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Prospects and problems using vanadium alloys as a structural material of the first wall and blanket of fusion reactors
ELSEVIER
Journal of Nuclear Materials 233-237 (1996) 370-375
journal of nuclear materials
Prospects and problems using vanadium alloys as a structural material of the first wall and blanket of fusion reactors S.N. Votinov a’* , M.I. Solonin ‘, Yu.1. Kazennov ‘I, V.P. Kondratjev ‘, A.D. Nikulin “, V.N. Tebus a, E.O. Adamov b, S.E. Bougaenko b, Yu.S. Strebkov ‘, A.V. Sidorenkov b, V.B. Ivanov ‘, V.A. Kazakov ‘, V.A. Evtikhin ‘, I.E. Lyublinski ‘, V.M. Trojanov ‘, A.E. Rusanov ‘, V.M. Chernov ‘, G.A. Birgevoj ’
Abstract Vanadium-based alloys are most promising as low activation structural materials for DEMO. It was previously established that high priority is to be given to V-alloys of the V-Ti-Cr system as structural materials of a tritium breeding blanket and the first wall of a fusion reactor. However, there is some uncertainty in selecting a specific element ratio between the alloy components in this system. This is primarily explained by the fact that the properties of V-alloys are dictated not only by the ratio between the main alloying elements (here Ti and Cr), but also by impurities, both metallic and oxygen interstitials. Based on a number of papers today one can say that V-Ti-Cr alloys with insignificant variations in the contents of the main constituents within 5-10 mass% Ti and 4-6 mass% Cr must be taken as a base for subsequent optimization of chemical composition and thermomechanical working. However, the database is obviously insufficient to assess the ecological acceptability (activation), physical and mechanical properties, corrosion and irradiation resistance and, particularly, the commercial production of alloys. Therefore, there is a need for comprehensive studies of promising V-alloys, namely V-4Ti-4Cr and V-lOTi-SCr.
1. Introduction Initial technological, physical/mechanical, radiation and corrosion properties, low activation and a relatively fast decay of induced activity are important requirements to choose structural materials for promising fusion reactors. Meeting these requirements reduces potential safety risk of fusion reactors, facilitates its operation and allows a multiple use of materials. Vanadium-based alloys are the most promising as low-activation structural materials for
* Corresponding author. Tel.: + 7-095-1902360; 1964168; e-mail: [email protected]. 0022-3 I 15/96/S 15.00 Copyright PII SOO22-3 1 15(96)00284-X
fax: + 7-09S-
DEMO and subsequent power fusion reactors. These alloys have the highest consumer characteristics. In particular: - low initial level of induced activity; - high strength and plasticity at temperatures 700~800°C; - high radiation strength; - high corrosion resistance in Li: - good thermal conductivity. All these have determined the choice of vanadium alloys for power fusion reactors. It has been established by previous studies [ I-41 that vanadium alloys of the V-Ti-Cr system should be first considered as a structural material for a tritium-breeding blanket and the first wall of a fusion reactor. Alloys of the V-Ti-Cr system are the most promising for a self-cooling lithium blanket. This is caused by good compatibility with lithium, and apt combination of
8 1996 Elsevier Science B.V. All rights reserved
S.N. Votinou et ul./.lournol
ofNuclrcrr Materials 233-237 (19961370-375
371
Table 1 Physical and mechanical properties of some candidate materials for fusion reactors Material
?;;
Cr,,Mo,
400
Cr,,Ni,,
400
Cr,, Nt,c V-IOTi-6Cr-O.ISi
400 600
Thermal expansion coef. (10e6 K-‘)
Thermal conductivity coef. (W/m.K)
E modulus (GPa)
Poisson coef.
R,
II.5 17.65 17.2 IO.0
29.9 19.60 18.5 30
1.99 I .70 I .82 1.2
0.30 0.33 0,30 0,32
600-670 510-520 523 560
physical, mechanical and radiation properties. However, there is a noticeable uncertainty in the choice of the particular element ratio of components for alloys of this system. The report presents the results of evaluations of vanadium alloys V-4Ti-4Cr and V-lOTi-5Cr based on the following criteria: ecological acceptability (activation, tritium accumulation, storage and re-fabrication); thermal conductivity; strength, plasticity, creep, radiation strengthening and embrittlement, swelling; ductile-brittle transition temperature (DB’lT) and its shift at accumulation of hydrogen and neutron irradiation; corrosion resistance in Li; technological effectiveness (commercial melting, deformability, weldability).
Activation and re-fabrication Vanadium alloys mostly meet among all the structural materials [4] the criterium of ‘low activation’. Alloying of vanadium with chromium and titanium does not withdraw the alloys from the category of low activation, but here titanium, namely its isotopes &Ti and 47Ti, causes the greatest problem. The other Ti isotopes do not have longlived radionuclides. Activation characteristics of vanadium alloys are greatly worsened by impurities of Ni, Nb, MO, Co, Ag, Bi, Cu, Sm, etc. (total: 26 impurities). A way to improve characteristics is to use isotopes of ‘pure’ compo-
Table 2 Thermal conductivitv of vanadium and its allovs Material
f&c,
V V-lOTi-6Cr-O.ISi-O.OSZr V-4Ti-4Cr, annealed V-4Ti-4Cr, deformed V-8Ti-SCr, annealed
34 28.5 32.7 30.8 29.6
w/m K
Thermal stress parameter M (W/cm)
(MPa)
500-560 205-210 187 400
14-16 34-40 33.2 18
33 I5 I2 34
nents and to remove undesirable impurities during the metallurgical production. As far as the re-fabrication of materials after a reasonable period of cooling (50- 100 years) is concerned, this characteristic of materials is determined by a residual level of activity and residual isotopic composition of the materials. There is no clear picture yet, and to clarify this, a detailed and thorough analysis is required.
