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Validity of master curve for dynamic fracture toughness characteristics
Dlouhý, Ivo ; Lenkey, G. ; Holzmann, Miloslav
Master curve methodology have been investigated experimentally based on data obtained with cast ferritic steel being considered for storage containers of spent nuclear fuel. Standard pre-cracked specimens have been tested by means of instrumented drop weight tower, pre-cracked Charpy specimens by means of Instrumented impact tester. Different methodologies of reference temperature determination have been followed. Transferability of data from small pre-cracked specimens to standard ones has been also followed.
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Problems in Theories of Low Stress Creep Mechanisms
Kloc, Luboš ; Sklenička, Václav ; Fiala, J.
Viscous creep behaviour of many materials including creep resistant steels was observed at temperatures close to one half of absolute melting point and at very low stresses. The experimental results were interpreted as Coble diffusional creep and/or Harper-Dorn dislocation creep. Some of the results are in a very good agreement with the Coble theory of diffusional creep and support an idea that the diffusional creep is an important deformation mechanism under certain loading conditions. Nevertheless, another results are not fully compatible with the theoretical predictions. Some authors dispute the role or even the very existence of the diffusional creep and offer the Harper-Dorn creep mechanisms instead. In fact, there are many theories trying to deescribe Harper-Dorn creep mechanism, but none of them is capable to explain all observed properties. Transition (or primary) stage observed at low stress creep regime exhibits also some properties which contradict all existing theories. The observed effects which cannot be explained by the current theories are: i) a large scatter of creep rates observed for coarse grain materials, ii) creep rates much higher than those predicted by the diffusional creep theory while the creep rate dependence on grain size is maintained, iii) transition stage duration is independent of stress, iv) transition strain is dependent on temperature. Despite the problems in theoretical description, the experiments show that the viscous creep regime in structural materials must be considered as an important behaviour of the structural materials.
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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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Dislocation structure in the HAZ of the HSS steel after shot peening
Kruml, Tomáš ; Buirette, C. ; Degallaix, G. ; Polák, Jaroslav
The welds of a high strength C-Mn microalloyed steel with ferritic-bainitic microstructure were shot peened in order to improve the fatigue resistance. Since the weakest point was the heat affected zone (HAZ), the dislocation structure in two depth off the zone was investigated before and after shot peening. No specific dislocation arrangement was found neither in the initial state nor after shot peening and the arrangement can be characterised as random. The dislocation density was elevated in the initial state due to bainitic transformation. The shot peening resulted in appreciable increase of the dislocation density with a very strong gradient. The results were discussed in terms of the effect of surface hard layer induced by shot peening on the fatigue life of the weld and on the initiation and growth of fatigue crack in HAZ.
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