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Space solidification experiment on Gd3Co compound
Acta Astronautica Vol. 9, No. 8, pp. 487-492, 1982 Printed in Great Britain.
0094-5765/82/0804874)6503.0010 Pergamon Press Ltd.
SPACE SOLIDIFICATION EXPERIMENT ON Gd3Co COMPOUND E. M. SAVITSKY and R. S. TORCHINOVA A. A. Baikov Institute of Metallurgy,Leninsky Prospect 49-117911 Moscow V-334, U.S.S.R.
(Received 13 January 1982) Abstract---Onboard the orbital complex "Salyut 6" during longterm 0-gravity space flight crystallizationof the polycrystal samples of the intermetallic compound Gd3Co has been performed. The object of the present experiment was to investigate the effects of microgravity on the solidificationprocess, final macro- and microstructure and magneticproperties of antiferromagneticGd3Cocompound. The experiment included the meltingof Gd3Co under the isothermal or thermal gradient conditions followedby passive cooling.The change of meniscus form from the cave form which is characteristic of the ground-based samples to the concave form for the flight samples has been observed. The formation of the reaction layers has been found as a result of the mutual contact between liquid Gd3Co and Ta-container in all samples investigated.The temperature dependences of the magnetization with Neel peaks have been observed for the ground-basedand flight samples of GdaCo. Magnetization values of the flight samples were appreciablyhigher than those of the ground-basedsamples.
I. INTRODUCTION
During long-term near zero-gravity space flight the possibility appears to affect the solidification process and the final structure of the different kinds of materials in a desired direction. There are theoretical approaches indicating possible future developments of materials processing in space. But some special features of the influence of microgravity on the properties of the materials processed in space need to be revealed by means of comparative analysis of the ground-based and flight samples produced under identical conditions. There are various types of materials which can be of interest from this point of view[l--6]. They include certain potentially useful alloys which are difficult to synthesize on Earth, immicible alloy systems, multicomponent alloys consisting of the components with different physical properties, directionally solidified eutectics, chemical compounds which appear to be a real source of new materials. The attempts to produce in space magnetic materials with superior magnetic characteristics became of great interest over the past few years. The results of the first experiments demonstrated a substantial improvement in the chemical homogeneity and magnetic properties [7, 8]. 2. EXPERIMENTAL
The experiment on crystallization of the intermetallic compound Gd3Co has been performed on board the orbital complex "Salyut 6". Gd3Co is an example of a material with weak antiferromagnetic coupling. It has the highest magnetic moment (8.1t~a) and the transition temperature (130K) among the other compounds of R3Co type [9]. Gd3Co (88.9 wt.% Gd) is isostructural with Fe3C (orthorombic)[10] and formed by a peritectic reaction at 780°C [11]. The raw materials were the distilled gadolinium and electrowon cobalt of 99.9 and 99.99% purity, respectively. The initial alloys of stoichiometric composition have been prepared on Earth by arc melting on a watercooled copper hearth under a helium atmosphere of
8 x 104 Nm -2. Prior to their use, they were analyzed for gadolinium. The alloys were placed in the tantalum containers and sealed. Then, the containers were put into the quartz capsules. The capsules were evactiated and installed in the furnace. The tantalum containers were applied for protection of Gd3Co from contamination by quartz. The temperature in hot isothermal zone was 980°C and the alloy was in molten state during 18 hr. The temperature of the cold end of the capsule heated in a temperature gradient of approx. 60°C/cm was 670°C.
3. RF~ULTS AND DISCUSSION
We observed a specific surface meniscus in the solidified flight samples which was never observed in the ground-based samples (Fig. I). The type of surface morphology in ground-based samples indicated the wetting of the tantalum walls of the container by the melt Gd3Co during melting and crystallization. The surface meniscus of the ground-based sample is of cave form and the wetting angle is less than 90°. The samples solidified on Earth exhibited surface features that reflected in detail the inner walls of the tantalum containers. The surface meniscus of the samples crystallized in near zero-gravity conditions is of concave form. It appears that in space the tendency for the metal Gd3Co fluid to assume a lowest energy configuration prevails over the process of wetting of the Ta-surface and the character of the interface changes. It is obvious that only the cylinder form of the tantalum container and the volume of Gd~Co melt inhibited the formation of the ideal sphere form. We observed a great number of macro- and micropores in the flight samples in contrast to the ground-based samples (Figs. I, 2). The mean sizes of the pores formed varied from 40 to 250 #m. They are characterised by an ideal sphere form. As a result of the partial interaction of liquid Gd3Co with Ta-container the reaction layer was formed along the Ta-surface (Fig. 3). The thickness of the diffusion layer in microns obtained from micrographs was 3-5 487
488
E.M. SAVlTSKYand R. S. TORCHINOVA
Fig. 1. Macrographs of metallographicallypolished longitudinalsections of the ground-based (up) and two flight (down) samples of Gd3Cocompound contained in tantalum. The containers were put vertically, x 2.0.
