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Effect of stress on radiation-induced hardening of A533B and Fe–Mn model alloys
Journal of Nuclear Materials 442 (2013) S776–S781
Contents lists available at SciVerse ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Effect of stress on radiation-induced hardening of A533B and Fe–Mn model alloys H. Watanabe a,⇑, A. Hiragane b, S. Shin b, N. Yoshida a, Y. Kamada c a
Research Institute for Applied Mechanics, Kyushu University, 6-1, Kasuga-kouenn, Kasugashi, Fukuoka 816-8580, Japan Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1, Kasuga-kouenn, Kasugashi, Fukuoka 816-8580, Japan c Faculty of Engineering, Iwate University, 4-3-5, Ueda, Morioka-shi, Morioka 020-8551, Japan b
a r t i c l e
i n f o
Article history: Available online 19 April 2013
a b s t r a c t In this study, a small tensile test machine was inserted in the beam line of a tandem-type accelerator. After 2.4 MeV Fe2+ ion irradiation, the microstructure and hardness changes of the specimens with and without applied stress were studied. Without stress, the formation of small interstitial-type dislocation loops was prominent in the matrix and also in the vicinity of dislocations in the Fe–1.4 wt.%Mn alloy. At room temperature, radiation-induced hardening was more prominent in samples with stress than in samples without stress. However, at 563 K, the effect of stress on hardness changes was minor. TEM observations showed that the applied stress reduced loop nucleation and enhanced loop growth to a degree corresponding to the microstructure at higher-temperature irradiation. This study revealed that the formation of interstitial-type dislocation loops enhanced by Mn addition was essential for irradiation hardening of these samples both with and without applied stress at higher dose levels. Ó 2013 Elsevier B.V. All rights reserved.
1. 1.Introduction The pressure vessel steels of a thermal reactor undergo mechanical property changes during service; these property changes are clearly very important for the safe operation of the reactor and play a major role in studying a plant’s life extension. Neutron irradiation of the steels increases the ductile-to-brittle transition temperature (DBTT) and decreases the upper shelf energy [1,2]. Copper has a strong effect on the embrittlement phenomenon, and copper-rich precipitates have been thought to be responsible for the embrittlement. Furthermore, studies on the mechanical properties of steels with different copper levels have shown that the so-called matrix defect is dominant for the embrittlement in low-copper steels [3] and in high-copper steels at high fluences [4]. Watanabe et al. [5] revealed that the majority of matrix defects formed after ion irradiation at higher dose levels were interstitial-type dislocation loops by using additional electron irradiation in a high-voltage electron microscope (HVEM). It was also shown that the contribution of dislocation loops to hardening was essential. Pressurized-water (PWR)-PWR type nuclear reactor pressure vessels operate in condition where the pressure vessel is under tension, however, surveillance specimens to simulate irradiation embrittlement are not irradiated under tensile stress conditions. Therefore, consideration of the difference in irradiation embrittlement between a pressure vessel and a surveillance spec⇑ Corresponding author. Tel.: +81 925837717; fax: +81 925837690. E-mail address: [email protected] (H. Watanabe). 0022-3115/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2013.04.029
imen might further our understanding of the mechanical properties involved. However, knowledge about dislocation loops formed at higher dose levels under tensile stress conditions in pressure vessel steels is very limited. Hawthorne et al. [6] showed that the experimental data suggest no synergistic effect of applied stress and radiation on mechanical properties. Applied stress levels used in the study were in excess of 75% of yield strengths. Recently, Fujii et al. [7] revealed that after ion irradiation on A533B steels (0.16 wt.%Cu) at 563 K to 1 dpa, radiation-induced hardening decreased with increasing stress to 500 MPa, which was near the yield strength. Fujii et al. used bent specimens of A533B steels to apply the tensile stresses. However, by this method, the estimation of stress changes during irradiation is difficult. Recent ion and neutron irradiation of pressure vessel steels and their model alloys has demonstrated that Mn addition has a significant effect on radiation-induced hardening [5,7–9]. Therefore, in this study, a small tensile test machine was newly developed for heavy-ion irradiation experiments using tensile stress conditions; the effect of stress on material properties and dislocation loop nucleation of a A533B steel with low-copper levels and of an Fe–Mn model alloy were studied. 2. Experimental procedure Fig. 1 shows the new tensile test machine used in the present study. Irradiation temperature was controlled by an infrared temperature sensor. To adjust the temperature and correct the emissivity of the samples, a thermocouple spot-welded to the
H. Watanabe et al. / Journal of Nuclear Materials 442 (2013) S776–S781
S777
Fig. 1. The miniature-sized tensile machine newly developed for the present study.
