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Heavy fast neutron irradiation effects in Al-Mg-Li alloys
Journal of Nuclear Materials 155 - 157 (1%X) YY6
996
IO00
North-Holland. Amsterdam
HEAVY FAST NEUTRON I. YOSHIZAWA’,
IRRADIATION
K. KAMADA
EFFECTS
2, H. KAYANO
IN Al-Mg-Li
‘, Y. KATANO
ALLOYS 4 and H. KATSUTA
4
’ Department of Scrence, Faculty of Education, Iharakl University, Mito 310, Japan ’ Insiltute of Plasma Physics, Nagoya University, Nagoya 464, Japan ’ Oarai Branch, The Research Institute for Iron, Steel and Other Metals, Tohoku Umversity. Oarai 31 l-13, Japan 4 Japan Atomic Energy Research Institute, Tokai 319-I 1, Japan The changes in the mechanical properties of Al-4.1 wt% Mg-1. 1 wt% Li alloys and the radiation induced microstructures after fast neutron irradiation have been studied. The irradiation was carried out at about 400 o C up to a fluence of 1.0 x 1O25 n/ m*. The results of tensile tests showed increases in proof stress, tensile strength and elongation in comparison with control samples annealed at 400° C for 200 h. Transmission electron microscope (TEM) observations revealed small b’-phase precipitates and radiation-induced interstitial loops with a diamond shape and Burgers vector (i) (llO), respectively. It was concluded that the loops are responsible for the increases of mechanical strengths, and the change of size of S’-precipitates correlates with the increase of elongation.
1. Introduction Aluminum alloys have been considered as one of the candidate materials for future D-T burning devices because of these low induced radioactivity after 14 MeV neutron irradiation [l-3]. For practical applications, it is very important to clarify the changes of mechanical properties after fast neutron irradiation, and also to characterize the irradiation-induced defects which are responsible for such changes. In a previous paper [3], we reported that no appreciable changes in proof stress, tensile strength and elongation were observed in Al-Mg-Li alloys after 14 MeV neutron irradiation in RTNS-II with neutron fluence up to 4 X 1O22 n/m* at room temperatures. The present paper reports the changes of mechanical properties and results of observations by transmission electron microscope (TEM) of Al-4.1 wt% Mg-1.1 wt% Li alloys after fast neutron irradiation in JOY0 with a neutron fluence up to 1.0 X 1O25 n/m* at 400°C.
Fuel Development Corporation. The irradiation was performed for 1146 h in a vacuum tight vessel filled with He gas after pumping. Neutron fluences were about 1.1 x 1O24 and 1.0 X 1O25 n/m’, z 0.1 MeV. The irradiation temperatures estimated from the amount of gas flow were 412 + 13” C and 402 + 6 o C for high fluence and low fluence specimens, respectively. The reliability of these temperatures is not so high. After the neutron irradiation, tensile tests were carried out on an Instron type tensile testing machine, Shimadzu Autograph DSC 500, at room temperatures with a strain rate of 6.67 x 1O-4,‘s. TEM observations were performed by use of a JEOL 200CX electron microscope with an accelerating voltage of 200 kV. The specimens were thinned in a mixture of 20% perchloric acid in ethanol at O’C using a tenupol apparatus at 20 V and 50 mA. 3. Results
3.1. Mechanical
Properties
2. Experimental The fabrication of the Al-Mg-Li alloys, the physical properties and microstructures after processing, and impurity contents of these alloys have been reported in detail in previous papers [1,2]. The alloy selected in the present study is Al-4.1 wt% Mg-1.1 wt% Li, because the precipitate size was suitable for observation in the TEM. The alloy was cold rolled and aged at 360 o C for 1 h. After chemical polishing, tensile specimens were cut along the rolling direction into dimension of 12.5 mm length, 2.3 mm width and 0.13 mm thickness. The gauge section had a length of 5 mm and a width of 1.2 mm. Hereafter, these specimens are called “mini tensiles”. TEM disks with 3 mm diameter were also obtained from the same sheet. Both mini tensiles and TEM