microns for the flight samples compared to 1-3 microns for the samples crystallized on Earth. In both cases the diffusion layers consisted of the Ta-Co and Gd-Co phases (see line profiles of Co, Gd and Ta in Fig. 3). The composition of the phases varied a little. It seems that a complex structure of the diffusion layer is emphasized by a scanning electron microscope image. In contrast to the groundbased samples only a partial type of contact between the layer formed and solidified Gd3Co was observed in the flight samples, while the diffusion layer was always present on the Ta-walls. An oscillating specimen magnetometer was used to measure the temperature dependences of magnetization of the ground-based and flight samples. The results of these measurements are presented in Fig. 4. The magnetic measurements are not completed up to the moment. In Fig. 4 the temperature dependences are characterized by pronounced Neel peaks on the curves of all samples corresponding to the transition from antiferromagnetic to paramagnetic state. The flight sample solidified in the thermal gradient zone is characterized by
the highest value of magnetization (curve 2) compared to the ground-based sample and the flight sample crystallized in hot isothermal zone of the furnace. In one of the previous experiments space solidification of Gd3Co was performed in quartz capsules instead of Ta-containers. The shift of the Neel peak from 131 to 128.5 K was observed for the flight sample, the magnetization peak being less sharp at the ordering temperature~ We observed that all flight samples investigated were characterized by certain broadening of Neel peaks and the magnetization values of the flight samples are higher than those of the ground-based samples at the ordering temperature region. The Neel peaks observed in low fields became less sharp for high fields. We observed the formation of Gd203 crystals in the form of well-developed single crystals on the surface of the macropore in one of the flight samples solidified directly in the quartz capsules (Fig. 5). Crystallographic perfection of the crystal grown in a microgravity environment is obvious. It seems that these crystals were condensed from a vapor phas e.
Space solidification experiment on Gd3Co compound
Fig. 2. Scanning electron micrograph of the pores in the flight sample of Gd3Co.
Fig. 3. Scanning electron micrographs of the diffusion layers formed in the flight samples along the Ta-surface. The line profiles of Co, Gd and Ta are shown.
489
0
o
Space solidification experiment on Gd3Co compound
491
40
_
~E
H = I 7 kOe
b
3 2O
~o[
L
L
1
0
-I00
I
-160
-140
-120
-I00 Tern p e r a t u r e ,
-80
-60
l
-40
--
-20
°C
Fig. 4. The temperature dependences of the magnetization of the ground-based (1) and flight (2, 3) samples.
Fig. 5. Scanning electron micrographs of space-grown Gd203 crystals on the surface of Gd3Coflight sample.
492
E.M. SAVITSKYand R. S. TORCHINOVA
Fig. 5. (cont.) 4. SUMMARYANDCONCLUSIONS The preliminary results concerning the space experiment on solidification of the antiferromagnetic Gd3Co compound are reported. Although the work is not yet complete, the following conclusions can be drawn. (1) The surface meniscus form in the solidified flight samples of Gd3Co was appreciably different than in the ground-based samples. (2) The formation of the reaction layers for the ground-based and flight samples consisted of Ta-Co and Gd-Co phases was observed. The thickness of the diffusion layers was greater for the flight samples. A partial contact between the solidified Gd3Co and Ta-container was observed in the flight samples. (3) The magnetic properties of Gd3Co are sensitive to the specific conditions of solidification in space. The flight samples investigated were characterized by certain broadening of Neel peaks. The magnetization values of the flight samples were considerably higher compared to the ground-based samples at the temperatures T ~< TNee~. REFERENCES
1, V. S. Avduijevsky, I. V. Barmin, S. D. Grishin, L. V. Leskov, A. M. Petrov, V. I. Polezhaev and V. V. Savichev Space Technology Problems. Mashinostroenie, Moscow (1980) (in Russian).
