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Influence of high dose neutron irradiation at 385 and 750°C on the microhardness of MgAl2O4 spinel
ELSEVIER
Journal of Nuclear Materials 212-215 (1994) 1096-1100
Influence of high dose neutron irradiation at 385 and 750°C on the microhardness of MgAl,O, spine1 * C.A. Black a, F.A. Garner b, R.C. Bradt a a University of Nevada-Rena, Rena, NV89557 USA b Pacific Northwest Laboratory, Richland, WA 99352, USA
Abstract
High-purity specimens of stoichiometric MgAIZO, single crystal spine1 and a hot-pressed polycrystalline ceramic spine1 were irradiated to exposures as large as 24.9 X 102’ n cm -’ (E > 0.1 MeV) in FFTF at 385 and 750°C. The specimens did not develop any brittleness or fragility, and maintained their physical integrity. Microhardness measurements revealed that initially all specimens hardened a small amount and then recovered slightly. At the lower irradiation temperature, the dependence of microhardness on orientation observed prior to irradiation tended to disappear. There was also some evidence that a secondary slip system was being activated. Following 750°C irradiation, the orientation dependence was not lost, and the evidence for activation of a secondary slip system was stronger.
1. Introduction
Stoichiometric MgAl,O, spine1 has been proposed as a potential electrical insulator ceramic for use in fusion reactors, primarily because, unlike other ceramics, it has demonstrated a remarkable insensitivity to neutron irradiation to levels as high as 2 x 102’ n cmd2 (E > 0.1 MeV) [l-9]. While spinels would most likely be employed in a dense hot-pressed polycrystalline form, it is advantageous to conduct fundamentally-oriented studies on the radiation response of this material using single crystal specimens in addition to hot-pressed specimens. Therefore, an extensive irradiation program using both types of specimens has been conducted in the Fast Flw Test Facility (FFTF), reaching exposure levels an order of magnitude or more larger than those of all earlier studies [lo]. The focus of the present paper is the effect of radiation on the microhardness of this material and its dependence on crystallographic orientation. This information will eventually be cou-
* Work supported by the US Department of Energy under Contract DE-AC06-76RL0 1830.
pled with the results of examination by electron microscopy to determine the microstructural origins of the radiation resistance of this ceramic. The single crystal spine1 used in this study has been previously examined in the unirradiated condition and the results published by Akimune and Bradt Ill]. They showed that the (100) plane exhibits a maximum microhardness in the [OOl] direction and a minimum in the [Oil] direction, consistent with a primary slip system of (111) (110). The microhardness on the (111) plane was shown to be independent of indenter orientation, which is also consistent with that primary slip system. It was anticipated that the hardness response might change as radiation-induced microstructure accumulated.
2. Experimental
High-purity stoichiometric MgAl,O, was obtained as transparent 2 cm boules with either [lOOI or [ill] growth direction from Union Carbide Corporation. Hot-pressed polycrystalline MgAl,O, of 100% theoretical density and comparable purity was obtained from the Coors Ceramic Corporation. The detailed composition of the three starting materials, the fabrication of
0022-3115/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0022-3115(94)00157-J
details
C.A. Black et al. /Journal
of Nuclear Materials 212-215
the individual specimens, and the details of the irradiation are presented in ref. [lo]. These materials were irradiated in the Materials Open Test Assembly (MOTA) of FFTF at 385°C to 2.2, 4.6, and 24.9 X 1O22 n cm-2 (E > 0.1 MeV), and at 750°C to 5.6, 13.7, and 21.7 x 1O22 n cmp2 (E > 0.1 MeV). Using a rough conversion factor of u 10 dpa per 1O22n cme2 (E > 0.1 MeV), the highest displacement level reached was N 250 dpa. Depending on the specific irradiation conditions, some specimens were in the form of 4.8 mm diameter cylinders, and some were in the form of square plates. The flat surfaces of the cylinders and the major flat surfaces of the plates were parallel to the (100) and (111) planes for the [lo01 and [ill] specimens, respectively. After irradiation, the specimens were vacuummounted with Buehler Epa-Thin epoxy, using three aluminum oxide cylindrical sheaths set around the specimen to insure that the original crystallographic orientation of the specimen surface was maintained during polishing. The mounted specimens were polished with progressively finer diamond pastes (30, 1.5, 1 urn) on a vibratory polishing machine. Unirradiated specimens were also prepared in an identical manner. The specimens did not develop any brittleness or fragility, and maintained their physical integrity during irradiation, polishing, and testing. While still mounted, the specimens were measured for microhardness. Since the microhardness is load-dependent in these materials, especially for loads less than 500 g, preliminary tests were conducted at 25, 50, 100, and 200 g load. Knoop microhardness was then measured at room temperature using 100 and 200 g loads at a load application rate of 17 urn/s for 15 s. The microhardness tester was a Tukon Model 300 fitted with a Wilson digital filar eyepiece accurate to 0.1 urn. A goniometer with f 1” of accuracy held the crystal on the hardness tester. For each orientation, 10 4000
. c
[Ill]-CwkntedSp6~lnte.n~
0 Hot-PressedPolycrystals
3ow
4_
Knoop Hardnet?& Kg mm** 2ooa
1000 0
50
*
I
,
I
100
130
200
2M
Load, gm
Fig. 1. Microhardness response versus applied load for unirradiated MgA1204 single crystal and hot-pressed polycrystal specimens.
