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Modification of microstructure and hardness for Cu–Ti alloy by means of energetic ion beam irradiation
Nuclear Instruments and Methods in Physics Research B 341 (2014) 53–57
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Modification of microstructure and hardness for Cu–Ti alloy by means of energetic ion beam irradiation D. Ueyama a,⇑, S. Semboshi b, Y. Saitoh c, F. Hori a, K. Nishida d, N. Soneda d, A. Iwase a a
Department of Materials Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan Kansai-Center, Institute of Materials Research, Tohoku University, Sakai, Osaka 599-8531, Japan c Takasaki Advanced Radiation Research Institute, Japan Atomic Energy Agency, Takasaki, Gunma 370-1292, Japan d Central Research Institute of Electric Power Industry, Komae, Tokyo 201-8511, Japan b
a r t i c l e
i n f o
Article history: Received 13 July 2013 Received in revised form 22 June 2014 Accepted 22 June 2014 Available online 2 August 2014 Keywords: Cu–Ti Ion-irradiation Vickers hardness Three-dimensional atom probe Deposited energy density
a b s t r a c t Cu–Ti alloys were irradiated with 5.4 MeV Al ions, 7.3 MeV Fe ions, 10 MeV I ions, and 16 MeV Au ions at room temperature and the Vickers microhardness was measured. The hardness once increases by the irradiation with a low fluence, and then it remains almost constant even with increasing the ion fluence. The change in hardness was well correlated with the density of energy deposited through the elastic collisions and not the electronic excitation. The observation of atom probe tomography (APT) did not show any Ti clusters in the irradiated specimens. This result suggests that not Ti clusters but lattic defects (interstitial atoms, vacancies and/or their aggregates) contributed to the increase in hardness of Cu–Ti alloys. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Irradiations of supersaturated alloys with energetic ions induce point defects such as interstitials and vacancies, and they promote the diffusion and the segregation of solute atoms. This phenomenon is well known as the radiation enhanced segregation. It has, however, been studied so far especially in terms of hardening and embrittlement of nuclear reactor materials [1]. In our previous studies, we have found that energetic ion irradiation causes a remarkable change in hardness in Al–Cu–Mg alloys (a typical duralumin) [2,3] and Al–Mg–Si alloy [4]. Therefore, it is interesting to examine whether this method is also effective for the hardness modification of other alloys. In the present study, we have investigated the microstructure and the hardness of ion-irradiated Cu–Ti alloy, and compared them with those of thermally-aged Cu–Ti alloy. The reason why we have chosen this alloy is as follows. In the case of thermal aging, the hardening of Al alloys is triggered by GP zones [5]. On the other hand, the hardening of Cu–Ti alloys is initially triggered by the spinodal decomposition [6]. It is interesting how such difference in hardening mechanism for the thermal aging between the two kinds of alloys is observed in the case of irradiation induced hardening. From an application point of view, Cu–Ti alloys have ⇑ Corresponding author. Tel./fax: +81 72 254 9810. E-mail address: [email protected] (D. Ueyama). http://dx.doi.org/10.1016/j.nimb.2014.06.033 0168-583X/Ó 2014 Elsevier B.V. All rights reserved.
