though it has excellent high temperature
properties, and the low temperature embrittlement can be
improved by Re addition.2,3) It was reported that the creep
strength of tungsten irradiated with thermal neutrons is
higher than that of unirradiated tungsten, and substitutional
solid solution rhenium transmuted from tungsten seems to be
the reason why the creep strength increased.4) Rhenium and
osmium also change the physical properties of tungsten. For
example, tungsten has high thermal conductivity, however
this decreases to half the value for pure tungsten by the
addition of 5 mass% Re.5,6) In terms of microstructural
evolution, it has been reported that phase precipitates
(Re3 W) were formed in W-xRe alloys after 0.5–0.7 dpa
irradiation at 600–1500 C without void formation, and the
bulk properties were affected by this precipitation.7) Heavy
neutron irradiation induced large irradiation hardening in W26Re alloy by irradiation-induced precipitation.8) However,
even though rhenium and osmium are the main transmutation
*1Graduate
Student, Tohoku University
*2Present address: Hachinohe Institute of Technology, Hachinohe 0318501, Japan
elements from tungsten, there are very few reports concerning irradiation effects on W-xRe and W-xRe-yOs alloys.
For materials used in fusion reactors, mechanical properties such as hardness and ductility are important when large
electromagnetic force and thermal stresses are induced in the
materials by instability of plasma. We have previously
reported differences in the irradiation hardening of W-xRe
and W-yOs alloys, formed primarily by irradiation precipitation.9) Thermal conductivity is also important for cooling
which is related to electrical conductivity. The electrical
resistivity, which is the inverse of the electrical conductivity,
depends on the solute content.
The purpose of this study was to investigate precipitation
behavior of rhenium and osmium in neutron-irradiated
tungsten alloys systematically as a function of the alloy
composition and irradiation dose in order to understand
irradiation-induced property changes of tungsten under
fusion reactor conditions.
2.
Experimental Procedure
In order to simulate composition changes due to transmutation, tungsten based model alloys containing rhenium
and/or osmium were fabricated. The nominal compositions
of the examined alloys are shown in Table 1. They were
selected basing compositional change predicted by calculation of transmutation1) in the solid solution area of W-xReyOs alloys. Model alloy fabrications were carried out using
an argon arc furnace. The raw materials were pure W
(99.96%) and W-26Re (W: 74:0 0:2%, Re: 26:0 0:2%)
rods supplied by Plansee Ltd. and Os (99.9%) powder
Table 1
Nominal compositions of the fabricated alloys (mass%).
W
bal.
bal.
bal.
bal.
bal.
bal.
bal.
bal.
bal.
bal.
bal.
Re
—
5
10
26
—
—
5
10
18
25
5
Os
—
—
—
—
3
5
3
3
3
3
5
2260
T. Tanno et al.
supplied by Kojundo Chemical Laboratory Co., Ltd. Interstitial impurity levels of the fabricated alloys were in the
range of 40–200 wppm for carbon (C), 20–40 wppm for
oxygen (O) and <12 wppm for nitrogen (N). Disk-shaped
specimens with a diameter of 3 mm and a thickness of
0.3 mm were cut from the ingots using an electro discharge
machine, mechanically ground and polished to 0.2 mm
thickness, and finally annealed at 1400 C for 1 h in vacuum
(<105 Pa).10)
Neutron irradiation was carried out in the JOYO fast test
reactor in Japan Atomic Energy Agency (JAEA). The
irradiation conditions used are shown in Table 2. Displacement damage (dpa) was calculated using the NPRIM-1.3
code11) with a displacement threshold energy of 90 eV.12)
Greenwood et al. calculated the compositional change due
to transmutation during irradiation in the fast reactor.13)
According to their results, transmutation of tungsten is
negligibly small even after 1.54 dpa irradiation in JOYO.
After the irradiation, microstructural observations were
carried out with a transmission electron microscope (TEM)
operating at 200 kV. Electrical resistivity measurements were
carried out using the four-probe method at 20 C with a load
current I of 100 mA, distance between the probes S of
Table 2
Neutron irradiation conditions.