3. Physical and mechanical alloys in the initial state
properties
of vanadium
A comparative analysis of mechanical and physical properties of representatives of different classes of materials (Table 1) shows that by such integral characteristics as parameter of thermal strength as yield strength of material (MPa), thermal conductivity (W/m. K), Poisson’s ratio coefficient, thermal expansion coefficient (K- ‘1, Young modulus (MPa), thermal stress parameter M (W/cm), vanadium alloys are on a par with ferritic steels and noticeably (by a factor of 5-10) outperform austenitic steels. An important property for fusion reactor materials is thermal conductivity. With respect to this criteria pure vanadium is the best (Table 2). The higher the concentration of alloying elements, the more thermal conductivity of vanadium is reduced by alloying. At 20°C the thermal conductivity of some alloys can make up one half or even one third of pure vanadium thermal conductivity; at higher temperatures this difference is not as much (as a rule, not more than 25%). Alloying strengthens vanadium considerably. Adding titanium and chromium to vanadium greatly strength, when their concentrations are up to 5-7%. Mechanical characteristics of alloys differently alloyed are considerably different. Thus, values of Rti,z and R, of alloy V-IOTi-5Cr are higher by 150-170 MPa than those for alloy V-4Ti4Cr (Fig. 1). The temperature dependence of R, =flT,,,,) for both alloys is of different character than Rp0,2 =flT,,,). Thus, for both alloys the R, reduces with increase of testing temperature from 20 to 200°C within the range of 200-300
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DBTI increases by IOO-200°C for each 0. I at% oxygen or nitrogen. Carbon, due to its low solubility in vanadium, has a weak effect. Such negative effect of interstitial impurities on mechanical properties of alloys is the main detrimental factor for vanadium. Better solubilities of oxygen and nitrogen in vanadium (0, - 6 at% IOOO”C,N, - 8 at% lOOO”C) compared with those in case of steel (lOme at% O,, 0.1 at% N,) shows that the development of radiation, corrosion and embrittlement resistant alloys of vanadium is one of the conditions of their use.
4. Neutron irradiation chanical properties
Fig. I. Temperature dependence of strength and ductility ties of V-4Ti-40 (a) and V- IOTi-50 (b).
proper-
the ultimate strength, does not change practically, and at temperatures above 300°C the strength increases monotonically up to 650°C. We consider the process of dynamic deformation ageing (DDAJ to be responsible for this effect. Appearance of separation expansion diagram (discontinuous), resulted from dislocations fixing by interstitial impurities and their separation from impurities with load increase at specimen deformation demonstrates When performing mechanical elongation tests of binary V-Ti alloys, a tendency of DB’IT is observed with increase of titanium content. During impact tests, the tendency of DB’IT change is different. Thus, vanadium alloys with 10% titanium have DB’IT < 200°C. Increase of titanium content even up to 20% causes an increase in DBTT, up to 40°C at impact tests. At low temperatures of testing, vanadium and its alloys are sensitive to the hydrogen effect. With this, the lower the hydrogen content, at a lower temperature a transition from the plastic state to brittle state is observed. Impact tests of 3.3 X 3.3 mm* specimens with 45” notch of V-4Ti-4Cr and V- IOTi-5Cr alloys have shown (Fig. 2) that a considerable reduction of impact properties occurs for V-4Ti-40 alloy at temperatures below - 7O”C, and for V-IOTi-Cr alloy below 0°C. Even at temperature of - 2Oo”C, all specimens have some energy-intensity, and with this impact values for V-4Ti-4Cr alloy are higher than these for V-IOTi-5Cr. Impact tests of vanadium alloy V-4Ti-40 of 2 sizes IO X IO X 5.5 mm3 and 3.3 X 3.3 X 5.4 mm3 have shown that when a specimen size is reduced, DBTT is shifted by 30-50°C towards positive temperatures, and an upper level of values of absorbed energy E is reduced by N 2 J. Hydrogen, oxygen and nitrogen have a rather considerable and negative effect on the DBTT value. When nitrogen and oxygen content is higher than 0.1-0.2 at%, vanadium plasticity reduces and
effects on vanadium
alloys me-
Temperature dependence of mechanical properties change for irradiated and non-irradiated vanadium and alloys of systems V-Ti, V-Ti-Cr, etc. (chromium content was changing from 5 to 8 at%) was studied (Fig. 3). Irradiation tests were carried out in the BOR-60 reactor (fast neutron total fluence 1.25 X lO23 n/cm* (- 45 dpa); temperature mode (I) 740-830°C. 4X lO22 n/cm2, (2) 350-400°C. 8 X IO** n/cm*; in niobium ampoules, filled with lithium). Before irradiation, all the specimens were annealed at 1000°C for I h. It was shown that after irradiation the yield strength and ultimate tensile strength of all the alloys are increased and their plasticity reduced compared to initial values. The level of change is determined by the degree of alloying of the alloys. Within the range of low temperatures (to - 300°C) a total embrittlement is characteristic for pure vanadium. Alloying of vanadium by titanium suppresses a low-temperature embrittlement, (it is more vividly revealed when titanium content is more than IO at%> At test temperatures of 700- IOOOT, a high-temperature radiation embrittlement is revealed with alloys of increased titanium content. Also, the strength of irradiated alloys is reduced compared to initial ones. Alloying of vanadium alloys with > 7% chromium does not eliminate a low-temperature radiation embrittlement of vanadium; however, it has a rather favorable effect at test temperatures above 500°C. It should be noticed that a rather high level of uniform elongation and ultimate
Fig. 2. Impact toughness tions.
