- Email: [email protected]
Formation of nanocrystalline structures in a Co–Al system by mechanical alloying and leaching
Journal of Alloys and Compounds 351 (2003) 101–105
L
www.elsevier.com / locate / jallcom
Formation of nanocrystalline structures in a Co–Al system by mechanical alloying and leaching a, a b c c G.V. Golubkova *, O.I. Lomovsky , Y.S. Kwon , A.A. Vlasov , A.L. Chuvilin a
Institute of Solid State Chemistry, Russian Academy of Sciences, Siberian Branch, ul. Kutateladze 18, 630128 Novosibirsk, Russia b Research Center for Machine Parts and Materials Processing, University of Ulsan, Ulsan 680 -749, South Korea c Boreskov Institute of Catalysts, Russian Academy of Sciences, Siberian Branch, Prosp. Akad. Lavrentieva 5, 630090 Novosibirsk, Russia Received 21 May 2002; received in revised form 11 September 2002; accepted 11 September 2002
Abstract Phase composition of the materials obtained by mechanical alloying of system Co–Al (Al concentration ranges from 50 to 70 at.%) and removal of aluminum from such alloys was investigated by differential dissolution, X-ray phase analysis and TEM with a resolution of 0.4 nm. The intensive mechanical alloying provides formation of the nanocomposite material containing both amorphous phase Co 2 Al 5 and nanocrystalline particles of phase CoAl. Leaching of amorphous phase Co 2 Al 5 results in the amorphous cobalt containing admixtures of alumina and hydroxide. Nanocomposite amorphous phase Co 2 Al 5 and CoAl convert into nanocomposite amorphous Co and b.c.c. Co. 2002 Elsevier Science B.V. All rights reserved. Keywords: Mechanical alloying; Cobalt aluminides
1. Introduction A combination of mechanical alloying (MA) of metal aluminides with following chemical leaching is very promising for production of nanocrystal powders of metal phases with unusual structural, magnetic and catalytic properties. Thus, leaching of amorphous alloy Ni 40 Al 60 obtained by mechanical treatment results in the nominal amorphous nickel [1]. The method of transmission electron microscopy (TEM) was used to study the process of crystallization of amorphous Ni particles during in situ annealing in the electron beam column. The size of amorphous crystal particles is 10–30 nm. The reaction of interaction between MA aluminides and alkali provides a unique possibility to remove the major part of aluminum without changing the structure of the initial alloy. Thus, alloy B2 Ni (Raney Ni) was obtained by leaching of mechanical alloy Ni 35 Al 65 (B2-type) as a result of its topotaxial transformation [2]. Alloy Co 40 Al 60 was subjected to similar investigations [3]. It was found that the leached alloy preserved the structure of the precursor alloy. The goal of the present work is to study the phase *Corresponding author. Tel.: 1383-2-363-834. E-mail address: [email protected] (G.V. Golubkova).
composition of MA alloys of system Co–Al (Al concentration ranges within 50–70 at.%) and the materials obtained by removal of aluminum from the above alloys.
2. Experimental Alloying was performed in a high-energy planetary-type activator AGO-1. The acceleration of milling bodies was 100 m 2 / s and the ratio between a ball load and mass of the initial powder charge was 200:10. The time of mechanical activation was varied from 2 to 60 min. To remove aluminum atoms, the MA alloys were leached with a 20% KOH solution at 273 K. The leaching was followed by heating with a water bath to 373 K to provide maximal removal of aluminum. The degree of conversion versus time of mechanical activation was determined from XRD data and magnetic measurements [4]. Conversion degrees during MA were calculated from changes in the cobalt concentrations in the sample, which were determined by Faraday’s procedure [5] with an assumption that a decrease in the cobalt concentration is associated with formation of aluminides. XRD studies were performed in a DRON-3 instrument using Cu Ka radiation. Lattice parameters of phase CoAl were determined with
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )01083-6
102
G.V. Golubkova et al. / Journal of Alloys and Compounds 351 (2003) 101–105
a diffractometer at the second channel of synchrotron radiation at the Institute of Nuclear Physics (Novosibirsk). The samples were investigated by TEM using an EM100C microscope, the resolution was 0.4 nm and the accelerating voltage was 100 kW. The method of differential dissolving (DD) [6,7] was used to determine stoichiometric compositions of phases in the samples. To provide dissolution, the powder samples were treated with an aqueous solution; the concentration of HCl was linearly increased from 0 to 1.2 N. The main process of dissolution proceeds when H 2 SO 4 –H 3 PO 4 – HClO 4 –HNO 3 , 6:3:1.5:1 (acid mixture–water, 1:10) and it stops in the HF solution (diluted 1:5). The temperature was gradually increased from 20 to 70 8C.
