THE NIOBIUM—NIOBIUM CARBIDE SYSTEM¹

Abstract

The Niobium-Niobium Carbide System 1471 identically to the test samples showed that free molybdenum and tungsten were indeed present after the first visible melting.The periodic variation of the minimum solidus temperature may be seen in Fig. 1, where the ratio Ts/Tm from Table V is plotted as a function of the group of the periodic table. Group VII exhibits a minimum.A chemical analysis of the NbC melting point cylinder gave a empirical formula of NbCo-ses, with free carbon not detectable.Acknowledgments.-The authors thank Dr. E. VanKooten and Mr. G. C. Heasley for the chemical analyses, Messrs. O. R Simi and J. A. Mariner for the spectrographic analyses, and Mr. C. G. Hoffman for the metallographic examinations.

Preparation and Characterization of Niobium Carbide and Carbonitride

“…(i) A solid solution which keeps the body-centered cubic lattice of the metal and which can dissolve up to 6 at.% of carbon at about 2608 K, the eutectic temperature (3)(4)(5)(6), the lattice parameter increasing with the carbon content.…”

“…The high temperature -Nb C phase presents a hexagonal structure (disordered L3 type) (3)(4)(5)(6). At a lower temperature or in presence of impurities such as oxygen (7) another crystal structure can occur by ordering of carbon atoms.…”

“…(iii) A monocarbide NbC V , which is an homogeneous phase from approximatively NbC to NbC. The high temperature NbC phase, also designated by -NbC (4,5), shows a random face-centered cubic structure whose lattice parameter increases from 4.412 to 4.470 A s with carbon content (12)(13)(14). As with hemicarbide, ordered structures could exist (15) such as -Nb C (16)(17)(18)(19)(20)(21).…”

Physical Properties of the NbC Carbide

“…There are several versions of the C-Nb phase diagram published in the literature, both from experimental information [15][16][17][18], as well as from thermodynamic assessments using the CALPHAD (Computer Coupling of Phase Diagrams and Thermochemistry) protocol [19][20][21][22][23][24][25]. Some features of the C-Nb system are not completely understood, especially the order-disorder transformations present in this system.…”

Interdiffusion in the carbides of the Nb-C system

“…Although an ε-carbide near that composition was observed by Crane and Ostermann, [8] it is not clear whether the ε-carbide is a stable phase. The work of Viswanadham and Wert [5] suggests that it is not, and Storms and Krikorian [9] also failed to find such a phase. The phase diagram in Reference 6 shows it as a tentative phase.…”