Synthesis of ultrafine particles of intermetallic compounds by the vapor-phase magnesium reduction of chloride mixtures:
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I.
INTRODUCTION
INTERMETALLIC compounds are attracting much attention.[1] Titanium and nickel aluminides have attracted a great deal of attention from the aerospace as well as automobile industries because of their potential for high-temperature application (e.g., low density, good oxidation resistance, and high-temperature strength retention[2,3]). Gamma TiAl and two-phase (TiAl 1 Ti3Al) alloys[4] are used for gas turbine engine rotational components as well as automobile exhaust valves. TiAl3 is one of the most extensively investigated aluminum-rich intermetallics. Nickel aluminides are attractive because of their low densities, relatively high melting point, and good high-temperature strength properties. For example, Ni3Al shows increasing rather than decreasing yield strength with increasing temperature.[5] In this research, a new synthesis process of intermetallic compounds, which involves the vapor-phase reduction of metal chlorides by magnesium forming ultrafine particles of intermetallic compounds, has been investigated. A typical reaction for intermetallic compound formation by this method can be written as mMClx (g) 1 n NCly (g) 1 0.5 (mx 1 ny) Mg (g) 5 Mm Nn (s) 1 0.5 (mx 1 ny)MgCl2 (1,g)
[1]
where M and N represent two different metals, with x and y being their valences, and MmNn is the intermetallic compound formed. In this and the Part II articles,[6] the synthesis of titanium aluminides and nickel aluminides, respectively,
H.Y. SOHN, Professor, is with the Department of Metallurgical Engineering, University of Utah, Salt Lake City, UT 84112-0114. S. PALDEY, formerly Graduate Student, Department of Metallurgical Engineering, University of Utah, is Visiting Faculty Member, Indian Institute of Technology, Madras, India 600036. Manuscript submitted March 31, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
according to this reaction, is described. In this laboratory, the preparation of ultrafine particles of nickel aluminide[7] and molybdenum nickelide[8] (also coating on a nickel substrate in this case) by hydrogen reduction of constituent metal chlorides has also been investigated. The preparation of titanium aluminide by hydrogen reduction of constituent chlorides, however, is not feasible thermodynamically (large positive DG7) up to 2500 K. The reduction of metal chlorides by Mg is the basis of the Kroll process.[9] Uranium was prepared by reducing UF4 by Ca.[10] Titanium sponge is prepared by chlorination of rutile, followed by the dehalogenation of the resulting titanium chloride by liquid or gaseous Mg.[11] No work has previously been done on the preparation of ultrafine particles of intermetallic compounds by reducing metal chloride vapors with Mg vapor. In this research, equilibrium calculation of the gaseous and condensed phases was first carried out for the Ti-AlMg-Cl-Ar system. Equilibrium compositions of the solid metallic and intermetallic phases were calculated as functions of reactant partial pressures and temperature. The effects of reaction conditions on the formation of different
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