3. Results and discussion Phases Co 2 Al 5 and CoAl exist in the equilibrium alloys of the investigated concentration range [8]. Note that a subtraction solid solution with vacant positions of cobalt atoms forms in the homogeneity region of phase CoAl (50–53 at.% Al) [8]. XRD shows the presence of only CoAl in the samples when the concentration increases from 50 to 70 at.% Al. MA results first in the gradual weakening of the basic reflections of cobalt and aluminum. On the appearance of phase CoAl, only traces of reflections of the basic lines of Co and Al are maintained (Fig. 1). This suggests that phase CoAl begins to form after
Fig. 1. The dependence of phase content of Co 33 Al 67 mixture on the time of MA: (a) after MA during 2 min, (b) after MA during 15 min, (c) after MA during 20 min, (d) after MA during 60 min.
some induction period. The presence of the induction period in such solid-phase reactions is usually associated with the initial stage of mechanical synthesis in metal systems, which is accompanied by formation of layered composite structures and dispersion of the initial components to nanometer-sized grains. It is believed that [9,10] the performance of a mechanochemical reaction in such ultrafine composite particles is determined by high rates of the solid-phase diffusion, whose driving force is the negative mixing enthalpy. In this case, the induction period depends on the value of deviation of the initial charge composition from a 1:1 stoichiometry. XRD and magnetic measurements of the samples containing 50 and 70 at.% Al show that phase CoAl appears after 9 and 20 min of activation of the samples, respectively. The lattice parameter of phase CoAl, measured through the center of gravity of line [211], is 0.28960.003 nm and remains practically constant in all samples. In Ref. [3], the lattice parameter of phase CoAl (60 at.% Al) was 0.28660.001 nm. Note that the lattice parameters of the as-milled and leached samples are similar. On leaching of the samples, we have observed only one broad line at 2u 5448. Since broadening of the homogeneity region of phase CoAl up to 75 at.% Al is highly improbable, we suggest that the products of mechanical activation contain both CoAl and amorphous phase Co 2 Al 5 . A sample of Co 33 Al 67 was investigated by TEM to elucidate the possibility of existence of an amorphous phase. As found, the sample is built of irregular particles, the cross-section is about 600 nm. The micrograph shows (Fig. 2) that the contrast of the particle is not uniform: there are dark spots 10–30 nm in diameter against the gray background. Microdiffraction of the particle is ring-like, however, bright point reflections and diffusion regions present on the rings at a time (Fig. 2). The data suggest that the particle is a nanocomposite formed by the amorphous matrix containing nanocrystal particles of CoAl. To verify this suggestion, the sample was recorded using the dark field procedure when the image was formed by point reflections from nanoparticles only. Actually, only nanocrystal inclusions found in the reflecting position glow in the dark-field image of the particle (Fig. 3), while the contrast of the amorphous matrix is very weak. Therefore, the presence of nanocrystal phase CoAl in the MA product is supported by both XRD and TEM data. To specify the phase composition of the amorphous matrix, one can use the DD method. The ratio between components in the dissolved phases was determined from stoichiograms (a molar ratio of elements on the kinetic curves), as in Ref. [11]. It was found that sample Co 332 Al 67 contains two cobalt aluminide phases. As follows from Fig. 4, the phase having a Co-to-Al ratio of 1:2.5 dissolves first. Then both phases dissolve at a time. The phase with a 1:1 ratio
G.V. Golubkova et al. / Journal of Alloys and Compounds 351 (2003) 101–105
103
Fig. 2. TEM image of as-milled Co 33 Al 67 alloy.