disks were irradiated with fast neutron using JOY0 nuclear reactor facility managed by Power Reactor and Nuclear 0022-3115/88/$03.50 Q Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Fig. 1 shows the stress-elongation curves of Al-4.1 wt% Mg-1.1 wt% Li alloys deformed at room temperature after annealing at 400 o C for 200 h (B), and after irradiation with fast neutrons to a fluence of 1.1 X 1O24 n/m2 (A), respectively. As seen in this figure, the proof stress, tensile strength and elongation of the specimen irradiated to 1.1 x 1O24 n/m2 were higher than those of the unirradiated specimen annealed at 400 o C for 200 h. As seen in fig. 1, serrations were observed on the stress-elongation curve after neutron irradiation at 400’ C. Similar serrated flow was also observed in unirradiated alloys, the magnitude and number of the serrations decreasing with increasing annealing time at 400 o c. Fig. 2A shows the changes of the tensile strength and 0.2% proof stress of unirradiated specimen on annealing time at 400 o C. The fluence dependence for the irradiated alloys is also shown. Fig. 2B, on the other hand,
I. Yoshirawa et al. / irradiation effects in Al-Mg-
s \
26-
(A)
--
~AN~ALED AT 4a
/NEUTRON-IRRADIATj AT 400°C .__i !-
200
j li__ r E
997
Li allays
< al
5
E
Elongation I%
LargePrecipitate
100’
Fig. 1. Typical stress-elongation curves of Al-4.1 wt% Mg-1.1 wt% Li alloys deformed at room temperature. (A) Irradiated with 1.1X 1O24 n/m2 at 400 o C; (B) annealed at 400 o C for
z 2
*
65
200 h.
161
0 shows the dependence of density of three kinds of defects observed in this alloy on the same annealing time and the neutron fluence. In figs. 2 and 3, mechanical properties such as tensile strength, 0.2% proof stress and elongation, and defect characters such as the density and size observed with TEM are shown as functions of the annealing time for
50 too 150 AnnealingTimelhr
I
200 lo24 IOZ5 Fluence/n/m*
Fig. 3. The relationships of elongation % versus annealing time or neutron fluence (A), and the size of precipitates vs annealing time or neutron fluence (B).
unirradiated specimens, and also as functions of the neutron fluence. These figures enable the mutual relationship to be established between each mechanical property and a particular type of defects. 3.2. TEM observation
lO'e
0
I
I
/
50 100 150 Annealing Time/ hr
In
200
I
1
to'" to"" Fluence/n/m'
Fig. 2. The relationships of tensile strength and 0.2% proof stress versus annealing time or neutron fluence (A), and densities of precipitates versus annealing time or neutron fluence (B).
Fig. 4 shows typical examples of TEM micrographs of as-received specimen (A), after annealing at 400 * C for 200 hours (B), after the irradiation at 400 o C to the fluences of 1.0 x 1O24 n/m’ (C) and 1.1 x 10” n/m* (D). For both unirradiated and irradiated specimens, both small spherical shape precipitates and large irregular shape precipitates were observed as indicated by symbols (S) and (L) in the figures. Precipitates of irregular shape are analyzed by using energy loss spectrometer of JEOL, JEM-2OOOFX-EDX. However, elements contained in the precipitates could not be identified. Fig. 5 shows selected area electron diffraction patterns taken from the small precipitates of the spherical shape in fig. 4A. It is obvious from weak spots with indexes of diffraction patterns, OOi, li0, etc, that these precipitates have an ordered lattice structure. The lattice parameter calculated using an Au standard sample was 0.4040 nm. This value almost agrees with the value of 0.40418 nm (at 20 * C) observed in 8 ‘-phase by Sung et al. [4]. It is also well-known that 6’-phase forms spherical particles [5]. Therefore, it is concluded that these precipitates are &‘-phase of Al,Li with structure of Ll, type. Figs. 6A and 6B show a high magnification micro-
Fig. 4. TEM micrographs of Al-4.1 wt% Mg-1.1 wt% Li alloys. (A) As-received, g = 200; (B) annealed at 400 o C for 200 h. g = 200; (C) irradiated to 1.1 x lO24 n/m*. g = 200; (D) 1.0 X 1O25n/m’. g = 200.
