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Effects of neutron irradiation on physical and mechanical properties of Mo-Re alloys
journal of
nuclear materials
Journal of Nuclear Materials 191-194 (1992) 426-429 North-Holland
Effects of neutron irradiation on physical and mechanical properties of Mo-Re alloys S.A Fabritsiev a, V.A Gosudarenkova a, V.A Potapova v.P. Chakin b, AS. Pokrovsky band V.R. Barabash
a,
v.v. Rybin
a,
L.S. Kosachev
b,
C
]93]67 St. Petersburg, Russia Lenin Scientific Research Institute of Atomic Reactors, 4335]0 Dimitrougrad, Russia Electrophysical Apparatus D. V. Efremov Institute, 189631 St. Petersburg, Russia
a Central Research Institute of Structural Materials "Prometey': h C
v.I.
The effects of neutron irradiation on mechanical strength, plasticity and electric conductivity of Mo-Re alloys are presented. Pure Mo and Mo-O.5%Re to Mo-30%Re alloys were studied. Samples were irradiated in the reactor SM-2 eel> - 5 x 10 21 n/cm 2 , Tin - 240-330°C). Irradiation at 300°C indicates that the radiation embrittlement results in reduced strength and ductility characteristics of Mo- Re alloys. Measurement of electric resistance shows that low-temperature irradiation of Mo-alloys results in 20~30% reduced conductivity. SEM studies of fracture surfaces show that after irradiation brittle intergranular fracture takes places that correlates with an abrupt reduction of strength.
1. Introduction
Molybdenum and copper alloys are now considered as candidate materials for the heat-sink system of the divertor of the International Thermonuclear Experimental Reactor (ITER). The design which includes graphite tiles on molybdenum substrata is preferred for the physics stage of the divcrtor [1]. The main advantage of such a solution is the similarity of the coefficients of thermal expansion for molybdenum and graphite, which permits significant reduction in the levels of thermal stresses. A high level of strength, high resistance to radiation swelling, and good thermal conductivity are among the merits of molybdenum alloys. The principal disadvantage of molybdenum-base alloys is a tendency to low-temperature embrittJement. It was necessary to find a solution to reduce the ductile to brittle transition temperature (DBIT) for Mo-alloys, and to provide a minimal shift of the DBIT following low-temperature neutron irradiation. It has been known for a long time that Mo-Re alloys provide satisfactory properties arising from the rhenium addition to molybdenum. Mo-Re alloys at 3-7%Re concentrations exhibit a significant increase in ductility in low-temperature tests [2]. Due to the refining effects of Re on microstructure, the strength of welded joints of Mo-Re alloys is improved significantly (due to grain size reductions in the heat-affected zone with 5% Re) [3]. Prohlems from the effect of Re addition on irradiation-resistant properties of molybdenum alloys remain Elsevier Science Publishers B.V.
unidentified. There is, however, an encouraging observation [4] of good resistance of alloys with 5% Re content to radiation embrittlement. It should bc noted, however, that a wide operating range of temperature and dose for Mo pipes in the divertor [T ~ 100 to 350°C and
2. Experimental procedure In irradiated conditions, the physical and mechanical properties of the Mo-Re alloys have been studied in recrystallized (R) and deformed (D) stages. The recrystallization temperatures for Mo-Re alloys were chosen so that the grain size for all alloys was approximately 80 iJom (for example, Mo-0.5Re alloys 1000°C, and Mo-30Re alloy 1350 C for 1 h). The sample size was: gauge length 25 mm, width 4 mm, thickness 1 mm. This sample geometry was used D
427
SA. Fabritsiev et at. j Effects o( neutron irradiation on Mo-Re alloys
'" E;
160 ,---.,.---r---r---'---,---,
'" t;
120 l-!=::::~:::::::~=~=~=~---I
::>
i:;~' " . _ _-
.'" ,5
.'-
'" 34 ;;
B 80 A
o
)-
.