1097
(1994) 1096-1100
100 gram
2000 Y E 9 3 ; 1600 tii I
-1
12oo
ll2A.LJ 0
5
10
15
20 x 1022
Neutron Fluence, n crnm2(E>O.lMeV)
Fig. 2. Evolution of microhardness of hot-pressed polycrystals irradiated at 38s”C. parallel indents were made with u 100 urn between each measurement. Indentations resulting in cracks were not recorded, and repeat measurements were made. The tendency to occasionally develop cracks radiating from the hardness indents was minimal and did not change with irradiation. From the long diagonal length (d) in mm, the Knoop microhardness (Hk) was calculated using the standard equation where the load P (in g) and Hk = P/O.7028 d2 in kg mmp2. True hardness was calculated from the slope of a P/d versus d graph after applying the above equation to calculate the hardness value of H,. On the cylindrical (100) single crystal specimens, nine equally spaced orientations were used over a total range of 90“. Because of the crystal symmetry, these were sufficient to characterize the microhardness anisotropy profile for the entire (100) plane. Indents on the (111) single crystals were also performed over 90” at five equally spaced orientations. The square (100) specimens were cut so that the [OOl] direction was parallel to the edge on the specimen, and in this manner, the orientation of the specimen was preserved. After confirming the already known anisotropy of the (100) plane on the square specimens, a random starting point was used on the cylindrical specimens. Later analysis of the (100) cylindrical specimen data assumed the maximum measured hardness to be the [OOl] direction and the minimum to be the [Oil] direction. Since the (111) plane showed no orientation dependence, the random starting point on the cylindrical specimens was chosen as the 0” position.
3. Results Fig. 1 shows the nearly identical hardness versus load response of the hot-pressed polycrystalline ce-
CA. Black et al./Joumal
1098
of Nuclear Materials 212-215
(1994) 1096-1100 (100) Plane
(111) Plane WOgm
100 gm 2000
1600
1600 I
1
I
I
\
unlrradlated
/
I
200 gm
9
’
d 1200 t 0 2 2000 I
unirradiated
200 gm
1600
1600
B
i( /I
1200
0
22.5
45
I
/
67.6
90
1200
/ 0
I
I
/
/
22.6
45
67.5
90
Angle, degrees
Angle, degrees
Fig. 3. Changes in microhardness induced in single crystal specimens irradiated to 2.2 x 10z2 n cmm2 (E > 0.1 MeV) at 385°C. Solid circles denote measurements on irradiated specimens.
ramic spine1 and the [ill] single crystal specimens in the unirradiated condition. A similar dependence on load was also observed as the specimens hardened with irradiation, as shown in Figs. 2 and 3. Note in Fig. 2 that a saturation level of hardness was reached in the hot-pressed specimens irradiated at 385°C for each load level, but the duration of the transient regime of hardening was found to be load-dependent, being slower to saturate at the higher load. Similar levels of hardening were observed in the
single crystal specimens irradiated at 385”C, as shown in Figs. 3 and 4. In the unirradiated condition, the [lOO] specimens exhibited the same dependence on crystalline orientation that was observed earlier by Akimune and Bradt ill]. The degree of anisotropy of the orientation dependence on the (1001 plane was observed to decrease as the specimens hardened dur-
2000
1600 Hardness,
4.6 X 1022 n cma
I
I
I
I
I 100 gram
1200 Kg/mm’ 2000
1600
24.9 x 1 022 n cmv2 I I I 0 22.5 46
I 67.5
I 90
Angie, degrees Fig. 4. Microhardness versus orientation for higher exposure levels at 385°C.
on the (100) plane
I
0
1
22.5
45
67.5
‘__ _~
90
Angle, degrees
Fig. 5. Hardening and softening observed in single crystal specimens irradiated at 750°C and tested with a 100 g load.