of vanadium
alloys under initial condi-
S.N. Votinov rt al./Journul
0
0
20
60 0
l
20
-v-m f-Jm - v-ZOIi-iocr-is
Fig. 3. Irradiation
40
oj’Nuclear Muterids
0
20
d0 O
233-237
20
(1996) 370-375
d0 O
2O
d0
373
0
2O
a0
Ti content, at.%
A A - v-301[i-i?iP0,3c 0 * - V-3rl-IOcr-izr-O,JC
induced hardening and loss of ductility as a function of titanium content in binary (V-Ti) and complex alloyed (V-Ti-Cr to fast neutron fluence of 1.25 X 10z3 cm- * under variable temperature (740-830°C) conditions.
etc.) alloys. BOR-60 irradiation
tensile strength/yield strength ratio is an evidence that no plastic instability is available with the irradiated alloys studied. In-core creep of V-4Cr-4Ti alloy was studied under conditions of irradiation of a specimen in reactor BR-10 at 450°C and under neutron flux intensity 8.6 X lOI n/cm2 s and fluence -6.1 X 102'n/cm2. The results of tests have shown (Fig. 4) that the alloy under study is susceptible to radiation stimulated creep within the whole range of the studied strains up to - 0.8 from the materials yield strength under nonirradiated state. Under strains lower than - 0.6 from the initial yield strength the rate of the radiation creep varies directly with strain values. Radiation creep modulus is defined to be equal to 3.3 . (10-12) dpa-’ Pa-‘. Under higher strains, the creep rate abruptly
increased (Fig. 4). The effect of radiation creep acceleration after reaching a certain threshold strain value had been observed earlier during in-core tests of austenitic steels. However, an abrupt increase of the creep rate for steels occurred under strains close to or even exceeding the initial yield strength which supposedly was associated with possible change of the deformation mechanism. For the studied vanadium alloy, the effect of radiation creep acceleration was observed under unexpectedly low strains. Experimental data [2,4,.5] show that radiation swelling of vanadium alloys can vary within very large limits. Binary alloys of V-Cr, V-Fe increase in volume up to 80-90% (irradiation of 420°C - 50 dpa). Titanium is most effective for suppressing of swelling. The swelling of V-IOTi-5Cr alloy under irradiation to fluence 50 dpa at 420°C was equal to 0.1%. The swelling of V-15Ti-1OCr alloy after irradiation at 450°C in reactor BN-600 up to irradiation damage - 45 dpa also does not exceed 0.1%.
5. Corrosion coolants
Fig. 4. Irradiation creep rate for V-4Ti-4Cr stress during BR- IO irradiation.
alloy as a function of
- compatibility
with lithium
and other
The most important advantage of vanadium-based alloys as low-activation structural materials of the fusion reactor blanket is their high potential in corrosion resistance under condition of direct contact with liquid lithium. Solubility of pure vanadium in Li is substantially lower than comparative solubility of basic components of steel and equals < 0.003 wt.% under 1000°C. A hazardous type of corrosion effect of lithium on vanadium-based alloys (as
374
S.N. Votinou et ul./Jourd
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well as Nb, Ta) is permeability due to interaction with impurities of oxygen and nitrogen. However, for vanadium a threshold concentration of oxygen, which is necessary for the initiation of this process, is much higher than required for Nb or Ta and is equal to approximately 600°C > 0.2 wt%. Thus this type of corrosion can be overlooked in practice for vanadium-based alloys. Hydrogen is also accumulated in liquid lithium [6]. Due to this circumstance, vanadium whose mechanical properties are extremely sensitive to the impurities of hydrogen will not be prone to the effect of tritium accumulation in liquid lithium. The most significant effect is imposed by nitrogen on the compatibility of vanadium-based alloys with lithium. This impurity, which rather rapidly propagates over the cross-section of metal, decreases the elastic properties of the alloys. Alloying of vanadium and alloys of V-Cr with titanium leads to localization of nitrogen impurity absorbed from lithium in fine surface layers of metal where it is present in the form of titanium nitride. This film of nitride, apparently, is protective and favors the decrease of the rate of the metal mass transfer [7]. Experimental studies indicate that mass transfer of the V-Ti-Cr system alloy by means of lithium flow containing the nitrogen impurity up to 400-500 wtppm is substantially lower than permissible values at temperatures up to 45O”C, and mechanical properties of these alloys are stable under such conditions [7]. However, these advantages of vanadium alloys can be realized by sophisticated production of these materials, beginning from melting and production of stock material and finishing by manufacturing full-scale structures and application of electric-insulation and corrosion-resistant coatings which in a lithium environment possess the properties of self-healing. Practical experience in creating a circulating lithium loop, made of V-4Ti-4Cr alloy, and its successful operation in May-June 1995 during > 1000 h at temperatures 350-700°C and flow rates of lithium up to 1 m/s confirms in principle the possibility in using vanadium-based alloys and compliance of the welding techniques with the requirements of creation of the liquid-metal blanket of ITER, DEMO. The demonstrated possibility of creating electricinsulation coating based on AIN on vanadium alloys allowed at the present time to pass to a creation of in-core loops and investigate the stability of their properties under conditions of simultaneous effect of liquid lithium, mechanical loads and neutron irradiation [8]. Alloying of vanadium by titanium and chromium also decreases weight gain by specimens (from 12-14 g/m2 for pure V down to 1-2 g/m2 for alloys) under conditions of testing in a water-steam environment at 300°C. Alloys of V- lOTi-5Cr type have unchanged properties of strength and elasticity after tests under these conditions. Apparently, this is connected with the presence of fine protective Ti and Cr oxides films which impedes the process of hydrogen saturation of alloys [9].