300
400
500
600 Tj,sl COC)
Fig. 1. Thermal conductivity of Mo-Re alloys versus test temperature (R-cond). 0 - Mo, - - Mo-IRe, f). - Mo-5Re, x - Mo-13Re, 0 - Mo-20Re, .... - Mo-30Re.
for measurements of electrical conductivity too. The samples were irradiated in mixed spectrum reactor SM-2 up to doses - 5 X 10 21 n/cm 2 at irradiation temperature 240-330°C. Irradiated and control specimens were subjected to uniaxial tensile tests over the temperature range 20 to sooae. Strength (yield strength, ultimate tensile strength) and ductility (uniform and total elongation) characteristics were obtained. Electrical conductivity for the Mo alloys was also measured. Fracture mode of the irradiated alloy was investigated using optical metallography techniques and scanning electron microscopy (SEM). Swelling in the alloys was determined using the immersion method. 3. Results The study of physical properties of Mo-Re alloys showed that molybdenum alloying with rhenium results in monotonic decreases in electrical conductivity and thermal conductivity (A) (fig. 1) and increases in the coefficient of thermal expansion (a) (fig. 2). Young's modulus determination showed that for low Re concentrations. modulus decreases with Re content but above 9% Re, modulus increases (fig. 3). Therefore, it was possible to evaluate the effect of Re on physical properties, A, a and heat capacity (C p ), of molybdenum alloys (fig. 4). The tensile strength as a function of Re content generally increases with Re concentration (fig. 5). The
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DBTT as a function of Re content is plotted on the basis of tensile test results and shows that the DBTT of alloys decreases with increasing Re content. At an Re content larger than 10% DBTT is lower than - 1S0a C. Analyses of the relationships obtained lead to the conclusion that an alloy containing 5-10% Rc possesses a complex of satisfactory properties including thermal conductivity: 90 W m - I °C, high strength, and low DBTT.
fO
20 ~HENlUM
15
~
~,
:z:
3Q
sa
40
CONTENT (0(0)
r___
<:> ;:: 40
-r--o
cz: c
0
400
30
280
RHENlLiM CONTENT (-;.)
(b)
E~~~ --0...-,.,-0200
20
6,0
Fig. 4. Effect of rhenium on physical properties (A, a, CD) for Mo-Re alloys at room temperature (R-cond).
~
o
--
_d" 1'"'-
W
6.6
-
Ku. ...--
20
~\-o....., ~.-,--.
6. 0
50
40
Fig. 3. Rhenium effect on Young's modulus for Mo-Re alloys. 0 - CR-cond); - - (D-cond).
;:
8t:
3D
RHENIUM CONTENT Co/.)
"" ~ oL-_J...-_..L-_..L._-l-_-'-:.,--:J. 200
20
m
~ 40 ~=t==*,=*""""~--+--1 iOO
--
33
mJO
~.1--*
10 Ties! ("c)
Fig. 2. Thermal expansion coefficient of Mo-Re alloys versus test temperature (R-cond). + - Mo-20Re, .... - Mo-13Re, D. - Mo-5Re, _ - Mo-IRe, 0 - Mo-O.5Re.
20
ao
40
•
50
RHENIUM CONTENT (%)
Fig. 5. Effect of rhenium on mechanical properties for Mo-Re 21 2 alloys CD-cond.). 0 - unirradiated; cf> - 5 X10 njcm , Tirr - 240-330·C, Tie'i - 300·C. * - brittle fracture.
-* -
428
SA. Fabritsiev et al. / Effects of neutron irradiation on Mo-Re alloys
In order to estimate radiation limits for Mo-Re, alloy specimens have been irradiated in the SM-2 reactor to doses up to 5 X 10 21 n/cm 2 at 240-330°C. The choice of a mixed spectrum reactor was dictated by thc necessity to have data for low irradiation temperatures, which cannot be obtained in fast neutron reactors. Aftcr irradiation in the SM-2 reactor at 240-330°C to 5 X 10 21 n/cm 2 , it was shown that the strength and ductility of Mo-Re alloys depend on Re content (fig. 5), and the lowest decrease in ductility was observed for pure Mo. Note that for Mo-Re alloys, as for W [5], the brittle fracture of irradiated samples was accompanied by a decrease in strength. The SEM investigation of Mo-Re alloy specimens showed that irradiation leads to the appearance of subgrain and intcrgranular fracture, fig. 6. The effect of neutron irradiation on swelling in Mo-Re alloys produced a density change of not more than 0.3%.
