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A new method of estimation of diffusion characteristics in ternary systems
Čermák, Jiří ; Rothová, Věra ; Gazda, Aleš
A method was proposed based on an estimation of components of diffusion matrix, Dij, in ternary systems. It makes possible to obtain the concentration dependence of DijŽs in a wide range of concentrations using one diffusion couple only. It is in contrast to the conventional procedure that needs two diffusion couples and provides DijŽs for a single composition related to an intersection of diffusion paths. The method was applied to gammaŽ one-phase interdiffusion in Ni3Al-X (X = Cr, Fe, Nb, Ti) in temperature interval 1173 ů 1533 K.
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In-situ monitoring of phase transformation in Ni3Al based alloys by means of hydrogen permeation measurement
Rothová, Věra ; Čermák, Jiří
Nickel aluminide Ni3Al crystallizes in the cubic L12 structure (Cu3Au type). It is known from the literature that this structure remains unchanged up to the melting point. However, under certain circumstances, it can change from L12 to other non-cubic ordered crystal structures. There have been reported some data which indicate that the presence of hydrogen may facilitate the change in the structure. Present detailed study of hydrogen permeation behavior in both the Ni3Al and the CMSX-4 has shown very sensitive changes of permeation rate depending upon temperature variations.
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Diffusion of Ni and Ga along High-Diffusivity Paths in polycrystalline Ni3Ga
Čermák, Jiří ; Rothová, Věra
The volume and grain boundary (GB) diffusion of 63Ni and 67Ga radiotracers in polycrystals of Ni3Ga intermetallic was studied in the vicinity of stoichiometric composition A3B (23.5 - 28.3 at. % Ga). Both volume- and GB diffusivity was measured in the temperature interval 773 - 1373 K by residual activity method. Obtained Ga volume diffusivity increases with increasing Ga concentration x(Ga); the increasing tendency of Ni volume diffusivity with increasing x(Ga) is pronounced especially at lower temperatures. GB diffusivity of both Ga and Ni in Ni3Ga are close one to another and contrary to diffusion behavior of components in Ni3Al - they do not show any significant dependence on x(Ga).
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Comparison of Al/Ni and Co/Ni Interdiffusion Coefficients in Pseudobinary Sections of Ni3Al-Co alloys
Gazda, Aleš ; Rothová, Věra ; Čermák, Jiří
Interdiffusion in Ni3Al intermetallic alloyed by 0 - 19 at % Co was studied in three pseudo-binary sections: c(Al) = 22, c(Co) = 3.5, and c(Co) = 16 (c in at. %). The experiments were carried out at temperatures between 900 and 1200°C. Since no detectable shift of tungsten inert markers was observed, it was concluded that the intrinsic diffusivities of element pairs with varying concentration (Al/Ni and Co/Ni) in respective pseudo-binary sections are approximately equal one to another. The interdiffusion in pseudo-binary alloys Al/Ni (c(Co) = const.) runs slightly faster than that in pseudo-binary alloys Ni/Co (c(Al) = const.). The relations between the measured interdiffusion coefficients in the Ni3Al-Co ordered gamma prime phase agree well with literature data for the interdiffusion coefficients in both pure gamma prime and Co alloyed gamma prime phases and also for the disordered Al-Ni and Co-Ni solid solutions. They are qualitatively explained by interaction between constituents and/or by their interaction with vacancies.
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Degradation of protective effect of surface layer on Ni3Al-Cr in hydrogen environment
Čermák, Jiří ; Rothová, Věra
Protective effect of surface layer on Cr-doped (4 and 8 at. % Cr) Ni3Al intermetallic was studied by high-temperature hydrogen permeation tests that were carried out at temperatures in the interval 775-1223 K. Hydrogen with about 1 vol. ppm O2 and 2 vol. ppm of H2O was used for the gas-permeation tests. It was observed that the addition of Cr degrades the - otherwise very good - high-temperature protective ability of oxide-containing surface layer of Ni3Al-Cr at strongly reducing hydrogen environment.
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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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