Journal of the Korean Physical Society, Vol. 63, No. 7, October 2013, pp. 1414∼1417
Microstructure Evolution of Beryllium during Proton Irradiation
Suk Hoon Kang,∗ Jinsung Jang, Yong Hwan Jeong and Tae Kyu Kim
Nuclear Materials Division, Korea Atomic Energy Research Institute, Daejeon 305-353, Korea
Jae Sang Lee
Proton Engineering Frontier Project, Korea Atomic Energy Research Institute, Gyeongju 780-904, Korea
Yong Seok Choi and Kyu Hwan Oh
Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Korea
(Received 26 December 2012, in final form 7 February 2013)
The effects of proton irradiation on a beryllium reflector in terms of microstructure evolution have
been studied to emulate the effects of neutron irradiation. Protons were irradiated on a beryllium
sample with the acceleration voltage of 120 keV and the fluence of 2.0 × 1018 ions/cm2 at room
temperature. The size of the irradiation damaged layer was estimated through a Monte Carlo
simulation (SRIM2012 software) and transmission electron microscopy (TEM) observation. While
the irradiated sample was observed by using TEM, the size of the damaged layer was roughly 1
µm, and the value was coincident with the simulation result. The most severely damaged area was
occurred at 600 nm in depth; tens-of-nanometer-sized voids were distributed. Multiple voids were
observed in the entire damaged area, and were preferentially distributed along the grain boundaries,
and the interfaces between the matrix and the BeO particles. Equi-axed voids, 10 nm in diameter,
were observed in the grain boundary, and planar voids were observed at the interfaces. The voids
were also distributed in the grains; the evolutions of the voids were observed to have been affected
by the grain orientation rather than the irradiation direction. The selective area diffraction pattern
(SADP) from TEM showed that the arrays of multiple voids were considerably longer along the
basal plane. The beryllium atoms could be easily dislocated by proton irradiation while the basal
plane was aligned along a direction perpendicular to the irradiation.
PACS numbers: 81.05.Bx
Keywords: Beryllium, Proton, Irradiation, Basal plane, Void
DOI: 10.3938/jkps.63.1414
I. INTRODUCTION
Pure beryllium has a very low mass absorption coefficient and has been used as a reflector element material
in research reactors. The lifetime of the beryllium reflector elements is usually determined by radiation-induced
swelling [1–4]; this swelling leads to a directional change
in the reflector’s frame, which results in bending or cracking of the reflector’s parts. Therefore, the accumulation
of radiation data is very important for determining the
lifetime of the material, and a visualization of the microstructure evolution during radiation is helpful for understanding proper material usages.
Ion irradiation is widely used as means of introducing
radiation damage in materials [5,6]. Protons can be used
to simulate the effects of neutron irradiation on the mate∗ E-mail: [email protected]; Tel: +82-42-868-8644; Fax: +8242-868-8549
rial because the masses of protons and neutrons are very
similar (1.007 amu). Protons and neutrons have similar
physical characteristic such as size and mass, the only
difference is the polarity of charge (protons are positively
charged and neutrons are neutral in charge). The difference in corresponding irradiation characteristics between
protons and neutrons is definite; irradiated protons will
be stopped inside the target material because of the electron’s stopping power whereas irradiated neutrons penetrate the target material [7,8]. However, it is suspected
that the microstructure evolution during a nuclear stopping event will be similar between proton and neutron irradiation when the mass difference between the incident
ions and the target’s atoms is relatively small. Beryllium
atoms are expected to react actively with the protons because they also have a relatively low atomic mass (9.012
amu). When a sufficient acceleration voltage is applied,
the beryllium atoms will be dislocated by proton irradiation. The displacement of beryllium atoms will cause the
accumulation of voids; which are very harmful to metals
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Microstructure Evolution of Beryllium during Proton Irradiation – Suk Hoon Kang et al.
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Fig. 2. (Color online) (a) Collision depth of protons on the
S-200-F surface is estimated using a Monte Carlo simulation
(software named SRIM2012). (b) Cross-sectional image of
proton-irradiated beryllium, observed by TEM. The acceleration voltage and the number of protons are 120 keV and 2.0
× 1018 ions/cm2 , respectively.