Irradiation
temperature
( C)
Fluence
(En > 0:1 MeV)
(1025 n/m2 )
dpa
400
1.3
0.17
500
2.9
0.37
740
3.1
0.40
750
12
1.54
¼ ðV t=IÞ C:F:
3.
ð1Þ
Results
3.1 Microstructural observations
Figure 1 shows the microstructures of irradiated W-5Re,
W-3Os and W-5Re-3Os alloys. Voids and dislocation loops,
seen as black dots, were observed in W-5Re after 0.17 dpa
irradiation at 400 C. Plate or needle-like precipitates on the
{110} plane were observed instead of dislocation loops for a
higher dose and irradiation temperature (1.54 dpa irradiation
at 750 C). Though the mean diameter of the voids grew to
twice as large, the number density of voids decreased to onetenth that of the lower dose and temperature. Precipitates
were also observed in other W-xRe alloys after 0.40 dpa and
above irradiation at 750 C. These precipitates were identified
as the phase (Re3 W) from the electron diffraction pattern.8)
A special structure due to irradiation was not observed
in W-3Os after 0.17 dpa irradiation at 400 C. Needle-like
precipitates in the {110} plane were observed after 1.54 dpa
irradiation at 750 C. A few black dots were observed but
voids were not seen. The widths of the precipitates were
smaller than those in W-5Re alloys. The precipitates could
be not identified from the diffraction patterns, but they are
likely the phase (Osx W1x , x ¼ 0:20{0:35) because this is
the only intermetallic compound, according to the W-yOs
phase diagram.15)
W-3Os
W-5Re-3Os
1.54dpa/750°C
0.17dpa/400 °C
W-5Re
0.635 mm and load P of 0.98 N. Electrical resistivity was
calculated using eq. (1), where V is the measured voltage
drop, t is the specimen thickness, C:F: is the conversion
factor fixed by the specimen shape (circle, rectangle, etc.)
and the measurement conditions. In this studym C:F: was
3.5242.14)
Fig. 1 TEM bright field images of W-5Re, W-3Os and W-5Re-3Os alloys after 0.17 dpa irradiation at 400 C and 1.54 dpa at 750 C. The
electron incidence directions were Z = [111], only the direction of W-5Re-3Os at 750 C was Z = [001]. There were few visible
structures in alloys irradiated at lower dose conditions. Plate or needle-like precipitates in the {110} plane were observed.
Precipitation of Solid Transmutation Elements in Irradiated Tungsten Alloys
Table 3
2261
Summary of microstructural observations of alloys after 1.54 dpa irradiation at 750 C.
W
W-5Re
W-10Re
W-3Os
W-5Re-3Os
—
Void
Mean diameter
Density
[nm]
4.7
3.3
1.6
—
[1022 /m3 ]
12
0.65
3.1
—
—
[%]
0.72
0.01
0.01
—
—
6.8
Swelling
Precipitate
Mean length
[nm]
—
14
9.5
7.3
Density
[1022 /m3 ]
—
7.3
42
22
67
Volume
[%]
—
1.5
3.6
0.80
3.4
50
50
Electrical resistivity ρ /µΩ cm
Electrical resistivity ρ /µΩ cm
W-xRe-3Os
W-xRe
40
W-yOs
30
20
1.54dpa/750 °C
0.40dpa/740 °C
0.37dpa/500 °C
0.17dpa/400 °C
unirradiated
10
40
30
20
1.54dpa/750° C
0.40dpa/740° C
0.37dpa/500° C
0.17dpa/400° C
unirradiated
10
0
0
0
5
10
15
20
25
30
Re x or Os y /mass%
0
5
10
15
20
25
30
Re x /mass%
Fig. 2 Electrical resistivity measurement results of W-xRe and W-yOs
alloys before and after irradiation.
Fig. 3 Electrical resistivity measurement results of W-5Re-3Os alloys
before and after irradiation.