specimen was used. This device was inserted and tested in a beam line of the tandem-type accelerator at the Research Institute for Applied Mechanics at Kyushu University. A A533B steel with a low copper level (0.046 wt.%) and an Fe–1.4 wt.% Mn alloy were used in this study. Tensile specimens with a gage size of 1.2 mm (width) 5.0 mm (length) 0.1 mm (thickness) were punched from the material. A533B steel was annealed at 1153 K for 60 min and tempered at 943 K for 80 min., while the Fe–1.4 wt.%Mn alloy was homogenized at 1273 K for 24 h, annealed at 1073 K for 60 min, and air cooled. Fig. 2a and b shows stress–strain curves of the Fe–1.4 wt.%Mn alloy and A533B steel, respectively. In the figure, the test results at room temperature and 563 K are shown. The tensile stress rate was 93 MPa/min. The 2.4 MeV Fe2+ ion irradiations were performed at several tensile stress conditions (shown in the figure) at room temperature and 563 K. Hardness tests were conducted before and after the ion irradiations at room temperature using an Elionix ENT-1100 with a load of 1 gf. A triangular pyramidal diamond indentor (Berkovich type) with a semi-apex angle of 65° was used. The indenter load (L) and indenter displacement (d) were continuously monitored by a computer system. L and d are given by
L=d ¼ Ad þ B
ð1Þ
where A and B are dependent on materials but independent of indenter load and displacement. A is proportional to the Vickers hardness (Hv) and is given by
AðGPaÞ ¼ 0:287 Hv
ð2Þ
For TEM observations, the area near the peak damage region (at around 700 nm) was electro-polished on the tensile samples by a back-thinning method. The electrolyte was 50 mL perchloric acid and 950 mL acetic acid held at room temperature, and the polishing condition was 30 V. The damage rate in this region was about 2.5 104 dpa/s.
3. Results 3.1. Microstructure before and after ion irradiation Without irradiation, the dislocation density of the Fe–1.4 wt.%Mn alloy increased with increasing tensile stress. Fig. 3 shows bright-field transmission electron microscopy (TEM) images of the Fe–1.4 wt.%Mn alloy at different stress levels at room temperature with the tensile direction shown by arrows in the figure. Measured dislocation densities are also
Fig. 2. Stress–strain curves of samples irradiated at room temperature and 563 K. (a) Fe–1.4 wt.%Mn and (b) A533B steel.
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H. Watanabe et al. / Journal of Nuclear Materials 442 (2013) S776–S781
Fig. 3. Bright-field TEM images and measured dislocation density of the Fe–1.4 wt.%Mn alloy with different loads at room temperature. Tensile direction is shown by arrows. The measured network dislocation density is also shown in the figure.
Fig. 4. Microstructure of the Fe–1.4 wt.%Mn alloy after ion irradiation at room temperature and 563 K with different tensile stresses up to 1 dpa. Tensile direction is shown by arrows.
H. Watanabe et al. / Journal of Nuclear Materials 442 (2013) S776–S781
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Fig. 5. Applied stress dependence of measured dislocation loop size and density of Fe–1.4 wt.%Mn alloy irradiated at room temperature and 563 K. Irradiation dose is 1 dpa. (a) Room temperature and (b) 563 K.
