Surface tensions of liquid Fe-Cr and Fe-Cr-N alloys
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A. SHARAN, T. NAGASAKA, and A.W. CRAMB In the present work, surface tensions of liquid Fe-Cr and Fe-Cr-N alloys were determined. Previous studies on surface tensions of liquid Fe-base alloys show a large variation between investigations that could be due to the presence of surface-active impurities such as O or S in the alloy samples or due to inaccurate experimental techniques/ u Figure I, which is a plot of the surface tension of liquid Fe-Cr alloys determined by previous investigators, t2-61 shows the typical disagreement. The purpose of the present work is to determine accurate surfacetension values of liquid Fe-Cr and Fe-Cr-N alloys, using ultrapure samples and modern image processing techniques. The surface-tension experiments were carried out using a combination of a SiC resistance furnace and an X-ray radiography unit. X-ray pictures of the sessile metal droplet, contained in an alumina crucible, were taken at 1550 ~ The surface tension of the alloy was subsequently calculated from the image contour of the droplet, by determining the curve of best fit between the numerical solution to the Laplace equation and the experimentally measured points. Details of data acquisition and the computational technique are available in References 7, 8, and 9. The Fe-Cr alloys were made by melting solid pieces of Fe and Cr (both of 99.99 pet metallic purity) in alumina crucibles using an induction furnace, in an atmosphere of H-ultrahigh-purity Ar mixture. The alloy samples were chemically analyzed for S and were found to contain less than 10 parts per million (ppm) of the element. Conical pieces, each weighing about 1 g, were prepared by cutting the alloy. The alloy pieces were then polished with sandpaper to remove any surface oxide and cleaned using acetone. Details of the experimental setup and the technique are provided in References l0 and 11. Sessile drop experiments were performed on Fe-Cr alloys of compositions 0, 10, and 20 wt pct Cr. Highpurity alumina was chosen as the substrate material. The temperature during the experiment was maintained at 1550 ~ and an extremely reducing gas, 0.1 pct CO2balance CO, was chosen as the atmosphere for the experiment. The equilibrium concentration of O in pure liquid Fe under these conditions is 8 ppm. The results of the experiment are plotted in Figure 2. The surface tension of Fe-Cr alloy was found to decrease by about 130 m N / m over the range of 0 to 10 wt pct Cr under the CO2-CO atmosphere. The experiment for the 20 wt
A. SI-IARAN, Research Engineer, is with the Bethlehem Steel Corporation, Bethlehem, PA. A.W. CRAMB, Professor, is with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. T. NAGASAKA, Associate Professor, is with the Department of Metallurgy, Tohoku University, Sendai, 980 Japan. Manuscript submitted January 18, 1994. 626--VOI.UME 25B, AUGUST 1994
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