recently received a great deal of attention as electronic components because of their good mechanical strength and the electrical conductivity. To make wider use of these alloys, it is important that we increase its mechanical strength and electrical conductivity. In this report, we show the effect of energetic ion irradiation on the hardness of the Cu–Ti alloy and also discuss the difference in the ion-irradiation induced hardening between for the Al–Mg–Si alloys and for the Cu–Ti alloys. 2. Experimental procedure We prepared Cu-4.2 at.% Ti alloys as specimens for the present experiment. The alloys were rolled to 250 lm thick. According to the Cu–Ti phase diagram [7], the solubility limit of Ti atoms in an fcc-Cu phase is larger than 4.2 at.% at 1223 K. To obtain supersaturated specimens, therefore, they were heat-treated at 1223 K for 15 min and then quenched into iced water. The alloys were cut to 10 10 mm2 in dimension by a shearing machine. Surfaces of the specimens were mechanically-polished by emery paper and buffing compound just before ion-irradiations. We irradiated the specimens with 5.4 MeV Al ions (Al2+), 7.3 MeV Fe ions (Fe2+), 10 MeV I ions (I3+) and 16 MeV Au ions (Au5+) by using a tandem accelerator at Takasaki Advanced Radiation Research Institute, Japan Atomic Energy Agency. All the irradiations were performed at room temperature. The beam current was less than 260 nA/cm2 in order to avoid the beam heating. Actually, the temperature rise
D. Ueyama et al. / Nuclear Instruments and Methods in Physics Research B 341 (2014) 53–57
by beam heating was negligibly small. For comparison, some supersaturated specimens were thermally aged at 723 K in air. The lattice structure of the specimens was evaluated for unirradiated and irradiated specimens by using an X-ray diffraction measurement (XRD). The Vickers hardness was measured with an applied load of 98.07 mN and a holding time of 10 s at room temperature. We used the micro hardness tester (HMV-2TADW-XYJ(344-04215-15), SHIMADZU Corporation). The microstructure for unirradiated (supersaturated) specimen, ion-irradiated specimens, and thermally-aged specimen was analyzed by using the atom probe tomography (APT) at Central Research Institute of Electric Power Industry. The general explanation about the APT method is described in Ref. [8]. We prepared tips for the APT measurements in following procedures; for the ion-irradiated specimen, extremely sharp needle-like tips were picked up from the depth region of 2 lm from the irradiated surface using a focused ion beam (FIB) system. For the supersaturated and thermally-aged specimens, we cut the specimens into bars and then electro-polished to obtain tips with a sharp edge. The atom probe measurements were performed at the specimen temperature of 50 K with an ultraviolet (UV) laser pulse assistance, the power of which was about 40 pJ.
(a) 200 190
180
Hv
54
170
unirradiated 16MeV Au ions 10MeV I ions 7.3MeV Fe ions 5.4MeV Al ions
160
150 0
2
4
6
8
13
10
12
2
Ion fluence(x10 /cm ) (b) 200
3. Results and discussion 190
180
Hv
Fig. 1 shows the change in Vickers hardness as a function of ion fluence. As shown in Fig. 1(a), for the four kinds of ions, Vickers hardness once increases by the irradiation with a low fluence, and then it tends to be strongly saturated with increasing the ion fluence. At a given fluence, however, the change in Vickers hardness is not the same but it is larger for the irradiation with heavier ions. This trend is more clearly seen in Fig. 1(b) which abscissa is the logarithmic scale. Fig. 1(a) and (b) show that the ion fluence is not a good parameter for describing the hardness change by the energetic ion irradiation. It is well known that there are two processes of energy transfer from energetic ions to targets. One is the energy transfer through the electronic excitation. The other is the energy transfer through the elastic collisions. Fig. 2(a) shows the depth profile of the energy deposited through electronic excitation for 16 MeV Au ion, 10 MeV I ion 7.3 MeV Fe ion, and 5.4 MeV Al ion in Cu–Ti alloy. Fig. 2(b) shows the depth profile of the energy deposited through elastic collisions for the four kinds of ions in Cu–Ti alloy. These profiles were calculated by using SRIM2008 code [9]. As shown in the both figures, irradiated depths are about same for all the ions. Therefore a comparison of the Vickers hardness for the four kinds of irradiation is meaningful. To evaluate the effects of the ion irradiation more quantitatively, we discuss the Vickers hardness in terms of the energy deposited in the specimen by the irradiation. Fig. 3(a) shows the Vickers hardness as a function of density of energy deposited through electronic excitation. Fig. 3(b) shows the Vickers hardness as a function of density of energy deposited through elastic collisions. As shown in the figures, Vickers hardness is correlated with the density of energy deposited through elastic collisions much better than with the density of energy deposited through electronic excitation. This result means that the elastically deposited energy mainly contributes to the increase in harness. The contribution of the energy elastically deposited by energetic ions and energetic electrons to the hardness increase in some metallic alloys has also been reported for example in Refs. [2,4,10]. During the past few decades, a lot of studies concerning the effects of high density electronic excitation due to swift heavy ions on lattice structures and physical properties have been reported such as amorphization and electronic sputtering in insulators [11]. In the
170
unirradiated 16 MeV Au ions 10 MeV I ions 7.3 MeV Fe ions 5.4 MeV Al ions
160
150 0
0. 1
1
13
10
2
Ion fluence(x10 /cm ) Fig. 1. Change in Vickers hardness of Cu–Ti alloy as a function of ion fluence for the irradiations with 5.4 MeV Al ions, 7.3 MeV Fe ions, 10 MeV I ions and 16 MeV Au ions. (a) The abscissa is linear scale. (b) The abscissa is logarithmic scale.