Fig. 1. (Color online) Microstructure of a beryllium reflector block (S-200-F). The grain shapes are shown through
(a) an SEM image of the etched surface, (b) the EBSD band
contrast, and (c) a crystal orientation map. The corresponding (d) 1010 pole-figure and (e) the grain size distributions
are shown.
owing to their strong interaction with lattice defects. As
a result, a local swelling and degradation of the mechanical properties of bulk materials take place as well.
In this study, the evolution of proton induced damage was visualized, and an anisotropic distribution of the
voids in a beryllium sample was investigated. We typically observed that the evolutions of voids were affected
by the grain orientation rather than by the irradiation
direction.
II. EXPERIMENTS
A column of loose beryllium powder (98.5% Be-1.2%
BeO-0.1% Fe-0.1% C-0.05% Al-etc.) was compacted under vacuum hot pressing (VHP), which operates in a vacuum by using the pressure of the opposed upper and
lower punches, bringing the billet to its final density.
VHP-processed commercial grade beryllium (S-200-F)
was sliced into a 1 cm × 1 cm square sheet. The surface of
the sheet was mechanically polished with diamond paste
and then electro-polished using 10% perchloric acid.
The microstructure of beryllium was observed by using
electron backscattered diffraction (EBSD, JSM 6500F &
INCA system), and the grain size and preferred orientations were estimated. The electro-polished surface was
irradiated with protons at room temperature by using
the gas ion implantation equipment of the Proton Engineering Frontier Project, Korea Atomic Energy Research
Institute; the acceleration voltage and number of proton
were 120 keV, and 2.0 × 1018 ions/cm2 , respectively. A
TEM sample of proton-irradiated beryllium was manufactured using a focused ion beam (FIB, NOVA200),
and the sample was sliced into thin foils for observation
along the direction perpendicular to the proton irradiation. Microstructure evolution during proton irradiation was observed in bright-field images and selective
area diffraction patterns (SADP) of TEM (JSM 2100F).
The collision depths of protons on the beryllium surface
were estimated using the Monte Carlo simulation code
SRIM2012. The estimated collision depth was compared
with the TEM observation.
III. RESULTS AND DISCUSSION
The microstructure of the beryllium reflector block
(S-200-F) was shown using EBSD (refer to figure 1).
An SEM image of the etched surface is shown in Fig.
1(a), the EBSD band contrast is shown in Fig. 1(b),
and a crystal orientation map is shown in Fig. 1(c),
respectively. Fig. 1(a) shows how deep furrows were
observed at the grain boundaries, which developed during the electro-polishing of the sample surface by using
10% perchloric acid. The etchant preferentially attacked
the interface between beryllium and BeO. The dip furrow was thought to be the location of the BeO particles.
In Fig. 1(b), a band contrast (BC) map of beryllium
is introduced, which is a qualitative factor of EBSD. It
was derived from the intensity of the diffraction bands;
it shows the detailed features of the microstructure, such
as the grain boundaries. The shape of the grains can also
be observed in the grain map. Fig. 1(c) shows the different colors of each grain; the blue, red, and green colors
suggest that the grains have different crystallographic
orientations. The random orientation distribution of the
beryllium sample is confirmed in the pole figure. In Fig.
1(d), the 1010 pole figure of a beryllium sample measured by using EBSD is shown, and the results show
is no preferred orientation. We can conclude that the
VHP process is quite effective in producing a beryllium
reflector block of random orientation. In Fig. 1(e), the
grain size distribution of the measured beryllium sample
is shown, and the results show that the grains are homo-
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Journal of the Korean Physical Society, Vol. 63, No. 7, October 2013
Fig. 5. A model of void evolution during proton irradiation
on beryllium is shown. As the amount of proton irradiation
increases, the number of voids on the basal plane will also
increase, as shown in (a) to (c).
Fig. 3. Cross-sectional image of proton-irradiated (120
keV, 2.0 × 1018 ions/cm2 ) beryllium, was observed by TEM.
Two different grains (Grains A and B) are irradiated simultaneously. The distribution of voids was affected by the crystal
orientation and the grain boudaries.
Fig. 4. Cross-sectional image of proton-irradiated (120
keV, 2.0 × 1018 ions/cm2 ) beryllium, was observed by TEM.
Small equi-axed voids were generated at the grain boundary.
The shapes of the voids were affected by the grain orientations. Selective area diffraction patterns indicate that the
length of these voids is relatively longer along the basal plane.
geneous, with the average size estimated as less than 20
µm.
