Metastable phases in the Tl-Sn alloy system
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IN recent
y e a r s , a broad survey of metastable alloy phases in binary B-metal alloy s y s t e m s has been c a r ried out; 145 the results of these works are s y s t e m a t i cally reviewed in Ref. 6 and, from a crystalchemical viewpoint, in Ref. 7. These metastable phases were prepared by rapid quenching from the melt (splat cooling); 8'9 about 50 new intermetallic phases have been described so far.~~ Metastable tin alloy s y s t e m s have been studied e s pecially thoroughly. The phase diagram of the TI-Sn system, Fig. l(a), shows a broad two-phase field between tin and the intermediate equilibrium phase a ; on the basis of crystalchemical analogies with related systems, it was likely that metastable phases could be retained in this composition range by splat cooling and the system was therefore studied. The equilibrium phase diagram shown in Fig. l(a) has been composed of the thallium-rich section given in Ref. 11 and the tin-rich section given in Ref. 12. EXPERIMENTAL METHODS AND RESULTS The preparation and examination of the quenched foils followed the procedure described in detail in Refs. 1, 2, and 10. Twenty alloys having compositions lying mostly between a and tin were prepared from metals of high purity (99.9+ pct) by melting in evacuated Pyrex capsules. Quantities of ~20 mg were splatcooled with the gun technique, i.e., they were blastatomized and propelled onto copper or platinum substrates maintained at -190~ Crystal structures and lattice p a r a m e t e r s were determined on a GE-XRD 5 diffractometer with CuKa radiation a t - 1 9 0 ~ and subsequently at room temperature. The duplication of the runs with both substrates permitted the elimination of overlap of the substrate diffraction pattern and that of the investigated substance. The XRD patterns were usually taken from sin20 = 0.03 to 0.50; the substrates were used as a means of internal calibration. The a c curacy of the lattice p a r a m e t e r s of metastable phases measured by this method z is approximately 0.1 pct. In the following, all compositions are in atomic percent. The stable and metastable phases found after rapid quenching to -190~ a r e shown in Fig. l(b), and are B. C. GIESSEN is Associate Professor, Department of Chemistry, Northeastern University, Boston, Mass. J. M. V1TEK and N. J. GRANT' are Research Assistant and Professor, respectively, Department of Metallurgy and Materials Science, Massachusetts Institute of Technology, Cambridge, Mass. Manuscript submitted January 5, 1972. METALLURGICAL TRANSACTIONS
listed in Table I together with estimated composition ranges and crystallographic data. The investigated alloys and the phases present in them are given in Table II. Mean atomic volumes of T1-Sn phases are plotted against composition in Fig. 2. The diffraction patterns taken after heating to room temperature agreed closely with those expected from the equilibrium phase diagram, Fig. l(a). This suggests that the new phases are not stable at room t e m perature, as previously observed for In-Bi, 2 Pb-Sb, 4 and other B-me
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