case of metallic alloys, however, we can consider that the effect of electronic excitation is very small because of the existence of free electrons, the resulting high thermal conductivity and so on. The present result and our previous reports [2–4] have confirmed that the hardness change by energetic ion irradiation in metallic alloys is little affected by the irradiation-induced electronic excitation. The results of the APT measurement are shown in Fig. 4(a)–(c). The figures show the distributions of Ti atoms in the unirradiated (therefore, supersaturated) specimen (Fig. 4(a)), in the specimen irradiated with 16 MeV Au ions at the fluence of 1 1014/cm2 (Fig. 4(b)) and in the specimen thermally-aged at 723 K for 12 h (Fig. 4(c)). As can be seen in Fig. 4(a), for the unirradiated specimen, Ti atoms are nearly homogeneously distributed. Fig. 4(b) shows that any Ti atom clusters are hardly found even in the ion-irradiated specimen. But, the hardness was increased to 195
55
D. Ueyama et al. / Nuclear Instruments and Methods in Physics Research B 341 (2014) 53–57
(a) 16MeV Au ions 10MeV I ions 7.3MeV Fe ions 16MeV Al ions
Hv
Energy deposition(eV/nm/ion)
(a)
unirradiated 16 MeV Au ions 10 MeV I ions 7.3 MeV Fe ions 5. 4 MeV Al ions 22
Deposited energy through electronic excitation(x10 eV/g)
(b)
depth(μm)
Hv
(b)
unirradiated 16 MeV Au ions 10 MeV I ions 7.3 MeV Fe ions 5.4 MeV Al ions
Density of energy deposited 22
through elastic collisions(x10 eV/g) Fig. 3. (a) Change in Vickers hardness as a function of density of energy deposited through the electronic excitation (i.e., the energy deposited per unit mass of the specimen (gram)), and (b) change in Vickers hardness as a function of density of energy deposited through the elastic collisions (i.e., the energy deposited per unit mass of the specimen (gram)). Fig. 2. (a) Depth profile of the energy deposited through the electronic excitation per unit path for 5.4 MeV Al ion, 7.3 MeV Fe ion, 10 MeV I ion, and 16 MeV Au ion in Cu–Ti alloys, and (b) depth profile of the energy deposited through the elastic collisions per unit path for 5.4 MeV Al ion, 7.3 MeV Fe ion, 10 MeV I ion, and 16 MeV Au ion in Cu–Ti alloys.
by the irradiation. Therefore, the experimental result suggests that not Ti clusters but lattice defects mainly contribute to the increase in hardness. The Vickers hardness of the thermally-aged specimen is 280. In Fig. 4(c), we can observe large Ti atom precipitates after the thermal aging at 723 K. As well as the irradiation induced segregation and lattice defect production, the amorphization is also well known as an irradiation effect on materials. However, we scarcely observed any change in XRD spectra by the ion irradiations. We can, therefore, conclude that the amorphization was not induced in the Cu–Ti specimen by the present ion irradiations. Next, we compare the effect of energetic ion irradiation on the hardness of Cu–Ti alloy with that on the hardness of Al–Mg–Si
alloy. Fig. 5 shows the relative change in Vickers hardness (DHv/Hv0) for the Cu–Ti alloy and Al–Mg–Si alloy as a function of 10 MeV I ion fluence, where Hv0 is the hardness for the unirradiated specimen, and DHv is the difference in hardness between irradiated and unirradiated specimens. The hardness of Al–Mg–Si alloy increases gradually with increasing the ion fluence. On the other hand, the hardness of the Cu–Ti alloy once rapidly increases by the irradiation with relatively low fluences, and then it remains almost constant even with increasing the ion fluence. In the case of Al–Mg–Si alloy, as our previous result shows [4], the main origin of the hardness increase by the ion irradiation is nanometer-scaled clusters of solute atoms (Mg and Si), which are produced by the irradiation enhanced segregation. In the case of Cu–Ti alloy, we do not observe any Ti atom clusters even after the irradiation. Therefore, the origin of the irradiation induced hardness for the Cu–Ti alloy is not solute clusters but lattice defects of the matrix, i.e., interstitial atoms, vacancies and/or their aggregates. If the effect of the lattice defects on hardness is saturated more rapidly than that of irradiation induced segregation
56
D. Ueyama et al. / Nuclear Instruments and Methods in Physics Research B 341 (2014) 53–57
1.35
(a)
(a) Cu-Ti
1.3
ΔHv/Hv0
1.25
1.2
1.15
1.1
1.05
1
0
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Ion-fluence(x10 /cm ) 1.35
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1.2
1.15
1.1
(c) 1.05
1 0
50
100
150
200 13
250 2
300
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Ion-fluence(x 10 /cm ) Fig. 4. Atom probe elemental mapping for (a) the specimen as-prepared (unirradiated, and therefore supersaturated), (b) the specimen irradiated with Au ions to the fluence of 1 1014/cm2, and (c) the specimen thermally-aged at 723 K for 12 h. Box sizes are: (a) 130 130 220 nm, (b) 52 53 29 nm, and (c) 120 120 60 nm.
