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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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Lomové chování kompozitů s křehkou matricí vyztuženou vlákny
Chlup, Zdeněk ; Dlouhý, Ivo ; Boccaccini, A. R. ; Chawla, K. K.
A number of examples exist that indicate the potential for increasing the fracture resistance of brittle matrix by dispersing brittle fibres. For applications development the real composite constituents response during mechanical loading under presence of flaws appears to be important. Theoretical and experimental knowledge sampled on glass matrix composites reinforced by fibres is shortly summarised in the paper. Borosilicate glass with continuous Nicalon fibres and the same matrix containing the chopped carbon fibres as well as the ZrO2 particles (hybrid composite) have been in focus of interests. Chevron notch technique was mainly used for the fracture toughness evaluation. High fracture toughness level of both composites has been proved when testing the as received state. Damage introduced by thermal ageing and/or thermal cycling has been discussed based on fracture toughness values and fracture behaviour analyses
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Přenositelnost lomové houževnatosti z malých těles na tělesa standardních rozměrů
Dlouhý, Ivo ; Chlup, Zdeněk ; Kozák, Vladislav
The capability of pre-cracked Charpy type specimens for prediction of fracture toughness characteristics for standard (1T) bend specimens has been analysed. The toughness scaling model and the master curve methodology have been combined for these purposes. Cast low-carbon manganese steel cut from midthickness of thick plate was used as an experimental material being tested as a candidate material for container casks of spent nuclear fuel. The Dodds-Anderson toughness scaling model was applied for toughness data correction from small scale pre-cracked Charpy specimens. This approach was combined with master curve methodology for the fracture toughness - temperature diagram and loading rate effects predictions supplying good correlation of predicted data with real behaviour
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Lomové chování kompozitů s křehkou (skelnou) matricí
Dlouhý, Ivo ; Chlup, Zdeněk ; Boccaccini, A. R.
A number of examples exist that indicate the potential for increasing the toughness of brittle matrix by dispersing different reinforcements. For further development of these advanced materials the real material response during mechanical loading under presence of flaws appears to be important. Theoretical and experimental knowledge sampled on different kinds of brittle matrix composites is summarised in the paper. These include glass matrix with metallic particles, alumina platelets, continuous Nicalon fibres, both the chopped fibres and ZrO2 particles (hybrid composite). Almost all of these composites were tested in as received state but also after thermal exposition and/or thermal shocking applied according to supposed composite application. Chevron notch technique was mainly used for the fracture toughness evaluation. Damage will be explained on base of fracture micromechanism
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