No specific microstructure formed by the irradiation was
observed by TEM in W-5Re-3Os after 0.17 dpa irradiation at
400 C. The microstructure after 1.54 dpa irradiation at 750 C
was similar to that of W-5Re irradiated under the same
conditions. However, the observed plate- or needle-like
precipitates on {110} were finer than those in W-5Re, and
voids were not observed in the W-5Re-3Os alloys. It was
also confirmed that same precipitates were formed in W25Re-3Os alloys irradiated to above 0.40 dpa at 750 C.
The microstructural features of W, W-5Re, W-10Re, W3Os and W-5Re-3Os alloys irradiated at 1.54 dpa at 750 C
are shown in Table 3. Rhenium and osmium addition over a
few mass% suppressed void formation and swelling. The
number density of precipitates increased with increasing
content of the transmutation elements.
as x increases. The values of the resistivity of the irradiated
alloys for x ¼ 5 to 10 tended to be smaller as the dose
increased though changes in resistivity due to irradiation
were slight. In contrast, the change in the resistivity of W26Re by irradiation showed the opposite tendency.
In the case of the W-yOs alloys, the resistivity increased
linearly as the osmium content y increased, and this linear
relationship was maintained after irradiation. The reduction
in resistivity by irradiation was much larger than those of
W-xRe alloys. In the case of 1.54 dpa irradiation at 750 C,
the reduction can be explained using eq. (2), wherein
is the reduction in resistivity by 1.54 dpa irradiation,
unirra. is the resistivity of unirradiated alloy, 1:54 dpa is the
resistivity of alloy after 1.54 dpa irradiation.
3.2 Electrical resistivity
Figure 2 shows the measured electrical resistivity of WxRe and W-yOs binary alloys before and after irradiation.
The resistivity changes in pure tungsten due to the irradiation
were less than 20%, and were smaller than the changes
caused by solution effects due to rhenium or osmium.
In the case of the W-xRe alloys, the variation in the
resistivity is not a simple linear relationship as a function of
the rhenium content x, rather the resistivity tends to saturate
¼ unirr. ðyÞ 1:54 dpa ðyÞ
2
ð2Þ
¼ ½unirr. ðyÞ unirr. ðy ¼ 0Þ
3
The reduction ratio decreased as the irradiation dose
decreased though the reduction was relative to the Os
content y.
Figure 3 shows the measured electrical resistivity of WxRe-3Os ternary alloys before and after irradiation. The
resistivity increase behavior of the unirradiated alloys was
similar to that of the W-xRe binary alloys. The resistivity
2262
T. Tanno et al.
tended to decrease as the irradiation dose increased however
the reduction became smaller as the rhenium content
increased. The electrical resistivity of W-5Re-yOs alloys
showed similar behavior to that of W-yOs alloys.
4.
Discussion
4.1 Microstructure
In the case of 0.17 dpa irradiation at 400 C, dislocation
loops were observed only in the W-5Re alloys. Thus,
rhenium and osmium might interrupt interstitial cluster
formation. Void swelling calculated by the size and density
of voids in the W-xRe alloys after 1.54 dpa irradiation at
750 C decreased drastically by the addition of rhenium. This
reduction suggests that rhenium interrupts vacancy cluster
formation. The interruption effects of osmium are much
stronger than those of rhenium because no voids were
observed, using TEM, in alloys containing osmium.