of solute atoms, we can explain the difference in ion fluence dependence of hardness between the two alloys. The temperature at which the effect of thermal aging on hardness is clearly observed for Al–Mg–Si alloy is much lower than that for Cu–Ti alloys. This difference can be explained as follows; as the melting temperature for Al alloys (about 650 °C) is much lower than that of Cu-4.2 at.% Ti alloy (about 1100 °C), the production and the diffusion of thermal vacancies, which cause the aging effect, are activated at lower temperature for Al alloys. Similarly, lattice defects produced by energetic ion irradiation can diffuse in Al alloys even at room temperature and the irradiation enhanced segregation is realized.
Fig. 5. Relative change in Vickers hardness (DHv/Hv0) for (a) Cu–Ti and (b) Al–Mg–Si alloys as a function of 10 MeV I ion fluence. Hv0 is the hardness for the unirradiated specimen, and DHv is the difference in hardness between irradiated and unirradiated specimens.
For Cu–Ti alloy, the diffusion of irradiation produced lattice defects is not enough at room temperature for the irradiation enhanced segregation. To clarify this point more quantitatively, the ion and electron irradiation experiments at elevated temperatures are now in progress for Cu–Ti alloys.
4. Summary We irradiated Cu–Ti alloys with some energetic ions at room temperature, and measured the Vickers hardness. The hardness was increased with increasing ion fluence. The change in Vickers hardness by the ion irradiations is well correlated with the density
D. Ueyama et al. / Nuclear Instruments and Methods in Physics Research B 341 (2014) 53–57
of energy deposited through elastic collisions. The APT measurement, however, revealed that Ti-enriched clusters were scarcely observed even after the irradiation. This result suggests that in the case of Cu–Ti alloy irradiated at room temperature, not solute atom clusters but lattice defects of the matrix mainly contribute to the increase in hardness. The results of the present study show that the energetic ion irradiation can systematically control the surface hardness of Cu–Ti alloys as well as Al–Mg–Si alloys. Acknowledgments This research was carried out under the collaboration program between Osaka Prefecture University and the Japan Atomic Energy Agency (JAEA) and under the collaboration program between Osaka Prefecture University and Central Research Institute of Electric Power Industry. A part of this research was financially supported by Osaka Nuclear Science Association (ONSA).