graph of the spherical precipitates and its high resolution lattice image, respectively. In fig. 6B Moire patterns with a spacing of 0.57 nm originating in the overlapping of 6’-phase and matrix, and also lattice images of [200] and [020] directions with 0.204 nm spacing are observable. From this lattice image and the Moire patterns, it is concluded that the coherency between matrix and precipitate is very good. It is also
Fig. 5. A selected area diffraction pattern from small precipitates in fig. 4A. The electron beam direction was near [llO].
concluded that S’-phase precipitate and matrix phase have the relation of &‘-phase (lOO)//matrix (100). The spacings between Moire fringes expected from the calculation is 0.57 nm. This value agrees with the measured value. It should be emphasized that the most significant difference between unirradiated and irradiated specimens is the generation of dislocations in the latter. This is illustrated in figs. 4C and 4D and fig. 7. Dislocations induced by the neutron irradiation were loops of diamond shape. The character of these loops was determined from the condition of g. b = 0, from the change of loop shape when the specimen is tilted and from the difference of contrast when changed the vector, S, indicating the deviation from the Bragg’s condition [6]. It was concluded by use of these conditions that the diamond-shaped loops lie on (110) plane and are perfect loops with the magnitude of Burgers vector, b = (i) (110). It was determined from the third condition that they are the dislocation loops of interstitial type which are surrounded by edges with (112) directions. Figs. 7A and 7B show the outside and inside contrasts, respectively, of the diamond-shaped loops as indicated by arrows. In these photographs the diffraction vectors were changed from g = 200 to g = %O with the same s > 0.
I. Yoshirawa et al. / Irradiation effects in AI-Mg-Li
4. Discussion
As seen in figs. 2A and 3A, it is evident that the increase in tensile strength and proof stress, even elongation increase, caused by irradiation is greater than the changes after annealing at 400°C for 200 h. However, contary to expectations an increase in neutron fluence by a factor of ten did not cause a further increase of the parameters. This may suggest that small changes in irradiation temperature have a significant effect on the
Fig. 6. Spherical small S/-phase precipitates,
999
alloys
mechanical properties. The neutron irradiation temperature for the high fluence irradiations was slightly higher than that of low fluence. Recovery of irradiation-induced defects and the decrease of precipitates density could proceed, as seen in fig. 2B, in the high fluence specimens. This is one possible explanation for the surprisingly small effect of irradiation fluence on the strength and elongation. From fig. 2B, it is very clear that the density of precipitates decreases with increasing annealing time.
g = 200 (A), and its TEM high resolution was near [OOl].
lattice image (B). The electron
beam direction
,20Onm,
Fig. 7. Microstructures of diamond-shaped loops of Al-4.1 I Mg-1.1 % Li alloy irradiated with fast neutrons to 1.1 x 10z4 n/m2 400 o C. The electron beam direction was near [013]. (A) Diffracting vector g = 200, (B) g = 200.
at
1000
I. Yoshmwa
et al. / Irradiatton
The increase of density in 6’-phase precipitates is generally believed to be responsible for the increase of both proof stress and tensile strength [7]. Conversely, the decreases of proof stress and tensile strength as seen in fig. 2A should be correlated with the decreases of S’precipitates shown in fig. 2B. The densities of small and large precipitates after 200 h annealing are almost identical to that after neutron-irradiation to a fluence of 1.1 X 1O24 n/m2 as seen in fig. 2B. However, the proof stress and strength after the neutron irradiation show higher values as seen in fig. 2A. This increase correlates with the generation of diamond-shaped loops introduced by the neutron irradiation. Loomis and Gerber [8] have demonstrated that the increase of strength due to the neutron irradiation is explained in terms of the interaction between dislocation loops and mobile dislocation. The slight decrease of the strength and the proof stress with increasing neutron fluence as seen in fig. 2A is explained by the decreases of densities of the precipitate and the diamond-shaped loops. The decrease of their densities for higher fluence may be due to the difference of neutron-irradiation temperature. As mentioned already, samples for higher fluence were situated close to the center of reactor during the neutron irradiation. Therefore, the temperature is about 10’ C higher than that of the low dose-irradiation. This might promote the recovery of the loops and precipitates. Generally speaking, the decrease of strength should be associated with the increase of the elongation. However, in the present results both strength and elongation for unirradiated specimens decrease on the annealing
