Interdiffusion in the carbides of the Nb-C system
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28,450 5 850 ; D (NbC12x ) 5 3.84 3 1029 exp (23.24x) exp 2 m2/s T where x is the site fraction of vacancies on the carbon sublattice. The interdiffusion coefficient in Nb2C is given by
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35,520 5 2190 ; D (Nb2C) 5 2.04 5 0.57 3 1024 exp 2 m2/s T Parabolic layer growth coefficients were estimated from the Nb|C diffusion couples as well. They are given by
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2.90 Kp (NbC) 5 2.65 2.41 3 1025 exp 2
37,600 5 1400 m2/s T
25 Kp (Nb2C) 5 1.57 1.92 exp 2 1.28 3 10
36,400 5 3100 m2/s T
˜ in NbC12x was found to be consistent with literature values for the tracer diffusivity The value of D of C in NbC12x via the thermodynamic factor, which was determined in two ways.
I.
INTRODUCTION
IN an effort to enhance the high-temperature stability of a Ti-alloy/Al2O3 fiber metal-matrix composite, we have been evaluating different fiber-coating schemes.[1] One such scheme is the sequence Al2O3 fiber|C|Nb metal|Y2O3|Ti-alloy matrix. We are, thus, interested in the kinetic stability of the Nb|C interface; this is best determined by finding the diffusivity of carbon in Nb2C and NbC12x. Several researchers[2,3,4] have prepared excellent reviews of the properties of the different phases in the Nb-C system. A brief overview of some of the intermediate phases will now be given. The Nb sublattice in Nb2C has the hexagonal close-packed structure, with C atoms occupying some of the octahedral interstitial sites. Storms[3] reported a possible low-temperature orthorhombic form, but further research[5] has not confirmed this. At the temperatures covered in the present study, Nb2C has a narrow range of solubility. The NaCl structures is in NbC12x with C atoms occupying octahedral sites in a face-entered cube Nb sublattice; x, the J. WOODFORD, Research Assistant, and Y.A. CHANG, Wisconsin Distinguished Professor, are with the Materials Science and Engineering Department, University of Wisconsin-Madison, Madison, WI 53706. Manuscript submitted May 20, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
site fraction of vacancies on the C sublattice, can be as high as 0.3 over the temperatures considered here[6] (Figure 1). Brauer and Lesser[7] postulated a third carbide phase, Nb4C32x, based on one weak powder pattern line at u 5 19.70 7. 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. Huang[10] has successfully modeled the major features of the Nb-C phase diagram (Figure 2). For the purposes of the model, the third carbide phase was ignored, as were all of the order-disorder transitions. At the temperatures of interest in this study, the only difference between the model results and the phase diagram given in Reference 6 was the shape of the low-carbon boundary on the NbC12x phase field, about which there is some doubt. Figure 3 contains plots of previousl
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