dissolves only at the end of the process. Since the kinetic curves of dissolution of CoAl and Co 2 Al 5 partially coincide, the phases were separated mathematically using a standard algorithm for calculating the number of phases with an assumption that the phases maintain their stoichiometry. The shape of the kinetic curves after phase separation counts favor the above approach. Fig. 5 shows the differential curves of dissolution of Co 2 Al 5 and CoAl after their separation. Amorphous phase Co 2 Al 5 is crystallized when the samples are annealed at 700 8C (Fig. 6).
Thus, the TEM method permitted us to prove that the samples contain an amorphous structure whose composition corresponds to phase Co 2 Al 5 , as it was found by the DD method. An increase in the induction period of appearance of phase CoAl, whose composition deviates from the 1:1 stoichiometry, can be associated with formation of amorphous phase Co 2 Al 5 , since its formation heat is higher than that of CoAl by a factor of 2.5. For this reason, formation of amorphous phase Co 2 Al 5 precedes that of crystal phase CoAl. Note that the differential dissolution method and chemical analysis evidence the appearance of iron in the samples as a material of the mechanochemical reactor. The concentration of iron varied from 0.7 to 1.1 mass% in the mechanical alloys and from 1.2 to 1.4 mass% in the leached samples. Almost all iron exists as a solid solution in the cobalt aluminides. X-ray diffraction patterns of the leaching samples exhibit one broad line (Fig. 6), which permits one to conclude that the resulting Raney-Co is amorphous. Differential dissolving data indicate that the removal of aluminum from amorphous alloy Co 2 Al 5 results in the formation of amorphous cobalt containing residual aluminum (CoAl 0.19 ), which probably exists as hydroxides (Fig. 7). The stoichiogram does not provide an unambigu-
Fig. 4. The DD data of as-milled alloy Co 33 Al 67 . (1) The kinetic curve of dissolution of Co. (2) The kinetic curve of dissolution of Al. (3) The stoichiogram of dissolution of alloy.
Fig. 5. The DD data of dissolution of Co 33 Al 67 sample after the separation of Co 2 Al 5 and CoAl phases. (1) The kinetic curve of dissolution of Co 2 Al 5 phase. (2) The kinetic curve of dissolution of CoAl phase.
Fig. 3. TEM image of as-milled Co 33 Al 67 alloy in the dark field.
104
G.V. Golubkova et al. / Journal of Alloys and Compounds 351 (2003) 101–105
considerably lower than that of the pyrometallurgic alloy of the same composition (6.03 g / cm 3 ). For this composition, the percentage of vacancies in the lattice, calculated from formulae 100(DPM 2DMA ) /DPM (D is density of samples) [13], is 12. One would expect an increase in the reactivity of defect phase CoAl obtained by MA and its leaching to yield metastable structure b.c.c. cobalt [3]. A broad peak in the region of 2u 5448 detected for the leached sample by XRD can be attributed to b.c.c. cobalt phase.