Rhenium and osmium are under-sized atoms compared to
tungsten. Under-sized solutes in alloys cause irradiation
effects that do not occur in the corresponding pure metal. The
first such effect is segregation toward a point defect sink,
such as a grain boundary, a dislocation etc. Over-sized
solutes migrate by exchange with vacancies. The direction of
migration for the over-sized solutes is thus opposite to that of
vacancy fluxes when the direction is toward a sink. On the
other hand, under-sized solutes readily bond with interstitial
atoms and form dumbbells. The interstitial atoms are thus
dragged toward the point defect sinks. As a result, undersized solutes are transported toward the sink with interstitial
atoms and cause segregation at the sink. Precipitation by
the interstitial mechanism has been observed at the surface
of Mo-Re alloys after irradiation at high temperatures.16)
Second, as a result of trapping of interstitial defects by
under-sized solutes, the interstitial defects cannot combine
with the vacancies and the survival rate of vacancies
increases. The surviving vacancies can enhance nucleation
and growth of voids. It has been reported that iron, which is
an under-sized atom compared to vanadium, enhanced void
swelling of irradiated vanadium-added iron.17)
Precipitation localized on the grain boundary was not
observed in this work. Therefore void swelling decreased due
to the addition of rhenium and osmium. This suggests that the
sinks, which are recombination sites of interstitial atoms and
vacancies, were in the grains. It has been suggested that
vacancy clusters or micro-voids could be the nuclei for
precipitation for under-sized atoms, and the interfaces
between the precipitates and the matrix could be the sinks
of interstitial atoms and vacancies.18) Although this idea was
not verified experimentally, it is consistent with results of this
work. Microstructural observations after 0.17 dpa irradiation
at 400 C suggested that rhenium and osmium enhanced
migration of interstitial atoms toward micro-vacancy clusters, and the nucleation of voids was interrupted. As a result,
rhenium and osmium transported with the interstitial atoms
segregate and precipitate at the site of the micro-vacancy
cluster. The interfaces between the matrix and precipitates of
rhenium and osmium would be a stronger sink for vacancies
and interstitial atoms. Thus, precipitation of under-sized
rhenium and osmium would be enhanced; in addition,
nucleation and growth of voids might be suppressed as seen
for the alloys after 1.54 dpa irradiation at 750 C. Osmium
could precipitate with a lower dose because its solubility
is lower than that of rhenium. As a result, precipitates might
be finer with a smaller mean space in irradiated alloys
containing osmium.
4.2 Solution content
Electrical resistivity can be explained by Matthiessen’s
rule with parameters of temperature, impurity concentration
and deformation rate. In this study, electrical resistivity
measurements were carried out at approximately the same
temperature and no deformation was induced. Therefore, any
resistivity changes were only due to the impurity levels. The
resistivity change due to impurities is related to atomic
content of the impurities. The mass content is nearly equal to
the atomic masses in the model alloys, because tungsten,
rhenium and osmium are close to each other in the periodic
table and have approximately the same mass. Thus, the
electrical resistivity (mcm) can be explained using eq. (3),
wherein 0 is the electrical resistivity of pure tungsten,
Ci is the solution content (mass%) and Ai is a parameter
which depends on the concentration of impurity elements
in tungsten.
X
¼ 0 þ
Ai ðCi =100Þð1 Ci =100Þ
ð3Þ
i
0 at 20 C was 5.9 mcm, which was obtained by measuring
pure tungsten before irradiation. Ai s of the binary and ternary
alloys were obtained, as shown in Table 4, by measurements
and curve fitting for the results of this work. The values
suggest that the effects of rhenium and osmium on electrical
resistivity are independent of each other, and the effect of
osmium is five times as large as that of rhenium.
Equation (3) can also estimate the solution content Ci
from the results of resistivity measurements. The predicted
solution contents in irradiated binary alloys are shown in
Table 5. In the case of W-5, 10Re alloys, CRe tended to
decrease as the dose increased. CRe s of alloys irradiated at
Table 4
Impurity parameters for electrical resistivity.
in Binary
in Ternary
ARe
138
108
AOs
602
583
Table 5 Solution content changes estimated by the resistivity.
Nominal
content
(mass%)
Estimated solution content Ci (mass%)
unirrad.
