Constitutional studies of cobalt-tin alloys
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I.
INTRODUCTION
THE equilibrium diagram of the Co-Sn system as presented in current assessmentsl~.21 is shown in Figure 1. It remains incompletely established, particularly in the Co-rich regions of the solid state. The solid solubility of Sn in Co has not been systematically investigated so that the form of the solvus line between the a-Co solid solution and the (a + y) region is not defined. Similarly, neither the homogeneity range of the y-phase, especially the variation of its boundaries at higher temperatures, nor the temperature of its transformation to the low-temperature ordered y ' form is known. Discrepancies also exist among the values suggested by various investigations 13"4's~for the temperatures of peritectic formation of the CoSn and CoSn2 phases. Further information on the constitution of the Co-Sn system is thus clearly desirable; resolution of many of the above uncertainties was obtained in the course of a recent thermodynamic investigation of Co-Sn alloys. 161 Metallographic, X-ray diffraction, and differential thermal analysis methods were used in the characterization of the alloy samples involved and the results obtained using these techniques, together with those arising from the solid electrolyte electromotive force (EMF) studies, have been used to establish phase boundaries. The thermodynamic study of the system has been described in detail elsewhere; t61 the present article, therefore, is confined to constitutional aspects and the proposed revisions of the phase diagram. II,
E X P E R I M E N T A L DETAILS
A. Alloy Preparation The alloys were prepared from 99.99 pct pure cobalt (Centre d'Information du Cobalt, Brussels) and 99.999 pct pure tin (Capper Pass Ltd., England), using two different procedures. Cobalt-rich alloys, containing less than 40 at. pct tin, were prepared by induction melting of the elements in a recrystallized alumina boat supported by a water-cooled hearth and under an atmosphere
of titanium-gettered argon. They were rapidly cooled after melting by the water-cooled hearth so as to yield finegrained samples and minimize segregation. The resulting ingots were resealed in argon-filled silica capsules and homogenized at 1000 ~ for 14 days before further annealing treatments. Initially, alloys containing more than 40 at. pct tin were prepared using the same method as used for the cobaltrich alloys. However, metallographic examination and the observation of anomalous electromagnetic force values in galvanic cell measurements showed that inhomogeneity persisted even after prolonged annealing at the highest practicable solid state temperatures. An alternative method of preparation, termed the "rotatingmelting" technique, was therefore adopted; the apparatus has been described in detail elsewhere. 17j Weighed, chemically cleaned, and dried quantities of the pure metals were sealed in silica capsules after evacuating to - 1 0 -5 atm and refilling with purified argon to a pressure of 0.3 to 0.4 atm. Capsules were then held in a horizontal resistance-heated furnace where they w
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