4. Conclusion
Fig. 6. The XRD patterns of samples: (a) as-milled Co 33 Al 67 alloy (60 min of MA), (b) leached Co 33 Al 67 alloy, (c) Co 33 Al 67 alloy after annealing at 700 8C.
ous conclusion concerning the existence of a chemical bond between amorphous cobalt and the residual aluminum. Phase CoAl with a stoichiometric composition, obtained by traditional pyrometallurgic melting, is stable to alkali action [12]. However, it was shown earlier that the reactivity of MA alloys can be increased. For system Ni–Al, as an example, the formation of the subtraction solid solution based on phase NiAl [4], broadening of the homogeneity region, appearance of both vacancy-type nonstoichiometric defects and structural defects of plastic deformation provided leaching of phase NiAl [4]. In this case, the density of sample Co 47 Al 53 (measured with a helium auto-pycnometer) is 5.2860.02 g / cm 3 , which is
(1) The intensive MA of system Co–Al (50–70 at.% Al) results in the formation of the nanocomposite material, containing amorphous phase Co 2 Al 5 and nanocrystalline particles CoAl. (2) XRD, TEM and DD (differential dissolution) method proved the fact of formation of micro-crystal phase CoAl in the samples. (3) An amorphous phase was detected by TEM in the MA samples. According to the DD method, the composition of the phase is Co 2 Al 5 . (4) Formation of amorphous phase Co 2 Al 5 manifests itself as an increase in the induction period of formation of crystal phase CoAl during MA when deviation of the initial mixture composition from the 1:1 stoichiometry increases. (5) On leaching of the alloy, amorphous phase Co 2 Al 5 dissolves to yield amorphous cobalt that contains residual aluminum. The nanoparticles of phase CoAl are topotaxially converted to yield b.c.c. cobalt.
Acknowledgements This work has been supported by Korean Institute of S&T Evaluation and Planning (KISTEP) Program No. Lab.03-15-003 and also supported by the Korea Science and Engineering Foundation (KOSEF) through the ReMM at the University of Ulsan.
References
Fig. 7. The DD data of Co 33 Al 67 sample after leaching. (1) The kinetic curve of dissolution of Co. (2) The kinetic curve of dissolution of Al. (3) The stoichiogram of dissolution of leaching alloy.
[1] S.A. Makhlof, K. Sumiyama, K. Suzuki, J. Alloys Comp. 199 (1993) 119. [2] E. Ivanov, S.A. Makhlof, K. Sumiyama, H. Yamauchi, K. Suzuki, G. Golubkova, J. Alloys Comp. 185 (1992) 25. [3] S.A. Makhlof, E. Ivanov, K. Sumiyama, K. Suzuki, J. Alloys Comp. 189 (1992) 117. [4] G.V. Golubkova, T.F. Grigorieva, E.U. Ivanov, Izv. Sib. Otde. Akad. Nauk SSSR (in Russian) 5 (1989) 102. [5] V.T. Kalinnikov, U.V. Rakitin, Vvedenie v magnetochimiiu, Moskva, 1980 (in Russian).
G.V. Golubkova et al. / Journal of Alloys and Compounds 351 (2003) 101–105 [6] I.G. Vasilyeva, A.A. Vlasov, V.V. Malakhov, M.R. Predtechenskii, Thin Solid Films 292 (1997) 85–90. [7] V.V. Malakhov, J. Mol. Catal. A: Chem. 158 (2000) 143. [8] M.A. Khansen, K. Anderko, Strukturi dvoinich splavov, Metallurgiya, Moskva, 1962 (in Russian). [9] C. Koch, J.S.C. Jang, P.Y. Lee, Milling, in: Proc. DGM Conf. on New Materials by Mechanical Alloying Techniques, 1988, p. 101.
105
[10] C. Suryanarayana, Prog. Mater. Sci. 46 (2001) 1. [11] G.V. Golubkova, O.I. Lomovsky, A.A. Vlasov, L.S. Davlitova, E.Yu. Belyaev, V.V. Malakhov, J. Alloys Comp. 307 (2000) 131. [12] E.I. Gildebrand, A.B. Fasman, Skeletnie katalizatori v organicheskoi chimii. Nauka, Alma-Ata, 1982 (in Russian). [13] W. Hume-Rothery, G.V. Raynor, in: The Structure of Metals and Alloys, The Institute of Metals, London, 1956.
