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Chemical composition of (310) [100] symmetrical tilt grain boundary in Ni3Al
Čermák, Jiří
Structure of . .SIGMA.5 (310) [100] symmetrical tilt grain boundary in ordered intermetallic alloy Ni3Al was constructed and relaxed by Monte Carlo method using pair L-J - type interatomic potential. Three modified grain boundary structures were tested. They differed in chemical composition and in relative grain boundary energy. The configuration with the lowest grain boundary energy was enriched on nickel.
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Modification of transmissivity of hydrogen through the surface of Ni3Al by alloying
Čermák, Jiří ; Rothová, Věra
Hydrogen invasion into Ni3Al-M was studied by gas-permeation method in temperature range 573 - 1 223 K. The dependence of hydrogen permeation flux J(d) through the membrane-like samples on the samples thickness d enabled us to assess the influence of the surface upon the rate of permeation. The surface transmissivity was introduced as a quantitative characteristic describing the rate of the process. Influence of three modifying elements M = Fe, Cr and Zr was studied. It was found that Zr supports and, on the contrary, Cr eliminates the inhibiting effect of the surface. The iron did not alter significantly the hydrogen transmissivity, and it causes enhanced grain boundary brittleness in hydrogen environment.
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Creep Resistance and Microstructural Stability of TAF 650 Steel
Svoboda, Milan ; Fejfarová, Ivana ; Kroupa, Aleš ; Sklenička, Václav ; Mayer, K. H.
The quantification of the particles of M23C6 and Laves phase in TAF 650 steel by means of image analysis and thermodynamic calculation of the amount of equilibrium phases were performed. It is shown that the growth rate of Laves phase is substantially higher than that of M23C6 carbide. The total area occupied by Laves phase precipitates represents 52.1 % of all particles area after creep (5888h, 100 MPa, 650 °C). The equilibrium amount of secondary phases calculated by THERMO-Calc is in good agreement with experimental results.
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Comparison of site preferences of 57Co at grain boundaries in Fe-Si and Fe-Al alloys
Schneeweiss, Oldřich ; Jirásková, Yvonna ; Čermák, Jiří ; Lejček, P.
Emission Moessbauer spectroscopy was applied to the study of grain boundaries in the polycrystalline samples of iron rich Fe-Al and Fe-Si alloys. The results obtained on Fe-Al samples indicate complicated phase and chemical composition along grain boundaries (GBs). Neither homogeneous increase in Al concentration at GBs nor important site preference of Co atoms is observed. A large fraction of Co atoms occupy positions of lower symmetry. The results on Fe-Si show differencies in the chemical composition in grain interiors (GIs) and at GBs. An increase in Si concentration at GBs is observed there. In Fe-! & at.%Si sample a strong site preference of cobalt atoms at GBs is detected. The diffusing Co atoms prefer the lattice sites, which are expected to be occupied by structural vacancies in the D03 superstructure.
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