Effect of Ion Irradiation on the Hardness Properties of
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High and Low Copper
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A R T I C L E I N F O A B S T R A C T 8 9
AIJ use only:
Received date Revised date Accepted date Keywords:
Irradiation Hardness dpa Copper
An investigation into the effects of proton beam exposure on various aspects of nuclear reactors as structural materials was used high and low copper. The aim of this work presented here is to investigate the impact of the radiation damage in the materials by the proton energy irradiation. The damage parameter used in the evaluation is displacement per atom (dpa) in material as a function of proton energy. In addition, a TRIM programmer was used to identify the penetration depth when changes were administered to proton energy. The effect of the irradiation induced hardening and also of the different copper levels was investigated by Vickers Hardness tests, in order to examine microstructural changes. The proton beam incident energy was 3MeV and temperature kept approximately at 30ᴼC. A 25μm flat damage profile was achieved and 0.367dpa and 0.373dpa for low and high copper samples, respectively. The hardness variation with depth and also yield strength variation with dose (dpa) were then investigated.. Based on the results, the study found that the hardness test for high copper is higher than low copper.
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INTRODUCTION∗
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Ion irradiation experiments allow control of
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ion energy, dose, dose rate and temperature which
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enables very reproducible and specific results
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making them desirable for studying irradiated
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micro-structure and property changes of a material
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in a shorter time period [1,2].
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However the evolution of irradiated
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microstructure is dependent upon a combination of
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damage rate and irradiation temperature. So,
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damage cannot always be reproduced in a shortened
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time period just through increased displacement rate
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(dpa/s), although temperature shift relations have be
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created which allow for more correlation of one type
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of damage evolution from one irradiation
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environment to another using dose, dose rate and
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temperature [3]. Ion beams are also generally
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cheaper to produce a given dose compared to
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neutron sources.
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∗Corresponding author.
E-mail address:
Copper impurity has been found in older
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generation reactor pressure vessel (RPV) steels due
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to the use of copper coated welding rods [4]. It was
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found that 0.3wt% and 0.05wt% respective for high
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and low impurity levels of copper. [5] Zhang et al.
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suggest copper precipitates that are rich in other
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impurities (Ni, Al and Mn) that form under proton
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irradiation, remain stable with further irradiation.
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Clusters of these precipitates inhibit the movement
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of dislocations under stress, which causes a net
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increase in hardness after irradiation. [5] Zhang et
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al. also suggest that the mechanism of irradiation
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hardening is similar to that of ageing except in this
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scenario proton radiation increases the free
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movement of vacancies, which encourages the
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diffusion of solutes. Another similarity to thermal
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ageing is that under irradiation, instead of dissolving
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the precipitates coarsen. This acts to soften the
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material to some degree. The conclusions that can
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be drawn from Zhang et al. [5] are that hardness
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increases with segregation of impurities in RPV
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steels, namely copper. Their binding to point defects
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in the matrix furthers the movement of these Cu-rich
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precipitate clusters. These clusters migrate to areas
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of low energy such as grain boundaries and
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sometimes act as ‘sinks’ to which point defects
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continue to migrate. This process means that these
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sinks can also act as recombination sites, where
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interstitials and vacancies can combine and
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‘recover’ the hardness of the material by some
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fraction. This process can occur during irradiation
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and aid to improve the mechanical properties of the
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material.
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Further study was conducted by Shibamoto et
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al. [6]. They also suggested hardening was due to
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copper clustering in the matrix and that nickel
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additions aided this mechanism. Their work agrees
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that Frenkel pairs increase hardening, however they
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also suggested that after annealing above 400oC,
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microvoids were no longer a cause of hardening, as
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they disappeared above 200oC [6].
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Hardness is the capability of a substance to
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resist permanent shape change under applied strain.
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Historically it has been measured as a materials
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ability to resist scratching from other materials.
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Many different methods for measuring hardness
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have been developed over the years, all of which
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have their own intricacies [7,8]. The Vickers
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hardness test has the advantages of very high
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versatility for example it can be used on both the
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micro scale and the macro scale [9]. The Vickers
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hardness test was the technique employed in this
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paper.