L
www.elsevier.com / locate / jallcom
Formation of nanocrystalline structures in a Co–Al system by mechanical alloying and leaching a, a b c c G.V. Golubkova *, O.I. Lomovsky , Y.S. Kwon , A.A. Vlasov , A.L. Chuvilin a
Institute of Solid State Chemistry, Russian Academy of Sciences, Siberian Branch, ul. Kutateladze 18, 630128 Novosibirsk, Russia b Research Center for Machine Parts and Materials Processing, University of Ulsan, Ulsan 680 -749, South Korea c Boreskov Institute of Catalysts, Russian Academy of Sciences, Siberian Branch, Prosp. Akad. Lavrentieva 5, 630090 Novosibirsk, Russia Received 21 May 2002; received in revised form 11 September 2002; accepted 11 September 2002
Abstract Phase composition of the materials obtained by mechanical alloying of system Co–Al (Al concentration ranges from 50 to 70 at.%) and removal of aluminum from such alloys was investigated by differential dissolution, X-ray phase analysis and TEM with a resolution of 0.4 nm. The intensive mechanical alloying provides formation of the nanocomposite material containing both amorphous phase Co 2 Al 5 and nanocrystalline particles of phase CoAl. Leaching of amorphous phase Co 2 Al 5 results in the amorphous cobalt containing admixtures of alumina and hydroxide. Nanocomposite amorphous phase Co 2 Al 5 and CoAl convert into nanocomposite amorphous Co and b.c.c. Co. 2002 Elsevier Science B.V. All rights reserved. Keywords: Mechanical alloying; Cobalt aluminides
1. Introduction A combination of mechanical alloying (MA) of metal aluminides with following chemical leaching is very promising for production of nanocrystal powders of metal phases with unusual structural, magnetic and catalytic properties. Thus, leaching of amorphous alloy Ni 40 Al 60 obtained by mechanical treatment results in the nominal amorphous nickel [1]. The method of transmission electron microscopy (TEM) was used to study the process of crystallization of amorphous Ni particles during in situ annealing in the electron beam column. The size of amorphous crystal particles is 10–30 nm. The reaction of interaction between MA aluminides and alkali provides a unique possibility to remove the major part of aluminum without changing the structure of the initial alloy. Thus, alloy B2 Ni (Raney Ni) was obtained by leaching of mechanical alloy Ni 35 Al 65 (B2-type) as a result of its topotaxial transformation [2]. Alloy Co 40 Al 60 was subjected to similar investigations [3]. It was found that the leached alloy preserved the structure of the precursor alloy. The goal of the present work is to study the phase *Corresponding author. Tel.: 1383-2-363-834. E-mail address: [email protected] (G.V. Golubkova).
composition of MA alloys of system Co–Al (Al concentration ranges within 50–70 at.%) and the materials obtained by removal of aluminum from the above alloys.
2. Experimental Alloying was performed in a high-energy planetary-type activator AGO-1. The acceleration of milling bodies was 100 m 2 / s and the ratio between a ball load and mass of the initial powder charge was 200:10. The time of mechanical activation was varied from 2 to 60 min. To remove aluminum atoms, the MA alloys were leached with a 20% KOH solution at 273 K. The leaching was followed by heating with a water bath to 373 K to provide maximal removal of aluminum. The degree of conversion versus time of mechanical activation was determined from XRD data and magnetic measurements [4]. Conversion degrees during MA were calculated from changes in the cobalt concentrations in the sample, which were determined by Faraday’s procedure [5] with an assumption that a decrease in the cobalt concentration is associated with formation of aluminides. XRD studies were performed in a DRON-3 instrument using Cu Ka radiation. Lattice parameters of phase CoAl were determined with
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )01083-6
102
G.V. Golubkova et al. / Journal of Alloys and Compounds 351 (2003) 101–105
a diffractometer at the second channel of synchrotron radiation at the Institute of Nuclear Physics (Novosibirsk). The samples were investigated by TEM using an EM100C microscope, the resolution was 0.4 nm and the accelerating voltage was 100 kW. The method of differential dissolving (DD) [6,7] was used to determine stoichiometric compositions of phases in the samples. To provide dissolution, the powder samples were treated with an aqueous solution; the concentration of HCl was linearly increased from 0 to 1.2 N. The main process of dissolution proceeds when H 2 SO 4 –H 3 PO 4 – HClO 4 –HNO 3 , 6:3:1.5:1 (acid mixture–water, 1:10) and it stops in the HF solution (diluted 1:5). The temperature was gradually increased from 20 to 70 8C.