2. L. I. Ivanov, V. S. Zemskov, V. N. Kubasov, V. N. Pimenov, I. N. Belokurova, K. P. Gurov, E. V. Demina, A. N. Titkov and I. L. Shulpina Melting, Crystallization and Phase Formation in Space. Nauka, Moscow (1979) (in Russian). 3. E. M. Savitsky Problems of the Development o[ Physical Metallurgy. Nauka, Moscow (1972) (in Russian). 4. E. M. Savitsky, I. V. Burov, S. V. Pirogova and U. A. Saveliev The production of AI-Bi and AI-Mg pseudoalloys in low-gravity environment. Dokl. Akad. Nauk. USSR. 252(6), 1387-1389 (1980). 5. E. M. Savitsky, I. V. Burov, G. S. Burkhanov, B. P. Mikhailov, M. I. Bychkova, R. S. Torchinova, U. A, Saveliev, I. V. Vlasova and I. D. Giller, Investigation of the influence of near 0-gravity on the structure and physical properties of the alloys. Nauchnye chtenie po aviatzii i kosmonavtike. 288 (1981) (in Russian). 6. E. M. Savitsky, M. I. Bychkova and I. D. Giller, Crystallization of some superconducting compounds in space. Dokl. Akad. Nauk. USSR. 257(1) (1981). 7. D. J. Larson, Zero-gravity processing of magnets. ApolloSoyuz Test Project. Summary science Rept., NASA SP-412, 449-470 (1977). 8. P. Pant, Neuschutz, J. Potschke and H. Coenen Tech. Mitt. Krupp. Forsch.-Ber. Band (37), 69-78 (1979). 9. K. N. R. Taylor, Intermetallic Rare Earth Compounds. Mir, Moscow (1974) (in Russian). 10. K. H. J. Buschow and S. S. van der Goot, J. Less-Common Metals. 18, 309-311 (1969). 11. E. M. Savitsky, V. F. Terekhova and I. V. Burov, The Gd-Co phase diagram. J. Inorg. Chem. (USSR) 7, 1732-1735 (1962).
0094-5765/82/0804874)6503.0010 Pergamon Press Ltd.
SPACE SOLIDIFICATION EXPERIMENT ON Gd3Co COMPOUND E. M. SAVITSKY and R. S. TORCHINOVA A. A. Baikov Institute of Metallurgy,Leninsky Prospect 49-117911 Moscow V-334, U.S.S.R.
(Received 13 January 1982) Abstract---Onboard the orbital complex "Salyut 6" during longterm 0-gravity space flight crystallizationof the polycrystal samples of the intermetallic compound Gd3Co has been performed. The object of the present experiment was to investigate the effects of microgravity on the solidificationprocess, final macro- and microstructure and magneticproperties of antiferromagneticGd3Cocompound. The experiment included the meltingof Gd3Co under the isothermal or thermal gradient conditions followedby passive cooling.The change of meniscus form from the cave form which is characteristic of the ground-based samples to the concave form for the flight samples has been observed. The formation of the reaction layers has been found as a result of the mutual contact between liquid Gd3Co and Ta-container in all samples investigated.The temperature dependences of the magnetization with Neel peaks have been observed for the ground-basedand flight samples of GdaCo. Magnetization values of the flight samples were appreciablyhigher than those of the ground-basedsamples.
