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ISSN 1062 8738, Bulletin of the Russian Academy of Sciences: Physics, 2009, Vol. 73, No. 9, pp. 1199­1201. © Allerton Press, Inc., 2009. Original Russian Text © B.B. Straumal, A.S. Gornakova, A.A. Mazilkin, A.B. Straumal, A.N. Nekrasov, E.P. Condrea, A.S. Sidorenko, A.V. Surdu, 2009, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2009, Vol. 73, No. 9, pp. 1265­1267.

Grain Boundary Wetting in the Al­Mg System and Synthesis of Magnesium Diboride in Contact with Melt
B. B. Straumala, A. S. Gornakovaa, A. A. Mazilkina, A. B. Straumala, A. N. Nekrasovb, E. P. Condreac, A. S. Sidorenkoc, and A. V. Surduc
b

Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia e mail: a.str@bk.ru Institute of Experimental Mineralogy, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia c Institute of Electronic Engineering and Industrial Technologies, MD2028 Chisinau, Moldova Abstract--The interaction between a Mg containing melt and B under conditions of partial and complete wetting of Al/Al grain boundaries by Al­Mg melt has been investigated. The study was performed on Al poly crystals with Mg contents of 5, 10, 15, 18, and 25 wt %. Correspondingly, the Mg content in the melt was determined by the liquidus line and was in the range from 5 to 30 wt %. The obtained metal matrix compos ites were investigated by light and scanning electron microscopy, electron probe microanalysis, and X ray diffraction. The possibility of synthesizing MgB2 in the contact with a melt having a relatively low Mg content (from 15 to 30 wt %) has been demonstrated. DOI: 10.3103/S1062873809090056

a

INTRODUCTION In 2001 magnesium diboride (MgB2) was found to have a superconducting transition temperature of 39 K [1]. Since that time, this superconductor has figured prominently in the superconductor technology. Cur rently, MgB2 based superconducting wire up to few kilometers long has been commercially produced. Magnesium diboride is a brittle compound; therefore, it is important to develop the fundamental bases of various technologies making it possible to form a plas tic metal matrix of sufficiently high conductivity with MgB2 filled channels or interlayers inside. Currently, either MgB2 superconducting channels are formed from a previously synthesized powder or MgB2 is synthesized in situ. In the latter case, a mix ture of boron and magnesium is placed in a tube made of a metal with a sufficiently high melting temperature (copper, niobium, iron, etc.) to react upon heating with the formation of MgB2. The mixture is heated above the Mg melting point and then B reacts with the Mg melt. However, this method has a number of sig nificant drawbacks. First, heating should be performed to a tempera ture above the Mg melting temperature, which is fairly high (650°C). Second, high temperature synthesis of MgB2 is generally performed after rolling or drawing a metal rod or bar with the Mg­B reaction mixture inside. Since Mg has a hexagonal lattice, its deform ability is low. Recently, an original method has been proposed to increase the deformability of pieces with a metallic shell and a reaction mixture inside to synthe size MgB2 [2, 3].

In this method, not magnesium but its alloy with about 12 wt % lithium is placed in the mechanical shell before rolling or drawing. This alloy has a bcc lattice and is more plastic than pure magnesium. In this case, boron reacts not with molten magnesium but with the melt containing both Mg and Li. Nevertheless, it was shown in [2, 3] that this technique makes it possible to successfully synthesize continuous superconducting MgB2 layers. We proposed to use not only pure magnesium melt but also melts containing other elements, along with Mg, to synthesize MgB2 in situ. To obtain composites with a plastic metal matrix and superconducting MgB2 based interlayers, one can also use the effect of the so called grain boundary wetting, which has been recently found and investigated in detail by us. The essence of grain boundary wetting is that the liquid phase in a number of systems in the two phase region of the solid solution + melt phase diagrams can completely wet the solid phase grain boundaries. In this state (Fig. 1), liquid interlayers in equilibrium sep arate solid phase grains. Changing the temperature, one can pass from complete to partial wetting and thus change the morphology of liquid channels in the solid matrix (Figs. 1b, 1c). In particular, this effect was found by us in the Al­Mg system [4, 5]. Thus, in a cer tain temperature range an Al­Mg alloy may consist of solid grains poor in magnesium and interlayers or channels of Mg rich melt. The concept of this study is that boron would react not with pure magnesium but with a composite where an Al­Mg melt is confined between solid metal grains having a fairly high plas ticity.

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STRAUMAL et al. Mg content, at % 20 30

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Twmax = 610°C Twmin = 540°C

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Fig. 1. Microstructure of Al­Mg polycrystals from the two phase region (S + L): (a) Al­10 wt % Mg, 610°, all grain boundaries are completely wetted by melt; (b) Al­10 wt % Mg, 581°, some grain boundaries are completely wetted and the others are incompletely wetted; and (c) Al­15 wt % Mg, 490°, completely wetted bound aries are absent.

Fig. 2. Al­Mg phase diagram with lines of the bulk phase transitions (bold lines, [6]) and two wetting phase transi tion tie lines at Twmax = 610° and Twmin = 540° (con tinuous thin lines). The closed and open circles indicate the annealing conditions under which MgB2 was formed and was not formed, respectively.

