Papers by Keyword: Diffusional 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 MoSi₂ has 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.

Abstract: High-temperature creep experiments were performed on a Cu-2.8 ат.% Co solid solution. Cylindrical foils of 18 micrometers thickness were used for this purpose. Creep tests were performed in a hydrogen atmosphere in the temperature range of about from 1233 K to 1343 K and at stresses lower than 0.25 MPa. For comparison, a foil of pure copper and Cu-20 at.% Ni solid solution were investigated on high temperature creep. Measurements on the Cu foil showed classical diffusional creep behavior. The activation energy of creep was defined and turned out to be equal 203 kJ/mol, which is close to the activation energy of bulk self-diffusion of copper. There was a significant increase in activation energy for the Cu-20 at.% Ni solid solution. Its activation energy was about 273 kJ/mol. The creep behavior of Cu-Co solid solution was more complicated. There were two stages of diffusional creep at different temperatures. The extremely large activation energy (about 480 kJ/mol) was determined at relatively low temperature and a small activation energy (about 105 kJ/mol) was found at high temperatures. The creep rate of Cu-Co solid solution was lower than that of pure copper at all temperatures. In addition, the free surface tension of Cu-2.8 ат.% Co was measured at different temperatures from 1242 K to 1352 K. The surface tension increases in this temperature range from 1.6 N/m to 1.75 N/m. There were no features on the temperature dependence of the surface tension.

Abstract: The reaction induced phase separation aimed for the distribution of nano-structured particles has been investigated by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) in ternary Ti-Si-N film via N+ implantation. The fabrication of Ti-20at%Si film has made on Si substrates by ion beam sputtering (IBS), and then N+ implantation with 50 keV has been conducted on these films. The selected area electron diffraction (SAED) from as-deposited film shows amorphous Ti-Si. As-deposited Ti-Si film exhibited high stability even for the heat treatment at 773K for 3600s. N+ implantation induced the direct formation of nano crystalline of fcc-TiNx within the Ti-Si film. The XPS depth profiling and chemical shift suggest that the preferential nitriding of Ti accompanied with the segregation of SiNx occurred during N-implantation.

Abstract: In order to analyze high temperature deformation behavior of NiAl alloys, deformation maps were constructed for stoichiometric NiAl materials with grain sizes of 4 and 200 µm. Relevant constitute equations and calculation method will be described in this paper. These maps are particularly useful in identifying the location of testing domains, such as creep and tensile tests, in relation to the stress-temperature-strain rate domains experienced by NiAl.