Irradiation damage of single crystal, coarse-grained, and
nanograined copper under helium bombardment at 450 °C
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Citation
Han, Weizhong, E.G. Fu, Michael J. Demkowicz, Yongqiang
Wang, and Amit Misra. “Irradiation Damage of Single Crystal,
Coarse-Grained, and Nanograined Copper Under Helium
Bombardment at 450 °C.” J. Mater. Res. 28, no. 20 (October
2013): 2763–2770. © 2013 Materials Research Society
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http://dx.doi.org/10.1557/jmr.2013.283
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Cambridge University Press (Materials Research Society)
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INVITED FEATURE PAPERS
Irradiation damage of single crystal, coarse-grained, and
nanograined copper under helium bombardment at 450 °C
Weizhong Hana) and E.G. Fu
Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
Michael J. Demkowicz
Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge,
Massachusetts 02139
Yongqiang Wang and Amit Misra
Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
(Received 11 June 2013; accepted 12 September 2013)
The irradiation damage behaviors of single crystal (SC), coarse-grained (CG), and nanograined
(NG) copper (Cu) films were investigated under Helium (He) ion implantation at 450 °C with
different ion fluences. In irradiated SC films, plenty of cavities are nucleated, and some of them
preferentially formed on growth defects or dislocation lines. In the irradiated CG Cu, cavities
formed both in grain interior and along grain boundaries; obvious void-denuded zones can be
identified near grain boundaries. In contrast, irradiation-induced cavities in NG Cu were observed
mainly gathering along grain boundaries with much less cavities in the grain interiors. The grains in
irradiated NG Cu are significantly coarsened. The number density and average radius of cavities
in NG Cu was smaller than that in irradiated SC Cu and CG Cu. These experiments indicate that
grain boundaries are efficient sinks for irradiation-induced vacancies and highlight the important
role of reducing grain size in suppressing radiation-induced void swelling.
I. INTRODUCTION
Metallic materials that are able to survive extreme irradiation doses are of critical importance for advanced nuclear
energy and related applications.1–3 Electron and light ion
irradiation of crystalline solids leads to the creation of large
numbers of vacancies and interstitials that agglomerate to
form dislocation loops, stacking-fault tetrahedra, voids,
or gas-filled bubbles.4–14 The irradiation-induced defect
agglomerates contribute to swelling, hardening, and
embrittlement, which accelerate material degradation
and failure in radiation environments.4–14 To prolong
the operating limits and lifetimes of metallic materials in
advanced nuclear reactors or under other extreme irradiation
conditions, the formation of these defect agglomerates must
be curtailed.
One approach to suppressing accumulation of point
defects in irradiated materials is to annihilate freely migrating point defects, such as interstitials and vacancies, at sinks,
such as grain or interphase boundaries.15–29 Because grain
boundaries are point defect sinks, defect-denuded zones are
observed to form in their vicinity, even when grain interiors
posses a large number of radiation-induced defects.30–37
In Cu irradiated at 450 °C by Helium (He) ions, the
typical width of void-denuded zones (VDZs) is between
40 and 70 nm for large-angle grain boundaries.37 If all
the grains inside a material have diameters smaller than
twice the characteristic width of VDZs, then one might
expect that irradiation-induced voids would be suppressed throughout the grain interiors. However, this
long-anticipated effect has never been rigorously demonstrated experimentally.
In the current contribution, we compare the irradiation
damage behaviors of single crystal (SC), coarse-grained
(CG), and nanograined (NG) copper (Cu) films under
identical He ion bombardment conditions. Through these
systematic experimental studies, we intend to probe the
role of grain boundaries and grain size on the irradiation
damage behavior of metallic materials. In addition, the
effect of irradiation dose on their damage behaviors will
be considered as well. The current experimental studies
indicate that the cavities’ size and number density can be
effectively reduced in NG Cu compared with its SC Cu or
CG Cu counterpart and increase with the irradiation dose.
In Sec. II, we introduce the experimental design and
procedures, the main results and discussion are presented
in Sec. III, and the key findings are summarized in Sec. IV.
II. EXPERIMENTAL DESIGN AND PROCEDURES
a)
Address all correspondence to this author.
e-mail: weizhong@lanl.gov
This paper has been selected as an Invited Feature Paper.
