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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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Investment casting of gamma TiAl intermetallics
Zemčík, L. ; Dlouhý, Antonín
The paper deals with the physical and chemical analyses of the above processes and gives their evaluation based on the results of experimental melts of the type of Ti48Al2Cr2Nb1B (at.%) obtained in vacuum induction furnace. In present study introduces a 1D model of heat flow in the system composed of a TiAl intermetallic casting, a ceramic shell mould and the vacuum furnace environment. Furthermore, a coupled 1D model of thermal strain evolves in the casting ů mould system during cooling to room temperature is also presented. The parametric study aimed at the minimization of thermal stresses provided optimum process parameters used later in investment casting of TiAl turbocharger wheels. The semi-optimized initial mould temperatures and cooling kinetics resulted in macroscopically sound castings that were free of misruns, macrocracks and internal porosity.
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Study of transition layers in a gamma TiAl - CaO - Al2O3 SYSTEM
Dočekalová, Kateřina ; Dlouhý, Antonín ; Zemčík, L. ; Fiala, J.
The main attention in the present study focuses on the reactions between a ceramic crucible and the molten intermetallic alloy. The qualitative and quantitative metallographic analysis investigates the transition layers which are built during the melting process on the CaO-coated surface of the Al2O3 crucible. Preliminary results suggest that relevant chemical reactions are limited to a boundary layer with a characteristic thickness of 200 mm. In this layer, the high-temperature diffusion contributes to the nucleation and growth of complex phases the origin of which could result in an enrichment of the melt by oxygen.
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Quantitative image analysis of precipitates in Ni-rich NiTi shape memory alloys
Bojda, Ondřej ; Dlouhý, Antonín ; Fiala, J.
A microstructure of a Ti-50.7at%Ni shape memory alloy resulting from solution annealing and subsequent aging at 500°C/3.6ks/8MPa is quantitatively analysed. The TEM study is assisted by Adaptive Contrast Control (ACC) routines embedded in a recent image processing system. The assessment focuses on the number density, size and distribution of crystallographic variants of Ni4Ti3 precipitates. It was found that the distribution of crystallographic particle variants in the microstructure is inhomogeneous. The influence of the heterogeneous Ni4Ti3 precipitation on the characteristics of the martensitic transformation B2®B19´ is briefly discussed.
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