Thermodynamics of formation of Y-Co alloys
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INTRODUCTION
W A T S O N and Bennett ~ have suggested that the thermodynamics of phase formation for the intermediate phases in the Y-Fe, Y-Co, and Y-Ni systems should provide a good basis for evaluating the predictions of their simplified electron band theory model for estimating enthalpies of formation of binary phases that form from elements both of which utilize d-states in their bonding interactions. Pursuant to the accumulation of the desired data, emf measurements on the Y-Fe system 2 have been completed and published. The present study is a continuation of that effort and deals with the thermodynamics of formation of the phases in the Y-Co system. Phase diagrams for the complete Y-Co system have been proposed by Pelleg and Carlson 3 with nine intermediate phases, by Strnat et al. 4 with later minor modification by Ray 5 with eight intermediate phases, and by Buschow 6 with eight intermediate phases. These investigations agree as to the existence of phases at stoichiometries of Y2C0~7, YCos, YCo3, YCo2, and Y3Co. There now seems little doubt that a phase originally indicated by Pelleg and Carlson to be YCo4 is actually the Y2Co7 phase of Strnat et al. and of Buschow. With regard to YzC07, crystallographic evidence4.6 9 indicates the phase to be dimorphic though a transition temperature has not been established. So also is YC03 indicated to be dimorphic, 4'6'8-12 but again no transition temperature has been established. In both instances the polymorphic forms are simple stacking variations analogous to the stacking difference between the cubic closestpacked and hexagonal closest-packed elemental structures. Lemaire~0.,3 has discussed the structural relationships among the Co-rich Y-Co phases from Y2Col7 through YC02; the crystal structures of these phases all result primarily from atomic packing considerations and, on this basis, the enthalpies of phase formation per mole of Y should be relatively insensitive to stoichiometry. A phase, not found in any of the forementioned phase diagram investigations, was reported by Khan 7 as occurring P.R. SUBRAMANIANis Research Associate, Department of Metallurgical Engineeringand Materials Science, 3325 Science Hall, CarnegieMellon University, Pittsburgh, PA 15213. J. E SMITH is Professor and Senior Metallurgist, Ames Lab and Departmentof Materials Science and Engineering, 122 Metallurgy Building, Iowa State University, Ames, IA 50011. Manuscript submittedJuly 30, 1984. METALLURGICALTRANSACTIONSA
at a stoichiometry of Y5Co19with a temperature range of stability at elevated temperatures. If this phase is an equilibrium phase in the pure binary system, it represents unusual behavior for Y because Y normally exhibits alloying behavior analogous to that of the heavier rare earths such as Tb, Ho, and Dy. For none of the heavier rare earths has a T5CoI9 phase (T representing a rare earth) been reported, though such phases occur s in the Co systems of the lighter rare earths: La, Ce, Pr, and Nd. In any case the crystal structures of TsCoI9 phases share many of the structu
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