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Metallurgy of gamma TiAl based alloys
Zemčík, L. ; Dlouhý, Antonín
The present paper deals with the basic metallurgical processes associated with the investment casting of intermetallic alloys based on gamma TiAl. The analysis focuses on the evaporation of the aluminium from the melt, chemical reactions between the molten intermetallic, the ceramic mould and crucible and also on the reactions between the melt and the residual gases present in the atmosphere of the vacuum furnace. Results of the thermodynamic models are applied to the experimental melts of the Ti-48Al-2Cr-2Nb-1B intermetallic alloy in the vacuum induction furnace.
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Grain boundaries in Ni3Ga as paths of rapid diffusion
Čermák, Jiří ; Růžičková, Jana ; Konecny, M. ; Ipser, H.
Grain boundary diffusivity P (P is a triple product of diffusion coefficient in grain boundary D, grain boundary width .delta. and segregation coefficient s) of nickel was measured by serial sectioning and residual activity methods in temperature interval 973-1373 K. Measurements are compared with our previous results on Ni grain boundary diffusivity in Ni3Al. It was obtained that the values of P for nickel in Ni3Ga are slightly higher than those for nickel in Ni3Al. The observed relation between Ps was interpreted by different affinity of diffusing Ni to different components X in the two Ni3X alloys.
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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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Microstructure and Anisotropy of Creep in Ti-46Al-2W-0.5Si Castings
Dlouhý, Antonín ; Kuchařová, Květa ; Svoboda, Milan ; Válek, Robert ; Březina, Josef
The present study investigates microstructure and creep behaviour of the cast alloy Ti-46A1-2W-0.5Si (at-%) at temperatures 700, 750 and 800°C and applied stresses between 250-420 MPa. Solidification of the alloy during precision casting is associated with two processes: (i) the growth of the [0001].alpha. oriented columnar grains from the mould wall into the cast interior and (ii) the enrichment of the central part of the cast with aluminium. The first process results in a fully lamellar microstructure where lamellae are preferentially oriented parallel to the cast surface. The second process contributes to the development of the duplex microstructure in the central part of the cast. It is shown that distinct microstructural types in different regions of the cast result in different creep strengths. Specimens taken from inside of the cast and oriented parallel to the cast bar axis exhibited the lowest creep strength while similarly oriented specimens cut from the fully lamellar region near the cast surface were the strongest. The third type of the creep specimens cut out of the central region of the cast and oriented perpendicular to the cast bar axis showed the intermediate creep strength. The observed variations of the creep strength are associated with the preferential orientation of the lamellas and with the fraction of equiaxed .gamma.-grains in the duplex regions of the cast.
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