The study of electrical resistivity of irradiated specimens shows that electrical resistivity of pure Mo increases by 10 to 15% and that for alloys having high Re content by 30 to 50%. Since alloys with additions of Re have relatively low thermal and electrical conductivity, this increase will significantly limit potential applications of Mo-Rc alloys. 4. Conclusion
The study of physical and mcchanical propertics of Mo-Re alloys in recrystallized and deformed states have shown that Mo-5.9%Re alloys in the deformed state possess the optimum set of high thermal conductivity, high strength, and low DBTT. The investigations on the effect of irradiation on properties of Mo alloys in the deformed state have
Mo-S%Rp+300'
24510
30
~
~
fig. 6. Effect of neutron irradiation on fracture in Mo alloys (D-cond). (a, b) Mo-S%Re; (c, d) Mo-47%Re; (a, c) unirradiated; (b, d) irradiated. (a) T;cst = 100°C; (b, c) TICS[ = 300°C; (d) Tle" = SaQ°e.
SA. Fabritsiev et al. / Effects of neutron irradiation on Mo-Re alloys
shown that Re alloying does not result in increased radiation resistance. References [1] ITER Concept Definition, vol. 1 (JAEA, Vienna, 1989). [2) N. Igata, A. Kohyuma and S. Nomura, J. Nue!. Mater. 103 & 104 (1981) 1157.
429
(3\ Y. Hiraoka, M. Okada and H. !ri!, J. Nue!. Mater. 155-157 (1988) 381. [4) D. Smith, l.V. Altovsky, V.R. Barabash et aI., ITER materials evaluation and data base, ITER Documentation Series no. 34 (lAEA, Vienna, 1990). [5) SA Fabritsiev, VA 19natov, V.V. Rybin, V.P. Chakin, V.A. Kasakov, A.S. Pokrovskiy, V.R. Barabash and G.L. Saksaganskiy, in: USSR Contribution to the ITER Materials Data Base Meeting (1990) ITER-IC-NE-1-02.
nuclear materials
Journal of Nuclear Materials 191-194 (1992) 426-429 North-Holland
Effects of neutron irradiation on physical and mechanical properties of Mo-Re alloys S.A Fabritsiev a, V.A Gosudarenkova a, V.A Potapova v.P. Chakin b, AS. Pokrovsky band V.R. Barabash
a,
v.v. Rybin
a,
L.S. Kosachev
b,
C
]93]67 St. Petersburg, Russia Lenin Scientific Research Institute of Atomic Reactors, 4335]0 Dimitrougrad, Russia Electrophysical Apparatus D. V. Efremov Institute, 189631 St. Petersburg, Russia
a Central Research Institute of Structural Materials "Prometey': h C
v.I.
The effects of neutron irradiation on mechanical strength, plasticity and electric conductivity of Mo-Re alloys are presented. Pure Mo and Mo-O.5%Re to Mo-30%Re alloys were studied. Samples were irradiated in the reactor SM-2 eel> - 5 x 10 21 n/cm 2 , Tin - 240-330°C). Irradiation at 300°C indicates that the radiation embrittlement results in reduced strength and ductility characteristics of Mo- Re alloys. Measurement of electric resistance shows that low-temperature irradiation of Mo-alloys results in 20~30% reduced conductivity. SEM studies of fracture surfaces show that after irradiation brittle intergranular fracture takes places that correlates with an abrupt reduction of strength.
1. Introduction
Molybdenum and copper alloys are now considered as candidate materials for the heat-sink system of the divertor of the International Thermonuclear Experimental Reactor (ITER). The design which includes graphite tiles on molybdenum substrata is preferred for the physics stage of the divcrtor [1]. The main advantage of such a solution is the similarity of the coefficients of thermal expansion for molybdenum and graphite, which permits significant reduction in the levels of thermal stresses. A high level of strength, high resistance to radiation swelling, and good thermal conductivity are among the merits of molybdenum alloys. The principal disadvantage of molybdenum-base alloys is a tendency to low-temperature embrittJement. It was necessary to find a solution to reduce the ductile to brittle transition temperature (DBIT) for Mo-alloys, and to provide a minimal shift of the DBIT following low-temperature neutron irradiation. It has been known for a long time that Mo-Re alloys provide satisfactory properties arising from the rhenium addition to molybdenum. Mo-Re alloys at 3-7%Re concentrations exhibit a significant increase in ductility in low-temperature tests [2]. Due to the refining effects of Re on microstructure, the strength of welded joints of Mo-Re alloys is improved significantly (due to grain size reductions in the heat-affected zone with 5% Re) [3]. Prohlems from the effect of Re addition on irradiation-resistant properties of molybdenum alloys remain Elsevier Science Publishers B.V.
