The Ce-Mg-Y system
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UCTION
THE considerable improvement of the mechanical strength of magnesium-based alloys by additions of rareearth metals has naturally lead to an increased interest in the constitution of ternary and higher order magnesiumbased systems containing rare earths. In order to provide proper understanding of phase-relation-dependent alloy properties, the present article is concerned with the isothermal section of the ternary cerium-magnesium-yttrium system at 500 7C. The experimental results of this article are based on a simultaneous critical assessment of all thermodynamic phase equilibria, crystallographic, and solid-solution data obtained for the Ce-Mg-Y ternary. Early investigations in literature essentially concerned two partial isotherms at 300 7C and 500 7C in the Mg-rich corner for more than '75 mass pct Mg.[1] Three isopleths were constructed for the very Mg-rich region, employing X-ray diffraction (XRD) analysis, differential thermal analysis, (DTA), scanning electron microscopy (SEM), and optical microscopy: Mg-30 mass pct Y to Mg-23 mass pct Ce,[2] Mg-19 mass pct Y to Mg-16 mass pct Ce,[1] and Mg15 mass pct Ce to Mg-16 mass pct Y.[1] The potential of Ce-Mg-Y alloys with respect to intermediate valence effects on the cerium atoms was investigated in a few selected alloys, prepared from the solid solutions Ce12xYxMg3[3–6] and Ce12xYxMg.[5,6,7] II.
sealed by arc welding under pure argon. The samples, each with a total weight of approximately 0.6 g, were melted and, in order to attain proper homogenization, were remelted in an induction furnace under continuous shaking of the crucibles in a stream of 99.999 pct pure argon. The tantalum crucibles were then sealed in iron cylinders, wherein the samples were annealed in a resistance furnace at 500 7C for 72 to 168 hours, and finally quenched in cold water. Thermodynamic equilibrium in the Ce-Y binary, as well as for the ternary region with low magnesium contents, was only attained after prolonged heat treatments of more than 3000 hours at 500 7C. Some of the Ce-Y binary alloys were simply fused in an argon arc furnace prior to annealing in silica tubes, which were vacuum sealed and which
EXPERIMENTAL DETAILS
The metals used were magnesium (rod, ∅ 5 6.0 mm, 99.9 mass pct) and cerium (rod, ∅ 5 12.5 mm, 99.9 mass pct), supplied by Koch Chemicals Ltd. (Hertford Herts, United Kingdom), and yttrium (ingots, 99.9 mass pct), supplied by Strem Chemicals (Newbury Port, MA). Ingots of the pure elements were enclosed in small cylindrical tantalum crucibles with an internal diameter of ∅i ; 9 mm,
H. FLANDORFER, Researche Assistent, is with the Inst. of Physih. Chemie, Vienna, M. GIOVANNINI, Researche Assistent, is with the Istituto de Chimica Generale, Genova, A. SACCONE, Assistant Professor, is with Istituto di Chimica Generale, Genova, R. ROGL, Assistant Professor, is with the Institut of Physih Chemie, Vienna and R. FERRO, Professor, is with the Istituto di Chimica Generale, Universita` di Genova, I-16132 Genova, Italy. Manuscript submitted November 1, 1995. METALLURGICA
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