C.A. Black et al. /Journal
of Nuclear Materials 212-215
200 gram
N
1600
a
I
I
Y” 1200 z? 2000- 13.7 x 1O** n cm-*
I
I
I 200 gram
6 5 I 1600 -
0
22.5
45
67.5
90
Angle, degrees
Fig. 6. Microhardness versus (100) orientation dependence for single crystal specimens irradiated at 75O”C,observed using a 200 g load.
ing irradiation.
The majority of the hardening occurred during the first irradiation increment. In addition, there is some suggestion that a second minimum in hardness versus orientation was developing at the highest exposure level, suggesting that a secondary slip system may have been activated. At an irradiation temperature of 75o”C, the microhardness first increased with irradiation and then subsequently decreased, as shown in Fig. 5. Even more important, however, the orientation dependence on the (100) plane was not reduced as for irradiation at 385°C. The tendency to develop a secondary minimum appeared to be stronger for irradiation at 75o”C, especially when higher indentation test loads were applied, as shown in Fig. 6.
(1994) 1096-1100
1099
isting grain boundaries and random grain orientations in the polycrystals seems to have produced a somewhat softer material before irradiation. However, the sink strength for point defects associated with these grain boundaries may also compete with the self-healing process and may therefore not allow the radiation-induced hardening to relax as quickly as was observed in the single crystals. The loss of the orientation dependence on the (100) plane at 385°C can be understood in terms of radiation-induced formation of defect aggregates that block the movement of dislocations on the primary slip system. The possible activation of a secondary slip system may have been facilitated by the development of radiation-induced dislocation microstructure. Microscopy analysis of these specimens at the highest fluence levels is in progress. Early results at 385°C show formation of high density of $ Ill01 dislocation loops (15 to 70 nm diameter) and tangled dislocations [12]. At 75o”C, there is a small number of voids (u 7 nm diameter) and extended i [llOl stacking faults. The substantially lower density of radiation-induced microstructure at 75O”C, compared to that at 385”C, probably explains why the orientation dependence of hardness on the (100) plane persists to very high fluences. The relatively small effect of radiation in hardening of this material at such high neutron exposure levels confirms its overall radiation resistance and its suitability for fusion use. As noted in Ref. [lo], the elastic properties and dimensional stability of these specimens were also unaffected by radiation.
1500 365 “C irradiation
4. Discussion Fig. 7 shows the calculated true hardness values derived from the data generated at 100 and 200 g loads. The tendency to first harden and then soften with increasing exposure occurred for all three types of specimens, with the largest changes observed in the hot-pressed polycrystals at 385°C. In each case, the radiation-induced increases in microhardness were relatively small and of no significance for fusion application. The softening may be evidence for a self-healing process that may be unique to this spine1 and is operating throughout the irradiation to limit the cumulative damage. It is interesting to note that the microhardness of the polycrystalline ceramics at 385°C appears to be approaching that of single crystals. The role of pre-ex-
NeutronFluence, n/c& (b0.1 MeV) Fig. 7. Influence of radiation on “true” hardness determined from the slope of a P/d versus d graph for 100 and 200 g.
1100
CA. Black et al. /Journal of Nuclear Materials 212-215 (1994) 1096-1100
5. Conclusions High temperature irradiation of high-purity stoichiometric MgAl,O, spine1 to very high neutron exposures induces a small amount of hardening that does not impair its application to fusion needs. The hardening peaks early in the irradiation and then relaxes somewhat at higher exposure levels. The details of the hardening process are somewhat sensitive to applied load, crystal orientation, irradiation temperature, and the exposure level. The dimensional and mechanical stability to displacements levels as large as 250 dpa suggests that an efficient self-healing process that may not operate in other ceramics may be operating in this material.