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6. Weldability
(1996) 370-375
of V-based alloys
Several investigations have been undertaken to study characteristics of weldability of vanadium alloys of the V-4Ti-4Cr and V-lOTi-4Cr types. The authors attempted to determine resistance to cracking of welded joints, obtained on metal sheets 1 and 5 mm width by means of argon-arc welding with enhanced protection, as well as electron-beam welding. The authors studied microstructures and microhardness. Welded joints are rather resistant to the formation of hot cracks. At the same time joints made by argon-arc welding exhibit a tendency towards formation of cold cracks. Apparently, the occurrence of cold cracks is enhanced as a result of the increase of impurities implantation at the expense of their ingress into welded seam in the course of welding. The structure in all areas of the welded joints is one-phase. Grain size near the welding zone increased in response to the welding heating. This was especially observed under conditions of argon-arc welding performed with a larger energy than under electron beam welding. The bend angle of the specimens with a cross-section 8 X 4 mm* with welds exceeds 80”, and for the specimens with a cross-section I X 20 mm2 amounts to 60-90”. In terms of mechanical properties, the welded joints of the studied alloys immediately following welding are somewhat inferior to similar properties of the base metal. The performances of strength and elasticity of welddamaged specimens are lower than those of base-metaldamaged ones. However, in both cases the level of uniformity and the total percentage elongation of welded joint is rather high and amounts to 8-13.5% and IO-20%. respectively. At the same time the strength properties and elongation changes little within the temperature range of 20-700°C. Impact tests of V-4Ti-4Cr and V- lOTi-5Cr specimen alloys with a notch in the area of the weld showed substantial decrease of the impact value of the metal of weld as compared to the base metal (see Fig. 5). Nevertheless, a clearly defined threshold of cold brittleness is not observed even in this case, which indicates a certain safety margin in terms of elasticity and welded joints of V-4Ti-
/
V-dli-4G
o
L/F5G
OP
-200
Fig. 5. Impact toughness
> 200 v
0 of weldments
of vanadium
alloys.
S.N. Votinou et ul./.Journul
4Cr and V-lOTi-5Cr bient) temperatures.
alloys under relatively
of’Nucleur
low (am-
7. Large scale industrial production of V-based alloys At the present time there are several industrially suitable technologies patterns for production of metallic vanadium with a high degree of purity. The most promising and industrially mastered technological pattern envisages aluminothermic reduction of vanadium pentoxide with subsequent double electron-beam remelting of vanadium into ingots 80-150 mm in diameter and up to 1.5 m in length. At present, vanadium is produced as ingots with weights up to 1 ton. The industry has mastered the production of vanadium alloys products in the form of sheets (80 X 1000 X 12000 mm3, 5 X 1000 X 12000 mm’) and tubes with diameters from 5 to 100 mm. Electric arc vacuum furnaces used in titanium production industry allow production ingots weighing 10 tons and more. In general terms, the technology of production of ingots and products from vanadium-based alloys does not differ from that existing for alloys based on Ti, Zr, Nb. This circumstance gives grounds for future obtaining of largescale ingots weighing 10 tons and more.
8. Conclusions (I) These studies once again proved that titanium and chromium as basic alloying components enhance vanadium based alloys significantly for practical application. These alloys possess high deformability (Stolal _ 20%). Basic gains in strength are provided by the components in concentrations of 5-8 at%. (2) In-pile tests of V-Ti-Cr alloys in fast-neutron reactors (BOR-60, BN-600) at temperatures of 3.50-800°C up to 60 dpa showed that titanium alloying (up to 10 wt% of Ti) suppress low-temperature radiation embrittlement, and the elasticity of the alloy is maintained at the level of 7-10%. Vanadium alloying with chromium contributes to the stability properties of the irradiated alloys under high
Muterials
233-237
(1996) 370-375
375
temperatures and under these conditions, the swelling of V-Ti-Cr alloys is not observed. (3) DB’IT is minimized (< -200°C) for vanadium alloys containing 5-10 wt% of titanium or a summarized content of Ti -I- Cr = 10 wt%. (4) Results of corrosion tests of vanadium alloys in lithium indicate that the effect of oxygen and hydrogen (tritium) in liquid lithium can be overlooked, in terms of its compatibility with these alloys. Substantial effects of nitrogen impurity can be decreased by alloying vanadium alloys with titanium. It is compatible with nitrogen containing lithium up to temperatures _ 700°C. (5) Vanadium alloys are weldable. Welded joints of V-Ti-Cr alloys, as a rule, do not need additional heat treatment. (6) In the process of industrial production (melting, forging, rolling, etc.) of various products, the V-Ti-Cr alloys have perfect parameters of adaptability to streamlined manufacture under conditions minimizing implantation of detrimental impurities (0, N, C, H). References
[II S.N. Votinov, L.I. Gomozov, A.I. Dedurin and L.P. Zavialski, Questions of Nuclear Science and Technique, Issue Nuclear Fusion (Moscow), V3 (1989) 56. Dl S.N. Votinov, V.A. Kazakov, V.A. Evrikhin, I.E. Lyublinski and A.I. Dedurin. IEA Workshop on Vanadium Alloys for Fusion Application, Salem, Oregon, USA. June 15-17, 1994.
131D.L. Smith, H.M.
Chug, B.A. Loomis, H. Matsui, S. Votinov and W. Van Witzenburg, Fusion. Eng. Des. 27 (1995). [41 B.A. Loomis and D.L. Smith, 1. Nucl. Mater. 191-194 (1992) 84. [51 H. Matsui and D.S. Gelles, Large swelling in V-Fe alloy after irradiation in FITF, ANI., 1989, p. 112. (61 G.M. Gryaznov, V.A. Evtikhin, I.E. Lyublinski et al., Material Science for Liquid Metal Systems of Fusion Reactor (Energoatomizdat, Moscow, 1989). [71V.A. Evtikhin and I.E. Lyublinski, .I. Adv. Mater. I, VI (1994) 60. k31I.E. Lyublinski, V.A. Evtikhin, V.B. Ivanov, V.A. Kazakov et al., J. Nucl. Mater. (Ref. N 270010-O). I91 A.I. Dedyurin, L.I. Gomozov and S.N. Votinov, J. Phys. Chem. Mater. Work. 5 (1983) 22.