EXPERIMENTAL Al alloys with different Mg contents (5, 10, 15, 18, and 25 wt %) were prepared for the experiments. Then cylindrical samples 7 mm in diameter and 4 mm in height were cut from each alloy piece. Holes 2 mm in diameter were drilled in the samples to be filled with boron powder. The annealings were performed in quartz ampoules with a residual gas pressure of 4 â 10­4 Pa. The Al­Mg alloy samples in contact with the boron powder were annealed in the two phase Al + L region of the bulk phase diagram at temperatures below liquidus for 60 min. The annealing tempera tures are indicated by circles in Fig. 2. After the annealing, the samples were ground and polished. The sample structure was studied by optical microscopy (Zeiss Axiophot microscope) and scanning electron microscopy (Tescan Vega TS5130 MM scanning elec tron microscope equipped with an energy dispersive Oxford Instruments LINK spectrometer). The phase composition of the samples was analyzed by X ray dif

fraction on a Siemens 500 diffractometer (FeK radia tion). RESULTS AND DISCUSSION Figure 1 shows how the grain boundary wetting phase transition affects the microstructure of Al­Mg polycrystals in the two phase region (S + L) by the example of Al­10 wt % Mg and Al­15 wt % Mg alloys. The phase grains (Al), which remained solid at the annealing temperature appear bright in the lap. The sample regions that were molten at the annealing temperature undergo the L Al + Zn eutectic trans formation upon quenching and appear gray in the lap. For example, at 610° (Fig. 1a) all boundaries are completely wetted by the melt, at 581° (Fig. 1b) some grain boundaries are completely wetted (about 26%) and the others are wetted incompletely, and at 490° (Fig. 1c) there are no completely wetted boundaries in the polycrystal. Grain boundaries com pletely wetted by the melt are absent in the polycrystals
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GRAIN BOUNDARY WETTING IN THE AL­MG SYSTEM Intensity, rel. units (Al) Al3Mg MgB2
2

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500 µm

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Fig. 3. Al­15 wt % Mg alloy with cylindrical holes filled with boron to react with Mg containing melt after anneal ing at 520°. A system of channels along the grain bound aries in the aluminum matrix can be seen, which were filled with the melt before the reaction with boron. Panel (b) is the enlarged fragment framed by a bright square in panel (a).

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at temperatures below Twmin = 540°. In the range from Twmin = 540° to Twmax = 610°, the fraction of completely wetted grain boundaries gradually increases from 0 to 100% with an increase in tempera ture and all boundaries become wetted above Twmax = 610°. These data made it possible to construct tie lines of the wetting phase transition at Twmax = 610° and Twmin = 540° (continuous thin lines) on the bulk phase diagram of Al­Mg (Fig. 2). The annealing temperatures of Al­Mg samples contacting with boron powder are indicated in Fig. 2 by circles. The Mg content in the melt at the reaction temperature corresponded to the liquidus in the Al­Mg diagram and ranged from 5 to 30 wt %. Figure 3 shows a typical sample microstructure after annealing in contact with boron. It can be seen that the Mg rich melt partially flew out from the intergranular space into the holes filled with boron powder. The X ray dif fraction data (Fig. 4) indicate that MgB2 can be pro duced as a result of the reaction of Mg rich melt with boron. The annealing temperatures and the sample composition at which MgB2 formation in contact with the Al­Mg melt was observed are indicated by closed circles in Fig. 2. It can be seen that MgB2 was not formed at a Mg content in the melt below 15 wt %. Thus, the possibility of MgB2 synthesis in contact with an Al based melt with a relatively low Mg content (from 15 to 30 wt %) is demonstrated. Furthermore, we planning to optimize the process conditions in order to increase the reaction yield and obtain MgB2 at

Fig. 4. X ray diffraction pattern of the Al­25 wt % Mg alloy after reaction with boron at 500°: (Al) peaks of the magnesium solid solution in aluminum with a lattice con stant of ~0.41 nm (corresponds to a Mg content of 10.5 wt %, according to [7]); (Al3Mg2) intermetallic compound with a cubic lattice; and (MgB2) hexagonal magnesium diboride (space group P6mmm).

interaction with the melt located directly between the grains of the Al rich solid phase. ACKNOWLEDGMENTS This study was supported by the Russian Founda tion for Basic Research, project no. 08 08 90105. REFERENCES
1. Nagamatsu, J., Nakagawa, N., Muranaka, T., et al., Nature, 2001, vol. 410, p. 63. 2. Togano, K., Fujii, H., Takeuchi, T., and Kumakura, H., Supercond. Sci. Technol., 2007, vol. 20, p. 629. 3. Togano, K., Fujii, H., Takeuchi, T., and Kumakura, H., Supercond. Sci. Technol., 2007, vol. 20, p. 239. 4. Kogtenkova, O.A., Protasova, S.G., Straumal, B.B., and Lopes, G., Izv. Ross. Akad. Nauk, Ser. Fiz., 2005, vol. 69, no. 9, p. 1324. 5. Straumal, B.B., Fazovye perekhody na granitsakh zeren (Phase Transitions at Grain Boundaries), Moscow: Nauka, 2003. 6. Lyakishev, N.P., Diagrammy sostoyaniya dvoinykh metallicheskikh sistem (Phase Diagrams of Binary Metallic Systems), Moscow: Mashinostroenie, 1996. 7. Ellwood, E.C., J. Inst. Met., 1952, vol. 80, p. 217.

BULLETIN OF THE RUSSIAN ACADEMY OF SCIENCES: PHYSICS

Vol. 73

No. 9

2009