DOI: 10.1557/jmr.2013.283
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A. Sample preparation
High purity (99.99%) CG Cu samples were prepared in
two steps to obtain finer initial grain structure: they were
! Materials Research Society 2013
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rolled to ;50% thickness reduction (;1 mm final
thickness) and annealed at 500 °C for 10 min. Postcharacterization show that the grain size of the annealed Cu is
;20–30 lm. SC Cu and NG Cu film samples were
prepared by physical vapor deposition on polished NaCl
substrates by electron beam evaporation at 350 and
200 °C, respectively. These deposited Cu films are fully
dense and have a total thickness of ;80 nm.
probe the effect of irradiation dose, the Cu TEM foils
were irradiated using 200 keV He ions to fluences of
2 ! 1016, 1 ! 1017, and 2 ! 1017 ions/cm2 at 450 °C. The
damage rate is approximately 2.8 ! 10"4 dpa/s. Before
and after irradiation experiments, the Cu TEM samples
were examined under focus and defocus imaging conditions by using a Tecnai F30 (Hillsboro, OR) operated at
300 keV with a field emission gun.
B. Irradiation design and microstructure
characterization
III. RESULTS AND DISCUSSION
He implanted in Cu precipitates out in the form of
nanometer-sized bubbles.38,39 Since our goal was to study
the formation of voids due to agglomeration of radiationinduced vacancies and not He bubbles, we wish to
minimize the ratio of implanted He atoms to the number
of collision-induced vacancies. According to the stopping
and range of ions in matter (SRIM)40 calculation shown in
Fig. 1, for a He ion energy of 200 keV, the peak He
concentration occurs at a depth of ;650 nm, whereas the He
concentration is near zero for the initial 200 nm range.
Therefore, for CG Cu sample, we first prepare transmission
electron microscopy (TEM) samples by conventional twinjet polishing and then perform He irradiation directly on the
thin CG Cu foils. In general, the regions in these foils that are
electron transparent in a 300 keV TEM are less than 100 nm
in thickness.41 Hence, the average implanted He concentration in the electron transparent regions is much less
(only 0.08 at.% in some discrete locations for a dose of
2 ! 1017 ions/cm2) based on Fig. 1(a) and the damage is
roughly uniform across the sample. For irradiation experiments on SC or NG Cu film, we also perform irradiation
directly on TEM samples as well. The TEM samples of SC
and NG Cu were prepared by dissolving the NaCl substrate in
water, and capturing the released Cu film on a TEM grid for
the following implantation and microstructural observation.
Ion irradiation was performed at the Ion Beam Materials Laboratory at Los Alamos National Laboratory. To
A. He concentration and damage profile
The He concentration and the damage profiles in
irradiated SC, CG, and NG Cu were calculated by SRIM
and shown in Figs. 1(a) and 1(b). The peak He concentration occurs at a depth of ;650 nm for all the irradiations
performed with 200 keV He ions, whereas for the initial
100 nm range (approximate the TEM foil thickness shown
as the gray areas in Fig. 1), the average He concentration is
very low, only 0.08 at.% at some discrete locations for
maximum fluence irradiation, as highlighted in the inset in
Fig. 1(a). Numerous studies show that small amount of
implanted He or solid transmuted He has an effect in
simulating the formation of cavities.42–48 In our experiment, due to the very low concentration of He, the formed
cavities are most likely void.42–48 Our estimation show that
even for an average He concentration of 0.08 at.%, the ratio
of vacancy to He in a 5 nm cavities is larger than 33 in
irradiated Cu. Therefore, the TEM observations will show
voids due to the agglomeration of irradiation-induced
vacancies and not pressurized He bubbles. For accuracy,
the term cavity will be used to describe these defects in
following sections. The damage levels for the three different
doses used in the experiments are distinct, varying from
;3 dpa for a dose of 2 ! 1017 ions/cm2 to ;0.3 dpa for
irradiation with a dose of 2 ! 1016 ions/cm2 as shown in
Fig. 1(b). In addition, the irradiation damages are approximately uniform across the TEM foils. In this way, we can
FIG. 1. SRIM calculation of (a) He concentration and (b) damage profile for 2 ! 1016, 1 ! 1017, and 2 ! 1017 ions/cm2 fluences of 200 keV He
irradiation on the 100-nm Cu foil and bulk Cu sample. Gray areas in the figures represent the TEM foils. The insets are the detailed calculation of the
He concentration and damage for the 100-nm-thick Cu foil.
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W. Han et al.: Irradiation damage of single crystal, coarse-grained, and nanograined copper
study the effect of irradiation dose on the agglomeration
behaviors of irradiation-induced vacancies in SC, CG, and
NG Cu.