shown in the figure. Fig. 4 shows the microstructure of the alloy after ion irradiation up to 1 dpa at room temperature and 563 K at different tensile stresses. Room temperature irradiations resulted in a relatively high density of interstitial-type dislocation loops. At 563 K, the number density of dislocation loops decreased and larger loops were formed. In the unstressed condition, formation of small interstitial-type dislocation loops was prominent in the matrix and also in the vicinity of dislocations in the Fe–1.4 wt.%Mn alloy. Fig. 5 shows the applied stress dependence of measured dislocation loop size and density, where Fig. 5a and b correspond to the results at room temperature and 563 K, respectively, at 1 dpa. Irradiation at room temperature and 563 K increased the diameter of the dislocation loops and decreased the dislocation number density when stress was applied. Fig. 6 shows the applied stress dependence of the measured dislocation density. At room temperature, the dislocation density increased with applied stress both with and without irradiation near the yield stress. The dislocation density increase was also detected in the sample irradiated at 563 K without irradiation, but the increase was not prominent with irradiation. 3.2. Hardness changes before and after ion irradiation Fig. 7 shows the tensile stress dependence of hardness changes for the Fe–1.4 wt.%Mn alloy. Fig. 7a and b correspond to the results at room temperature and 563 K, respectively. As a result of irradiation at room temperature, the measured hardness increased with applied load and strong radiation hardening occurred after the yield point was reached. On the other hand, at 563 K, the hardness increase was small regardless of the applied stress. Fig. 8 shows the tensile stress dependence of hardness changes for the A533B steel. Fig. 8a and b correspond to the results at room temperature and 563 K, respectively. Hardness from room temperature irradiation was increased by applied stress only at higher stress levels (namely, in the plastic region). However, at 563 K, no prominent increase due to applied stress was detected in any part of the tensile stress range. 4. Discussion In the present study, irradiation at room temperature and at 563 K increased the diameter of the dislocation loops and decreased the number density when an external stress was applied. Table 1 summarizes the average dislocation loop density and loop size formed by irradiation, and the average dislocation density introduced by the applied tensile stress in samples of Fe– 1.4 wt.%Mn irradiated at room temperature and 563 K. To calculate radiation hardening from dislocation loops, the following equation was used:
Fig. 6. Applied stress dependence of measured dislocation density of Fe– 1.4 wt.%Mn alloy with and without irradiation at room temperature and 563 K. Irradiation dose is 1 dpa.
DH v ¼
pffiffiffiffiffiffi 3:04 alb Nd 3
ð3Þ
where a is a constant set at 0.3, l is shear modulus, b is the Burgers vector set at 0.25 nm, N is the number density of dislocation loops, and d is the average dislocation loop diameter [10,11]. Using this equation, the estimated hardness and observed hardness at room temperature and 563 K are also shown in Table 1. As shown in the table, the contribution of dislocation loops to the total hardness change is relatively large at room temperature. About 60% of the observed radiation-induced hardness can be explained by dislocation loops and dislocations observable by TEM. As described in [12], Mn clusters formed by irradiation appear at the dislocation loop nucleation site. The total amount of radiation hardening can presumably be the sum of multiple operating mechanisms. These small Mn clusters, which are not visible by TEM, are also responsible for radiation-induced hardening of the materials. Related to the effects of stress on the material during irradiation, radiation-enhanced creep of austenitic stainless steels is a well-known phenomenon and an important deformation mechanism in fast breeder reactors and fusion reactors. The shape changes that occur in reactor components are the result of a sensitive balance between void swelling and irradiation-enhanced creep. Models of radiation-enhanced creep in austenitic steels, stress-induced preferential loop nucleation (SIPN) [13], and stress-induced preferential attraction (SIPA) [14] were proposed. Recently, the present authors showed the evolution of damage structure in stressed Fe–1.4 wt.%Mn alloy under electron irradiation in a HVEM, and also showed enhanced dislocation loop growth
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H. Watanabe et al. / Journal of Nuclear Materials 442 (2013) S776–S781
Fig. 7. Tensile stress dependence of hardness changes in the Fe–1.4 wt.%Mn alloy irradiated at room temperature and 563 K. Irradiation dose is 1 dpa. (a) Room temperature and (b) 563 K.
Fig. 8. Tensile stress dependence of hardness changes in A533B steel irradiated at room temperature and 563 K. Irradiation dose is 1 dpa. (a) Room temperature and (b) 563 K.
