Thermodynamic properties of liquid Al-Mg alloys measured by the Emf method
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INTRODUCTION
ALUMINUM-magnesium alloys are widely used in industry for their light weight and superior mechanical and corrosion properties. Therefore, the thermodynamic properties of these alloys are of considerable scientific and practical interest. Several investigators 1'2'3 have studied the A1-Mg system, but there are significant differences in their reported results. Besides, there are no definitive values for the lowmagnesium ( < 10 at. pct) aluminum alloys which are most commonly used in industry. Hence, an additional study of the A1-Mg system with particular attention to alloys containing less than 10 at. pct magnesium was deemed necessary. The low-concentration data generated in this study were used to develop the calibration relation for an electrochemical magnesium-sensor, 4 and they can also be used to evaluate the chemical equilibria involved in refining aluminum scrap. 5 Lukashenko and Pogodayev ~ used both emf and vapor pressure methods to measure the activity of magnesium in liquid AI-Mg alloys, and reported that the emf method gives more accurate values. The vapor pressure method yields a higher value for the magnesium activity, as magnesium is surface active and tends to concentrate at the carriergas/melt interface. Belton and Rao 3 have also expressed doubt about the accuracy of the data produced by the vapor pressure method. Therefore, we decided to use the emf method. In the present study, the activity of magnesium in liquid A1-Mg alloys is determined by measuring the emf of a magnesium concentration cell. The cell is composed of pure liquid magnesium as the reference electrode, a eutectic melt of MgC12 and CaC12 as the electrolyte, and a liquid AI-Mg alloy, in which the magnesium activity is to be determined, as the working electrode. The data have also been used to determine the free energy, entropy, enthalpy, and excess free energy for the liquid A1-Mg alloys.
BASANT L. TIWARI is Staff Research Scientist with the Metallurgy Department, General Motors Research Laboratories, Warren, MI 48090-9055. Manuscript submitted January 12, 1987. METALLURGICAL TRANSACTIONS A
EXPERIMENTAL
A. Materials
The A1-Mg alloys were prepared by melting pure aluminum in a clay-graphite crucible and adding desired amounts of magnesium. The melts were purged with dry nitrogen for twenty minutes to remove dross and hydrogen, and then cast into slabs. Pieces cut from the slabs were used as the working electrode material to conduct the tests. A eutectic mixture of calcium and magnesium chlorides, which melts at 610 ~ was used as the electrolyte. Magnesium chloride (Alfa Products) was 99 pct and calcium chloride (Ulrich Chemical, Inc.) was 99.8 pct pure. Both salts were further purified by melting in a quartz crucible and bubbling chlorine through the melt. The electrolyte was then prepared by mixing equal amounts of the anhydrous chlorides in a helium-atmosphere glove-box. B. Cell
The thermodynamic properties of the A1-Mg alloy system were determined from the potential of an electrochemical cell of the typ
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