Densification of nonstoichiometric niobium-carbide phases
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A study was made of the influence of temperature on the density of compacts produced from powders of nonstoichiometric NbCX phases of approximately the same specific surface area. It was established that, with increasing carbon deficiency, the relative density at any given temperature increases, because the diffusional mobility of the carbon and niobium atoms is greater in a “looser” crystalline lattice.
The energy of activation, q, for vacancy formation (per 1 g-form) in the NbCX alloys investigated was calculated. It was found that, with increasing defectiveness of the C sublattice, q decreases, from 110 kcal/g-form for NbC1.0 to 83.6 kcal/g-form for NbC0.72.
Self-diffusion coefficients for NbCX alloys have been calculated over a wide temperature range. It is demonstrated that, at low temperatures, self-diffusion coefficients increase with rise in carbon concentration in the alloys, which explains the increase in relative density observed experimentally during the sintering of alloys of lower carbon content. The parity of the self-diffusion coefficients at the melting points of the NbCX phases investigated indicates that transition from the solid state into liquid takes place at equal loss in the stability of the crystalline lattices of the alloys in the region of homogeneity of niobium monocarbide.
A comparison of relative density with calculated self-diffusion coefficients was instrumental in establishing that the sintering of nonstoichiometric niobium-carbide phases occurs at self-diffusion coefficients ranging from 10−11 to 10−6 cm2/sec. By analyzing the magnitude of the principal component parts of relative density, it was ascertained that, for the NbCX phases investigated, the observed difference in experimental values of relative density is apparently linked with difference in the capacity of these phases for plastic flow (diffusion creep); this flow is effected as a result of dislocation migration (climb) under the action of capillary forces and is governed by the diffusional mobility of the niobium and carbon atoms.
KeywordsRelative Density Niobium Carbon Concentration Plastic Flow Wide Temperature Range
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