Reaction diffusion and phase equilibria in the V-N system
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I.
INTRODUCTION
THE phase equilibria in the vanadium-nitrogen system have been reviewed by Carlson et al.,[1] who also published the first complete version of a tentative V-N phase diagram. Relevant studies, which were published later or were not considered by these authors, cover the preparation and properties of d-VN12x within the homogeneity range,[2] thermodynamic data of coexistence of b-V2N with d-VN12x,[3] and thermodynamic and kinetic effects of nitrogen diffusion in d-VN12x.[4,5] The d-VN12x attains a stoichiometric composition at relatively high nitrogen pressures (depending on the temperature) only.[2] In the temperature range 1400 7C to 1700 7C, the nitrogen-poor boundary was determined by Kieda et al.[3] to lie at [N]/[V] 5 0.65 to 0.60. For b-V2N there is substantial disagreement concerning the phase boundaries (Table I). According to Brauer and Schnell,[6] the nitrogen-rich composition of b-V2N is at 32.8 at. pct N, and according to Kieda et al.,[3] it is at 33.8 at. pct N ([N]/[V] 5 0.51), the boundary supposedly being independent of temperature. However, in the compilation of Carlson et al.,[1] a composition close to 31 at. pct N was adopted for the phase diagram based on data by Arbuzov et al.[7] and Khaenko and Fak.[8] The nitrogen-poor boundary was stated to be rather uncertain; values between 25 and 30 at. pct N were reported.[1] The aim of the present study was to describe phenomena occurring within the nitriding of vanadium metal and to investigate vanadium nitride layers that develop during the reaction of vanadium metal with nitrogen, in order to establish the homogeneity regions of the vanadium nitrides C. TEICHMANN, Graduate Student and High-School Teacher, W. LENGAUER, Associate Professor, and P. ETTMAYER, Professor, are with the Institute for Chemical Technology of Inorganic Material, Vienna University of Technology, A-1060 Vienna, Austria. J. BAUER, Research Scientist CNRS, is with the Laboratory of Metallurgy and Physical Chemistry of Materials, F-35043 Rennes, France. M. BOHN, Research Engineer CNRS, is with the IFREMER Brest, F-29280 Plouzane´, France. Manuscript submitted February 6, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
Table I.
Literature Data of Different Authors for the Phase Boundary of the b-V2N Phase
Temperature (7C) 1500 1408 to 1710 1500 700 to 1500 1500 700, 1500 1500 700 1500
N-Poor
N-Rich
28 to 30 — 28 to 30 — approx. 25 — 26.7 28.7 27
— 33.8 32.8 31.0 — 31.3 — — —
Author(s) Carlson et al.[1] Kieda et al.[3] Brauer and Schnell[6] Arbuzov et al.[7] Arbuzov et al.[7] Khaenko and Fak[8] Khaenko and Fak[8] Khaenko and Fak[8] Fromm and Gebhardt[10]
by electron probe microanalysis (EPMA) measurements of diffusion bands and to derive nitrogen diffusivities. No information on the nitridation of vanadium metal and the reactive diffusion-band formation in V-N2 diffusion couples was available in the literature. II.
EXPERIMENTAL PROCEDURES
Vanadium rods (10-mm diameter, purity .99.8 wt pct V, Heraeus, Hanau, Germany) were cut into 1- to 3-mmthick slices, the
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