0.17 dpa
0.37 dpa
0.40 dpa
1.54 dpa
400 C
500 C
740 C
750 C
W-xRe
5
5.3
6.4
6.1
6.4
4.7
10
11.6
14.4
12.0
—
10.6
26
24.7
24.9
26.6
29.0
36.7
4.3
6.0
2.1
3.4
1.6
2.4
1.4
2.4
1.3
1.7
W-yOs
3
5
Precipitation of Solid Transmutation Elements in Irradiated Tungsten Alloys
50
W-xRe-3Os
Electrical resistivity ρ /µΩ cm
a lower dose were higher than the nominal contents. The
increment by irradiation seemed to be a result of the
resistivity increment by irradiation-induced defects. However, there is not matter for relative comparison because the
increment of resistivity was independent of dose in this
work. Thus, the amount of precipitation of rhenium was up
to 25% of nominal content at the highest irradiation dose
condition. This value agrees with the calculated one from
the results of microstructural observations, however, the CRe
of W-26Re increased as the dose increased. If the volume
of precipitates was sufficiently high, the resistivity of the
precipitates affected the bulk resistivity related to the
volume fraction. However, precipitation occurred in other
alloys, therefore the opposite tendency could be not
explained only by precipitates. Osmium transmuted from
rhenium might be another source. In this study, the
predicted production of rhenium from pure tungsten was
0.7% at most, however, osmium production from W-26Re
reached about 1% because the cross-section of transmutation from rhenium to osmium might be larger than that from
tungsten to rhenium according to the calculation results.13)
The effect on resistivity of osmium is very large as
described above, those can explain the irregular CRe
increment in W-26Re with dose increasing.
The decrease of COs clearly depended on the irradiation
dose. The decrease reflects the solute content changes
because the resistivity change due to the solution osmium
is much larger than that due to other factors. The calculated
results showed that the solute osmium content decreased to
half of that of the unirradiated one after irradiation of just
0.17 dpa at 400 C. Only 30% of osmium existed as solute
atoms in the alloys irradiated to 1.54 dpa at 750 C. On the
other hand, precipitation of osmium estimated by microstructural observations of W-3Os was less than 10% of the
nominal content. In addition, no precipitates were observed
in alloys after 0.17 dpa irradiation at 400 C. Thus, the
observed precipitates were a part of the precipitated or
segregated osmium, and most of them would exist as
invisible micro-clusters.
The resistivity changes of ternary alloys irradiated at the
lowest and highest dose conditions are shown in Fig. 4. In
this figure, the symbols represent the measured values and
the dotted lines are calculated values from the results of
binary alloys obtained using eq. (3). Each dotted line is well
correlated with the measured values. Thus, it is considered
that the behavior of CRe and COs due to irradiation in ternary
alloys were similar to that in binary alloys irradiated at the
same conditions.
Based on the results obtained in this work, precipitation
behavior in tungsten under fusion operation conditions are
predicted, as follows. First, voids are formed by irradiation,
and nucleation and growth of the voids are suppressed by the
presence of rhenium as transmutation from tungsten to
rhenium progresses. When the rhenium content reaches a few
percent, precipitation starts and the precipitates increase as
the rhenium content increases. Osmium transmuted from
rhenium would form clusters rapidly, and finer precipitates
would form with a high number density. As a result, most
of the osmium existing as precipitates or micro-clusters could
be observed by TEM.
2263
40
30
1.54dpa/750 °C
0.17dpa/400 °C
unirradiated
1.54dpa(calc.)
0.17dpa(calc.)
unirrad. (calc.)
20
10
0
0
5
10
15
20
Re x /mass%
25
30
Fig. 4 Electrical resistivity measurement results and calculated results of
W-xRe-3Os alloys before and after irradiation.
5.
Conclusions
In order to investigate irradiation-induced precipitation
behavior of solid transmutation elements of tungsten, the
fabrication of tungsten-based model alloys was carried out,
and the alloys were irradiated with fast neutrons. The
following were obtained from microstructural observations
and electrical resistivity measurements.
(1) Rhenium and osmium precipitated in W-5Re and W3Os alloys after 1.54 dpa irradiation at 750 C, though
the contents were lower than the solubility limit. Void
formation was suppressed by the presence of rhenium
and osmium.
