Activities in carbon-saturated Fe-Al alloys and the stability of Al 4 C 3 at 1873 K
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was recently shown1 that Knudsen cell-mass spectrometer methodse could be usefully applied to the determination of the activities of the components in carbon-saturated liquid iron alloys. The present paper describes a study of carbon-saturated i r o n aluminum alloys. The principal motivation for this work was the d e s i r e t o have values for the activities of F e and A1 for application to l a t e r studies of s l a g m e t a l reactions in graphite crucibles, but the opportunity was also taken to establish the stability of aluminum c a r b i d e - a substance of current interest in analyses a of d i r e c t reduction processes for the production of aluminum. Our present knowledge of the activity of aluminum in carbon-saturated Fe-A1 alloys comes from the work of Chipman and Floridis 4 who m e a s u r e d the distribution of A1 between the molten alloy and silver. However, their results showed more scatter than those with carbon-free alloys and there was a significant intersolubility (up to about 4 wt pct) of Ag in the F e - r i c h l a y e r and F e in the A g - r i c h layer. In view of these factors it was decided to reinvestigate the system r a t h e r than derive activities by combination of the above distribution measurements with more recent values S for the activity of A1 in A1-Ag alloys. EXPERIMENTAL DETAILS The experimental apparatus and procedure were essentially the same as those described e a r l i e r .1 Orifice diameters in the graphite or alumina Knudsen c e i l s were less than 3 x 10-2 cm to give a minimum value for the ratio of the evaporation area to the orifice area of 450 : 1. Alloys were prepared in situ by m e l t ing together weighed amounts of aluminum wire (Alpha Inorganic, 99.999 pct pure) and either sponge iron (United Mineral, 99.99 pct pure) or approximately eutectic composition (analyzed) iron carbon alloy which had been previously prepared from the sponge iron. The t i m e s required to r e a c h steady readings for the ion-currents were the same as in the previous work1 except at compositions where aluminum carbide formed; h e r e , an additional 10 t o 15 minutes sufficed to give steady, time-independent readings. Weight losses of the c e i l s were determined and s m a l l c o r r e c tions to the composition were made, where necessary, U. V. CHOUDARY is presently Post-Doctoral Fellow, Department
of Materials, University of Wisconsin-Milwaukee, Milwaukee, WI53201. G. R. BELTON is Professor, Department of Metallurgy and Materials Science K1, University of Pennsylvania, Philadelphia, PA 19104,where Dr. Choudary was formerly a GraduateStudent. Manuscript submittedJanuary 24, 1977. METALLURGICAL
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by assuming that the r a t i o of the losses of the individual components (Fe and A1) was proportional to the r a t i o of the ion-currents, adjusted for isotopic abundances. The m a x i m u m such correction was 0.8 pct; accordingly, no subsequent chemical analyses were c a r r i e d out. An ionization potential of 10 V was used and the two detector circuits were cal
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