B. Irradiation damage of SC Cu
The microstructure of as-deposited and irradiated SC
Cu film is displayed in Fig. 2. As shown in Fig. 2(a), the
as-deposited SC film has clean structures and only
dislocation contrast can be seen in some locations. The
corresponding selected area diffraction pattern indicates
that the Cu film has an SC structure with its image normal
near [001]Cu. After He ion irradiation at 450 °C, visible
cavities can be found in the SC film while the cavities
number density varies a lot for different irradiation doses.
Under irradiation to a dose of 2 ! 1016 ions/cm2, cavities
are distributed heterogeneously, with preferential nucleation along the growth defects, such as dislocations, as
marked in Fig. 2(b). With the increase of dose to 1 ! 1017
ions/cm2, some cavities are also observed away from
dislocations as shown in Fig. 2(c). At the dose of 2 ! 1017
ions/cm2, cavities appear to be distributed homogeneously
[Fig. 2(d)], resulting from both heterogeneous and homo-
geneous nucleation of cavities across the sample in
contrast to the cavities nucleated primarily at dislocations
under lower dose irradiation [Fig. 2(b)]. These results
indicate that the cavities number density increased with the
irradiation dose, and the growth defects such as dislocation
lines are preferential sites for nucleation of cavities in the
SC Cu film under He irradiation.
C. Irradiation damage of CG Cu
Figures 3(a)–3(d) show the irradiation damage features
in the grain boundary regions in irradiated CG Cu with He
ion fluences from 2 ! 1016 to 2 ! 1017 ions/cm2. A large
number of irradiation-induced cavities were formed both
in the grain interior and along grain boundaries. The
cavities have a spherical shape with diameters smaller than
;10 nm in grain interior. However, the cavities formed in
the grain boundary are much larger than that in the grain
interior as shown in Fig. 3. A clear VDZ can be identified
along grain boundaries in Fig. 3, consistent with earlier
reports.29–35 Their presence indicates that grain boundaries
are efficient sinks for irradiation-induced vacancies. Under
the current elevated temperature irradiation conditions, no
FIG. 2. TEM images showing the SC Cu film (a) prior to irradiation and after fluences of (b) 2 ! 1016 ions/cm2, (c) 1 ! 1017 ions/cm2, and
(d) 2 ! 1017 ions/cm2. The image in (a) is in focus, whereas those in (b)–(d) are taken under a defocus of "3 lm.
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FIG. 3. Irradiation damage near grain boundary I in CG Cu under fluences of (a) 2 ! 1016 ions/cm2 and (b) 1 ! 1017 ions/cm2. Irradiation damage
near grain boundary II in CG Cu under fluences of (c) 2 ! 1016 ions/cm2 and (d) 2 ! 1017 ions/cm2. All images were taken with a defocus of "5 lm.
VDZ stands for “void-denuded zone.” The dashed ellipse in (d) shows cavities that were not present at the lower dose shown in (c).
radiation-induced dislocation loops or stacking-fault tetrahedra were observed.
As revealed in a previous study, VDZ widths depend
on the character of the grain boundaries along which they
form.37 However, It is unclear whether a critical irradiation
dose need to be achieved to form an obvious VDZs.
Therefore, we performed irradiation on the same grain
boundary and monitor the formation of VDZ under different
fluences of He ions. As shown in Fig. 3, we found that the
VDZs become clear with the increasing of irradiation
dose. For grain boundary I, irradiated to a fluence of
2 ! 1016 ions/cm2, no clear VDZ can be identified along
the grain boundary [Fig. 3(a)] due to the low cavities density.
In the mean time, some small cavities are nucleated and
decorated along the grain boundary I. By increasing the dose
to 1 ! 1017 ions/cm2, the number of cavities near grain
boundary I increased significantly, and the VDZ became
obvious and has a width of 38 nm, and the size of cavities in
grain boundary I become larger. These results show that
a critical irradiation dose is needed to form a clear VDZ.
A similar phenomenon can be observed in irradiated
grain boundary II [Figs. 3(c) and 3(d)]. For irradiation
with the dose of 2 ! 1016 ions/cm2, no clear VDZ can be
identified, such as no visible cavities can be found within
200 nm range in the upper part of grain boundary II
[Fig. 3(c)], whereas there are many small cavities in the
grain boundary interior. With the increasing irradiation
dose, the VDZ becomes obvious and the width is ;42 nm
as marked in Fig. 3(d). Furthermore, as marked by the
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dashed circle in Fig. 3(d), cavities are formed in the left
vertical grain boundary under high-dose irradiation,
while not in the low-dose experiment shown in Fig. 3(c).