Acknowledgement This work was funded by the US Department of Energy, Office of Fusion Energy. Pacific Northwest Laboratory is operated for the US Department of Energy by Battelle Memorial Institute under Contract DE-AC06-76RL0 1830. The work of C.A. Black and R.C. Bradt was sponsored by the Northwest College and University Association for Science under US Department of Energy Contract DE-FG06-89ER-75522.
References [ll G.F. Hurley and J.M. Bunch, Am. Ceram. Sot. Bull. 59 (1980) 456.
w F.W. Clinard Jr., G.F. Hurley and L.W. Hobbs, J. Nucl. Mater. 108 & 109 (1982) 655. [31 G.F. Hurley, J.C. Kennedy, F.W. Clinard, Jr., R.A. Youngman and W.R. McDoneIl, J. Nucl. Mater. 103 & 104 (1981) 761. [41 F.W. Clinard Jr., G.F. Hurley and R.W. Klaffky, Res. Mech. 8 (1983) 207. 61 F.W. Clinard Jr., G.F. Hurley, L.W. Hobbs, D.L. Rohr and R.A. Youngman, J. Nucl. Mater. 122 & 123 (1984) 1386. kl F.W. Clinard Jr., G.F. Hurley, R.A. Youngman and L.W. Hobbs, J. Nucl. Mater. 133 & 134 (1985) 701. [71 C.A. Parker, L.W. Hobbs, K.C. Russell and F.W. Clinard Jr., J. Nucl. Mater. 133 & 134 (1985) 741. Bl Y. Fukushima, T. Yano, T. Maruyama and T. Iseki, J. Nucl. Mater. 175 (1990) 203. [91 H. Suematsu, T. Iseki, T. Yano, Y. Saito, T. Suzuki and T. Mori, J. Am. Ceram. Sot. 75 (1992) 1742. WI F.A. Gamer, G.W. Hollenberg, J.L. Ryan, Z. Li, C.A. Black and R.C. Bradt, in these Proceedings (ICFRM-6), J. Nucl. Mater. 212-215 (1994). Dll Y. Akimune and R.C. Bradt, J. Am. Ceram. Sot. (1987) C84. WI C. Kinoshita, K. Fukumoto and F.A. Garner, Proc. 4th Symp. on the Fabrication and Properties of Ceramics for Fusion Energy, Indianapolis, IN, April 24-28, 1994, J. Nucl. Mater., in press.
Journal of Nuclear Materials 212-215 (1994) 1096-1100
Influence of high dose neutron irradiation at 385 and 750°C on the microhardness of MgAl,O, spine1 * C.A. Black a, F.A. Garner b, R.C. Bradt a a University of Nevada-Rena, Rena, NV89557 USA b Pacific Northwest Laboratory, Richland, WA 99352, USA
Abstract
High-purity specimens of stoichiometric MgAIZO, single crystal spine1 and a hot-pressed polycrystalline ceramic spine1 were irradiated to exposures as large as 24.9 X 102’ n cm -’ (E > 0.1 MeV) in FFTF at 385 and 750°C. The specimens did not develop any brittleness or fragility, and maintained their physical integrity. Microhardness measurements revealed that initially all specimens hardened a small amount and then recovered slightly. At the lower irradiation temperature, the dependence of microhardness on orientation observed prior to irradiation tended to disappear. There was also some evidence that a secondary slip system was being activated. Following 750°C irradiation, the orientation dependence was not lost, and the evidence for activation of a secondary slip system was stronger.
1. Introduction
Stoichiometric MgAl,O, spine1 has been proposed as a potential electrical insulator ceramic for use in fusion reactors, primarily because, unlike other ceramics, it has demonstrated a remarkable insensitivity to neutron irradiation to levels as high as 2 x 102’ n cmd2 (E > 0.1 MeV) [l-9]. While spinels would most likely be employed in a dense hot-pressed polycrystalline form, it is advantageous to conduct fundamentally-oriented studies on the radiation response of this material using single crystal specimens in addition to hot-pressed specimens. Therefore, an extensive irradiation program using both types of specimens has been conducted in the Fast Flw Test Facility (FFTF), reaching exposure levels an order of magnitude or more larger than those of all earlier studies [lo]. The focus of the present paper is the effect of radiation on the microhardness of this material and its dependence on crystallographic orientation. This information will eventually be cou-
* Work supported by the US Department of Energy under Contract DE-AC06-76RL0 1830.
pled with the results of examination by electron microscopy to determine the microstructural origins of the radiation resistance of this ceramic. The single crystal spine1 used in this study has been previously examined in the unirradiated condition and the results published by Akimune and Bradt Ill]. They showed that the (100) plane exhibits a maximum microhardness in the [OOl] direction and a minimum in the [Oil] direction, consistent with a primary slip system of (111) (110). The microhardness on the (111) plane was shown to be independent of indenter orientation, which is also consistent with that primary slip system. It was anticipated that the hardness response might change as radiation-induced microstructure accumulated.