unidentified. There is, however, an encouraging observation [4] of good resistance of alloys with 5% Re content to radiation embrittlement. It should bc noted, however, that a wide operating range of temperature and dose for Mo pipes in the divertor [T ~ 100 to 350°C and
2. Experimental procedure In irradiated conditions, the physical and mechanical properties of the Mo-Re alloys have been studied in recrystallized (R) and deformed (D) stages. The recrystallization temperatures for Mo-Re alloys were chosen so that the grain size for all alloys was approximately 80 iJom (for example, Mo-0.5Re alloys 1000°C, and Mo-30Re alloy 1350 C for 1 h). The sample size was: gauge length 25 mm, width 4 mm, thickness 1 mm. This sample geometry was used D
427
SA. Fabritsiev et at. j Effects o( neutron irradiation on Mo-Re alloys
'" E;
160 ,---.,.---r---r---'---,---,
'" t;
120 l-!=::::~:::::::~=~=~=~---I
::>
i:;~' " . _ _-
.'" ,5
.'-
'" 34 ;;
B 80 A
o
)-
.
300
400
500
600 Tj,sl COC)
Fig. 1. Thermal conductivity of Mo-Re alloys versus test temperature (R-cond). 0 - Mo, - - Mo-IRe, f). - Mo-5Re, x - Mo-13Re, 0 - Mo-20Re, .... - Mo-30Re.
for measurements of electrical conductivity too. The samples were irradiated in mixed spectrum reactor SM-2 up to doses - 5 X 10 21 n/cm 2 at irradiation temperature 240-330°C. Irradiated and control specimens were subjected to uniaxial tensile tests over the temperature range 20 to sooae. Strength (yield strength, ultimate tensile strength) and ductility (uniform and total elongation) characteristics were obtained. Electrical conductivity for the Mo alloys was also measured. Fracture mode of the irradiated alloy was investigated using optical metallography techniques and scanning electron microscopy (SEM). Swelling in the alloys was determined using the immersion method. 3. Results The study of physical properties of Mo-Re alloys showed that molybdenum alloying with rhenium results in monotonic decreases in electrical conductivity and thermal conductivity (A) (fig. 1) and increases in the coefficient of thermal expansion (a) (fig. 2). Young's modulus determination showed that for low Re concentrations. modulus decreases with Re content but above 9% Re, modulus increases (fig. 3). Therefore, it was possible to evaluate the effect of Re on physical properties, A, a and heat capacity (C p ), of molybdenum alloys (fig. 4). The tensile strength as a function of Re content generally increases with Re concentration (fig. 5). The
~f'6,5
~~
~~6. "'>-
z --'~ « ~t:5,
"'''"'w >-0 ~
o~L~_L+-'" ~
s
__1-.-
60
~
-3 =z:
to
'" ~
Ii;
1000
800
1000
220
4,8
IDO
blI0 400
-.
{a}
800
!:l 200
.. .
()
~
;;;:0- -"
.
loU
~
~
600
40
5,1,
DBTT as a function of Re content is plotted on the basis of tensile test results and shows that the DBTT of alloys decreases with increasing Re content. At an Re content larger than 10% DBTT is lower than - 1S0a C. Analyses of the relationships obtained lead to the conclusion that an alloy containing 5-10% Rc possesses a complex of satisfactory properties including thermal conductivity: 90 W m - I °C, high strength, and low DBTT.
fO
20 ~HENlUM
15
~
~,
:z:
3Q
sa
40
CONTENT (0(0)
r___
<:> ;:: 40
-r--o
cz: c
0
400
30
280
RHENlLiM CONTENT (-;.)
(b)
E~~~ --0...-,.,-0200
20
6,0
Fig. 4. Effect of rhenium on physical properties (A, a, CD) for Mo-Re alloys at room temperature (R-cond).