Table 1 Summary of dislocation loops, dislocation, and hardness increase in samples of Fe–1.4 wt.%Mn irradiated at room temperature and 563 K. Irradiation dose is 1 dpa. Stress (MPa)
Loop density (1020 m3)
Loop size (nm)
Dislocation density (l dpa) (1015 m2)
Estimat ed (DHv)
Observ ed (DHv)
Contribution of loops to total hardness changes (%)
Contribution of loops and dislocation to total hardness changes (%)
RT
0 41.6 104
174 145 72
2.7 4.9 10
3.0 7.6 70.3
61 75 73
92 112 201
66 67 36
66 74 46
563 K
0 41.6 70.7
3.6 1.8 0.5
31 36 133
3.0 9.3 14
29 22 21
51 76 50
57 29 42
57 30 45
and suppressed loop nucleation under an applied stress [12]. The results obtained from the HVEM study are consistent with the present study. The results show that stress reduced loop nucleation and enhanced loop growth to a degree corresponding to the microstructure at higher-temperature irradiation. This means that stress enhances the mobility of point defects (vacancy and/or interstitial). In Mn-containing alloys, Mn atoms act as nuclei for interstitialtype dislocation loops, and strong binding energy between interstitial and Mn atoms of about 0.22 eV (for Fe–0.6 0%Mn alloy) is estimated for the unstressed condition [12]. If the binding energy is decreased by an applied stress, the apparent mobility of the interstitial – Mn atom complex will increase.
(1) In the Fe–1.4 wt.%Mn alloy, hardness increased with applied stress and strong radiation hardening occurred after the yield point at room temperature. On the other hand, at 563 K, the hardness increase was small throughout the range of applied stress. (2) The results that stress reduced loop nucleation and enhanced loop growth were shown to correspond to an irradiated microstructure at higher-temperature irradiation. This suggests that stress enhances the apparent mobility of point defects. (3) In A5333B, the hardness at room temperature irradiation increased along with the applied stress only at higher stress levels. However, at 563 K, no prominent increase was detected due to the applied stress.
5. Conclusions Fe ion irradiation was conducted on A533B steel and Fe–1.4 wt.%Mn alloy with and without an applied stress at room temperature and 563 K to 1 dpa. The main results are summarized as follows.
Acknowledgments This work was supported by a Grant-in-Aid for Science Research (B), Nos. 20360420 and 23360418, from the Ministry of Education,
H. Watanabe et al. / Journal of Nuclear Materials 442 (2013) S776–S781
Culture, Sports, Science and Technology of Japan. This work was also supported in part by a JSPS bilateral program. This work was also supported in part by the Collaborative Research Program of Research Institute for Applied Mechanics, Kyushu University.
References [1] G.R. Odette, Scripta Meter. 11 (1983) p1183. [2] G.E. Lucas, G.R. Odette, P.M. Lombrozo, J.W. Sheckherd, Effects of radiation on materials, in: 12th International Symposium, ASTM STP 870, American Society for Testing Materials, Philadelphia, 1985, p. 900. [3] M. Suzuki, K. Onizawa, M. Kizaki, Effects of radiation on materials, in: 17th International Symposium, ASTM STP 1270, American Society for Testing Materials, Philadelphia, 1996, p. 351.
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[4] G.R. Odette, E.V. Mader, G.E. Lucas, W.J. Phythian, C.A. English, Effects of radiation on materials, in: 16th International Symposium, ASTM STP 1275, American Society for Testing Materials, Philadelphia, 1993, p. 373. [5] H. Watanabe, S. Masaki, S. Masubuchi, N. Yoshida, Y. Kamada, J. Nucl. Mater. 417 (2011) 932. [6] J.R. Hawthorne, F.J. Loss, Nucl. Eng. Des. 8 (1968) 108. [7] K. Fuji, K. Fukuya, R. Kasada, A. Kimura, T. Ohkubo, J. Nucl. Mater. 407 (2010) 151. [8] K. Yabuuchi, H. Yano, R. Kasada, H. Kishimoto, A. Kimura, J. Nucl. Mater. 417 (2011) 988. [9] K. Yabuuchi, M. Saito, R. Kasada, A. Kimura, J. Nucl. Mater. 414 (2011) 498. [10] R. Stoller, A. Calder, J. Nucl. Mater. 283–287 (2000) 746. [11] G. Lucas, J. Nucl. Mater. 206 (1993) 287. [12] H. Watanabe et al., J. Nucl. Mater. 439 (2013) 268. [13] P. Brailsford, R. Bullough, UKAEA T.P. 486 (1972). [14] P. Heald, M. Speight, Philos. Mag. 29 (1974) 1075.