These observations indicate that the grain boundaries are
efficient sinks for vacancies, and a critical cavities density is
needed to form clear VDZ.
D. Irradiation damage of NG Cu
The irradiation experiments in CG Cu show that the
grain boundaries are efficient sinks for irradiation-induced
vacancies, and the formation of clear VDZ need a critical
cavities density. The typical widths are in the range of
40–70 nm35 for general large-angle grain boundaries under
He ion irradiation with a dose of 2 ! 1017 ions/cm2. Hence,
if the grain size is smaller than twice the width of VDZ, such
as d , 80 nm, the cavities density in the grain interior would
be expected to be reduced, and cavities will mainly nucleate
along grain boundaries. Here, we have tested this hypothesis
in NG Cu.
For the irradiation experiments performed on NG Cu,
since the implantations were performed at 450 °C for
;6 h, the grain size of NG Cu was significantly coarsened.
As shown by the dark-field TEM image of the as-deposited
NG Cu in Figs. 4(a) and 4(b), the initial grain size is
;15 nm and has a relatively narrow size distribution. After
irradiation at 450 °C to a dose of 1 ! 1017 ions/cm2 by He
ions, the average grain size increased to approximately
35 nm, with a wider size distribution from 15 to 60 nm, as
displayed in Figs. 4(c) and 4(d). From the inserted selected
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W. Han et al.: Irradiation damage of single crystal, coarse-grained, and nanograined copper
FIG. 4. (a) Dark-field TEM image and (b) grain size distribution of as-deposited NG Cu. (b) Dark-field TEM image of NG Cu after irradiation at
450 °C to a fluence of 1 ! 1017 ions/cm2. (d) Grain size distribution of irradiated NG Cu in (c).
area diffraction pattern in Figs. 4(a) and 4(c), we note that
the continuous diffraction rings have become discontinuous consistent with an increase in grain size. Grain growth
under irradiation is a phenomenon of interest to several
applications, including the behavior of materials in nuclear
reactors and the behavior of thin-film solid-state devices
that are modified by ion implantation.49–51 In general, the
grain diameter is observed to increase linearly with the
dose, and the grain boundary mobility increases linearly
with deposited damage energy for a low-temperature
regime (below about 0.15–0.22 Tm), where grain growth
is independent of the irradiation temperature.52 For irradiation temperature higher than ;0.3 Tm, grain growth
may be dominated by the thermally activated process.52 In
the current study, the irradiation temperature is ;0.42Tm-Cu,
and hence the observed grain growth is primarily thermal.
Irradiation to all three doses were performed at the same time
by keeping all samples on the same heating stage and
masking off some of them when they were irradiated to the
desired dose (intend for easier experiment), and hence all the
Cu film samples were kept at 450 °C for the same durations,
so the extent of grain growth in three samples are identical.
The quickly coarsening of the grain in irradiated NG Cu
indicates the thermal instability of NG metals under high
temperature irradiation.
Figure 5 highlights the damage features of NG Cu
irradiated at 450 °C with the same range of He ion doses as
described above. As expected, few cavities are observed
within grain interior since the grain size (;50 nm or less
after irradiation) is smaller than twice the characteristic
widths of grain boundary VDZs shown in Fig. 3 and in
Ref. 37. Isolated cavities are observed within limited
regions inside the specific grain interiors possibly due to
the low sink efficiency of some nearby grain boundaries35
or trapping of cavities by some local defects. Numerous
cavities are nevertheless seen to form along the grain
boundaries possibly due to the large fluxes of radiationinduced vacancies expected to arrive at the grain boundaries from the neighboring grains. Unlike cavities within
grains, grain boundary cavities have an elliptical shape as
shown in Fig. 5(d). Grain boundary triple junctions appear
to be preferred sites for the formation of larger cavities.