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Microhardness for high and low copper
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samples was measured using a Vickers hardness
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machine. The indent is a diamond in the form of a
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square-based pyramid. The included angle of the
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force per area [10]. Hardness can be calculated by:
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(1)
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Additionally, it is possible to work out the indent
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depth:
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(2)
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Where HV is hardness value, d is dimeter of the
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indentation, F is load and h is indenter depth
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The Transport of Ions in Matter (TRIM) is a
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group of computer program that measure the
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stopping and range of ions into target matter by
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using a full quantum mechanical treatment of ion-
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atom collision (assuming a transferring atom as an
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ion, as well as all target atoms as an atom) [11].
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Furthermore, it is a significant program that can be
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used to gain information regarding the penetration
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depth of the ions into the surface of materials [12].
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In this paper, TRIM was used to explore the
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irradiation effect on high and low copper, moreover
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this programme was also useful in discovering the
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penetration depth of proton beams on high and low
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copper, by finding the penetration depth of protons;
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this helped to select a suitable load for the Vickers
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hardness test.
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EXPERIMENTAL METHOD
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The compositions of the high and low copper
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investigated in this paper are shown in Table 1.
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Hardness tests were performed on the high and low
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copper samples irradiated at 30oC. it was not
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possible to continue with the hardness tests because
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the black layer on the surface had a brittle profile.
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The indents were causing cracks in the material so
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the result was not valid because Vickers Hardness
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test is not supposed to be performed in brittle
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surfaces. Figure 1 shows the photos of invalid
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specimens.
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132 Fig. 1. A contamination in the vacuum system has invalided 133 specimens irradiated at 380ᵒC
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Different loads were used in order reach
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different indent depths in the sample. 3MeV protons
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138 It is possible to measure hardness until 25μm and
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have a good simulation of neutron damage profile.
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The indent depths for different loads were
141 approximately 3μm, 8μm, 11μm and 16μm. Indents
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were 0.5mm apart from each other, which avoids
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interference in the measurements. Moreover,
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measurement started and ended far away from the
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edges of the specimens.
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Table 1. Compositions of low high copper (in wt%) Element
Fe C Si Mn P Cr Mo Ni Cu
Low
copper 93.9 0.25 0.2 1.5 0.007 0.1 0.5 3.5 0.05 High
copper 100 0.2 0.2 1.5 0.007 0.1 0.5 3.5 0.3 148
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RESULTS AND DISCUSSION
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The Monte Carlo code TRIM was used to
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gain information about the displacement per atom
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into the surface of materials [12]. Displacement per
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atom is a strong function to measure the amount of
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radiation damage in irradiated materials [13]. The
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ion used was Hydrogen, with ions of energies of
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1.0MeV, 3.0MeV and 9.0MeV H PKA into high
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copper and low copper for 5000 incident ion.
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One of the greatest advantages of using
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proton beam is the easy variation of parameters.
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Accelerators, such as the cyclotron used in this
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work, offer a huge variety of irradiation
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environments. It is important to express the
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displacements in terms of ion/cm² due to the fact
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that this value can be easily changed for different
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irradiation conditions. The graphs for low copper
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compositions are shown in Figure 2. The results
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were pretty satisfactory as the dpa (Displacement
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per atom) behaviour is clear. For higher energies,
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the dose at the surface decreases because they pass
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through the material easily, the scattering cross
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section is lower. Moreover, the results was the same
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pattern for high copper.
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Fig. 2. Bragg peak in low copper, showing Target depth change 177
with increase displacement per atom at 1, 3, 9 MeV proton 178 179
TRIM calculation of proton penetration in
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low copper, The proton penetration depth varies
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strongly with its energy, and no difference was
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observed between low and high copper
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compositions. The penetration depth was
184 approximately 7μm for 1MeV protons, 38μm for
185 3MeV and 234μm for the 9MeV ones.