Contents lists available at SciVerse ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Effect of stress on radiation-induced hardening of A533B and Fe–Mn model alloys H. Watanabe a,⇑, A. Hiragane b, S. Shin b, N. Yoshida a, Y. Kamada c a
Research Institute for Applied Mechanics, Kyushu University, 6-1, Kasuga-kouenn, Kasugashi, Fukuoka 816-8580, Japan Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1, Kasuga-kouenn, Kasugashi, Fukuoka 816-8580, Japan c Faculty of Engineering, Iwate University, 4-3-5, Ueda, Morioka-shi, Morioka 020-8551, Japan b
a r t i c l e
i n f o
Article history: Available online 19 April 2013
a b s t r a c t In this study, a small tensile test machine was inserted in the beam line of a tandem-type accelerator. After 2.4 MeV Fe2+ ion irradiation, the microstructure and hardness changes of the specimens with and without applied stress were studied. Without stress, the formation of small interstitial-type dislocation loops was prominent in the matrix and also in the vicinity of dislocations in the Fe–1.4 wt.%Mn alloy. At room temperature, radiation-induced hardening was more prominent in samples with stress than in samples without stress. However, at 563 K, the effect of stress on hardness changes was minor. TEM observations showed that the applied stress reduced loop nucleation and enhanced loop growth to a degree corresponding to the microstructure at higher-temperature irradiation. This study revealed that the formation of interstitial-type dislocation loops enhanced by Mn addition was essential for irradiation hardening of these samples both with and without applied stress at higher dose levels. Ó 2013 Elsevier B.V. All rights reserved.
1. 1.Introduction The pressure vessel steels of a thermal reactor undergo mechanical property changes during service; these property changes are clearly very important for the safe operation of the reactor and play a major role in studying a plant’s life extension. Neutron irradiation of the steels increases the ductile-to-brittle transition temperature (DBTT) and decreases the upper shelf energy [1,2]. Copper has a strong effect on the embrittlement phenomenon, and copper-rich precipitates have been thought to be responsible for the embrittlement. Furthermore, studies on the mechanical properties of steels with different copper levels have shown that the so-called matrix defect is dominant for the embrittlement in low-copper steels [3] and in high-copper steels at high fluences [4]. Watanabe et al. [5] revealed that the majority of matrix defects formed after ion irradiation at higher dose levels were interstitial-type dislocation loops by using additional electron irradiation in a high-voltage electron microscope (HVEM). It was also shown that the contribution of dislocation loops to hardening was essential. Pressurized-water (PWR)-PWR type nuclear reactor pressure vessels operate in condition where the pressure vessel is under tension, however, surveillance specimens to simulate irradiation embrittlement are not irradiated under tensile stress conditions. Therefore, consideration of the difference in irradiation embrittlement between a pressure vessel and a surveillance spec⇑ Corresponding author. Tel.: +81 925837717; fax: +81 925837690. E-mail address: [email protected] (H. Watanabe). 0022-3115/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2013.04.029
imen might further our understanding of the mechanical properties involved. However, knowledge about dislocation loops formed at higher dose levels under tensile stress conditions in pressure vessel steels is very limited. Hawthorne et al. [6] showed that the experimental data suggest no synergistic effect of applied stress and radiation on mechanical properties. Applied stress levels used in the study were in excess of 75% of yield strengths. Recently, Fujii et al. [7] revealed that after ion irradiation on A533B steels (0.16 wt.%Cu) at 563 K to 1 dpa, radiation-induced hardening decreased with increasing stress to 500 MPa, which was near the yield strength. Fujii et al. used bent specimens of A533B steels to apply the tensile stresses. However, by this method, the estimation of stress changes during irradiation is difficult. Recent ion and neutron irradiation of pressure vessel steels and their model alloys has demonstrated that Mn addition has a significant effect on radiation-induced hardening [5,7–9]. Therefore, in this study, a small tensile test machine was newly developed for heavy-ion irradiation experiments using tensile stress conditions; the effect of stress on material properties and dislocation loop nucleation of a A533B steel with low-copper levels and of an Fe–Mn model alloy were studied. 2. Experimental procedure Fig. 1 shows the new tensile test machine used in the present study. Irradiation temperature was controlled by an infrared temperature sensor. To adjust the temperature and correct the emissivity of the samples, a thermocouple spot-welded to the
H. Watanabe et al. / Journal of Nuclear Materials 442 (2013) S776–S781
S777
Fig. 1. The miniature-sized tensile machine newly developed for the present study.