effectsrn AI- Mg-LI ulloy.~ time, as seen in figs. 2A and 3A. The elongation may be correlated with the changes of the size of precipitates. That is. the increase of size in 8’-phase may predominantly contribute to the decrease of the elongation. As seen in fig. 3B, the size of large precipitates after annealing at 400 o C for 200 h is almost identical to those after neutron irradiations to fluences of 1.1 x 1O24 and 1.0 X 1O25 n/m2. However, the size of the 6’-phase is smaller in comparison with that after the annealing for 200 h. Therefore, the increase of elongations after the irradiation can be interpreted in terms of the decrease of the size of S’-phase precipitates. References [l] K. Kamada, Y. Baba, T. Uno, H. Yoshida and Y. Shoji, J. Nucl. Mater. 122 & 123 (1984) 845. [2] K. Kamada, H. Kayano, Y. Baba, T. Uno, H. Yoshida, and Y. Shoji, J. Nucl. Mater. 133 & 134 (1985) 897. [3] K. Abe, I. Yoshizawa, K. Kamada and H. Kayano, J. Nucl. Mater. 141-143 (1986) 915. [4] C.M. Sung, H.M. Chan and D.B. Williams, in: Alminium-Lithium Alloys, Vol. III, Eds. C. Baker, P.J. Gregson, S.J. Harris and C.J. Peel (The Institute of Metals, London, 1986) p. 337. [5] D.B. Williams, in: Aluminium-Lithium Alloys, Eds. T.H. Sanders, Jr. and E.A. Starke, Jr. (Met. Sot. AIME, 1981) p.89. [6] M.H. Loretto and R.E. Smallman, Defect Analysis in Electron Microscopy (Chapman and Hall, London. 1975) p.75 [7] K. Dinsdale, S.J. Harris and B. Noble, in: AluminiumLithium alloys, Eds. T.H. Sanders, Jr. and E.A. Starke. Jr. (Met. Sot. AIME, 1981) p. 101. [8] B.A. Loomis and S.B. Gerber, Acta Metall. 21 (1973) 165.
996
IO00
North-Holland. Amsterdam
HEAVY FAST NEUTRON I. YOSHIZAWA’,
IRRADIATION
K. KAMADA
EFFECTS
2, H. KAYANO
IN Al-Mg-Li
‘, Y. KATANO
ALLOYS 4 and H. KATSUTA
4
’ Department of Scrence, Faculty of Education, Iharakl University, Mito 310, Japan ’ Insiltute of Plasma Physics, Nagoya University, Nagoya 464, Japan ’ Oarai Branch, The Research Institute for Iron, Steel and Other Metals, Tohoku Umversity. Oarai 31 l-13, Japan 4 Japan Atomic Energy Research Institute, Tokai 319-I 1, Japan The changes in the mechanical properties of Al-4.1 wt% Mg-1. 1 wt% Li alloys and the radiation induced microstructures after fast neutron irradiation have been studied. The irradiation was carried out at about 400 o C up to a fluence of 1.0 x 1O25 n/ m*. The results of tensile tests showed increases in proof stress, tensile strength and elongation in comparison with control samples annealed at 400° C for 200 h. Transmission electron microscope (TEM) observations revealed small b’-phase precipitates and radiation-induced interstitial loops with a diamond shape and Burgers vector (i) (llO), respectively. It was concluded that the loops are responsible for the increases of mechanical strengths, and the change of size of S’-precipitates correlates with the increase of elongation.
1. Introduction Aluminum alloys have been considered as one of the candidate materials for future D-T burning devices because of these low induced radioactivity after 14 MeV neutron irradiation [l-3]. For practical applications, it is very important to clarify the changes of mechanical properties after fast neutron irradiation, and also to characterize the irradiation-induced defects which are responsible for such changes. In a previous paper [3], we reported that no appreciable changes in proof stress, tensile strength and elongation were observed in Al-Mg-Li alloys after 14 MeV neutron irradiation in RTNS-II with neutron fluence up to 4 X 1O22 n/m* at room temperatures. The present paper reports the changes of mechanical properties and results of observations by transmission electron microscope (TEM) of Al-4.1 wt% Mg-1.1 wt% Li alloys after fast neutron irradiation in JOY0 with a neutron fluence up to 1.0 X 1O25 n/m* at 400°C.
