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Creepové zkoušky zirkoniových povlakových tyčí jaderného paliva a jejich interpretace
Sklenička, Václav ; Kuchařová, Květa ; Kloc, Luboš ; Dvořák, Jiří ; Král, Petr ; Kvapilová, Marie ; Vrtílková, V. ; Krejčí, J.
Časově závislá plastická deformace-creep je jedním z nejdůležitějších degradačních faktorů, určujících životnost zirkoniových slitin, určených k pokrytí jaderného paliva vodou chlazených reaktorů v jaderné energetice. Příspěvek je věnován creepovým zkouškám povlakových tyčí slitiny Zr1%Nb (modifikace slitiny E110) používaných v reaktorech typu VVER. Vedle standardní creepové zkoušky při konstantním tahovém napětí bude demonstrována nestandardní speciální creepová zkušební metoda umožňující měření velmi malých creepových deformací při napětích blížících se reálným podmínkám namáhání v reálné praxi.
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Influence of severe plastic deformation and subsequent annealing on creep behaviour of martensitic 9% Cr steel
Král, Petr ; Dvořák, Jiří ; Sklenička, Václav ; Horita, Z. ; Tokizawa, Y. ; Tang, Y. ; Kunčická, Lenka ; Kuchařová, Květa ; Kvapilová, Marie ; Svobodová, M.
The objective of the study is to evaluate the effects of severe plastic deformation (SPD) and annealing on creep behaviour of advanced tungsten modified creep-resistant 9 % Cr martensitic P92 steel. The as-received P92 steel was deformed by high-pressure torsion (HTP), high-pressure sliding (HPS) and rotary swaging (RS) at room temperature prior creep testing. These SPD methods imposed significantly different equivalent plastic strain in the range from 1 up to 20. Constant load creep tests in tension were performed in an argon atmosphere at 873 K and applied stress ranging from 50 to 200 MPa. The microstructure and phase composition of P92 steel were studied using a scanning electron microscope Tescan Lyra 3 and a transmission electron microscope Jeol 2100F. The results show that under the same creep loading conditions the HPT and HPS-processed P92 steel exhibited significantly faster minimum creep rates, creep fracture strain and the decrease in the value of the stress exponent of the creep rate in comparison with as-received P92 steel. However, it was revealed that the RS-processed specimens exhibited one order of magnitude lower minimum creep rate and lower ductility compared to commercial P92 steel. The creep curves for the HPT and HPS-processed states exhibited a pronounced minimum of strain rate. The pronounced minimum of strain rate disappeared when these states were annealed at 923K/500h before application of creep loading. The microstructure changes occurring during creep and different creep behaviour between as-received and deformed states are discussed.
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Development of Creep Damage in Similar Weld Joints of P92 Steel Pipe
Král, Petr ; Sklenička, Václav ; Kuchařová, Květa ; Svobodová, M. ; Kvapilová, Marie ; Dvořák, Jiří
The microstructure and creep behaviour of the welded joints of P92 steel pipe were\ninvestigated in order to determine the influence of orbital heat welding technology on the creep\nresistance. Creep specimens were machined from the welded joints. Tensile creep tests of welded\njoints were performed at 873 K using different stresses. The microstructure of tested specimens was\ninvestigated by scanning electron microscope Tescan equipped with an electron-back scatter\ndiffraction. The creep results showed that the creep fracture strain of the welded joints decreases\nwith decreasing value of applied stress. Microstructure investigation showed that fracture behaviour\nof welded joints is influenced by an enhanced cavity formation at grain boundaries in the heataffected\nzone causing lower fracture ductility.
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Creep damage tolerance factor lambda of selected creep-resistant steels
Sklenička, Václav ; Kuchařová, Květa ; Dvořák, Jiří ; Kvapilová, Marie ; Král, Petr
The creep damage tolerance factor lambda as an important outcome of the continuum damage mechanics approach has been used to asses the creep fracture mode and the susceptibility of material to localized cracking at strain concentrations. In this work, using sets of our earlier published creep data of three advanced ferritic creep-resistant steels (T23 low alloy steel, P91 and P92 chromium steels) are analysed in terms of the creep damage tolerance factor lambda. It was found that the value of the creep damage factor lambda is not constant and depends on the creep loading conditions. The data analysis is followed by fractographic investigations, which is used to identify the creep fracture mode(s) experimentally.