After the EBSD observation, the sample was irradiated by protons. The proton irradiation was carried out
at 120 keV and 2.0 × 1018 ions/cm2 . The collision depth
of protons on the beryllium surface were estimated using
the SRIM2012 code, and microstructure evolutions were
observed by using TEM. In Fig. 2(a), the size of the
damaged layer was roughly estimated as 1 µm, and the
most severely damaged area was occurred at 600 nm in
depth. A similar result was observed in the TEM brightfield image in Fig. 2(b). Voids were developed at up to
1 µm in depth, and their shapes and distributions are
coincident with the profile of the number of ions in the
simulation results.
In Fig. 3, a cross-sectional image of proton-irradiated
beryllium observed by using TEM is shown. Various
sizes of voids had developed in the damaged area; the
size distribution was roughly related to the profile of the
number of ions in Fig. 2(a). The largest voids were
observed at 600 nm depth, and other small voids were
distributed at up to 1 µm in depth. In the irradiation
damaged area within 600 ∼ 1000 nm, a grain boundary
was clearly observed, and two different grains (Grains
A and B) were irradiated simultaneously. Typically, the
distribution of voids was observed to be affected by the
crystal orientation and the grain boundaries. Definite
voids are shown in the grain boundaries while the population of voids is low in the other grain regions. Grains
A and B showed different directions of voids alignment,
which was common in the most severely damaged area.
A magnified image of the proton-irradiated region is
shown in Fig. 4. Small equi-axed voids were observed to
develop at a grain boundary. The micrograph consists of
two different grains, and the corresponding SADP was
measured for each grains. The shapes of the voids are
suspected to be affected by the grain orientations, regardless of the proton’s irradiation direction. SADP indicates that the length of the voids were relatively longer
along the basal plane. This suggests that the beryllium
atoms can be easily dislocated by the proton irradiation
while the basal plane is aligned along a direction perpendicular to the irradiation.
A model of the voids evolutions during proton irradiation is described in Fig. 5. First, a beryllium atom
will be displaced by an incident proton, as shown in Fig.
5(a). Because the HCP structure has a closely packed
plane (basal plane) in a particular direction, the beryllium atom can be easily displaced when the incident
direction of the proton is perpendicular to the closely
packed plane. The size of the voids is also suspected to
gradually increas along the closely packed plane as the
displacement of beryllium atoms will be active along the
basal plane. The evolution of voids on the basal plane
as the proton irradiation increases is described in Figs.
5(a) to 5(c).
In Fig. 6, the distribution of the voids around the
beryllium oxide (BeO) particles are shown. The BeO
particles are generally known to be distributed in the
grain boundary, and they become a source of embrittle-
Microstructure Evolution of Beryllium during Proton Irradiation – Suk Hoon Kang et al.
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the BeO particles. Equi-axed voids, 10 nm in diameter,
were observed in the grain boundary, and planar voids
were observed at the interfaces. Voids were also distributed in the grains; the evolutions of these voids were
observed to have been affected by the grain orientation
rather than the irradiation direction. A selective area
diffraction pattern (SADP) from TEM showed that the
arrays of multiple voids were considerably longer along
the basal plane. The beryllium atoms could be easily
dislocated through the proton irradiation while the basal
plane was aligned along a direction perpendicular to the
irradiation.
Fig. 6. Cross-sectional image of proton-irradiated (120
keV, 2.0 × 1018 ions/cm2 ) beryllium, was observed by TEM.
Beryllium-oxide (BeO) particles are shown in a line, and voids
are generated at the interface between the beryllium and the
BeO.
ment. In Fig. 6, the voids preferentially developed at
the interface between the beryllium matrix and the BeO
after proton irradiation, and it was confirmed that the
existence of BeO will accelerate the embrittlement and
swelling of a beryllium reflector.
IV. CONCLUSION
Effects of the proton irradiation on a beryllium reflector have been studied to emulate the effects of neutron
irradiation. Protons were irradiated on a beryllium sample by using 120 keV acceleration voltage at room temperature at up to 2.0 × 1018 ions/cm2 . The size of the
damaged layer was roughly 1 µm, and the most severely
damaged area was occurred at 600 nm in depth; tensof-nanometer-sized voids were also distributed. Multiple voids were preferentially distributed along the grain
boundaries and the interfaces between the matrix and
ACKNOWLEDGMENTS
This work was supported by the National Research
Foundation of Korea (NRF) grant funded by the Korea
government (MEST) (No. 2012M2A8A1027872).
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