Papers by Keyword: Dislocation Creep

Abstract: This work deals with the prediction of time-to-rupture (TTR) diagrams of martensitic 9-12% Cr steels. Martensitic 9-12% Cr steels are state of the art materials for powerplants due to their high creep strength and oxidation resistance. Since the experimental determination of TTR diagrams is costly and time-expensive (minimum 10 years), it is of particular interest to be able to model TTR diagrams and gradually replace experiments. Here, we approach the question to what extent we can generate a TTR diagram of a material out of a fraction of experimental results plus detailed understanding of the underlying microstructural/physical phenomena during creep. Our model is based on dislocation creep and includes multiple interactions between the microstructural constituents. We show the applicability of our approach by reproducing a TTR diagram of the well-known material P92. Input parameters are basic material data from literature, the starting microstructure before creep, chemical composition, some model parameters determined on the similar material P91, and one single creep curve of P92. The precipitate evolution is simulated by the software MatCalc, the other microstructural constituents (dislocation densities, subgrain boundaries etc.) by our creep model. By varying the stress between individual creep simulations whilst keeping all input parameters (starting microstructure, temperature and material parameters) constant, we produce multiple creep curves and thus generate the complete dataset for a TTR diagram. The model is of particular interest when it comes to the development of new materials, as the application range of these materials can be estimated quickly and with good reproducibility.

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 deformation behavior of AZ31 and AZ91 magnesium alloys was examined by compression tests over a wide strain rate range from 10–3 to 103 s–1 with emphasis on the behavior at high strain rates. The dominant deformation mechanism in the low strain rate range below 10–1 s–1 was suggested to be climb-controlled dislocation creep. On the other hand, experimental results indicated that the deformation at a high strain rate of ~103 s–1 proceeds by conventional plastic flow of dislocation glide and twinning even at elevated temperatures. The solid-solution strengthening was operative for high temperature deformation at ~103 s–1.

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.