Fuel Development Corporation. The irradiation was performed for 1146 h in a vacuum tight vessel filled with He gas after pumping. Neutron fluences were about 1.1 x 1O24 and 1.0 X 1O25 n/m’, z 0.1 MeV. The irradiation temperatures estimated from the amount of gas flow were 412 + 13” C and 402 + 6 o C for high fluence and low fluence specimens, respectively. The reliability of these temperatures is not so high. After the neutron irradiation, tensile tests were carried out on an Instron type tensile testing machine, Shimadzu Autograph DSC 500, at room temperatures with a strain rate of 6.67 x 1O-4,‘s. TEM observations were performed by use of a JEOL 200CX electron microscope with an accelerating voltage of 200 kV. The specimens were thinned in a mixture of 20% perchloric acid in ethanol at O’C using a tenupol apparatus at 20 V and 50 mA. 3. Results
3.1. Mechanical
Properties
2. Experimental The fabrication of the Al-Mg-Li alloys, the physical properties and microstructures after processing, and impurity contents of these alloys have been reported in detail in previous papers [1,2]. The alloy selected in the present study is Al-4.1 wt% Mg-1.1 wt% Li, because the precipitate size was suitable for observation in the TEM. The alloy was cold rolled and aged at 360 o C for 1 h. After chemical polishing, tensile specimens were cut along the rolling direction into dimension of 12.5 mm length, 2.3 mm width and 0.13 mm thickness. The gauge section had a length of 5 mm and a width of 1.2 mm. Hereafter, these specimens are called “mini tensiles”. TEM disks with 3 mm diameter were also obtained from the same sheet. Both mini tensiles and TEM disks were irradiated with fast neutron using JOY0 nuclear reactor facility managed by Power Reactor and Nuclear 0022-3115/88/$03.50 Q Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Fig. 1 shows the stress-elongation curves of Al-4.1 wt% Mg-1.1 wt% Li alloys deformed at room temperature after annealing at 400 o C for 200 h (B), and after irradiation with fast neutrons to a fluence of 1.1 X 1O24 n/m2 (A), respectively. As seen in this figure, the proof stress, tensile strength and elongation of the specimen irradiated to 1.1 x 1O24 n/m2 were higher than those of the unirradiated specimen annealed at 400 o C for 200 h. As seen in fig. 1, serrations were observed on the stress-elongation curve after neutron irradiation at 400’ C. Similar serrated flow was also observed in unirradiated alloys, the magnitude and number of the serrations decreasing with increasing annealing time at 400 o c. Fig. 2A shows the changes of the tensile strength and 0.2% proof stress of unirradiated specimen on annealing time at 400 o C. The fluence dependence for the irradiated alloys is also shown. Fig. 2B, on the other hand,
I. Yoshirawa et al. / irradiation effects in Al-Mg-
s \
26-
(A)
--
~AN~ALED AT 4a
/NEUTRON-IRRADIATj AT 400°C .__i !-
200
j li__ r E
997
Li allays
< al
5
E
Elongation I%
LargePrecipitate
100’
Fig. 1. Typical stress-elongation curves of Al-4.1 wt% Mg-1.1 wt% Li alloys deformed at room temperature. (A) Irradiated with 1.1X 1O24 n/m2 at 400 o C; (B) annealed at 400 o C for
z 2
*
65
200 h.
161
0 shows the dependence of density of three kinds of defects observed in this alloy on the same annealing time and the neutron fluence. In figs. 2 and 3, mechanical properties such as tensile strength, 0.2% proof stress and elongation, and defect characters such as the density and size observed with TEM are shown as functions of the annealing time for
50 too 150 AnnealingTimelhr
I
200 lo24 IOZ5 Fluence/n/m*
Fig. 3. The relationships of elongation % versus annealing time or neutron fluence (A), and the size of precipitates vs annealing time or neutron fluence (B).
unirradiated specimens, and also as functions of the neutron fluence. These figures enable the mutual relationship to be established between each mechanical property and a particular type of defects. 3.2. TEM observation
lO'e
0
I
I
/
50 100 150 Annealing Time/ hr
In
200
I
1
to'" to"" Fluence/n/m'
Fig. 2. The relationships of tensile strength and 0.2% proof stress versus annealing time or neutron fluence (A), and densities of precipitates versus annealing time or neutron fluence (B).
