Reactive diffusion and phase equilibria in the V-C, Nb-C, and Ta-C and Ta-N systems

Journal Article · Mon Jan 05 04:00:00 EST 1998 · Acta Materialia

DOI:https://doi.org/10.1016/S1359-6454(97)00204-8· OSTI ID:616201

Wiesenberger, H; Lengauer, W; Ettmayer, P ^[1]

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1. Vienna Univ. of Technology (Austria). Inst. for Chemical Technology of Inorganic Materials

The reactive formation of vanadium, niobium and tantalum carbides as well as tantalum nitrides was investigated by means of diffusion couples. For the preparation of the carbides the compact metals were reacted with graphite powder and the formation of the phases MeC{sub 1{minus}x} and Me{sub w}C could be observed. The slow growth rate of the Me{sub 4}C{sub 3} phases made it necessary to provide restricted diffusion geometry (wedge-type or plane-sheet samples) or, preferentially, diffusion couples of the type Me{sub 2}C/graphite. The phases {zeta}-V{sub 4}C{sub 3{minus}x}, {zeta}-Nb{sub 4}C{sub 3{minus}x} and {zeta}-Ta{sub 4}C{sub 3{minus}x} could be observed as distinct diffusion bands below 1,900, 1,575 and 2,170 C, respectively. The compositions of the phase boundaries as a function of temperature were measured by EPMA. The composition of {zeta}-V{sub 4}C{sub 3{minus}x} is 37.9 at.%C that of {zeta}-Nb{sub 4}C{sub 3{minus}x} is 40.1--40.7 at.%C and that of {zeta}Ta{sub 4}C{sub 3{minus}x} is 38.2--39.0 at.%C. For the {beta}-Me{sub 2}C phases the carbon content was found to be greater than the values given in recent assessments, whereas the homogeneity regions measured for the {delta}-MeC{sub 1{minus}x} phases are consistent with established data. In the Ta-N system the {var_epsilon}TaN and the {delta}-TaN{sub 1{minus}x} phases usually form very narrow phase bands. Here again wedge-type diffusion samples were applied in order to enlarge the thickness of the phase bands. {delta}-TaN{sub 1{minus}x} undergoes a eutectoid decomposition at 1,821 {+-} 19 C into {var_epsilon}-TaN and {beta}-Ta{sub 2}N. The formation of {var_epsilon}-TaN occurs at 1,888 {+-} 15 C at 25 bar N{sub 2}, its homogeneity range at 1,873 C is smaller than 0.4 at.%N. Re-investigated portions of the phase diagrams of all four systems are given.

• OSTI ID: 616201
• Journal Information: Acta Materialia, Journal Name: Acta Materialia Journal Issue: 2 Vol. 46; ISSN 1359-6454; ISSN ACMAFD
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
NIOBIUM CARBIDES
PHASE DIAGRAMS
PHASE STUDIES
TANTALUM CARBIDES
TANTALUM NITRIDES
TEMPERATURE DEPENDENCE
VANADIUM CARBIDES