57
References [1] G.S. Was, Fundamental of Radiation Materials Science, Springer, 2007. Chapters 12, 13. [2] T. Mitsuda, I. Kobayashi, S. Kosugi, Nao Fujita, Y. Saitoh, F. Hori, S. Semboshi, Y. Kaneno, K. Nishida, N. Soneda, S. Ishino, A. Iwase, J. Nucl. Mater. 408 (2011) 201. [3] T. Mitsuda, I. Kobayashi, S. Kosugi, Nao Fujita, Y. Saitoh, F. Hori, S. Semboshi, Y. Kaneno, K. Nishida, N. Soneda, S. Ishino, A. Iwase, Nucl. Instr. Meth. B 272 (2012) 49. [4] D. Ueyama, Y. Saitoh, F. Hori, Y. Kaneno, K. Nishida, K. Dohi, N. Soneda, S. Semboshi, A. Iwase, Nucl. Instr. Meth. B 314 (2013) 107–111. [5] S. Pogatscher, H. Antrekowitsch, H. Leitner, T. Ebner, P.J. Ugowitzer, Acta Mater. 59 (2011) 3352. [6] W.A. Soffa, D.E. Laughlin, Prog. Mater. Sci. 49 (2004) 347–366. [7] J.Y. Brun, S.J. Hamar-Thibault, C.H. Allibert, Z. Metallkd. 74 (1983) 525–529. [8] M.K. Miller, Atom Probe Tomography: Analysis at the Atomic Level, Kluwer Academic, Plenum Publishers, New York, 2000. [9] http://www.srim.org/. [10] T. Tobita, M. Suzuki, A. Iwase, K. Aizawa, J. Nucl. Mater. 299 (2001) 267–270. [11] For example M. Toulemonde, W. Assamann, C. Dufour, A. Meftah, C. Trautmann, Nucl. Instr. Meth. B 277 (2012) 28–39. and the references therein.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Modification of microstructure and hardness for Cu–Ti alloy by means of energetic ion beam irradiation D. Ueyama a,⇑, S. Semboshi b, Y. Saitoh c, F. Hori a, K. Nishida d, N. Soneda d, A. Iwase a a
Department of Materials Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan Kansai-Center, Institute of Materials Research, Tohoku University, Sakai, Osaka 599-8531, Japan c Takasaki Advanced Radiation Research Institute, Japan Atomic Energy Agency, Takasaki, Gunma 370-1292, Japan d Central Research Institute of Electric Power Industry, Komae, Tokyo 201-8511, Japan b
a r t i c l e
i n f o
Article history: Received 13 July 2013 Received in revised form 22 June 2014 Accepted 22 June 2014 Available online 2 August 2014 Keywords: Cu–Ti Ion-irradiation Vickers hardness Three-dimensional atom probe Deposited energy density
a b s t r a c t Cu–Ti alloys were irradiated with 5.4 MeV Al ions, 7.3 MeV Fe ions, 10 MeV I ions, and 16 MeV Au ions at room temperature and the Vickers microhardness was measured. The hardness once increases by the irradiation with a low fluence, and then it remains almost constant even with increasing the ion fluence. The change in hardness was well correlated with the density of energy deposited through the elastic collisions and not the electronic excitation. The observation of atom probe tomography (APT) did not show any Ti clusters in the irradiated specimens. This result suggests that not Ti clusters but lattic defects (interstitial atoms, vacancies and/or their aggregates) contributed to the increase in hardness of Cu–Ti alloys. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Irradiations of supersaturated alloys with energetic ions induce point defects such as interstitials and vacancies, and they promote the diffusion and the segregation of solute atoms. This phenomenon is well known as the radiation enhanced segregation. It has, however, been studied so far especially in terms of hardening and embrittlement of nuclear reactor materials [1]. In our previous studies, we have found that energetic ion irradiation causes a remarkable change in hardness in Al–Cu–Mg alloys (a typical duralumin) [2,3] and Al–Mg–Si alloy [4]. Therefore, it is interesting to examine whether this method is also effective for the hardness modification of other alloys. In the present study, we have investigated the microstructure and the hardness of ion-irradiated Cu–Ti alloy, and compared them with those of thermally-aged Cu–Ti alloy. The reason why we have chosen this alloy is as follows. In the case of thermal aging, the hardening of Al alloys is triggered by GP zones [5]. On the other hand, the hardening of Cu–Ti alloys is initially triggered by the spinodal decomposition [6]. It is interesting how such difference in hardening mechanism for the thermal aging between the two kinds of alloys is observed in the case of irradiation induced hardening. From an application point of view, Cu–Ti alloys have ⇑ Corresponding author. Tel./fax: +81 72 254 9810. E-mail address: [email protected] (D. Ueyama). http://dx.doi.org/10.1016/j.nimb.2014.06.033 0168-583X/Ó 2014 Elsevier B.V. All rights reserved.