I. INTRODUCTION
During long-term near zero-gravity space flight the possibility appears to affect the solidification process and the final structure of the different kinds of materials in a desired direction. There are theoretical approaches indicating possible future developments of materials processing in space. But some special features of the influence of microgravity on the properties of the materials processed in space need to be revealed by means of comparative analysis of the ground-based and flight samples produced under identical conditions. There are various types of materials which can be of interest from this point of view[l--6]. They include certain potentially useful alloys which are difficult to synthesize on Earth, immicible alloy systems, multicomponent alloys consisting of the components with different physical properties, directionally solidified eutectics, chemical compounds which appear to be a real source of new materials. The attempts to produce in space magnetic materials with superior magnetic characteristics became of great interest over the past few years. The results of the first experiments demonstrated a substantial improvement in the chemical homogeneity and magnetic properties [7, 8]. 2. EXPERIMENTAL
The experiment on crystallization of the intermetallic compound Gd3Co has been performed on board the orbital complex "Salyut 6". Gd3Co is an example of a material with weak antiferromagnetic coupling. It has the highest magnetic moment (8.1t~a) and the transition temperature (130K) among the other compounds of R3Co type [9]. Gd3Co (88.9 wt.% Gd) is isostructural with Fe3C (orthorombic)[10] and formed by a peritectic reaction at 780°C [11]. The raw materials were the distilled gadolinium and electrowon cobalt of 99.9 and 99.99% purity, respectively. The initial alloys of stoichiometric composition have been prepared on Earth by arc melting on a watercooled copper hearth under a helium atmosphere of
8 x 104 Nm -2. Prior to their use, they were analyzed for gadolinium. The alloys were placed in the tantalum containers and sealed. Then, the containers were put into the quartz capsules. The capsules were evactiated and installed in the furnace. The tantalum containers were applied for protection of Gd3Co from contamination by quartz. The temperature in hot isothermal zone was 980°C and the alloy was in molten state during 18 hr. The temperature of the cold end of the capsule heated in a temperature gradient of approx. 60°C/cm was 670°C.
3. RF~ULTS AND DISCUSSION
We observed a specific surface meniscus in the solidified flight samples which was never observed in the ground-based samples (Fig. I). The type of surface morphology in ground-based samples indicated the wetting of the tantalum walls of the container by the melt Gd3Co during melting and crystallization. The surface meniscus of the ground-based sample is of cave form and the wetting angle is less than 90°. The samples solidified on Earth exhibited surface features that reflected in detail the inner walls of the tantalum containers. The surface meniscus of the samples crystallized in near zero-gravity conditions is of concave form. It appears that in space the tendency for the metal Gd3Co fluid to assume a lowest energy configuration prevails over the process of wetting of the Ta-surface and the character of the interface changes. It is obvious that only the cylinder form of the tantalum container and the volume of Gd~Co melt inhibited the formation of the ideal sphere form. We observed a great number of macro- and micropores in the flight samples in contrast to the ground-based samples (Figs. I, 2). The mean sizes of the pores formed varied from 40 to 250 #m. They are characterised by an ideal sphere form. As a result of the partial interaction of liquid Gd3Co with Ta-container the reaction layer was formed along the Ta-surface (Fig. 3). The thickness of the diffusion layer in microns obtained from micrographs was 3-5 487
488
E.M. SAVlTSKYand R. S. TORCHINOVA
Fig. 1. Macrographs of metallographicallypolished longitudinalsections of the ground-based (up) and two flight (down) samples of Gd3Cocompound contained in tantalum. The containers were put vertically, x 2.0.
microns for the flight samples compared to 1-3 microns for the samples crystallized on Earth. In both cases the diffusion layers consisted of the Ta-Co and Gd-Co phases (see line profiles of Co, Gd and Ta in Fig. 3). The composition of the phases varied a little. It seems that a complex structure of the diffusion layer is emphasized by a scanning electron microscope image. In contrast to the groundbased samples only a partial type of contact between the layer formed and solidified Gd3Co was observed in the flight samples, while the diffusion layer was always present on the Ta-walls. An oscillating specimen magnetometer was used to measure the temperature dependences of magnetization of the ground-based and flight samples. The results of these measurements are presented in Fig. 4. The magnetic measurements are not completed up to the moment. In Fig. 4 the temperature dependences are characterized by pronounced Neel peaks on the curves of all samples corresponding to the transition from antiferromagnetic to paramagnetic state. The flight sample solidified in the thermal gradient zone is characterized by
the highest value of magnetization (curve 2) compared to the ground-based sample and the flight sample crystallized in hot isothermal zone of the furnace. In one of the previous experiments space solidification of Gd3Co was performed in quartz capsules instead of Ta-containers. The shift of the Neel peak from 131 to 128.5 K was observed for the flight sample, the magnetization peak being less sharp at the ordering temperature~ We observed that all flight samples investigated were characterized by certain broadening of Neel peaks and the magnetization values of the flight samples are higher than those of the ground-based samples at the ordering temperature region. The Neel peaks observed in low fields became less sharp for high fields. We observed the formation of Gd203 crystals in the form of well-developed single crystals on the surface of the macropore in one of the flight samples solidified directly in the quartz capsules (Fig. 5). Crystallographic perfection of the crystal grown in a microgravity environment is obvious. It seems that these crystals were condensed from a vapor phas e.