3. Results and discussion Phases Co 2 Al 5 and CoAl exist in the equilibrium alloys of the investigated concentration range [8]. Note that a subtraction solid solution with vacant positions of cobalt atoms forms in the homogeneity region of phase CoAl (50–53 at.% Al) [8]. XRD shows the presence of only CoAl in the samples when the concentration increases from 50 to 70 at.% Al. MA results first in the gradual weakening of the basic reflections of cobalt and aluminum. On the appearance of phase CoAl, only traces of reflections of the basic lines of Co and Al are maintained (Fig. 1). This suggests that phase CoAl begins to form after
Fig. 1. The dependence of phase content of Co 33 Al 67 mixture on the time of MA: (a) after MA during 2 min, (b) after MA during 15 min, (c) after MA during 20 min, (d) after MA during 60 min.
some induction period. The presence of the induction period in such solid-phase reactions is usually associated with the initial stage of mechanical synthesis in metal systems, which is accompanied by formation of layered composite structures and dispersion of the initial components to nanometer-sized grains. It is believed that [9,10] the performance of a mechanochemical reaction in such ultrafine composite particles is determined by high rates of the solid-phase diffusion, whose driving force is the negative mixing enthalpy. In this case, the induction period depends on the value of deviation of the initial charge composition from a 1:1 stoichiometry. XRD and magnetic measurements of the samples containing 50 and 70 at.% Al show that phase CoAl appears after 9 and 20 min of activation of the samples, respectively. The lattice parameter of phase CoAl, measured through the center of gravity of line [211], is 0.28960.003 nm and remains practically constant in all samples. In Ref. [3], the lattice parameter of phase CoAl (60 at.% Al) was 0.28660.001 nm. Note that the lattice parameters of the as-milled and leached samples are similar. On leaching of the samples, we have observed only one broad line at 2u 5448. Since broadening of the homogeneity region of phase CoAl up to 75 at.% Al is highly improbable, we suggest that the products of mechanical activation contain both CoAl and amorphous phase Co 2 Al 5 . A sample of Co 33 Al 67 was investigated by TEM to elucidate the possibility of existence of an amorphous phase. As found, the sample is built of irregular particles, the cross-section is about 600 nm. The micrograph shows (Fig. 2) that the contrast of the particle is not uniform: there are dark spots 10–30 nm in diameter against the gray background. Microdiffraction of the particle is ring-like, however, bright point reflections and diffusion regions present on the rings at a time (Fig. 2). The data suggest that the particle is a nanocomposite formed by the amorphous matrix containing nanocrystal particles of CoAl. To verify this suggestion, the sample was recorded using the dark field procedure when the image was formed by point reflections from nanoparticles only. Actually, only nanocrystal inclusions found in the reflecting position glow in the dark-field image of the particle (Fig. 3), while the contrast of the amorphous matrix is very weak. Therefore, the presence of nanocrystal phase CoAl in the MA product is supported by both XRD and TEM data. To specify the phase composition of the amorphous matrix, one can use the DD method. The ratio between components in the dissolved phases was determined from stoichiograms (a molar ratio of elements on the kinetic curves), as in Ref. [11]. It was found that sample Co 332 Al 67 contains two cobalt aluminide phases. As follows from Fig. 4, the phase having a Co-to-Al ratio of 1:2.5 dissolves first. Then both phases dissolve at a time. The phase with a 1:1 ratio
G.V. Golubkova et al. / Journal of Alloys and Compounds 351 (2003) 101–105
103
Fig. 2. TEM image of as-milled Co 33 Al 67 alloy.