Journal of Nuclear Materials 233-237 (1996) 370-375
journal of nuclear materials
Prospects and problems using vanadium alloys as a structural material of the first wall and blanket of fusion reactors S.N. Votinov a’* , M.I. Solonin ‘, Yu.1. Kazennov ‘I, V.P. Kondratjev ‘, A.D. Nikulin “, V.N. Tebus a, E.O. Adamov b, S.E. Bougaenko b, Yu.S. Strebkov ‘, A.V. Sidorenkov b, V.B. Ivanov ‘, V.A. Kazakov ‘, V.A. Evtikhin ‘, I.E. Lyublinski ‘, V.M. Trojanov ‘, A.E. Rusanov ‘, V.M. Chernov ‘, G.A. Birgevoj ’
Abstract Vanadium-based alloys are most promising as low activation structural materials for DEMO. It was previously established that high priority is to be given to V-alloys of the V-Ti-Cr system as structural materials of a tritium breeding blanket and the first wall of a fusion reactor. However, there is some uncertainty in selecting a specific element ratio between the alloy components in this system. This is primarily explained by the fact that the properties of V-alloys are dictated not only by the ratio between the main alloying elements (here Ti and Cr), but also by impurities, both metallic and oxygen interstitials. Based on a number of papers today one can say that V-Ti-Cr alloys with insignificant variations in the contents of the main constituents within 5-10 mass% Ti and 4-6 mass% Cr must be taken as a base for subsequent optimization of chemical composition and thermomechanical working. However, the database is obviously insufficient to assess the ecological acceptability (activation), physical and mechanical properties, corrosion and irradiation resistance and, particularly, the commercial production of alloys. Therefore, there is a need for comprehensive studies of promising V-alloys, namely V-4Ti-4Cr and V-lOTi-SCr.
1. Introduction Initial technological, physical/mechanical, radiation and corrosion properties, low activation and a relatively fast decay of induced activity are important requirements to choose structural materials for promising fusion reactors. Meeting these requirements reduces potential safety risk of fusion reactors, facilitates its operation and allows a multiple use of materials. Vanadium-based alloys are the most promising as low-activation structural materials for
* Corresponding author. Tel.: + 7-095-1902360; 1964168; e-mail: [email protected]. 0022-3 I 15/96/S 15.00 Copyright PII SOO22-3 1 15(96)00284-X
fax: + 7-09S-
DEMO and subsequent power fusion reactors. These alloys have the highest consumer characteristics. In particular: - low initial level of induced activity; - high strength and plasticity at temperatures 700~800°C; - high radiation strength; - high corrosion resistance in Li: - good thermal conductivity. All these have determined the choice of vanadium alloys for power fusion reactors. It has been established by previous studies [ I-41 that vanadium alloys of the V-Ti-Cr system should be first considered as a structural material for a tritium-breeding blanket and the first wall of a fusion reactor. Alloys of the V-Ti-Cr system are the most promising for a self-cooling lithium blanket. This is caused by good compatibility with lithium, and apt combination of
8 1996 Elsevier Science B.V. All rights reserved
S.N. Votinou et ul./.lournol
ofNuclrcrr Materials 233-237 (19961370-375
371
Table 1 Physical and mechanical properties of some candidate materials for fusion reactors Material
?;;
Cr,,Mo,
400
Cr,,Ni,,
400
Cr,, Nt,c V-IOTi-6Cr-O.ISi
400 600
Thermal expansion coef. (10e6 K-‘)
Thermal conductivity coef. (W/m.K)
E modulus (GPa)
Poisson coef.
R,
II.5 17.65 17.2 IO.0
29.9 19.60 18.5 30
1.99 I .70 I .82 1.2
0.30 0.33 0,30 0,32
600-670 510-520 523 560
physical, mechanical and radiation properties. However, there is a noticeable uncertainty in the choice of the particular element ratio of components for alloys of this system. The report presents the results of evaluations of vanadium alloys V-4Ti-4Cr and V-lOTi-5Cr based on the following criteria: ecological acceptability (activation, tritium accumulation, storage and re-fabrication); thermal conductivity; strength, plasticity, creep, radiation strengthening and embrittlement, swelling; ductile-brittle transition temperature (DB’lT) and its shift at accumulation of hydrogen and neutron irradiation; corrosion resistance in Li; technological effectiveness (commercial melting, deformability, weldability).
Activation and re-fabrication Vanadium alloys mostly meet among all the structural materials [4] the criterium of ‘low activation’. Alloying of vanadium with chromium and titanium does not withdraw the alloys from the category of low activation, but here titanium, namely its isotopes &Ti and 47Ti, causes the greatest problem. The other Ti isotopes do not have longlived radionuclides. Activation characteristics of vanadium alloys are greatly worsened by impurities of Ni, Nb, MO, Co, Ag, Bi, Cu, Sm, etc. (total: 26 impurities). A way to improve characteristics is to use isotopes of ‘pure’ compo-
Table 2 Thermal conductivitv of vanadium and its allovs Material
f&c,
V V-lOTi-6Cr-O.ISi-O.OSZr V-4Ti-4Cr, annealed V-4Ti-4Cr, deformed V-8Ti-SCr, annealed
34 28.5 32.7 30.8 29.6
w/m K
Thermal stress parameter M (W/cm)
(MPa)
500-560 205-210 187 400
14-16 34-40 33.2 18
33 I5 I2 34
nents and to remove undesirable impurities during the metallurgical production. As far as the re-fabrication of materials after a reasonable period of cooling (50- 100 years) is concerned, this characteristic of materials is determined by a residual level of activity and residual isotopic composition of the materials. There is no clear picture yet, and to clarify this, a detailed and thorough analysis is required.