~
o
--
_d" 1'"'-
W
6.6
-
Ku. ...--
20
~\-o....., ~.-,--.
6. 0
50
40
Fig. 3. Rhenium effect on Young's modulus for Mo-Re alloys. 0 - CR-cond); - - (D-cond).
;:
8t:
3D
RHENIUM CONTENT Co/.)
"" ~ oL-_J...-_..L-_..L._-l-_-'-:.,--:J. 200
20
m
~ 40 ~=t==*,=*""""~--+--1 iOO
--
33
mJO
~.1--*
10 Ties! ("c)
Fig. 2. Thermal expansion coefficient of Mo-Re alloys versus test temperature (R-cond). + - Mo-20Re, .... - Mo-13Re, D. - Mo-5Re, _ - Mo-IRe, 0 - Mo-O.5Re.
20
ao
40
•
50
RHENIUM CONTENT (%)
Fig. 5. Effect of rhenium on mechanical properties for Mo-Re 21 2 alloys CD-cond.). 0 - unirradiated; cf> - 5 X10 njcm , Tirr - 240-330·C, Tie'i - 300·C. * - brittle fracture.
-* -
428
SA. Fabritsiev et al. / Effects of neutron irradiation on Mo-Re alloys
In order to estimate radiation limits for Mo-Re, alloy specimens have been irradiated in the SM-2 reactor to doses up to 5 X 10 21 n/cm 2 at 240-330°C. The choice of a mixed spectrum reactor was dictated by thc necessity to have data for low irradiation temperatures, which cannot be obtained in fast neutron reactors. Aftcr irradiation in the SM-2 reactor at 240-330°C to 5 X 10 21 n/cm 2 , it was shown that the strength and ductility of Mo-Re alloys depend on Re content (fig. 5), and the lowest decrease in ductility was observed for pure Mo. Note that for Mo-Re alloys, as for W [5], the brittle fracture of irradiated samples was accompanied by a decrease in strength. The SEM investigation of Mo-Re alloy specimens showed that irradiation leads to the appearance of subgrain and intcrgranular fracture, fig. 6. The effect of neutron irradiation on swelling in Mo-Re alloys produced a density change of not more than 0.3%.
The study of electrical resistivity of irradiated specimens shows that electrical resistivity of pure Mo increases by 10 to 15% and that for alloys having high Re content by 30 to 50%. Since alloys with additions of Re have relatively low thermal and electrical conductivity, this increase will significantly limit potential applications of Mo-Rc alloys. 4. Conclusion
The study of physical and mcchanical propertics of Mo-Re alloys in recrystallized and deformed states have shown that Mo-5.9%Re alloys in the deformed state possess the optimum set of high thermal conductivity, high strength, and low DBTT. The investigations on the effect of irradiation on properties of Mo alloys in the deformed state have
Mo-S%Rp+300'
24510
30
~
~
fig. 6. Effect of neutron irradiation on fracture in Mo alloys (D-cond). (a, b) Mo-S%Re; (c, d) Mo-47%Re; (a, c) unirradiated; (b, d) irradiated. (a) T;cst = 100°C; (b, c) TICS[ = 300°C; (d) Tle" = SaQ°e.
SA. Fabritsiev et al. / Effects of neutron irradiation on Mo-Re alloys
shown that Re alloying does not result in increased radiation resistance. References [1] ITER Concept Definition, vol. 1 (JAEA, Vienna, 1989). [2) N. Igata, A. Kohyuma and S. Nomura, J. Nue!. Mater. 103 & 104 (1981) 1157.
429
(3\ Y. Hiraoka, M. Okada and H. !ri!, J. Nue!. Mater. 155-157 (1988) 381. [4) D. Smith, l.V. Altovsky, V.R. Barabash et aI., ITER materials evaluation and data base, ITER Documentation Series no. 34 (lAEA, Vienna, 1990). [5) SA Fabritsiev, VA 19natov, V.V. Rybin, V.P. Chakin, V.A. Kasakov, A.S. Pokrovskiy, V.R. Barabash and G.L. Saksaganskiy, in: USSR Contribution to the ITER Materials Data Base Meeting (1990) ITER-IC-NE-1-02.