Synthesis of ultrafine particles of intermetallic compounds by the vapor-phase magnesium reduction of chloride mixtures:
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I.
INTRODUCTION
THE increasing attention paid to intermetallic compounds and some of their useful properties was discussed in Part I.[1] Part I also presented a new synthesis process of intermetallic compounds, which involves the vapor-phase reduction of metal chlorides by magnesium forming ultrafine particles of intermetallic compounds, as applied to the synthesis of titanium aluminide particles. In Part II, the results of applying this new process to the synthesis of nickel aluminide particles are presented. In this research, equilibrium calculation of the gaseous and condensed phases was first carried out for the Ni-AlMg-Cl-Ar system. Equilibrium compositions of the solid metallic and intermetallic phases were calculated as functions of reactant concentration and temperature. The effect of reactant partial pressures on the formation of different intermetallic phases was then studied experimentally. The size and shape of the fine particles were observed by transmission electron microscopy (TEM). II.
THERMODYNAMIC EQUILIBRIUM ANALYSIS
The method of thermodynamic calculation has been described in detail in Part I. Thirteen gaseous species and six condensed phases were considered in the equilibrium calculation of the Ni-Al-Mg-Cl-Ar system. They are listed in Table I. The input molar ratios AlCl3/NiCl2 and Mg/NiCl2 were found to have significant effects on the contents of intermetallic phases between Ni and Al. Figure 1(a) shows the effect of the input AlCl3/NiCl2 partial pressure ratio on the equilibrium contents of solid phases. The input ratio Mg/NiCl2 was constant at 4.0. As the AlCl3/NiCl2 ratio varies between 0.1 and 10, the conH.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 August 13, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
tents of Ni3Al and NiAl reach a maximum value of 99.5 mol pct at the AlCl3/NiCl2 ratios of 0.33 and 1.0, respectively. Beyond the ratio of 1.0, Ni2Al3 starts forming at the expense of NiAl and reaches a maximum value of 53 mol pct at the AlCl3/NiCl2 ratio 1.5. Beyond this point, the content of Ni2Al3 decreases and NiAl becomes more favorable. The corresponding equilibrium gas-phase composition, as shown in Figure 1(b), indicates that beyond the ratio of 1.0, the gas phase contained a large excess of AlCl3 and AlCl. The amount of Mg decreases to a negligible value beyond the AlCl3/NiCl2 ratio of 2.0. Thus, Mg is not available to reduce the excess AlCl3 to form metallic Al. The effect of the input Mg/NiCl2 partial pressure ratio on the equilibrium solid-phase contents is presented in Figure 2. The input ratio AlCl3/NiCl2 was constant at 1.0. As the Mg/NiCl2 ratio varies from 1 to 10, the composition changes from Ni and Ni3Al in the lower range of the ratio to Ni3Al and NiAl in the higher range. Ni2Al3 or
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