3. Physical and mechanical alloys in the initial state
properties
of vanadium
A comparative analysis of mechanical and physical properties of representatives of different classes of materials (Table 1) shows that by such integral characteristics as parameter of thermal strength as yield strength of material (MPa), thermal conductivity (W/m. K), Poisson’s ratio coefficient, thermal expansion coefficient (K- ‘1, Young modulus (MPa), thermal stress parameter M (W/cm), vanadium alloys are on a par with ferritic steels and noticeably (by a factor of 5-10) outperform austenitic steels. An important property for fusion reactor materials is thermal conductivity. With respect to this criteria pure vanadium is the best (Table 2). The higher the concentration of alloying elements, the more thermal conductivity of vanadium is reduced by alloying. At 20°C the thermal conductivity of some alloys can make up one half or even one third of pure vanadium thermal conductivity; at higher temperatures this difference is not as much (as a rule, not more than 25%). Alloying strengthens vanadium considerably. Adding titanium and chromium to vanadium greatly strength, when their concentrations are up to 5-7%. Mechanical characteristics of alloys differently alloyed are considerably different. Thus, values of Rti,z and R, of alloy V-IOTi-5Cr are higher by 150-170 MPa than those for alloy V-4Ti4Cr (Fig. 1). The temperature dependence of R, =flT,,,,) for both alloys is of different character than Rp0,2 =flT,,,). Thus, for both alloys the R, reduces with increase of testing temperature from 20 to 200°C within the range of 200-300
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DBTI increases by IOO-200°C for each 0. I at% oxygen or nitrogen. Carbon, due to its low solubility in vanadium, has a weak effect. Such negative effect of interstitial impurities on mechanical properties of alloys is the main detrimental factor for vanadium. Better solubilities of oxygen and nitrogen in vanadium (0, - 6 at% IOOO”C,N, - 8 at% lOOO”C) compared with those in case of steel (lOme at% O,, 0.1 at% N,) shows that the development of radiation, corrosion and embrittlement resistant alloys of vanadium is one of the conditions of their use.
4. Neutron irradiation chanical properties
Fig. I. Temperature dependence of strength and ductility ties of V-4Ti-40 (a) and V- IOTi-50 (b).
proper-
the ultimate strength, does not change practically, and at temperatures above 300°C the strength increases monotonically up to 650°C. We consider the process of dynamic deformation ageing (DDAJ to be responsible for this effect. Appearance of separation expansion diagram (discontinuous), resulted from dislocations fixing by interstitial impurities and their separation from impurities with load increase at specimen deformation demonstrates When performing mechanical elongation tests of binary V-Ti alloys, a tendency of DB’IT is observed with increase of titanium content. During impact tests, the tendency of DB’IT change is different. Thus, vanadium alloys with 10% titanium have DB’IT < 200°C. Increase of titanium content even up to 20% causes an increase in DBTT, up to 40°C at impact tests. At low temperatures of testing, vanadium and its alloys are sensitive to the hydrogen effect. With this, the lower the hydrogen content, at a lower temperature a transition from the plastic state to brittle state is observed. Impact tests of 3.3 X 3.3 mm* specimens with 45” notch of V-4Ti-4Cr and V- IOTi-5Cr alloys have shown (Fig. 2) that a considerable reduction of impact properties occurs for V-4Ti-40 alloy at temperatures below - 7O”C, and for V-IOTi-Cr alloy below 0°C. Even at temperature of - 2Oo”C, all specimens have some energy-intensity, and with this impact values for V-4Ti-4Cr alloy are higher than these for V-IOTi-5Cr. Impact tests of vanadium alloy V-4Ti-40 of 2 sizes IO X IO X 5.5 mm3 and 3.3 X 3.3 X 5.4 mm3 have shown that when a specimen size is reduced, DBTT is shifted by 30-50°C towards positive temperatures, and an upper level of values of absorbed energy E is reduced by N 2 J. Hydrogen, oxygen and nitrogen have a rather considerable and negative effect on the DBTT value. When nitrogen and oxygen content is higher than 0.1-0.2 at%, vanadium plasticity reduces and
effects on vanadium
alloys me-
Temperature dependence of mechanical properties change for irradiated and non-irradiated vanadium and alloys of systems V-Ti, V-Ti-Cr, etc. (chromium content was changing from 5 to 8 at%) was studied (Fig. 3). Irradiation tests were carried out in the BOR-60 reactor (fast neutron total fluence 1.25 X lO23 n/cm* (- 45 dpa); temperature mode (I) 740-830°C. 4X lO22 n/cm2, (2) 350-400°C. 8 X IO** n/cm*; in niobium ampoules, filled with lithium). Before irradiation, all the specimens were annealed at 1000°C for I h. It was shown that after irradiation the yield strength and ultimate tensile strength of all the alloys are increased and their plasticity reduced compared to initial values. The level of change is determined by the degree of alloying of the alloys. Within the range of low temperatures (to - 300°C) a total embrittlement is characteristic for pure vanadium. Alloying of vanadium by titanium suppresses a low-temperature embrittlement, (it is more vividly revealed when titanium content is more than IO at%> At test temperatures of 700- IOOOT, a high-temperature radiation embrittlement is revealed with alloys of increased titanium content. Also, the strength of irradiated alloys is reduced compared to initial ones. Alloying of vanadium alloys with > 7% chromium does not eliminate a low-temperature radiation embrittlement of vanadium; however, it has a rather favorable effect at test temperatures above 500°C. It should be noticed that a rather high level of uniform elongation and ultimate
Fig. 2. Impact toughness tions.
