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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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MICROSTRUCTURE/PROPERTIES RELATIONSHIP IN ULTRAFINE GRAINED ALUMINIUM PROCESSED BY ECAP TACHNIQUE
Král, Petr ; Dvořák, Jiří ; Svoboda, Milan ; Sklenička, Václav
Experiments were conducted on pure ultrafine grained aluminium processed by severe plastic deformation using the equal ů channel angular pressing ů ECAP technique. By this technique bulk ultrafine grained material was prepared. The propertis of this material considerably differs from properties of material with standard grain size. It was found that microstructure of ultrafine grained aluminium after one pass by the ECAP die contains subgrains and these subgrains are changed by subsequent ECAP passes in grains separated by high angle grain boundaries. In this work special attention is given to the stability of microstructure and the relationship between microstructure and mechanical properties of ECAP material.
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Creep of intermetallic alloy TiAl at very low strain rates
Mareček, P. ; Kloc, Luboš ; Fiala, J.
Intermetallic alloys are very advanced materials for high-temperature applications. There are series of attractive properties exceed coventional up to now used creep resistance steels. The creep behaviour of TiAl alloy was investigated under an applied stress lower than 100 MPa at temperatures 700-850 °C. The strain rate was lower than 10-9 s-1 under testing conditions, which have importance for technical applications. In relation to results, the creep behavior of intermetallic alloy TiAl shows a transition to a mechanism with low stress exponent in the range of minimum strain rates, similar to creep resistance steels with evidence change of deformation mechanism under strain rate 10-10 s-1 . However, this transition in TiAl alloys is not as clear as in creep resistant steels.
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Vliv zatěžovací historie na creep 9% chromové oceli
Kloc, Luboš ; Sklenička, Václav
The creep life assessment is currently based on the phenomenological models mostly derived from the constant stress or constant load creep experiments. Since the loading history of the parts in industry used to be complicated and frequently unknown, the model data can be invalid. Stress change creep experiments were done on the P-91 type creep resistant steel at loading conditions corresponding to the low stress creep regime. The experiment was interrupted, that is cooled down and heated up again several times. The interrupts were done during the ``quasi-steady state'' when the creep rate is stabilized. The effect of both stress changes and temperature cycles on creep properties was analyzed. While the temperature cycle introduce some additional creep strain, the stress changes reduce the creep rate considerably. Taking into account the proportionality between the creep strain and creep damage nucleation, it is possible to conclude that small stress changes during primary stage can improve the creep strength of the steel, while the temperature changes have opposite effect.
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MECHANISMY CREEPOVÉ DEFORMACE U UTRAJEMNOZRNNÉHO HLINÍKU PŘIPRAVENÉHO METODOU ECAP
Dvořák, Jiří ; Sklenička, Václav ; Král, Petr
Extremely coarse-grained aluminium (99,99%) was subjected to severe plastic deformation (ECAP)for obtain an ultrafine-grained microstructure. Creep tensile and compression tests were conducted on such material and for comparison reasons also on coarse-grained aluminium. The results indicate an increased creep resistant of ultrafine-grained aluminium compared to its coarse-grained state. It was proposed, that creep of ultrafine-grained aluminium is probably control by the same creep mechanisms as coarse-grained one. However, the role of grain boundary sliding in creep considerably differs in both ECAP and coarse-grained materials
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