Fig. 4 shows typical examples of TEM micrographs of as-received specimen (A), after annealing at 400 * C for 200 hours (B), after the irradiation at 400 o C to the fluences of 1.0 x 1O24 n/m’ (C) and 1.1 x 10” n/m* (D). For both unirradiated and irradiated specimens, both small spherical shape precipitates and large irregular shape precipitates were observed as indicated by symbols (S) and (L) in the figures. Precipitates of irregular shape are analyzed by using energy loss spectrometer of JEOL, JEM-2OOOFX-EDX. However, elements contained in the precipitates could not be identified. Fig. 5 shows selected area electron diffraction patterns taken from the small precipitates of the spherical shape in fig. 4A. It is obvious from weak spots with indexes of diffraction patterns, OOi, li0, etc, that these precipitates have an ordered lattice structure. The lattice parameter calculated using an Au standard sample was 0.4040 nm. This value almost agrees with the value of 0.40418 nm (at 20 * C) observed in 8 ‘-phase by Sung et al. [4]. It is also well-known that 6’-phase forms spherical particles [5]. Therefore, it is concluded that these precipitates are &‘-phase of Al,Li with structure of Ll, type. Figs. 6A and 6B show a high magnification micro-
Fig. 4. TEM micrographs of Al-4.1 wt% Mg-1.1 wt% Li alloys. (A) As-received, g = 200; (B) annealed at 400 o C for 200 h. g = 200; (C) irradiated to 1.1 x lO24 n/m*. g = 200; (D) 1.0 X 1O25n/m’. g = 200.
graph of the spherical precipitates and its high resolution lattice image, respectively. In fig. 6B Moire patterns with a spacing of 0.57 nm originating in the overlapping of 6’-phase and matrix, and also lattice images of [200] and [020] directions with 0.204 nm spacing are observable. From this lattice image and the Moire patterns, it is concluded that the coherency between matrix and precipitate is very good. It is also
Fig. 5. A selected area diffraction pattern from small precipitates in fig. 4A. The electron beam direction was near [llO].
concluded that S’-phase precipitate and matrix phase have the relation of &‘-phase (lOO)//matrix (100). The spacings between Moire fringes expected from the calculation is 0.57 nm. This value agrees with the measured value. It should be emphasized that the most significant difference between unirradiated and irradiated specimens is the generation of dislocations in the latter. This is illustrated in figs. 4C and 4D and fig. 7. Dislocations induced by the neutron irradiation were loops of diamond shape. The character of these loops was determined from the condition of g. b = 0, from the change of loop shape when the specimen is tilted and from the difference of contrast when changed the vector, S, indicating the deviation from the Bragg’s condition [6]. It was concluded by use of these conditions that the diamond-shaped loops lie on (110) plane and are perfect loops with the magnitude of Burgers vector, b = (i) (110). It was determined from the third condition that they are the dislocation loops of interstitial type which are surrounded by edges with (112) directions. Figs. 7A and 7B show the outside and inside contrasts, respectively, of the diamond-shaped loops as indicated by arrows. In these photographs the diffraction vectors were changed from g = 200 to g = %O with the same s > 0.
I. Yoshirawa et al. / Irradiation effects in AI-Mg-Li
4. Discussion
As seen in figs. 2A and 3A, it is evident that the increase in tensile strength and proof stress, even elongation increase, caused by irradiation is greater than the changes after annealing at 400°C for 200 h. However, contary to expectations an increase in neutron fluence by a factor of ten did not cause a further increase of the parameters. This may suggest that small changes in irradiation temperature have a significant effect on the
Fig. 6. Spherical small S/-phase precipitates,
999
alloys
mechanical properties. The neutron irradiation temperature for the high fluence irradiations was slightly higher than that of low fluence. Recovery of irradiation-induced defects and the decrease of precipitates density could proceed, as seen in fig. 2B, in the high fluence specimens. This is one possible explanation for the surprisingly small effect of irradiation fluence on the strength and elongation. From fig. 2B, it is very clear that the density of precipitates decreases with increasing annealing time.