of vanadium
alloys under initial condi-
S.N. Votinov rt al./Journul
0
0
20
60 0
l
20
-v-m f-Jm - v-ZOIi-iocr-is
Fig. 3. Irradiation
40
oj’Nuclear Muterids
0
20
d0 O
233-237
20
(1996) 370-375
d0 O
2O
d0
373
0
2O
a0
Ti content, at.%
A A - v-301[i-i?iP0,3c 0 * - V-3rl-IOcr-izr-O,JC
induced hardening and loss of ductility as a function of titanium content in binary (V-Ti) and complex alloyed (V-Ti-Cr to fast neutron fluence of 1.25 X 10z3 cm- * under variable temperature (740-830°C) conditions.
etc.) alloys. BOR-60 irradiation
tensile strength/yield strength ratio is an evidence that no plastic instability is available with the irradiated alloys studied. In-core creep of V-4Cr-4Ti alloy was studied under conditions of irradiation of a specimen in reactor BR-10 at 450°C and under neutron flux intensity 8.6 X lOI n/cm2 s and fluence -6.1 X 102'n/cm2. The results of tests have shown (Fig. 4) that the alloy under study is susceptible to radiation stimulated creep within the whole range of the studied strains up to - 0.8 from the materials yield strength under nonirradiated state. Under strains lower than - 0.6 from the initial yield strength the rate of the radiation creep varies directly with strain values. Radiation creep modulus is defined to be equal to 3.3 . (10-12) dpa-’ Pa-‘. Under higher strains, the creep rate abruptly
increased (Fig. 4). The effect of radiation creep acceleration after reaching a certain threshold strain value had been observed earlier during in-core tests of austenitic steels. However, an abrupt increase of the creep rate for steels occurred under strains close to or even exceeding the initial yield strength which supposedly was associated with possible change of the deformation mechanism. For the studied vanadium alloy, the effect of radiation creep acceleration was observed under unexpectedly low strains. Experimental data [2,4,.5] show that radiation swelling of vanadium alloys can vary within very large limits. Binary alloys of V-Cr, V-Fe increase in volume up to 80-90% (irradiation of 420°C - 50 dpa). Titanium is most effective for suppressing of swelling. The swelling of V-IOTi-5Cr alloy under irradiation to fluence 50 dpa at 420°C was equal to 0.1%. The swelling of V-15Ti-1OCr alloy after irradiation at 450°C in reactor BN-600 up to irradiation damage - 45 dpa also does not exceed 0.1%.
5. Corrosion coolants
Fig. 4. Irradiation creep rate for V-4Ti-4Cr stress during BR- IO irradiation.
alloy as a function of
- compatibility
with lithium
and other
The most important advantage of vanadium-based alloys as low-activation structural materials of the fusion reactor blanket is their high potential in corrosion resistance under condition of direct contact with liquid lithium. Solubility of pure vanadium in Li is substantially lower than comparative solubility of basic components of steel and equals < 0.003 wt.% under 1000°C. A hazardous type of corrosion effect of lithium on vanadium-based alloys (as
374
S.N. Votinou et ul./Jourd
of’Nucleur Materids
well as Nb, Ta) is permeability due to interaction with impurities of oxygen and nitrogen. However, for vanadium a threshold concentration of oxygen, which is necessary for the initiation of this process, is much higher than required for Nb or Ta and is equal to approximately 600°C > 0.2 wt%. Thus this type of corrosion can be overlooked in practice for vanadium-based alloys. Hydrogen is also accumulated in liquid lithium [6]. Due to this circumstance, vanadium whose mechanical properties are extremely sensitive to the impurities of hydrogen will not be prone to the effect of tritium accumulation in liquid lithium. The most significant effect is imposed by nitrogen on the compatibility of vanadium-based alloys with lithium. This impurity, which rather rapidly propagates over the cross-section of metal, decreases the elastic properties of the alloys. Alloying of vanadium and alloys of V-Cr with titanium leads to localization of nitrogen impurity absorbed from lithium in fine surface layers of metal where it is present in the form of titanium nitride. This film of nitride, apparently, is protective and favors the decrease of the rate of the metal mass transfer [7]. Experimental studies indicate that mass transfer of the V-Ti-Cr system alloy by means of lithium flow containing the nitrogen impurity up to 400-500 wtppm is substantially lower than permissible values at temperatures up to 45O”C, and mechanical properties of these alloys are stable under such conditions [7]. However, these advantages of vanadium alloys can be realized by sophisticated production of these materials, beginning from melting and production of stock material and finishing by manufacturing full-scale structures and application of electric-insulation and corrosion-resistant coatings which in a lithium environment possess the properties of self-healing. Practical experience in creating a circulating lithium loop, made of V-4Ti-4Cr alloy, and its successful operation in May-June 1995 during > 1000 h at temperatures 350-700°C and flow rates of lithium up to 1 m/s confirms in principle the possibility in using vanadium-based alloys and compliance of the welding techniques with the requirements of creation of the liquid-metal blanket of ITER, DEMO. The demonstrated possibility of creating electricinsulation coating based on AIN on vanadium alloys allowed at the present time to pass to a creation of in-core loops and investigate the stability of their properties under conditions of simultaneous effect of liquid lithium, mechanical loads and neutron irradiation [8]. Alloying of vanadium by titanium and chromium also decreases weight gain by specimens (from 12-14 g/m2 for pure V down to 1-2 g/m2 for alloys) under conditions of testing in a water-steam environment at 300°C. Alloys of V- lOTi-5Cr type have unchanged properties of strength and elasticity after tests under these conditions. Apparently, this is connected with the presence of fine protective Ti and Cr oxides films which impedes the process of hydrogen saturation of alloys [9].