2. Experimental
High-purity stoichiometric MgAl,O, was obtained as transparent 2 cm boules with either [lOOI or [ill] growth direction from Union Carbide Corporation. Hot-pressed polycrystalline MgAl,O, of 100% theoretical density and comparable purity was obtained from the Coors Ceramic Corporation. The detailed composition of the three starting materials, the fabrication of
0022-3115/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0022-3115(94)00157-J
details
C.A. Black et al. /Journal
of Nuclear Materials 212-215
the individual specimens, and the details of the irradiation are presented in ref. [lo]. These materials were irradiated in the Materials Open Test Assembly (MOTA) of FFTF at 385°C to 2.2, 4.6, and 24.9 X 1O22 n cm-2 (E > 0.1 MeV), and at 750°C to 5.6, 13.7, and 21.7 x 1O22 n cmp2 (E > 0.1 MeV). Using a rough conversion factor of u 10 dpa per 1O22n cme2 (E > 0.1 MeV), the highest displacement level reached was N 250 dpa. Depending on the specific irradiation conditions, some specimens were in the form of 4.8 mm diameter cylinders, and some were in the form of square plates. The flat surfaces of the cylinders and the major flat surfaces of the plates were parallel to the (100) and (111) planes for the [lo01 and [ill] specimens, respectively. After irradiation, the specimens were vacuummounted with Buehler Epa-Thin epoxy, using three aluminum oxide cylindrical sheaths set around the specimen to insure that the original crystallographic orientation of the specimen surface was maintained during polishing. The mounted specimens were polished with progressively finer diamond pastes (30, 1.5, 1 urn) on a vibratory polishing machine. Unirradiated specimens were also prepared in an identical manner. The specimens did not develop any brittleness or fragility, and maintained their physical integrity during irradiation, polishing, and testing. While still mounted, the specimens were measured for microhardness. Since the microhardness is load-dependent in these materials, especially for loads less than 500 g, preliminary tests were conducted at 25, 50, 100, and 200 g load. Knoop microhardness was then measured at room temperature using 100 and 200 g loads at a load application rate of 17 urn/s for 15 s. The microhardness tester was a Tukon Model 300 fitted with a Wilson digital filar eyepiece accurate to 0.1 urn. A goniometer with f 1” of accuracy held the crystal on the hardness tester. For each orientation, 10 4000
. c
[Ill]-CwkntedSp6~lnte.n~
0 Hot-PressedPolycrystals
3ow
4_
Knoop Hardnet?& Kg mm** 2ooa
1000 0
50
*
I
,
I
100
130
200
2M
Load, gm
Fig. 1. Microhardness response versus applied load for unirradiated MgA1204 single crystal and hot-pressed polycrystal specimens.
1097
(1994) 1096-1100
100 gram
2000 Y E 9 3 ; 1600 tii I
-1
12oo
ll2A.LJ 0
5
10
15
20 x 1022
Neutron Fluence, n crnm2(E>O.lMeV)
Fig. 2. Evolution of microhardness of hot-pressed polycrystals irradiated at 38s”C. parallel indents were made with u 100 urn between each measurement. Indentations resulting in cracks were not recorded, and repeat measurements were made. The tendency to occasionally develop cracks radiating from the hardness indents was minimal and did not change with irradiation. From the long diagonal length (d) in mm, the Knoop microhardness (Hk) was calculated using the standard equation where the load P (in g) and Hk = P/O.7028 d2 in kg mmp2. True hardness was calculated from the slope of a P/d versus d graph after applying the above equation to calculate the hardness value of H,. On the cylindrical (100) single crystal specimens, nine equally spaced orientations were used over a total range of 90“. Because of the crystal symmetry, these were sufficient to characterize the microhardness anisotropy profile for the entire (100) plane. Indents on the (111) single crystals were also performed over 90” at five equally spaced orientations. The square (100) specimens were cut so that the [OOl] direction was parallel to the edge on the specimen, and in this manner, the orientation of the specimen was preserved. After confirming the already known anisotropy of the (100) plane on the square specimens, a random starting point was used on the cylindrical specimens. Later analysis of the (100) cylindrical specimen data assumed the maximum measured hardness to be the [OOl] direction and the minimum to be the [Oil] direction. Since the (111) plane showed no orientation dependence, the random starting point on the cylindrical specimens was chosen as the 0” position.