specimen was used. This device was inserted and tested in a beam line of the tandem-type accelerator at the Research Institute for Applied Mechanics at Kyushu University. A A533B steel with a low copper level (0.046 wt.%) and an Fe–1.4 wt.% Mn alloy were used in this study. Tensile specimens with a gage size of 1.2 mm (width) 5.0 mm (length) 0.1 mm (thickness) were punched from the material. A533B steel was annealed at 1153 K for 60 min and tempered at 943 K for 80 min., while the Fe–1.4 wt.%Mn alloy was homogenized at 1273 K for 24 h, annealed at 1073 K for 60 min, and air cooled. Fig. 2a and b shows stress–strain curves of the Fe–1.4 wt.%Mn alloy and A533B steel, respectively. In the figure, the test results at room temperature and 563 K are shown. The tensile stress rate was 93 MPa/min. The 2.4 MeV Fe2+ ion irradiations were performed at several tensile stress conditions (shown in the figure) at room temperature and 563 K. Hardness tests were conducted before and after the ion irradiations at room temperature using an Elionix ENT-1100 with a load of 1 gf. A triangular pyramidal diamond indentor (Berkovich type) with a semi-apex angle of 65° was used. The indenter load (L) and indenter displacement (d) were continuously monitored by a computer system. L and d are given by
L=d ¼ Ad þ B
ð1Þ
where A and B are dependent on materials but independent of indenter load and displacement. A is proportional to the Vickers hardness (Hv) and is given by
AðGPaÞ ¼ 0:287 Hv
ð2Þ
For TEM observations, the area near the peak damage region (at around 700 nm) was electro-polished on the tensile samples by a back-thinning method. The electrolyte was 50 mL perchloric acid and 950 mL acetic acid held at room temperature, and the polishing condition was 30 V. The damage rate in this region was about 2.5 104 dpa/s.
3. Results 3.1. Microstructure before and after ion irradiation Without irradiation, the dislocation density of the Fe–1.4 wt.%Mn alloy increased with increasing tensile stress. Fig. 3 shows bright-field transmission electron microscopy (TEM) images of the Fe–1.4 wt.%Mn alloy at different stress levels at room temperature with the tensile direction shown by arrows in the figure. Measured dislocation densities are also
Fig. 2. Stress–strain curves of samples irradiated at room temperature and 563 K. (a) Fe–1.4 wt.%Mn and (b) A533B steel.
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Fig. 3. Bright-field TEM images and measured dislocation density of the Fe–1.4 wt.%Mn alloy with different loads at room temperature. Tensile direction is shown by arrows. The measured network dislocation density is also shown in the figure.
Fig. 4. Microstructure of the Fe–1.4 wt.%Mn alloy after ion irradiation at room temperature and 563 K with different tensile stresses up to 1 dpa. Tensile direction is shown by arrows.
H. Watanabe et al. / Journal of Nuclear Materials 442 (2013) S776–S781
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Fig. 5. Applied stress dependence of measured dislocation loop size and density of Fe–1.4 wt.%Mn alloy irradiated at room temperature and 563 K. Irradiation dose is 1 dpa. (a) Room temperature and (b) 563 K.
