Growth of niobium-niobium carbide (Nb2C) eutectic and hypereutectic composites by zone melting

Abstract

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 primarily by growth rate and composition. The carbide morphology may be either rod-like or plate-like. 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 inter-rod spacings vary 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 {fx2309-1} = constant, withn = 0.8. For alloys four at. pct above the eutectic, primary Nb2C dendrites nucleate and grow ahead of the still well aligned two phase solid plus liquid interface.