3. Results Fig. 1 shows the nearly identical hardness versus load response of the hot-pressed polycrystalline ce-
CA. Black et al./Joumal
1098
of Nuclear Materials 212-215
(1994) 1096-1100 (100) Plane
(111) Plane WOgm
100 gm 2000
1600
1600 I
1
I
I
\
unlrradlated
/
I
200 gm
9
’
d 1200 t 0 2 2000 I
unirradiated
200 gm
1600
1600
B
i( /I
1200
0
22.5
45
I
/
67.6
90
1200
/ 0
I
I
/
/
22.6
45
67.5
90
Angle, degrees
Angle, degrees
Fig. 3. Changes in microhardness induced in single crystal specimens irradiated to 2.2 x 10z2 n cmm2 (E > 0.1 MeV) at 385°C. Solid circles denote measurements on irradiated specimens.
ramic spine1 and the [ill] single crystal specimens in the unirradiated condition. A similar dependence on load was also observed as the specimens hardened with irradiation, as shown in Figs. 2 and 3. Note in Fig. 2 that a saturation level of hardness was reached in the hot-pressed specimens irradiated at 385°C for each load level, but the duration of the transient regime of hardening was found to be load-dependent, being slower to saturate at the higher load. Similar levels of hardening were observed in the
single crystal specimens irradiated at 385”C, as shown in Figs. 3 and 4. In the unirradiated condition, the [lOO] specimens exhibited the same dependence on crystalline orientation that was observed earlier by Akimune and Bradt ill]. The degree of anisotropy of the orientation dependence on the (1001 plane was observed to decrease as the specimens hardened dur-
2000
1600 Hardness,
4.6 X 1022 n cma
I
I
I
I
I 100 gram
1200 Kg/mm’ 2000
1600
24.9 x 1 022 n cmv2 I I I 0 22.5 46
I 67.5
I 90
Angie, degrees Fig. 4. Microhardness versus orientation for higher exposure levels at 385°C.
on the (100) plane
I
0
1
22.5
45
67.5
‘__ _~
90
Angle, degrees
Fig. 5. Hardening and softening observed in single crystal specimens irradiated at 750°C and tested with a 100 g load.
C.A. Black et al. /Journal
of Nuclear Materials 212-215
200 gram
N
1600
a
I
I
Y” 1200 z? 2000- 13.7 x 1O** n cm-*
I
I
I 200 gram
6 5 I 1600 -
0
22.5
45
67.5
90
Angle, degrees
Fig. 6. Microhardness versus (100) orientation dependence for single crystal specimens irradiated at 75O”C,observed using a 200 g load.
ing irradiation.
The majority of the hardening occurred during the first irradiation increment. In addition, there is some suggestion that a second minimum in hardness versus orientation was developing at the highest exposure level, suggesting that a secondary slip system may have been activated. At an irradiation temperature of 75o”C, the microhardness first increased with irradiation and then subsequently decreased, as shown in Fig. 5. Even more important, however, the orientation dependence on the (100) plane was not reduced as for irradiation at 385°C. The tendency to develop a secondary minimum appeared to be stronger for irradiation at 75o”C, especially when higher indentation test loads were applied, as shown in Fig. 6.
(1994) 1096-1100
1099
isting grain boundaries and random grain orientations in the polycrystals seems to have produced a somewhat softer material before irradiation. However, the sink strength for point defects associated with these grain boundaries may also compete with the self-healing process and may therefore not allow the radiation-induced hardening to relax as quickly as was observed in the single crystals. The loss of the orientation dependence on the (100) plane at 385°C can be understood in terms of radiation-induced formation of defect aggregates that block the movement of dislocations on the primary slip system. The possible activation of a secondary slip system may have been facilitated by the development of radiation-induced dislocation microstructure. Microscopy analysis of these specimens at the highest fluence levels is in progress. Early results at 385°C show formation of high density of $ Ill01 dislocation loops (15 to 70 nm diameter) and tangled dislocations [12]. At 75o”C, there is a small number of voids (u 7 nm diameter) and extended i [llOl stacking faults. The substantially lower density of radiation-induced microstructure at 75O”C, compared to that at 385”C, probably explains why the orientation dependence of hardness on the (100) plane persists to very high fluences. The relatively small effect of radiation in hardening of this material at such high neutron exposure levels confirms its overall radiation resistance and its suitability for fusion use. As noted in Ref. [lo], the elastic properties and dimensional stability of these specimens were also unaffected by radiation.