g = 200 (A), and its TEM high resolution was near [OOl].
lattice image (B). The electron
beam direction
,20Onm,
Fig. 7. Microstructures of diamond-shaped loops of Al-4.1 I Mg-1.1 % Li alloy irradiated with fast neutrons to 1.1 x 10z4 n/m2 400 o C. The electron beam direction was near [013]. (A) Diffracting vector g = 200, (B) g = 200.
at
1000
I. Yoshmwa
et al. / Irradiatton
The increase of density in 6’-phase precipitates is generally believed to be responsible for the increase of both proof stress and tensile strength [7]. Conversely, the decreases of proof stress and tensile strength as seen in fig. 2A should be correlated with the decreases of S’precipitates shown in fig. 2B. The densities of small and large precipitates after 200 h annealing are almost identical to that after neutron-irradiation to a fluence of 1.1 X 1O24 n/m2 as seen in fig. 2B. However, the proof stress and strength after the neutron irradiation show higher values as seen in fig. 2A. This increase correlates with the generation of diamond-shaped loops introduced by the neutron irradiation. Loomis and Gerber [8] have demonstrated that the increase of strength due to the neutron irradiation is explained in terms of the interaction between dislocation loops and mobile dislocation. The slight decrease of the strength and the proof stress with increasing neutron fluence as seen in fig. 2A is explained by the decreases of densities of the precipitate and the diamond-shaped loops. The decrease of their densities for higher fluence may be due to the difference of neutron-irradiation temperature. As mentioned already, samples for higher fluence were situated close to the center of reactor during the neutron irradiation. Therefore, the temperature is about 10’ C higher than that of the low dose-irradiation. This might promote the recovery of the loops and precipitates. Generally speaking, the decrease of strength should be associated with the increase of the elongation. However, in the present results both strength and elongation for unirradiated specimens decrease on the annealing
effectsrn AI- Mg-LI ulloy.~ time, as seen in figs. 2A and 3A. The elongation may be correlated with the changes of the size of precipitates. That is. the increase of size in 8’-phase may predominantly contribute to the decrease of the elongation. As seen in fig. 3B, the size of large precipitates after annealing at 400 o C for 200 h is almost identical to those after neutron irradiations to fluences of 1.1 x 1O24 and 1.0 X 1O25 n/m2. However, the size of the 6’-phase is smaller in comparison with that after the annealing for 200 h. Therefore, the increase of elongations after the irradiation can be interpreted in terms of the decrease of the size of S’-phase precipitates. References [l] K. Kamada, Y. Baba, T. Uno, H. Yoshida and Y. Shoji, J. Nucl. Mater. 122 & 123 (1984) 845. [2] K. Kamada, H. Kayano, Y. Baba, T. Uno, H. Yoshida, and Y. Shoji, J. Nucl. Mater. 133 & 134 (1985) 897. [3] K. Abe, I. Yoshizawa, K. Kamada and H. Kayano, J. Nucl. Mater. 141-143 (1986) 915. [4] C.M. Sung, H.M. Chan and D.B. Williams, in: Alminium-Lithium Alloys, Vol. III, Eds. C. Baker, P.J. Gregson, S.J. Harris and C.J. Peel (The Institute of Metals, London, 1986) p. 337. [5] D.B. Williams, in: Aluminium-Lithium Alloys, Eds. T.H. Sanders, Jr. and E.A. Starke, Jr. (Met. Sot. AIME, 1981) p.89. [6] M.H. Loretto and R.E. Smallman, Defect Analysis in Electron Microscopy (Chapman and Hall, London. 1975) p.75 [7] K. Dinsdale, S.J. Harris and B. Noble, in: AluminiumLithium alloys, Eds. T.H. Sanders, Jr. and E.A. Starke. Jr. (Met. Sot. AIME, 1981) p. 101. [8] B.A. Loomis and S.B. Gerber, Acta Metall. 21 (1973) 165.