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Kinetics of precipitate formation in (TixWyCrz)B2 solid solutions: influence of Cr concentration and Co impurities

  • Harald Schmidt EMAIL logo , Roland E. Fotsing , Günter Borchardt , Clemens Schmalzried and Rainer Telle
Published/Copyright: January 11, 2022
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Abstract

Supersaturated (Ti0.5 – xW0.5Crx)B2 (x = 0, 0.1, 0,2) solid solutions were produced by reaction sintering of transition metal diboride powders at a pressure of 60 MPa at 1800 °C. The formation kinetics of W2B5 precipitates were investigated at lower temperatures by X-ray diffractometry (XRD) and secondary electron microscopy (SEM). The precipitation rate constants as a function of reciprocal temperature show non-monotonic behaviour in the range between 1450 and 1700 °C with a maximum at about 1650 °C. At low temperatures, Arrhenius behaviour is approximately valid, where the precipitation kinetics are dominated by the diffusion of the transition metals. In contrast, at temperatures higher than 1650 °C, the precipitation rates decrease rapidly, since supersaturation, and consequently nucleation, is strongly reduced. The rate constants of precipitation are identical within error limits for Cr concentrations of x = 0.1 and x = 0.2, however, they are significantly lower for x = 0. This means that the addition of small amounts of Cr accelerates the precipitate formation, while further addition has no further influence. Samples resulting from powders which were high-energy milled with WC/Co hard balls before reaction sintering show an increased density of precipitated crystallites but no increased rate constants of precipitation. This result can be explained under the assumption that for these samples a significant amount of additional pre-existing nuclei was present before precipitation starts, which were formed by heterogeneous nucleation at Co impurities, introduced by the milling process.


Priv.-Doz. Dr. Harald Schmidt TU Clausthal IMET/AG Thermochemie und Mikrokinetik Robert-Koch-Str. 42, D-38678 Clausthal-Zellerfeld, Germany Tel.: +49 5323 72 2094 Fax: +49 5323 72 3184

  1. The authors would like to thank E. Ebeling for ceramographic preparation of the samples and for technical support and B. Mühlhan and S. Lenk for their assistance in SEM analysis. This work has been funded by the German Research Foundation (DFG).

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Received: 2005-08-18
Accepted: 2006-02-06
Published Online: 2022-01-11

© 2006 Carl Hanser Verlag, München

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  29. Microstructure and mechanical behavior of Pt-modified NiAl diffusion coatings
  30. Evolution of C-rich SiOC ceramics
  31. Evolution of C-rich SiOC ceramics
  32. Nanostructured SiC/BN/C ceramics derived from mixtures of B3N3H6 and [HSi(Me)C≡C]n
  33. Thermodynamic analysis of structural transformations induced by annealing of amorphous Si–C–N ceramics derived from polymer precursors
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  35. Thermodynamic assessment of the Ce–O system in solid state from 60 to 67 mol.% O
  36. Phase transformations of iron nitrides at low temperatures (< 700 K) – application of mechanical mixtures of powders of nitrides and iron
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  41. High-temperature plasticity of SiC sintered with Lu2O3-AlN additives
  42. Interaction of functionalised surfaces on silica with dissolved metal cations in aqueous solutions
  43. XRD and TEM study of NiO–LSGM reactivity
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