recently received a great deal of attention as electronic components because of their good mechanical strength and the electrical conductivity. To make wider use of these alloys, it is important that we increase its mechanical strength and electrical conductivity. In this report, we show the effect of energetic ion irradiation on the hardness of the Cu–Ti alloy and also discuss the difference in the ion-irradiation induced hardening between for the Al–Mg–Si alloys and for the Cu–Ti alloys. 2. Experimental procedure We prepared Cu-4.2 at.% Ti alloys as specimens for the present experiment. The alloys were rolled to 250 lm thick. According to the Cu–Ti phase diagram [7], the solubility limit of Ti atoms in an fcc-Cu phase is larger than 4.2 at.% at 1223 K. To obtain supersaturated specimens, therefore, they were heat-treated at 1223 K for 15 min and then quenched into iced water. The alloys were cut to 10 10 mm2 in dimension by a shearing machine. Surfaces of the specimens were mechanically-polished by emery paper and buffing compound just before ion-irradiations. We irradiated the specimens with 5.4 MeV Al ions (Al2+), 7.3 MeV Fe ions (Fe2+), 10 MeV I ions (I3+) and 16 MeV Au ions (Au5+) by using a tandem accelerator at Takasaki Advanced Radiation Research Institute, Japan Atomic Energy Agency. All the irradiations were performed at room temperature. The beam current was less than 260 nA/cm2 in order to avoid the beam heating. Actually, the temperature rise
D. Ueyama et al. / Nuclear Instruments and Methods in Physics Research B 341 (2014) 53–57
by beam heating was negligibly small. For comparison, some supersaturated specimens were thermally aged at 723 K in air. The lattice structure of the specimens was evaluated for unirradiated and irradiated specimens by using an X-ray diffraction measurement (XRD). The Vickers hardness was measured with an applied load of 98.07 mN and a holding time of 10 s at room temperature. We used the micro hardness tester (HMV-2TADW-XYJ(344-04215-15), SHIMADZU Corporation). The microstructure for unirradiated (supersaturated) specimen, ion-irradiated specimens, and thermally-aged specimen was analyzed by using the atom probe tomography (APT) at Central Research Institute of Electric Power Industry. The general explanation about the APT method is described in Ref. [8]. We prepared tips for the APT measurements in following procedures; for the ion-irradiated specimen, extremely sharp needle-like tips were picked up from the depth region of 2 lm from the irradiated surface using a focused ion beam (FIB) system. For the supersaturated and thermally-aged specimens, we cut the specimens into bars and then electro-polished to obtain tips with a sharp edge. The atom probe measurements were performed at the specimen temperature of 50 K with an ultraviolet (UV) laser pulse assistance, the power of which was about 40 pJ.
(a) 200 190
180
Hv
54
170
unirradiated 16MeV Au ions 10MeV I ions 7.3MeV Fe ions 5.4MeV Al ions
160
150 0
2
4
6
8
13
10
12
2
Ion fluence(x10 /cm ) (b) 200
3. Results and discussion 190
180
Hv
Fig. 1 shows the change in Vickers hardness as a function of ion fluence. As shown in Fig. 1(a), for the four kinds of ions, Vickers hardness once increases by the irradiation with a low fluence, and then it tends to be strongly saturated with increasing the ion fluence. At a given fluence, however, the change in Vickers hardness is not the same but it is larger for the irradiation with heavier ions. This trend is more clearly seen in Fig. 1(b) which abscissa is the logarithmic scale. Fig. 1(a) and (b) show that the ion fluence is not a good parameter for describing the hardness change by the energetic ion irradiation. It is well known that there are two processes of energy transfer from energetic ions to targets. One is the energy transfer through the electronic excitation. The other is the energy transfer through the elastic collisions. Fig. 2(a) shows the depth profile of the energy deposited through electronic excitation for 16 MeV Au ion, 10 MeV I ion 7.3 MeV Fe ion, and 5.4 MeV Al ion in Cu–Ti alloy. Fig. 2(b) shows the depth profile of the energy deposited through elastic collisions for the four kinds of ions in Cu–Ti alloy. These profiles were calculated by using SRIM2008 code [9]. As shown in the both figures, irradiated depths are about same for all the ions. Therefore a comparison of the Vickers hardness for the four kinds of irradiation is meaningful. To evaluate the effects of the ion irradiation more quantitatively, we discuss the Vickers hardness in terms of the energy deposited in the specimen by the irradiation. Fig. 3(a) shows the Vickers hardness as a function of density of energy deposited through electronic excitation. Fig. 3(b) shows the Vickers hardness as a function of density of energy deposited through elastic collisions. As shown in the figures, Vickers hardness is correlated with the density of energy deposited through elastic collisions much better than with the density of energy deposited through electronic excitation. This result means that the elastically deposited energy mainly contributes to the increase in harness. The contribution of the energy elastically deposited by energetic ions and energetic electrons to the hardness increase in some metallic alloys has also been reported for example in Refs. [2,4,10]. During the past few decades, a lot of studies concerning the effects of high density electronic excitation due to swift heavy ions on lattice structures and physical properties have been reported such as amorphization and electronic sputtering in insulators [11]. In the
170
unirradiated 16 MeV Au ions 10 MeV I ions 7.3 MeV Fe ions 5.4 MeV Al ions
160
150 0
0. 1
1
13
10
2
Ion fluence(x10 /cm ) Fig. 1. Change in Vickers hardness of Cu–Ti alloy as a function of ion fluence for the irradiations with 5.4 MeV Al ions, 7.3 MeV Fe ions, 10 MeV I ions and 16 MeV Au ions. (a) The abscissa is linear scale. (b) The abscissa is logarithmic scale.
