Thermodynamics of aluminum-barium alloys
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I.
INTRODUCTION
A knowledge of activities in liquid A1-Ba alloys is useful for the analysis of the physical chemistry of the reduction of barium oxide by aluminum in vacuum. During this process, an A1-Ba alloy is formed, which decreases the efficiency of utilization of aluminum and recovery of barium. The intermetallic compound AlaBa was widely used in thermionic valves as the main component of evaporated barium getters. Notin e t al. rl~ have measured the partial enthalpy of mixing of barium in A1-Ba alloys by drop calorimetry at 1215 K. Their measurements spanned over the composition range 0.013 < X~a < 0.466. Measurements in the concentration range 0.061 < XB, < 0.320 lie in the two-phase region comprising liquid and AlaBa. Notin e t al. t~] have subsequently derived the integral enthalpy of mixing by successive summation. For liquid alloys between 0.34 < XBa < 0.47, the enthalpy of mixing is - 3 1 . 6 (---0.2) k J / g . atom with respect to pure liquid aluminum and barium as standard states, almost independent of composition. The values for integral enthalpy derived by this procedure are subject to uncertainties, especially at higher concentrations of barium, because of the accumulation of the errors associated with each measurement of the partial property. Burylev e t a l . t21 have measured the vapor pressure of barium over liquid AI-Ba alloys by Knudsen effusionmass loss analysis between 1223 and 1373 K for three compositions. The values for the activity of barium at 1373 K are 9.504 x 10-4 at 10 at. pct Ba, 3.455 x 10 -3 at 20 at. pct Ba, and 0.0355 at 33.3 at. pct Ba with respect to liquid barium as the standard state. Vigdorovich e t a l . ~3~ have also measured the vapor pressure of barium over A1-Ba alloys using the Knudsen effusion-mass loss technique in the temperature range of 870 to 1370 K for 0.1 < XBa < 0.75. Most of the measurements lie in the two-phase regions. The results suggest strong negative S. SRIKANTH, formerly with the Department of Metallurgy, Indian Institute of Science, is Scientist, Nonferrous Process Division, National Metallurgical Laboratory, Jamshedpur 831 007, India. K.T. JACOB, Professor and Chairman, is with the Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India. Manuscript submitted September 10, 1990. METALLURGICAL TRANSACTIONS B
deviations from ideal behavior for Al-rich alloys. Activity data for liquid alloys are not available over the entire concentration range. The phase diagram of the A1-Ba system given in the assessments of Elliott and S h a n k [4] and Massalski e t al. tSl is based on early thermal and microscopic studies of Alberti, E6] Flanigen, t7j and Iida. tsl It shows the presence of three intermetallic compounds - - A14Ba melting congruently at 1370 K and the compounds A12Ba and A1Ba melting incongruently at 1018 and 991 K, respectively. The Ba-rich eutectic occurs at 72.0 at. pct Ba and 801 K. More recent phase diagram studies by Bruzzone and Merlo 191show that the compound A12Ba decomposes peritectically at 1187 K, rather than
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