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METALLURGY AND PROPERTIES OF ADVANCED NiAl-Mo EUTECTICS
Barták, Tomáš ; Kuchařová, Květa ; Záležák, Tomáš ; Dlouhý, Antonín
A NiAl-Mo eutectic alloy was melt from 99,99% purity components and cast by the drop casting technique. The drop-cast ternary alloy (nominal composition of Ni-45Al-9Mo at. %), was re-melted and directionally solidified using a high temperature optical floating zone furnace. A resulting in-situ composite consists of Ni-45,2Al matrix and Mo-10Al-4Ni fibers, all in at. %. The volume fraction of 14% Mo-fibers stems from the eutectic composition. Spacing and a diameter of Mo-fibers can be controlled within certain limits using different growth rates of the crystals. Microstructural parameters of the as-cast crystals were assessed by light microscopy, scanning and transmission electron microscopy. Backscatter diffraction shows that the NiAl-matrix and the Mo-fibers are both < 001 >-oriented with respect to the axis of the cylindrical rods. Preliminary creep experiments confirmed an immense improvement of high temperature strength due to the fine distribution of Mo-fibres. The amount of strengthening in terms of minimum creep rate can be as high as 7 orders of magnitude. Post-mortem transmission electron microscopy experiments provided evidence that creep in the temperature range of 800-900 degrees C results in an extensive formation of subgrain boundaries. The strengthening effect is very likely associated with the reactions between subgrain boundaries and fine Mo-fibres.
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METALLURGY AND PROPERTIES OF ADVANCED NiAl-Mo EUTECTICS
Barták, Tomáš ; Kuchařová, Květa ; Záležák, Tomáš ; Dlouhý, Antonín
A NiAl-Mo eutectic alloy was melt from 99,99% purity components and cast by the drop casting technique. The drop-cast ternary alloy (nominal composition of Ni-45Al-9Mo at. %), was re-melted and directionally solidified using a high temperature optical floating zone furnace. A resulting in-situ composite consists of Ni-45,2Al matrix and Mo-10Al-4Ni fibers, all in at. %. The volume fraction of 14% Mo-fibers stems from the eutectic composition. Backscatter diffraction shows that the NiAl-matrix and the Mo-fibers are both < 001 >-oriented with respect to the axis of the cylindrical rods. Preliminary creep experiments confirmed an immense improvement of high temperature strength due to the fine distribution of Mo-fibres. The amount of strengthening in terms of minimum creep rate can be as high as 7 orders of magnitude. Post-mortem transmission electron microscopy experiments provided evidence that creep in the temperature range of 800 - 900 degrees C results in an extensive formation of subgrain boundaries. The strengthening effect is very likely associated with the reactions between subgrain boundaries and fine Mo-fibres.
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Comparison of Creep Behaviour and Microstructural Changes in Electrodeposited Ultrafine-grained Nickel and its Particle-reinforced Nanocomposite
Kvapilová, Marie ; Sklenička, Václav ; Kuchařová, Květa ; Vidrich, G.
On the basic of the creep experimental results the paper reports comparison creep behaviour of electrodeposited ultrafine-grained nickel and its particle-reinforced nanocomposite. The objective of this research was to further improve the knowledge of the creep behaviour of monolithic nickel and to explore the role of nano-sized SiO2 particles in the potential creep strengthening of electrodeposited Ni nanocomposite. The creep behaviour and microstructure of the pure ultrafine-grained nickel and its nanocomposite reinforced by 2 vol.% nano-sized SiO2 particles were studied at temperatures in the range from 293 to 573 K and at the tensile stresses between 100 to 800 MPa. It was found that the creep resistance of the nanocomposite might be noticeably improved compared to the monolithic nickel. Possible creep mechanisms controlling the creep rate during the creep loading were discussed. The Coble creep as creep rate controlling mechanism was founded improbable.
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Creep in NiAl-Mo Eutectics
Dudová, Marie ; Barták, Tomáš ; Kuchařová, Květa ; Dlouhý, Antonín
We report on creep in NiAl-Mo ternary eutectic and NiAl intermetallic, having respective nominal compositions Ni-45.5Al-9Mo and Ni-45.2Al (at.%). These alloys were directionally solidified in a high-temperature optical floating zone furnace. The eutectic alloy exhibited a well-aligned rod-like microstructure, consisting of NiAl matrix and 14% (by volume) continuous Mo-fibres oriented along a [001]-crystal growth direction. Cylindrical [001]-oriented specimens were loaded in compression at temperatures in a range 1073 – 1173 K. Formidable strengthening by regularly distributed fine fibres (typical diameter, 400 nm) was observed which resulted in minimum creep rates of the NiAl-Mo eutectic that were seven orders of magnitude lower than the corresponding minimum creep rates of the NiAl matrix. Preliminary microstructural investigations suggested that the strengthening effect is due to an interaction between Mo fibres and subgrain boundaries that form in the course of creep. This interaction leads to an increase of dislocation density in the vicinity of fibres and to an efficient redistribution of stresses in the microstructure.
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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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