(2) Few visible structures could be observed at lower
irradiation dose conditions. Only grown precipitates
were observed at higher irradiation dose conditions.
The interface between the matrix and precipitates could
be strong sink for interstitial atoms and vacancies.
(3) Electrical resistivity could be used to estimate the
decrease in the solute contents of the transmutation
elements in the alloys. The calculated decreases of
rhenium and osmium were 20% and 70% of the original
contents, respectively, after irradiation to 1.54 dpa at
750 C. Most osmium would exist as invisible microclusters after irradiation.
(4) In ternary alloys the behaviors of solute content change
by irradiation for rhenium and osmium were similar to
the behaviors for rhenium and osmium in binary alloys.
Rhenium and osmium would not affect each other in
the behavior at any irradiation condition.
Acknowledgements
This work was supported by the Institute for Materials
Research (IMR), Tohoku University. Post-irradiation experiments (PIE) were carried out at the International Research
Center for Nuclear Materials Science of IMR and the
2264
T. Tanno et al.
Laboratory of Alpha-ray Emitters of IMR. The authors
thank Mr. K. Obara for assistance with the arc melting
processes. The authors also thank Prof. M. Narui, Prof. Y.
Satoh, Dr. N. Nita (present: Kobelco Research Institute, Inc.)
and the IMR staff for PIE.
REFERENCES
1) T. Noda, M. Fujita and M. Okada: J. Nucl. Mater. 258–263 (1998)
934–939.
2) P. Makarov and K. Povarova: J. Refractory Metal & Hard Materials
20 (2002) 277–285.
3) H. P. Gao and R. H. Zee: J. Mater. Sci. Lett. 20 (2001) 885–887.
4) R. C. Rau, J. Moteff and R. L. Ladd: J. Nucl. Mater. 24 (1967) 164–173.
5) M. Fujitsuka, B. Tsuchiya, I. Mutou, T. Tanabe and T. Shikama:
J. Nucl. Mater. 283–287 (2000) 1148–1151.
6) T. Tanabe, C. Eamchotchawalit, C. Busabok, S. Taweethavorn, M.
Fujitsuka and T. Shikama: Mater. Lett. 57 (2003) 2950–2953.
7) R. K. Williams, F. W. Wiffen, J. Bentley and J. O. Stiegler: Metall.
Trans. A 14 (1983) 655–666.
8) Y. Nemoto, A. Hasegawa, M. Satou and K. Abe: J. Nucl. Mater. 283–
287 (2000) 1144–1147.
9) T. Tanno, A. Hasegawa, J. C. He, M. Fujiwara, S. Nogami, M. Satou,
T. Shishido and K. Abe: Mater. Trans. 48 (2007) 2399–2402.
10) J. C. He, A. Hasegawa, M. Fujiwara, M. Satou, T. Shishido and K. Abe:
Mater. Trans. 45 (2004) 2657–2660.
11) S. Shimakawa, N. Sekimura and N. Nojiri: Proc. the 2002 Sympo. on
Nuclear Data (2003) pp. 283–288.
12) C. H. M. Broeders and A. Yu. Konobeyev: J. Nucl. Mater. 328 (2004)
197–214.
13) L. R. Greenwood and F. A. Garner: J. Nucl. Mater. 252 (1998)
635–639.
14) M. A. Logan: Bell Syst. Tech. J. 40 (1961) 865–919.
15) A. Taylor, B. J. Kagle and N. J. Doyle: J. Less-Common MET. 3 (1961)
333–347.
16) R. A. Erck, C. M. Wayman and L. E. Rehn: Radiation-Induced Changes
in Microstructure 13th Int. Sympo. (Part I), ASTM STP 955, F. A. ed.
by Garner et al., (ASTM, Philadelphia, 1987) pp. 721–729.
17) H. Nakajima, S. Yoshida, Y. Kohno and H. Matsui: J. Nucl. Mater.
191–194 (1992) 952–955.
18) V. K. Sikka and J. Moteff: J. Appl. Phys. 43 (1972) 4942.