case of metallic alloys, however, we can consider that the effect of electronic excitation is very small because of the existence of free electrons, the resulting high thermal conductivity and so on. The present result and our previous reports [2–4] have confirmed that the hardness change by energetic ion irradiation in metallic alloys is little affected by the irradiation-induced electronic excitation. The results of the APT measurement are shown in Fig. 4(a)–(c). The figures show the distributions of Ti atoms in the unirradiated (therefore, supersaturated) specimen (Fig. 4(a)), in the specimen irradiated with 16 MeV Au ions at the fluence of 1 1014/cm2 (Fig. 4(b)) and in the specimen thermally-aged at 723 K for 12 h (Fig. 4(c)). As can be seen in Fig. 4(a), for the unirradiated specimen, Ti atoms are nearly homogeneously distributed. Fig. 4(b) shows that any Ti atom clusters are hardly found even in the ion-irradiated specimen. But, the hardness was increased to 195
55
D. Ueyama et al. / Nuclear Instruments and Methods in Physics Research B 341 (2014) 53–57
(a) 16MeV Au ions 10MeV I ions 7.3MeV Fe ions 16MeV Al ions
Hv
Energy deposition(eV/nm/ion)
(a)
unirradiated 16 MeV Au ions 10 MeV I ions 7.3 MeV Fe ions 5. 4 MeV Al ions 22
Deposited energy through electronic excitation(x10 eV/g)
(b)
depth(μm)
Hv
(b)
unirradiated 16 MeV Au ions 10 MeV I ions 7.3 MeV Fe ions 5.4 MeV Al ions
Density of energy deposited 22
through elastic collisions(x10 eV/g) Fig. 3. (a) Change in Vickers hardness as a function of density of energy deposited through the electronic excitation (i.e., the energy deposited per unit mass of the specimen (gram)), and (b) change in Vickers hardness as a function of density of energy deposited through the elastic collisions (i.e., the energy deposited per unit mass of the specimen (gram)). Fig. 2. (a) Depth profile of the energy deposited through the electronic excitation per unit path for 5.4 MeV Al ion, 7.3 MeV Fe ion, 10 MeV I ion, and 16 MeV Au ion in Cu–Ti alloys, and (b) depth profile of the energy deposited through the elastic collisions per unit path for 5.4 MeV Al ion, 7.3 MeV Fe ion, 10 MeV I ion, and 16 MeV Au ion in Cu–Ti alloys.
