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Precipitation of Solid Transmutation Elements in Irradiated Tungsten Alloys

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Precipitation of Solid Transmutation Elements in Irradiated Tungsten Alloys Materials Transactions, Vol. 49, No. 10 (2008) pp. 2259 to 2264 #2008 The Japan Institute of Metals Precipitation of Solid Transmutation Elements in Irradiated Tungsten Alloys Takashi Tanno1;*1, Akira Hasegawa1, Mitsuhiro Fujiwara1, Jian-Chao He1;*1, Shuhei Nogami1, Manabu Satou1, Toetsu Shishido2 and Katsunori Abe1;*2 1Department of Quantum Science and Energy Engineering, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan 2Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Tungsten-based model alloys were fabricated to simulate compositional changes by neutron irradiation, performed in the JOYO fast test reactor. The irradiation damage range was 0.17–1.54 dpa and irradiation temperatures were 400, 500 and 750C. After irradiation, microstructural observations and electrical resistivity measurements were carried out. A number of precipitates were observed after 1.54 dpa irradiation. Rhenium and osmium were precipitated by irradiation, which suppressed the formation of dislocation loops and voids. Structures induced by irradiation were not observed so much after 0.17 dpa irradiation. Electrical resistivity measurements showed that the effects of osmium on the electrical resistivity, related to impurity solution content, were larger than that of rhenium. Measurements of electrical resistivity of ternary alloys showed that the precipitation behavior was similar to that in binary alloys. [doi:10.2320/matertrans.MAW200821] (Received April 23, 2008; Accepted August 8, 2008; Published September 18, 2008) Keywords: tungsten, solid transmutation element, precipitation, neutron irradiation, composition change, electrical resistivity 1. Introduction elements from tungsten, there are very few reports concern- ing irradiation effects on W-xRe and W-xRe-yOs alloys. Tungsten (W) is one of the candidate materials for fusion For materials used in fusion reactors, mechanical proper- reactors because of its high melting point, high sputtering ties such as hardness and ductility are important when large resistivity and high temperature strength. During fusion electromagnetic force and thermal stresses are induced in the reactor operation, it is anticipated that due to 14 MeV neutron materials by instability of plasma. We have previously irradiation, in addition to irradiation damage, solid trans- reported differences in the irradiation hardening of W-xRe mutation elements, such as rhenium (Re) and osmium (Os), and W-yOs alloys, formed primarily by irradiation precip- will be induced in pure tungsten. Consequently, with itation.9) Thermal conductivity is also important for cooling increasing neutron fluence, the solid transmutation elements which is related to electrical conductivity. The electrical will accumulate and the original pure tungsten will change resistivity, which is the inverse of the electrical conductivity, to W-xRe or W-xRe-yOs alloys. For example, calculations depends on the solute content. predict that pure W will change to a W-18Re-3Os alloy after The purpose of this study was to investigate precipitation 50 dpa irradiation, corresponding to a neutron fluence of behavior of rhenium and osmium in neutron-irradiated 10 MWy/m2.1) tungsten alloys systematically as a function of the alloy It is well known that tungsten shows brittle behavior at composition and irradiation dose in order to understand room temperature though it has excellent high temperature irradiation-induced property changes of tungsten under properties, and the low temperature embrittlement can be fusion reactor conditions. improved by Re addition.2,3) It was reported that the creep strength of tungsten irradiated with thermal neutrons is 2. Experimental Procedure higher than that of unirradiated tungsten, and substitutional solid solution rhenium transmuted from tungsten seems to be In order to simulate composition changes due to trans- the reason why the creep strength increased.4) Rhenium and mutation, tungsten based model alloys containing rhenium osmium also change the physical properties of tungsten. For and/or osmium were fabricated. The nominal compositions example, tungsten has high thermal conductivity, however of the examined alloys are shown in Table 1. They were this decreases to half the value for pure tungsten by the selected basing compositional change predicted by calcula- addition of 5 mass% Re.5,6) In terms of microstructural tion of transmutation1) in the solid solution area of W-xRe- evolution, it has been reported that phase precipitates yOs alloys. Model alloy fabrications were carried out using (Re3W) were formed in W-xRe alloys after 0.5–0.7 dpa an argon arc furnace. The raw materials were pure W irradiation at 600–1500C without