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6. Weldability
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of V-based alloys
Several investigations have been undertaken to study characteristics of weldability of vanadium alloys of the V-4Ti-4Cr and V-lOTi-4Cr types. The authors attempted to determine resistance to cracking of welded joints, obtained on metal sheets 1 and 5 mm width by means of argon-arc welding with enhanced protection, as well as electron-beam welding. The authors studied microstructures and microhardness. Welded joints are rather resistant to the formation of hot cracks. At the same time joints made by argon-arc welding exhibit a tendency towards formation of cold cracks. Apparently, the occurrence of cold cracks is enhanced as a result of the increase of impurities implantation at the expense of their ingress into welded seam in the course of welding. The structure in all areas of the welded joints is one-phase. Grain size near the welding zone increased in response to the welding heating. This was especially observed under conditions of argon-arc welding performed with a larger energy than under electron beam welding. The bend angle of the specimens with a cross-section 8 X 4 mm* with welds exceeds 80”, and for the specimens with a cross-section I X 20 mm2 amounts to 60-90”. In terms of mechanical properties, the welded joints of the studied alloys immediately following welding are somewhat inferior to similar properties of the base metal. The performances of strength and elasticity of welddamaged specimens are lower than those of base-metaldamaged ones. However, in both cases the level of uniformity and the total percentage elongation of welded joint is rather high and amounts to 8-13.5% and IO-20%. respectively. At the same time the strength properties and elongation changes little within the temperature range of 20-700°C. Impact tests of V-4Ti-4Cr and V- lOTi-5Cr specimen alloys with a notch in the area of the weld showed substantial decrease of the impact value of the metal of weld as compared to the base metal (see Fig. 5). Nevertheless, a clearly defined threshold of cold brittleness is not observed even in this case, which indicates a certain safety margin in terms of elasticity and welded joints of V-4Ti-
/
V-dli-4G
o
L/F5G
OP
-200
Fig. 5. Impact toughness
> 200 v
0 of weldments
of vanadium
alloys.
S.N. Votinou et ul./.Journul
4Cr and V-lOTi-5Cr bient) temperatures.
alloys under relatively
of’Nucleur
low (am-
7. Large scale industrial production of V-based alloys At the present time there are several industrially suitable technologies patterns for production of metallic vanadium with a high degree of purity. The most promising and industrially mastered technological pattern envisages aluminothermic reduction of vanadium pentoxide with subsequent double electron-beam remelting of vanadium into ingots 80-150 mm in diameter and up to 1.5 m in length. At present, vanadium is produced as ingots with weights up to 1 ton. The industry has mastered the production of vanadium alloys products in the form of sheets (80 X 1000 X 12000 mm3, 5 X 1000 X 12000 mm’) and tubes with diameters from 5 to 100 mm. Electric arc vacuum furnaces used in titanium production industry allow production ingots weighing 10 tons and more. In general terms, the technology of production of ingots and products from vanadium-based alloys does not differ from that existing for alloys based on Ti, Zr, Nb. This circumstance gives grounds for future obtaining of largescale ingots weighing 10 tons and more.
8. Conclusions (I) These studies once again proved that titanium and chromium as basic alloying components enhance vanadium based alloys significantly for practical application. These alloys possess high deformability (Stolal _ 20%). Basic gains in strength are provided by the components in concentrations of 5-8 at%. (2) In-pile tests of V-Ti-Cr alloys in fast-neutron reactors (BOR-60, BN-600) at temperatures of 3.50-800°C up to 60 dpa showed that titanium alloying (up to 10 wt% of Ti) suppress low-temperature radiation embrittlement, and the elasticity of the alloy is maintained at the level of 7-10%. Vanadium alloying with chromium contributes to the stability properties of the irradiated alloys under high
Muterials
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375
temperatures and under these conditions, the swelling of V-Ti-Cr alloys is not observed. (3) DB’IT is minimized (< -200°C) for vanadium alloys containing 5-10 wt% of titanium or a summarized content of Ti -I- Cr = 10 wt%. (4) Results of corrosion tests of vanadium alloys in lithium indicate that the effect of oxygen and hydrogen (tritium) in liquid lithium can be overlooked, in terms of its compatibility with these alloys. Substantial effects of nitrogen impurity can be decreased by alloying vanadium alloys with titanium. It is compatible with nitrogen containing lithium up to temperatures _ 700°C. (5) Vanadium alloys are weldable. Welded joints of V-Ti-Cr alloys, as a rule, do not need additional heat treatment. (6) In the process of industrial production (melting, forging, rolling, etc.) of various products, the V-Ti-Cr alloys have perfect parameters of adaptability to streamlined manufacture under conditions minimizing implantation of detrimental impurities (0, N, C, H). References
[II S.N. Votinov, L.I. Gomozov, A.I. Dedurin and L.P. Zavialski, Questions of Nuclear Science and Technique, Issue Nuclear Fusion (Moscow), V3 (1989) 56. Dl S.N. Votinov, V.A. Kazakov, V.A. Evrikhin, I.E. Lyublinski and A.I. Dedurin. IEA Workshop on Vanadium Alloys for Fusion Application, Salem, Oregon, USA. June 15-17, 1994.
131D.L. Smith, H.M.
Chug, B.A. Loomis, H. Matsui, S. Votinov and W. Van Witzenburg, Fusion. Eng. Des. 27 (1995). [41 B.A. Loomis and D.L. Smith, 1. Nucl. Mater. 191-194 (1992) 84. [51 H. Matsui and D.S. Gelles, Large swelling in V-Fe alloy after irradiation in FITF, ANI., 1989, p. 112. (61 G.M. Gryaznov, V.A. Evtikhin, I.E. Lyublinski et al., Material Science for Liquid Metal Systems of Fusion Reactor (Energoatomizdat, Moscow, 1989). [71V.A. Evtikhin and I.E. Lyublinski, .I. Adv. Mater. I, VI (1994) 60. k31I.E. Lyublinski, V.A. Evtikhin, V.B. Ivanov, V.A. Kazakov et al., J. Nucl. Mater. (Ref. N 270010-O). I91 A.I. Dedyurin, L.I. Gomozov and S.N. Votinov, J. Phys. Chem. Mater. Work. 5 (1983) 22.