Growth of niobium-niobium carbide (Nb 2 C) eutectic and hypereutectic composites by zone melting
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Growth from the melt of niobium-niobium carbide (Nb-Nb2C) composites at the eutectic and off the eutectic composition by electron beam floating zone melting and freezing was studied. Composite growth has been successfully achieved for the eutectic composition and for compositions up to four atomic percent above the eutectic. The volume fraction of the carbide phase was increased from 26 to 33 pct by growing off the eutectic composition. The alignment and morphology of the carbide phase are influenced p r i m a r i l y by growth rate and composition. The carbide morphology may be either rod-like or platelike. For a given composition, rod morphology is favored at higher growth rates. For constant growth rate, the plate-like morphology is promoted by increasing carbon content. The transition from plate to rod morphology is gradual. Lamellar and ihter-rod spacings v a r y from 1 to 6 microns for growth rates varying from 5 to 90 microns per second. Variation of lamellar spacing with freezing rate, R, produces a straight line (on a log-log plot) in agreement with a relationship of the form X2Rn = constant, with n = 0.8. For alloys four at. pet above the eutectic, p r i m a r y N'b2C dendrites nucleate and grow ahead of the still we11 aligned two phase solid plus liquid interface.
W I T H the advancement of high temperature technology, the need for materials with high strength, rigidity and good ductility at elevated temperatures has been growing. From this point of view, r e f r a c t o r y metalr e f r a c t o r y metal carbide systems seem a potential basis of high temperature composite materials. The controlled solidification of such eutectic and off-eutectic alloys seems to be a possible approach toward the achievement of such composites. The binary r e f r a c t o r y m e t a l - r e f r a c t o r y metal c a r bide systems have eutectic temperatures in the range of 2300 to 3000~ In an initial study of these systems, Lemkey and Salkind 1 mentioned that unidirectionally solidified composites of Nb-NbsC and Ta-Ta2C appear to have good strength and m i c r o s t r u c t u r a l stability at elevated temperatures. For the growth of eutectic composites from the melt many workers have employed the normal freezing technique z'3 because of the difficulty in c o n t r o l l i n g the struc ture of zone melted materials. However, fabrication of composites of reactive and r e f r a c t o r y metals and compounds suggests that a containerless melting technique be used, e . g . , floating zone melting and freezing. Zone melting and freezing has been used to grow composite structures in some binary eutectic alloys. 1,4,s The present investigation describes growth of Nb-Nb~C composites by electron beam floating z o n e melting and freezing at eutectic and hypereutectic compositions, and the effect of various operating par a m e t e r s and composition on the morphology and distribution of the carbide phase. A section of the Nb-C phase diagram 6 is shown in Fig. 1. In a set of c u r s o r y experiments, the eutectic
composition
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