Some grain boundary cavities appear to grow preferentially toward one side of the grain boundary, which might
be related to the orientation of the specific grains or grain
coarsening after irradiation. It can be seen that there is
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FIG. 5. NG Cu irradiated to fluences of (a) 2 ! 1017 ions/cm2, (b) 1 ! 1017 ions/cm2, and (c) 2 ! 1016 ions/cm2. (d) Elliptical cavities formed along
grain boundaries in NG Cu irradiated to a dose of 1 ! 1017 ions/cm2 (grain boundary contours are marked by white stars). (a)–(c) were taken under
a defocus of "2 lm and (d) is in focus.
a clear change in the number density and size of cavities
with the decreasing dose. These results show that irradiation-induced cavities are mainly formed along grain
boundaries in NG Cu, which is significantly different
from its SC Cu and CG Cu counterpart. Our experimental
studies clearly demonstrate that once the grain size is smaller
than twice the characteristic width of grain boundary VDZ,
the irradiation-induced defects mainly agglomerate along
grain boundary sinks.
The size and number density of cavities formed in SC,
CG, and NG Cu under the same ion irradiation conditions
are compared in Figs. 6(a) and 6(b). The number density
of cavities formed in SC and in grain interiors of CG and
NG Cu are plotted in Fig. 6(a) as a function of irradiation
dose. For the dose range investigated here, the number
density of cavities in NG Cu is smaller than that in SC and
CG Cu. In addition, the average size of cavities is smaller in
NG Cu. With decreasing dose, the number densities of
cavities decrease in irradiated SC, CG and NG Cu, as shown
in Fig. 6(a). With increasing dose, the average radius of
cavities formed in grain boundaries slightly increases for
both CG and NG Cu. In particular, the average radius of
cavities in NG Cu is smaller than that in CG Cu as shown in
Fig. 6(a). These results demonstrate that the number
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densities and sizes of cavities are slightly suppressed in
NG Cu than in SC and CG Cu under the same irradiation
conditions.
In the current studies, we performed irradiation experiments directly on the TEM foils with thickness less than
100 nm, and the top and bottom surfaces of TEM foils are
also sinks for irradiation-induced defects.53 However, all
the SC, CG, and NG Cu foils in our experiments have the
similar surface effect during irradiation, hence the observed irradiation damage difference in SC, CG, and NG
Cu are mainly due to the appearance of grain boundaries
and the increase of their density (reduction in grain size).
Due to certain effect of the free surface, the cavities
formation will be less than what would occur in a thick
bulk sample.53 Due to the large area per unit volume of
grain boundaries in NG Cu, radiation-induced vacancies
will largely migrate to grain boundaries. We expect that
a significant fraction of those vacancies may annihilate
with interstitials or with grain boundary defects, while the
remaining excess vacancies (interstitials are easy to diffuse
to dislocations, grain boundaries or free surface) agglomerate and form into cavities along grain boundaries. In this
case, the total cavities number density and size is reduced
obviously in NG Cu. However, in SC and CG Cu foils,
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FIG. 6. Comparison of the dose dependences of (a) the number density of cavities in SC Cu and in grain interiors of CG and NG Cu and (b) the
average radius of cavities formed at grain boundaries in CG Cu and NG Cu.
only a smaller portion of irradiation-induced vacancies
migrate to the grain boundaries and annihilate with
interstitials or agglomerate into cavities. Most of the
irradiation-induced vacancies remain inside grain interiors
and contribute to cavities growth. Hence, the number
density and the size of cavities in SC Cu and CG Cu are
larger than in NG Cu. The current experimental studies
demonstrate that synthesis of materials in the nanocrystalline
form is moderately effective for suppressing irradiationinduced void-swelling. Furthermore, the thermal instability
of NG Cu demonstrated in this study is one of the common
drawbacks in nanocrystalline metallic materials, and the
coarsening of the microstructures would be more significant
in irradiated bulk NC Cu.
IV. CONCLUSIONS
Irradiation damage was investigated in SC, CG, and NG
Cu exposed to He ion irradiation at elevated temperature. In
SC Cu, plenty of cavities are nucleated and some of them
are preferentially decorated along the growth defects and
dislocations. In irradiated CG Cu, VDZs can be identified
near most of the grain boundaries. In contrast, irradiationinduced cavities are observed predominantly along grain
boundaries in NG Cu and less in the grain interior. The
number densities and average size of cavities are smaller in
NG Cu than in SC and CG Cu under the same irradiation
conditions, suggesting a suppression of void-swelling.
ACKNOWLEDGMENTS
This work was supported by the Center for Materials in
Irradiation and Mechanical Extremes (CMIME), an
Energy Frontier Research Center (EFRC) funded by the
US Department of Energy, Office of Science, Office of
Basic Energy Sciences under Grant No. 2008LANL
1026. The authors thank J.K. Baldwin at the Center for
Integrated Nanotechnologies at LANL, a Basic Energy
Science sponsored user facility, for film deposition.
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