dissolves only at the end of the process. Since the kinetic curves of dissolution of CoAl and Co 2 Al 5 partially coincide, the phases were separated mathematically using a standard algorithm for calculating the number of phases with an assumption that the phases maintain their stoichiometry. The shape of the kinetic curves after phase separation counts favor the above approach. Fig. 5 shows the differential curves of dissolution of Co 2 Al 5 and CoAl after their separation. Amorphous phase Co 2 Al 5 is crystallized when the samples are annealed at 700 8C (Fig. 6).
Thus, the TEM method permitted us to prove that the samples contain an amorphous structure whose composition corresponds to phase Co 2 Al 5 , as it was found by the DD method. An increase in the induction period of appearance of phase CoAl, whose composition deviates from the 1:1 stoichiometry, can be associated with formation of amorphous phase Co 2 Al 5 , since its formation heat is higher than that of CoAl by a factor of 2.5. For this reason, formation of amorphous phase Co 2 Al 5 precedes that of crystal phase CoAl. Note that the differential dissolution method and chemical analysis evidence the appearance of iron in the samples as a material of the mechanochemical reactor. The concentration of iron varied from 0.7 to 1.1 mass% in the mechanical alloys and from 1.2 to 1.4 mass% in the leached samples. Almost all iron exists as a solid solution in the cobalt aluminides. X-ray diffraction patterns of the leaching samples exhibit one broad line (Fig. 6), which permits one to conclude that the resulting Raney-Co is amorphous. Differential dissolving data indicate that the removal of aluminum from amorphous alloy Co 2 Al 5 results in the formation of amorphous cobalt containing residual aluminum (CoAl 0.19 ), which probably exists as hydroxides (Fig. 7). The stoichiogram does not provide an unambigu-
Fig. 4. The DD data of as-milled alloy Co 33 Al 67 . (1) The kinetic curve of dissolution of Co. (2) The kinetic curve of dissolution of Al. (3) The stoichiogram of dissolution of alloy.
Fig. 5. The DD data of dissolution of Co 33 Al 67 sample after the separation of Co 2 Al 5 and CoAl phases. (1) The kinetic curve of dissolution of Co 2 Al 5 phase. (2) The kinetic curve of dissolution of CoAl phase.
Fig. 3. TEM image of as-milled Co 33 Al 67 alloy in the dark field.
104
G.V. Golubkova et al. / Journal of Alloys and Compounds 351 (2003) 101–105
considerably lower than that of the pyrometallurgic alloy of the same composition (6.03 g / cm 3 ). For this composition, the percentage of vacancies in the lattice, calculated from formulae 100(DPM 2DMA ) /DPM (D is density of samples) [13], is 12. One would expect an increase in the reactivity of defect phase CoAl obtained by MA and its leaching to yield metastable structure b.c.c. cobalt [3]. A broad peak in the region of 2u 5448 detected for the leached sample by XRD can be attributed to b.c.c. cobalt phase.
