Incongruent reduction of tungsten carbide by a zirconium-copper melt
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The reduction of tungsten carbide (WC) to elemental tungsten by reaction with a Zr–Cu melt was examined. Dense WC disks were immersed in a vertical orientation in molten Zr2Cu at 1150–1400 °C for 1.5–24 h. Continuous, adherent layers of W and ZrC formed at WC/melt interfaces. The rates of thickening of the W and ZrC product layers were examined as a function of reaction time and temperature and position along the vertical WC surface. Such kinetic data, along with microstructural analyses, indicate that the incongruent reduction of tungsten carbide is likely to be controlled by carbon diffusion through one or both of the product layers.
I. INTRODUCTION
Composite materials comprised of refractory metals (e.g., W, Mo, Ta, Re) and carbides (e.g., HfC, ZrC, TiC, TaC) can possess attractive combinations of chemical, thermal, and mechanical properties.1–9 Such composites can have higher hardness, greater resistance to wear and creep, and reduced weight relative to refractory metals, and can also exhibit higher fracture strengths, higher toughnesses, and improved thermal shock resistance relative to monolithic carbides. Hence, these materials can be attractive for a variety of applications in the aerospace/ aircraft, automotive, energy production, materials processing, defense, and other industries.7–11 Among the most extreme environments in aerospace applications can be found in the throat regions of solidfueled rockets, where combustion products can exceed 2500 °C and flow at supersonic velocities.9,10 Although refractory metals, such as W and Re (Tm[W] ⳱ 3422 °C, Tm[Re] ⳱ 3186 °C), may be used under such conditions, these materials possess high specific densities ([W] ⳱ 19.3 × 103 kg/m3, [Re] ⳱ 21.0 × 103 kg/m3).9,12,13 Refractory metal/carbide composites, such as W/ZrC composites, can be attractive alternatives to pure refractory metals. A composite comprised of equal volumes of W and ZrC is 33% lighter than pure W (12.9 × 103 versus 19.3 × 103 kg/m3).13 Such composites are also thermally, chemically, and mechanically compatible. Unlike most a)
Present address: Solid State Chemistry Department, Faculty of Materials Science & Ceramics, University of Mining & Metallurgy, al. A. Mickiewicza 30, 30-059 Krakow, Poland. b) Address all correspondence to this author. Present address: School of Materials Science and Engineering, Georgia Institute of Technology, 711 Ferst Drive, Atlanta, GA 30332-0245. e-mail: [email protected] J. Mater. Res., Vol. 18, No. 9, Sep 2003
metal/ceramic composites, the average linear thermal expansion coefficients of W and ZrC are similar at 20 °C (4.5 × 10−6/K versus 4.0 × 10−6/K, respectively) and at 2700 °C (9.2 × 10−6/K versus 10.2 × 10−6/K, respectively).4,5 As a result, W/ZrC composites have exhibited excellent resistance to thermal shock at surface heating rates of ≈2000 °C/s.8 Like tungsten, zirconium carbide melts at a high temperature (i.e., up to 3445 °C).3 ZrC and W exhibit limited mutual solid solubilities and do not form intermediate compounds.3 ZrC can endow a coconti
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