void formation, and the (99.96%) and W-26Re (W: 74:0 Æ 0:2%, Re: 26:0 Æ 0:2%) bulk properties were affected by this precipitation.7) Heavy rods supplied by Plansee Ltd. and Os (99.9%) powder neutron irradiation induced large irradiation hardening in W- 26Re alloy by irradiation-induced precipitation.8) However, even though rhenium and osmium are the main transmutation Table 1 Nominal compositions of the fabricated alloys (mass%). W bal. bal. bal. bal. bal. bal. bal. bal. bal. bal. bal. *1 Graduate Student, Tohoku University Re — 5 10 26 — — 5 10 18 25 5 *2Present address: Hachinohe Institute of Technology, Hachinohe 031- Os————3533335 8501, Japan 2260 T. Tanno et al. supplied by Kojundo Chemical Laboratory Co., Ltd. Inter- 0.635 mm and load P of 0.98 N. Electrical resistivity was stitial impurity levels of the fabricated alloys were in the calculated using eq. (1), where V is the measured voltage range of 40–200 wppm for carbon (C), 20–40 wppm for drop, t is the specimen thickness, C:F: is the conversion oxygen (O) and <12 wppm for nitrogen (N). Disk-shaped factor fixed by the specimen shape (circle, rectangle, etc.) specimens with a diameter of 3 mm and a thickness of and the measurement conditions. In this studym C:F: was 0.3 mm were cut from the ingots using an electro discharge 3.5242.14) machine, mechanically ground and polished to 0.2 mm ¼ðV  t=IÞC:F: ð1Þ thickness, and finally annealed at 1400C for 1 h in vacuum (<10À5 Pa).10) Neutron irradiation was carried out in the JOYO fast test 3. Results reactor in Japan Atomic Energy Agency (JAEA). The irradiation conditions used are shown in Table 2. Displace- 3.1 Microstructural observations ment damage (dpa) was calculated using the NPRIM-1.3 Figure 1 shows the microstructures of irradiated W-5Re, code11) with a displacement threshold energy of 90 eV.12) W-3Os and W-5Re-3Os alloys. Voids and dislocation loops, Greenwood et al. calculated the compositional change due seen as black dots, were observed in W-5Re after 0.17 dpa to transmutation during irradiation in the fast reactor.13) irradiation at 400C. Plate or needle-like precipitates on the According to their results, transmutation of tungsten is {110} plane were observed instead of dislocation loops for a negligibly small even after 1.54 dpa irradiation in JOYO. higher dose and irradiation temperature (1.54 dpa irradiation After the irradiation, microstructural observations were at 750C). Though the mean diameter of the voids grew to carried out with a transmission electron microscope (TEM) twice as large, the number density of voids decreased to one- operating at 200 kV. Electrical resistivity measurements were tenth that of the lower dose and temperature. Precipitates carried out using the four-probe method at 20C with a load were also observed in other W-xRe alloys after 0.40 dpa and current I of 100 mA, distance between the probes S of above irradiation at 750C. These precipitates were identified 8) as the phase (Re3W) from the electron diffraction pattern. A special structure due to irradiation was not observed Table 2 Neutron irradiation conditions. in W-3Os after 0.17 dpa irradiation at 400C. Needle-like precipitates in the {110} plane were observed after 1.54 dpa Irradiation Fluence temperature (En > 0:1 MeV) dpa irradiation at 750 C. A few black dots were observed but (C) (1025 n/m2) voids were not seen. The widths of the precipitates were 400 1.3 0.17 smaller than those in W-5Re alloys. The precipitates could 500 2.9 0.37 be not identified from the diffraction patterns, but they are 740 3.1 0.40 likely the phase (OsxW1Àx, x ¼ 0:20{0:35) because this is 750 12 1.54 the only intermetallic compound, according to the W-yOs phase diagram.15) W-5Re W-3Os W-5Re-3Os C ° 0.17dpa/400 C ° 1.54dpa/750 Fig. 1 TEM bright field images of W-5Re, W-3Os and W-5Re-3Os alloys after 0.17 dpa irradiation at 400C and 1.54 dpa at 750C. The electron incidence directions were Z = [111], only the direction of W-5Re-3Os at 750C was Z = [001]. There were few visible structures in alloys irradiated at lower dose conditions. Plate or needle-like precipitates in the {110} plane were observed. Precipitation of Solid Transmutation Elements in Irradiated Tungsten Alloys 2261 Table 3 Summary of microstructural observations of alloys after 1.54 dpa irradiation at 750C. W W-5Re W-10Re W-3Os W-5Re-3Os Void Mean diameter [nm] 4.7 3.3 1.6 — — Density [1022/m3] 12 0.65 3.1 — — Swelling [%] 0.72 0.01 0.01 — — Precipitate Mean length [nm] — 14 9.5 7.3 6.8 Density [1022/m3] — 7.3 42 22 67 Volume [%] — 1.5 3.6 0.80 3.4 50 50 W-xRe-3Os W-xRe cm cm 40 40 /µΩ /µΩ ρ ρ 30 W-yOs 30 20 20 ° 1.54dpa/750°C 1.54dpa/750 C ° 0.40dpa/740°C 0.40dpa/740 C 10 0.37dpa/500°C 10 0.37dpa/500°C Electrical resistivity Electrical resistivity 0.17dpa/400°C 0.17dpa/400°C unirradiated unirradiated 0 0 0 5 10 15 20 25 30 0 5 10 15 20 25 30 Re x or Os y /mass% Re x /mass% Fig.
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