Space solidification experiment on Gd3Co compound
Fig. 2. Scanning electron micrograph of the pores in the flight sample of Gd3Co.
Fig. 3. Scanning electron micrographs of the diffusion layers formed in the flight samples along the Ta-surface. The line profiles of Co, Gd and Ta are shown.
489
0
o
Space solidification experiment on Gd3Co compound
491
40
_
~E
H = I 7 kOe
b
3 2O
~o[
L
L
1
0
-I00
I
-160
-140
-120
-I00 Tern p e r a t u r e ,
-80
-60
l
-40
--
-20
°C
Fig. 4. The temperature dependences of the magnetization of the ground-based (1) and flight (2, 3) samples.
Fig. 5. Scanning electron micrographs of space-grown Gd203 crystals on the surface of Gd3Coflight sample.
492
E.M. SAVITSKYand R. S. TORCHINOVA
Fig. 5. (cont.) 4. SUMMARYANDCONCLUSIONS The preliminary results concerning the space experiment on solidification of the antiferromagnetic Gd3Co compound are reported. Although the work is not yet complete, the following conclusions can be drawn. (1) The surface meniscus form in the solidified flight samples of Gd3Co was appreciably different than in the ground-based samples. (2) The formation of the reaction layers for the ground-based and flight samples consisted of Ta-Co and Gd-Co phases was observed. The thickness of the diffusion layers was greater for the flight samples. A partial contact between the solidified Gd3Co and Ta-container was observed in the flight samples. (3) The magnetic properties of Gd3Co are sensitive to the specific conditions of solidification in space. The flight samples investigated were characterized by certain broadening of Neel peaks. The magnetization values of the flight samples were considerably higher compared to the ground-based samples at the temperatures T ~< TNee~. REFERENCES
1, V. S. Avduijevsky, I. V. Barmin, S. D. Grishin, L. V. Leskov, A. M. Petrov, V. I. Polezhaev and V. V. Savichev Space Technology Problems. Mashinostroenie, Moscow (1980) (in Russian).
2. L. I. Ivanov, V. S. Zemskov, V. N. Kubasov, V. N. Pimenov, I. N. Belokurova, K. P. Gurov, E. V. Demina, A. N. Titkov and I. L. Shulpina Melting, Crystallization and Phase Formation in Space. Nauka, Moscow (1979) (in Russian). 3. E. M. Savitsky Problems of the Development o[ Physical Metallurgy. Nauka, Moscow (1972) (in Russian). 4. E. M. Savitsky, I. V. Burov, S. V. Pirogova and U. A. Saveliev The production of AI-Bi and AI-Mg pseudoalloys in low-gravity environment. Dokl. Akad. Nauk. USSR. 252(6), 1387-1389 (1980). 5. E. M. Savitsky, I. V. Burov, G. S. Burkhanov, B. P. Mikhailov, M. I. Bychkova, R. S. Torchinova, U. A, Saveliev, I. V. Vlasova and I. D. Giller, Investigation of the influence of near 0-gravity on the structure and physical properties of the alloys. Nauchnye chtenie po aviatzii i kosmonavtike. 288 (1981) (in Russian). 6. E. M. Savitsky, M. I. Bychkova and I. D. Giller, Crystallization of some superconducting compounds in space. Dokl. Akad. Nauk. USSR. 257(1) (1981). 7. D. J. Larson, Zero-gravity processing of magnets. ApolloSoyuz Test Project. Summary science Rept., NASA SP-412, 449-470 (1977). 8. P. Pant, Neuschutz, J. Potschke and H. Coenen Tech. Mitt. Krupp. Forsch.-Ber. Band (37), 69-78 (1979). 9. K. N. R. Taylor, Intermetallic Rare Earth Compounds. Mir, Moscow (1974) (in Russian). 10. K. H. J. Buschow and S. S. van der Goot, J. Less-Common Metals. 18, 309-311 (1969). 11. E. M. Savitsky, V. F. Terekhova and I. V. Burov, The Gd-Co phase diagram. J. Inorg. Chem. (USSR) 7, 1732-1735 (1962).