shown in the figure. Fig. 4 shows the microstructure of the alloy after ion irradiation up to 1 dpa at room temperature and 563 K at different tensile stresses. Room temperature irradiations resulted in a relatively high density of interstitial-type dislocation loops. At 563 K, the number density of dislocation loops decreased and larger loops were formed. In the unstressed condition, formation of small interstitial-type dislocation loops was prominent in the matrix and also in the vicinity of dislocations in the Fe–1.4 wt.%Mn alloy. Fig. 5 shows the applied stress dependence of measured dislocation loop size and density, where Fig. 5a and b correspond to the results at room temperature and 563 K, respectively, at 1 dpa. Irradiation at room temperature and 563 K increased the diameter of the dislocation loops and decreased the dislocation number density when stress was applied. Fig. 6 shows the applied stress dependence of the measured dislocation density. At room temperature, the dislocation density increased with applied stress both with and without irradiation near the yield stress. The dislocation density increase was also detected in the sample irradiated at 563 K without irradiation, but the increase was not prominent with irradiation. 3.2. Hardness changes before and after ion irradiation Fig. 7 shows the tensile stress dependence of hardness changes for the Fe–1.4 wt.%Mn alloy. Fig. 7a and b correspond to the results at room temperature and 563 K, respectively. As a result of irradiation at room temperature, the measured hardness increased with applied load and strong radiation hardening occurred after the yield point was reached. On the other hand, at 563 K, the hardness increase was small regardless of the applied stress. Fig. 8 shows the tensile stress dependence of hardness changes for the A533B steel. Fig. 8a and b correspond to the results at room temperature and 563 K, respectively. Hardness from room temperature irradiation was increased by applied stress only at higher stress levels (namely, in the plastic region). However, at 563 K, no prominent increase due to applied stress was detected in any part of the tensile stress range. 4. Discussion In the present study, irradiation at room temperature and at 563 K increased the diameter of the dislocation loops and decreased the number density when an external stress was applied. Table 1 summarizes the average dislocation loop density and loop size formed by irradiation, and the average dislocation density introduced by the applied tensile stress in samples of Fe– 1.4 wt.%Mn irradiated at room temperature and 563 K. To calculate radiation hardening from dislocation loops, the following equation was used:
Fig. 6. Applied stress dependence of measured dislocation density of Fe– 1.4 wt.%Mn alloy with and without irradiation at room temperature and 563 K. Irradiation dose is 1 dpa.
DH v ¼
pffiffiffiffiffiffi 3:04 alb Nd 3
ð3Þ
where a is a constant set at 0.3, l is shear modulus, b is the Burgers vector set at 0.25 nm, N is the number density of dislocation loops, and d is the average dislocation loop diameter [10,11]. Using this equation, the estimated hardness and observed hardness at room temperature and 563 K are also shown in Table 1. As shown in the table, the contribution of dislocation loops to the total hardness change is relatively large at room temperature. About 60% of the observed radiation-induced hardness can be explained by dislocation loops and dislocations observable by TEM. As described in [12], Mn clusters formed by irradiation appear at the dislocation loop nucleation site. The total amount of radiation hardening can presumably be the sum of multiple operating mechanisms. These small Mn clusters, which are not visible by TEM, are also responsible for radiation-induced hardening of the materials. Related to the effects of stress on the material during irradiation, radiation-enhanced creep of austenitic stainless steels is a well-known phenomenon and an important deformation mechanism in fast breeder reactors and fusion reactors. The shape changes that occur in reactor components are the result of a sensitive balance between void swelling and irradiation-enhanced creep. Models of radiation-enhanced creep in austenitic steels, stress-induced preferential loop nucleation (SIPN) [13], and stress-induced preferential attraction (SIPA) [14] were proposed. Recently, the present authors showed the evolution of damage structure in stressed Fe–1.4 wt.%Mn alloy under electron irradiation in a HVEM, and also showed enhanced dislocation loop growth
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H. Watanabe et al. / Journal of Nuclear Materials 442 (2013) S776–S781
Fig. 7. Tensile stress dependence of hardness changes in the Fe–1.4 wt.%Mn alloy irradiated at room temperature and 563 K. Irradiation dose is 1 dpa. (a) Room temperature and (b) 563 K.
Fig. 8. Tensile stress dependence of hardness changes in A533B steel irradiated at room temperature and 563 K. Irradiation dose is 1 dpa. (a) Room temperature and (b) 563 K.