by the irradiation. Therefore, the experimental result suggests that not Ti clusters but lattice defects mainly contribute to the increase in hardness. The Vickers hardness of the thermally-aged specimen is 280. In Fig. 4(c), we can observe large Ti atom precipitates after the thermal aging at 723 K. As well as the irradiation induced segregation and lattice defect production, the amorphization is also well known as an irradiation effect on materials. However, we scarcely observed any change in XRD spectra by the ion irradiations. We can, therefore, conclude that the amorphization was not induced in the Cu–Ti specimen by the present ion irradiations. Next, we compare the effect of energetic ion irradiation on the hardness of Cu–Ti alloy with that on the hardness of Al–Mg–Si
alloy. Fig. 5 shows the relative change in Vickers hardness (DHv/Hv0) for the Cu–Ti alloy and Al–Mg–Si alloy as a function of 10 MeV I ion fluence, where Hv0 is the hardness for the unirradiated specimen, and DHv is the difference in hardness between irradiated and unirradiated specimens. The hardness of Al–Mg–Si alloy increases gradually with increasing the ion fluence. On the other hand, the hardness of the Cu–Ti alloy once rapidly increases by the irradiation with relatively low fluences, and then it remains almost constant even with increasing the ion fluence. In the case of Al–Mg–Si alloy, as our previous result shows [4], the main origin of the hardness increase by the ion irradiation is nanometer-scaled clusters of solute atoms (Mg and Si), which are produced by the irradiation enhanced segregation. In the case of Cu–Ti alloy, we do not observe any Ti atom clusters even after the irradiation. Therefore, the origin of the irradiation induced hardness for the Cu–Ti alloy is not solute clusters but lattice defects of the matrix, i.e., interstitial atoms, vacancies and/or their aggregates. If the effect of the lattice defects on hardness is saturated more rapidly than that of irradiation induced segregation
56
D. Ueyama et al. / Nuclear Instruments and Methods in Physics Research B 341 (2014) 53–57
1.35
(a)
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Ion-fluence(x10 /cm ) 1.35
(b) Al-Mg-Si 1.3
(b) ΔHV/Hv0
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(c) 1.05
1 0
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300
350
Ion-fluence(x 10 /cm ) Fig. 4. Atom probe elemental mapping for (a) the specimen as-prepared (unirradiated, and therefore supersaturated), (b) the specimen irradiated with Au ions to the fluence of 1 1014/cm2, and (c) the specimen thermally-aged at 723 K for 12 h. Box sizes are: (a) 130 130 220 nm, (b) 52 53 29 nm, and (c) 120 120 60 nm.
of solute atoms, we can explain the difference in ion fluence dependence of hardness between the two alloys. The temperature at which the effect of thermal aging on hardness is clearly observed for Al–Mg–Si alloy is much lower than that for Cu–Ti alloys. This difference can be explained as follows; as the melting temperature for Al alloys (about 650 °C) is much lower than that of Cu-4.2 at.% Ti alloy (about 1100 °C), the production and the diffusion of thermal vacancies, which cause the aging effect, are activated at lower temperature for Al alloys. Similarly, lattice defects produced by energetic ion irradiation can diffuse in Al alloys even at room temperature and the irradiation enhanced segregation is realized.
Fig. 5. Relative change in Vickers hardness (DHv/Hv0) for (a) Cu–Ti and (b) Al–Mg–Si alloys as a function of 10 MeV I ion fluence. Hv0 is the hardness for the unirradiated specimen, and DHv is the difference in hardness between irradiated and unirradiated specimens.
For Cu–Ti alloy, the diffusion of irradiation produced lattice defects is not enough at room temperature for the irradiation enhanced segregation. To clarify this point more quantitatively, the ion and electron irradiation experiments at elevated temperatures are now in progress for Cu–Ti alloys.
4. Summary We irradiated Cu–Ti alloys with some energetic ions at room temperature, and measured the Vickers hardness. The hardness was increased with increasing ion fluence. The change in Vickers hardness by the ion irradiations is well correlated with the density
D. Ueyama et al. / Nuclear Instruments and Methods in Physics Research B 341 (2014) 53–57
of energy deposited through elastic collisions. The APT measurement, however, revealed that Ti-enriched clusters were scarcely observed even after the irradiation. This result suggests that in the case of Cu–Ti alloy irradiated at room temperature, not solute atom clusters but lattice defects of the matrix mainly contribute to the increase in hardness. The results of the present study show that the energetic ion irradiation can systematically control the surface hardness of Cu–Ti alloys as well as Al–Mg–Si alloys. Acknowledgments This research was carried out under the collaboration program between Osaka Prefecture University and the Japan Atomic Energy Agency (JAEA) and under the collaboration program between Osaka Prefecture University and Central Research Institute of Electric Power Industry. A part of this research was financially supported by Osaka Nuclear Science Association (ONSA).
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