1500 365 “C irradiation
4. Discussion Fig. 7 shows the calculated true hardness values derived from the data generated at 100 and 200 g loads. The tendency to first harden and then soften with increasing exposure occurred for all three types of specimens, with the largest changes observed in the hot-pressed polycrystals at 385°C. In each case, the radiation-induced increases in microhardness were relatively small and of no significance for fusion application. The softening may be evidence for a self-healing process that may be unique to this spine1 and is operating throughout the irradiation to limit the cumulative damage. It is interesting to note that the microhardness of the polycrystalline ceramics at 385°C appears to be approaching that of single crystals. The role of pre-ex-
NeutronFluence, n/c& (b0.1 MeV) Fig. 7. Influence of radiation on “true” hardness determined from the slope of a P/d versus d graph for 100 and 200 g.
1100
CA. Black et al. /Journal of Nuclear Materials 212-215 (1994) 1096-1100
5. Conclusions High temperature irradiation of high-purity stoichiometric MgAl,O, spine1 to very high neutron exposures induces a small amount of hardening that does not impair its application to fusion needs. The hardening peaks early in the irradiation and then relaxes somewhat at higher exposure levels. The details of the hardening process are somewhat sensitive to applied load, crystal orientation, irradiation temperature, and the exposure level. The dimensional and mechanical stability to displacements levels as large as 250 dpa suggests that an efficient self-healing process that may not operate in other ceramics may be operating in this material.
Acknowledgement This work was funded by the US Department of Energy, Office of Fusion Energy. Pacific Northwest Laboratory is operated for the US Department of Energy by Battelle Memorial Institute under Contract DE-AC06-76RL0 1830. The work of C.A. Black and R.C. Bradt was sponsored by the Northwest College and University Association for Science under US Department of Energy Contract DE-FG06-89ER-75522.
References [ll G.F. Hurley and J.M. Bunch, Am. Ceram. Sot. Bull. 59 (1980) 456.
w F.W. Clinard Jr., G.F. Hurley and L.W. Hobbs, J. Nucl. Mater. 108 & 109 (1982) 655. [31 G.F. Hurley, J.C. Kennedy, F.W. Clinard, Jr., R.A. Youngman and W.R. McDoneIl, J. Nucl. Mater. 103 & 104 (1981) 761. [41 F.W. Clinard Jr., G.F. Hurley and R.W. Klaffky, Res. Mech. 8 (1983) 207. 61 F.W. Clinard Jr., G.F. Hurley, L.W. Hobbs, D.L. Rohr and R.A. Youngman, J. Nucl. Mater. 122 & 123 (1984) 1386. kl F.W. Clinard Jr., G.F. Hurley, R.A. Youngman and L.W. Hobbs, J. Nucl. Mater. 133 & 134 (1985) 701. [71 C.A. Parker, L.W. Hobbs, K.C. Russell and F.W. Clinard Jr., J. Nucl. Mater. 133 & 134 (1985) 741. Bl Y. Fukushima, T. Yano, T. Maruyama and T. Iseki, J. Nucl. Mater. 175 (1990) 203. [91 H. Suematsu, T. Iseki, T. Yano, Y. Saito, T. Suzuki and T. Mori, J. Am. Ceram. Sot. 75 (1992) 1742. WI F.A. Gamer, G.W. Hollenberg, J.L. Ryan, Z. Li, C.A. Black and R.C. Bradt, in these Proceedings (ICFRM-6), J. Nucl. Mater. 212-215 (1994). Dll Y. Akimune and R.C. Bradt, J. Am. Ceram. Sot. (1987) C84. WI C. Kinoshita, K. Fukumoto and F.A. Garner, Proc. 4th Symp. on the Fabrication and Properties of Ceramics for Fusion Energy, Indianapolis, IN, April 24-28, 1994, J. Nucl. Mater., in press.