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According to details of hardness results are
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possible to notice that it does not have the same
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number of points for each condition. This is because
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the work was held throughout the surface and the
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interface between non irradiated and irradiated sides
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was not exactly in the half of the row. Additionally,
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the indent depth was approximately the same for all
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specimens and conditions, so this value was also
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averaged in order to make comparisons easier. It is
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possible to notice the influence of Copper in the
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specimen. It is slighter than expected. The surface
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was pretty rough and doing polish was complex due
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to safety issues related to the residual activity of the
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samples. Shown by Figure 3c, the surface of the
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sample in what assumed to be the iraddiated zones
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appears more dirty than the surface imaged in
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Figures 3a and b.
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Different depths on the material receive
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different damage accumulation due their different
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volumes. The deeper, the greater is the dose
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received. The hardness profile through sample
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thickness can be seen in Figures 4 and 5. The values
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are averages for the non-irradiated and irradiated
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parts for both samples and the error bars express the
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standard deviation. Figures 4 and 5 show that
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irradiated values were slightly higher than non-
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irradiated values.
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Fig. 4. Vickers hardness values profile through sample 216
thickness. For non-irradiated and irradiated low copper 217
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Fig. 5. Vickers hardness values profile through sample 220
thickness. For non-irradiated and irradiated high copper 221
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The profile was not the best expected,
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because hardness should increase with dose, namely
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depth. However, it is possible to notice that the
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standard deviation is large and comprehends a great
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variation among the values for both samples. Also,
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the surfaces were not ideal to do hardness tests
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because they were rough.
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The hardening result from proton irradiation
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is shown in terms of yield strength in Figures 6 and
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7. It has been calculated from hardness results using
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the following relation [14,15]:
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Δ𝜎𝑦 =3.55 Δ𝐻𝑉 (3)
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Where Δ𝜎𝑦 is the increment in yield strength and
237 ΔHV is the increment in hardness.
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Gary Was et al [16] have calculated the yield
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strength variation for protons and neutrons
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irradiations for two different types of austenitic
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steels and higher doses, 1-6dpa. Their results were
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in excellent agreement and the increase of yield
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strength with dose is clear for each experiment. The
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increase of yield strength with dose is expected. The
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errors bars in Figures 6 and 7 represent the standard
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deviation for yield strength average values and they
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comprehend a large variation of values.
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250 Fig. 6. Yield Strength verss Dose for non-irradiated and 251
irradiated high copper 252 253
254 Fig. 7. Yield Strength verss Dose for non-irradiated and 255
irradiated low copper 256
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The yield strength variation through depth
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was not in great agreement with others experiments.
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However, the dose applied in this experiment was
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much lower ~150mdpa. It is expected to have better
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results and cleaner profiles with higher doses.
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Additionally, the surfaces of the samples were not
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very smooth and shiny as expected for a hardness
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test.
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CONCLUSION
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Nanoindentation experiments were performed
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to invistigate the effect of ion irradiation on the
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hardness properties of high and low Copper.
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Irradiated values for hardness were higher than non-
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irradiated ones as expected from radiation
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hardening. Furthermore, it was expected that high
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copper samples had a higher increase in hardness.
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This Copper influence was not very perceptive in
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the results.
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it is clear that the hardness results did not
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follow the predictions. The values had great
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variation so the standard deviations for those
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averages were high. Additionally, the total dose in
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the first 25 microns of the flat damage profile were
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0.367dpa for low copper and 0.373dpa for high
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copper samples. The hardness results appear to be
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clearer for higher doses, such as 1-6dpa. Also, the
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surfaces were rough and not ideal to do hardness
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tests.
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ACKNOWLEDGMENT
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The author would like to thank the Ministry
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of Higher Education and Scientific
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Research/ Kurdistan of Iraq for the financial support
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and providing the facilities to this study.
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Fig. 3. Images from the High Copper hardness analysis. (a) Indent on the non-irradiated surface (b) Indent on the irradiated surface 359
(c) Example of invalid indent 360 361
362 363 364 365
