Interdiffusion in the carbides of the Nb-C system

Abstract

Interdiffusion coefficients in Nb 2 C and NbC 1Ϫx were measured using bulk diffusion couples in the temperature range from 1400 ЊC to 1700 ЊC. Marker experiments were used to show that carbon is the only component undergoing significant diffusion in both carbides. Carbon concentrations were measured by difference using electron probe microanalysis, and interdiffusion coefficients were taken from Boltzmann-Matano analyses of the resulting concentration profiles. This analysis clearly showed that, in NbC 1Ϫx , interdiffusion coefficient varies with carbon concentration, and is expressed by 28,450 ‫ע‬ 850 ϳ Ϫ9 2 D (NbC ) ϭ 3.84 ϫ 10 exp (23.24x) exp Ϫ m /s 1Ϫx

Thin film solid-state reactions forming carbides as contact materials for carbon-containing semiconductors

“…The graphite formation on top of the carbide layer ͑for Fe-C and Cr-C͒ supports the concept that the small carbon atoms are diffusing fast through the metal ͑or carbide͒. Woodford and Chang 40 evidenced that carbon is the only species undergoing significant diffusion in the Nb-C system.…”

“…One would expect that C atoms will move more easily through a layer with more vacancies, which may explain the lower activation energy. Many authors 40,41 pose that the activation energy for diffusion is independent of the composition. Others 30,32,42 have shown that different carbon diffusion mechanisms are active, and the dominance of a particular mechanism is dependent on the composition and on the temperature range.…”

Joining of ultra-refractory carbides

“…C is the faster diffuser in these systems. 27,41,42 One would thus envision the possibility of C from the monocarbide combining with Nb and C in the liquid to form a mixed carbide layer. In the joints involving ZrC and HfC, mixed monocarbide (Zr,Nb)C and (Hf,Nb)C solid solutions were observed in contact with the pure carbide.…”

Cavitation erosion of NiAl

“…[38] Such bubbles, created during each upward motion of the vibrating tip (Figure 3), collapse during the following half cycle. [40] Fatigue striations were observed by Gould [29] on the cavitated surface of a polycrystalline titanium alloy specimen. Earlier investigations of ductile metallic materials with such a vibratory apparatus suggest that the implosions of the cavitation zone produce dynamic stresses which lead to plastic deformation and eventual fracture of material at the surface exposed to cavitation testing.…”