4. Conclusion
Fig. 6. The XRD patterns of samples: (a) as-milled Co 33 Al 67 alloy (60 min of MA), (b) leached Co 33 Al 67 alloy, (c) Co 33 Al 67 alloy after annealing at 700 8C.
ous conclusion concerning the existence of a chemical bond between amorphous cobalt and the residual aluminum. Phase CoAl with a stoichiometric composition, obtained by traditional pyrometallurgic melting, is stable to alkali action [12]. However, it was shown earlier that the reactivity of MA alloys can be increased. For system Ni–Al, as an example, the formation of the subtraction solid solution based on phase NiAl [4], broadening of the homogeneity region, appearance of both vacancy-type nonstoichiometric defects and structural defects of plastic deformation provided leaching of phase NiAl [4]. In this case, the density of sample Co 47 Al 53 (measured with a helium auto-pycnometer) is 5.2860.02 g / cm 3 , which is
(1) The intensive MA of system Co–Al (50–70 at.% Al) results in the formation of the nanocomposite material, containing amorphous phase Co 2 Al 5 and nanocrystalline particles CoAl. (2) XRD, TEM and DD (differential dissolution) method proved the fact of formation of micro-crystal phase CoAl in the samples. (3) An amorphous phase was detected by TEM in the MA samples. According to the DD method, the composition of the phase is Co 2 Al 5 . (4) Formation of amorphous phase Co 2 Al 5 manifests itself as an increase in the induction period of formation of crystal phase CoAl during MA when deviation of the initial mixture composition from the 1:1 stoichiometry increases. (5) On leaching of the alloy, amorphous phase Co 2 Al 5 dissolves to yield amorphous cobalt that contains residual aluminum. The nanoparticles of phase CoAl are topotaxially converted to yield b.c.c. cobalt.
Acknowledgements This work has been supported by Korean Institute of S&T Evaluation and Planning (KISTEP) Program No. Lab.03-15-003 and also supported by the Korea Science and Engineering Foundation (KOSEF) through the ReMM at the University of Ulsan.
References
Fig. 7. The DD data of Co 33 Al 67 sample after leaching. (1) The kinetic curve of dissolution of Co. (2) The kinetic curve of dissolution of Al. (3) The stoichiogram of dissolution of leaching alloy.
[1] S.A. Makhlof, K. Sumiyama, K. Suzuki, J. Alloys Comp. 199 (1993) 119. [2] E. Ivanov, S.A. Makhlof, K. Sumiyama, H. Yamauchi, K. Suzuki, G. Golubkova, J. Alloys Comp. 185 (1992) 25. [3] S.A. Makhlof, E. Ivanov, K. Sumiyama, K. Suzuki, J. Alloys Comp. 189 (1992) 117. [4] G.V. Golubkova, T.F. Grigorieva, E.U. Ivanov, Izv. Sib. Otde. Akad. Nauk SSSR (in Russian) 5 (1989) 102. [5] V.T. Kalinnikov, U.V. Rakitin, Vvedenie v magnetochimiiu, Moskva, 1980 (in Russian).
G.V. Golubkova et al. / Journal of Alloys and Compounds 351 (2003) 101–105 [6] I.G. Vasilyeva, A.A. Vlasov, V.V. Malakhov, M.R. Predtechenskii, Thin Solid Films 292 (1997) 85–90. [7] V.V. Malakhov, J. Mol. Catal. A: Chem. 158 (2000) 143. [8] M.A. Khansen, K. Anderko, Strukturi dvoinich splavov, Metallurgiya, Moskva, 1962 (in Russian). [9] C. Koch, J.S.C. Jang, P.Y. Lee, Milling, in: Proc. DGM Conf. on New Materials by Mechanical Alloying Techniques, 1988, p. 101.
105
[10] C. Suryanarayana, Prog. Mater. Sci. 46 (2001) 1. [11] G.V. Golubkova, O.I. Lomovsky, A.A. Vlasov, L.S. Davlitova, E.Yu. Belyaev, V.V. Malakhov, J. Alloys Comp. 307 (2000) 131. [12] E.I. Gildebrand, A.B. Fasman, Skeletnie katalizatori v organicheskoi chimii. Nauka, Alma-Ata, 1982 (in Russian). [13] W. Hume-Rothery, G.V. Raynor, in: The Structure of Metals and Alloys, The Institute of Metals, London, 1956.