Table 1 Summary of dislocation loops, dislocation, and hardness increase in samples of Fe–1.4 wt.%Mn irradiated at room temperature and 563 K. Irradiation dose is 1 dpa. Stress (MPa)
Loop density (1020 m3)
Loop size (nm)
Dislocation density (l dpa) (1015 m2)
Estimat ed (DHv)
Observ ed (DHv)
Contribution of loops to total hardness changes (%)
Contribution of loops and dislocation to total hardness changes (%)
RT
0 41.6 104
174 145 72
2.7 4.9 10
3.0 7.6 70.3
61 75 73
92 112 201
66 67 36
66 74 46
563 K
0 41.6 70.7
3.6 1.8 0.5
31 36 133
3.0 9.3 14
29 22 21
51 76 50
57 29 42
57 30 45
and suppressed loop nucleation under an applied stress [12]. The results obtained from the HVEM study are consistent with the present study. The results show that stress reduced loop nucleation and enhanced loop growth to a degree corresponding to the microstructure at higher-temperature irradiation. This means that stress enhances the mobility of point defects (vacancy and/or interstitial). In Mn-containing alloys, Mn atoms act as nuclei for interstitialtype dislocation loops, and strong binding energy between interstitial and Mn atoms of about 0.22 eV (for Fe–0.6 0%Mn alloy) is estimated for the unstressed condition [12]. If the binding energy is decreased by an applied stress, the apparent mobility of the interstitial – Mn atom complex will increase.
(1) In the Fe–1.4 wt.%Mn alloy, hardness increased with applied stress and strong radiation hardening occurred after the yield point at room temperature. On the other hand, at 563 K, the hardness increase was small throughout the range of applied stress. (2) The results that stress reduced loop nucleation and enhanced loop growth were shown to correspond to an irradiated microstructure at higher-temperature irradiation. This suggests that stress enhances the apparent mobility of point defects. (3) In A5333B, the hardness at room temperature irradiation increased along with the applied stress only at higher stress levels. However, at 563 K, no prominent increase was detected due to the applied stress.
5. Conclusions Fe ion irradiation was conducted on A533B steel and Fe–1.4 wt.%Mn alloy with and without an applied stress at room temperature and 563 K to 1 dpa. The main results are summarized as follows.
Acknowledgments This work was supported by a Grant-in-Aid for Science Research (B), Nos. 20360420 and 23360418, from the Ministry of Education,
H. Watanabe et al. / Journal of Nuclear Materials 442 (2013) S776–S781
Culture, Sports, Science and Technology of Japan. This work was also supported in part by a JSPS bilateral program. This work was also supported in part by the Collaborative Research Program of Research Institute for Applied Mechanics, Kyushu University.
References [1] G.R. Odette, Scripta Meter. 11 (1983) p1183. [2] G.E. Lucas, G.R. Odette, P.M. Lombrozo, J.W. Sheckherd, Effects of radiation on materials, in: 12th International Symposium, ASTM STP 870, American Society for Testing Materials, Philadelphia, 1985, p. 900. [3] M. Suzuki, K. Onizawa, M. Kizaki, Effects of radiation on materials, in: 17th International Symposium, ASTM STP 1270, American Society for Testing Materials, Philadelphia, 1996, p. 351.
S781
[4] G.R. Odette, E.V. Mader, G.E. Lucas, W.J. Phythian, C.A. English, Effects of radiation on materials, in: 16th International Symposium, ASTM STP 1275, American Society for Testing Materials, Philadelphia, 1993, p. 373. [5] H. Watanabe, S. Masaki, S. Masubuchi, N. Yoshida, Y. Kamada, J. Nucl. Mater. 417 (2011) 932. [6] J.R. Hawthorne, F.J. Loss, Nucl. Eng. Des. 8 (1968) 108. [7] K. Fuji, K. Fukuya, R. Kasada, A. Kimura, T. Ohkubo, J. Nucl. Mater. 407 (2010) 151. [8] K. Yabuuchi, H. Yano, R. Kasada, H. Kishimoto, A. Kimura, J. Nucl. Mater. 417 (2011) 988. [9] K. Yabuuchi, M. Saito, R. Kasada, A. Kimura, J. Nucl. Mater. 414 (2011) 498. [10] R. Stoller, A. Calder, J. Nucl. Mater. 283–287 (2000) 746. [11] G. Lucas, J. Nucl. Mater. 206 (1993) 287. [12] H. Watanabe et al., J. Nucl. Mater. 439 (2013) 268. [13] P. Brailsford, R. Bullough, UKAEA T.P. 486 (1972). [14] P. Heald, M. Speight, Philos. Mag. 29 (1974) 1075.