Molybdenum Disilicide - Diffusion, Defects, Diffusion Correlation, and Creep

Helmut Mehrer; Hans Eckhardt Schaefer; Irina V. Belova; Graeme E. Murch

doi:10.4028/www.scientific.net/DDF.322.107

Paper Titles

Interface Controlled Diffusional Creep of Cu + 2.8 at.% Co Solid Solution
p.33

Reactive Diffusion at the Contact of a Solid Phase with the Solder Melt
p.41

Determination of Intrinsic Diffusion Coefficients in Binary Alloys with Variable Molar Volume by the M-M Method
p.73

The Coke and Iron Ore Materials Kinetic Characteristics and Quantitative Indicators of Blast Furnace Process
p.87

Molybdenum Disilicide - Diffusion, Defects, Diffusion Correlation, and Creep
p.107

Original Methods for Diffusion Measurements in Polycrystalline Thin Films
p.129

Influence of Deformation on Precipitation Kinetics in Mg-Tb Alloy
p.151

Duplex Stainless Steels: A Dozen of Significant Phase Transformations
p.163

Hydrogen and Electric Field Effect on Iron Impurities Removal from Molten Zirconium
p.175

HomeDefect and Diffusion ForumDefect and Diffusion Forum Vol. 322Molybdenum Disilicide - Diffusion, Defects,...

Molybdenum Disilicide - Diffusion, Defects, Diffusion Correlation, and Creep

Abstract:

Molybdenum disilicide (MoSi₂) is an interesting material for high-temperature applications. It has a high melting temperature, good thermal and electrical conductivity and an excellent oxidation resistance. For many years the primary use of MoSi₂has been in heating elements, which can be used for temperatures up to 1800°C. Since the 1990s the potential of MoSi₂ as a high-temperature structural material has been recognized as well. Its brittleness at lower temperatures and a poor creep resistance above 1200°C have hindered its use as in load-bearing parts. These disadvantages may be offset at least partly by using it together with a second material in a composite or an alloy. Projected applications of MoSi₂-based materials include, e.g. stationary hot section components in gas turbine engines and glow plugs in diesel engines. For future research and development directions of MoSi₂-based composites diffusion is a crucial property because creep is closely connected with diffusion. This paper is devoted to the basic diffusion and defect properties of MoSi₂. Data of Si and Mo as well as Ge diffusion from the Münster laboratory for both principal directions are briefly summarized. For all three kinds of atoms diffusion perpendicular to the tetragonal axis is faster than parallel to it. The diffusivities of Mo in both directions are many orders of magnitude slower than those of Si and Ge. The huge asymmetry between Mo and Si (or Ge) diffusion suggests that atomic motion of each constituent is restricted to its own sublattice. Positron annihilation studies on MoSi₂ from the Stuttgart laboratory are reviewed as well. They show that formation of thermal vacancies occurs primarily on the Si sublattice but cannot exclude vacancy formation on the Mo sublattice at higher temperatures. Correlation factors for Si and Mo diffusion via sublattice vacancies in the respective sublattices of MoSi₂ have been calculated recently mainly by Monte Carlo simulation techniques and are also briefly described. Diffusion, in particular self-diffusion, is discussed in connection with literature data on high-temperature creep, which is diffusion-controlled. Grain-size effects of creep have been reported and can be attributed to Nabarro-Herring and Coble creep. Power-law creep is attributed to diffusion-controlled dislocation creep. Some details are, however, not completely understood, presumably due to a lack of theoretical concepts for creep in uniaxial, stochiometric compounds and due to missing information on grain-boundary diffusion.