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Surface relief evolution in cast superalloy In738LC fatigued at room temperature
Samek, Petr ; Obrtlík, Karel (referee) ; Juliš, Martin (advisor)
Low cycle fatigue is an important valving parameter of materiale which are exposed random alternate strain during their operation. The alternate strain in that material is caused by temperature fluctuations during operation and outages such as aircraft engines. Tests of low cycle fatigue were performed on samples of superalloy Inconel 738LC at stable room temperature at 23°C. The actual experiment took place at certain intervals, consisting of cycling itself, and observing changes in surface relief by light and electron microscopy. There was observed significant surface relief at an early stage of low cycle fatigue. We compared results of measurement with other different observation methods.
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Determination of Shortcut Cyclic Stress-strain Curves of Superalloy Inconel 738LC at Elevated Temperatures
Šmíd, Miroslav ; Obrtlík, Karel (referee) ; Petrenec, Martin (advisor)
Multiple step tests under cyclic strain control have been performed using cylindrical specimens of cast polycrystalline Inconel 738LC superalloy at 23, 700, 500, 800 and 900 °C in laboratory atmosphere to obtain cyclic stress-strain curves. During cyclic straining of specimen were obtained cyclic hardening-softening curves. Their progress changed with temperature and strain amplitude. Evaluated cyclic stress-strain curves are shifted to lower stresses with increasing temperature. Surface relief was observed in fatigued specimens under SEM and metalography under optic microscopy. Slip markings were studied on specimen surface fatigued at 700 °C .Stress-strain response is compared and discussed in relation to the surface observations - persistent slip markings.
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THE EFFECT OF MO AND/OR C ADDITION ON MICROSTRUCTURE AND PROPERTIES OF TIAL ALLOYS
Chlupová, Alice ; Kruml, Tomáš ; Roupcová, Pavla ; Heczko, Milan ; Obrtlík, Karel ; Beran, Přemysl
Cast TiAl alloys with high Nb content are subject of extensive research with the aim to develop material with low density, good corrosion resistance and high strength at elevated temperatures. Disadvantage of their broad applications is restricted workability, machinability and low fracture toughness especially at room temperature. Improvement of properties of TiAl based materials can be achieved by tailoring the microstructure by modification of chemical composition. For this purpose 5 types of TiAl alloys with 7 % of Nb were prepared having variable content of Mo and/or C. Addition of Mo and/or C resulted in three types of microstructure and different phase composition. All modified alloys contain colonies consisting of thin lamellae of a and g phases sometimes complemented by g and/or b phase at the grain boundaries. Variable microstructure and phase composition resulted in differences in mechanical behaviour. The most promising tensile properties at both room and elevated temperature were observed for alloy doped with 2 % of Mo having the mixed microstructure containing b phase and for alloy doped with 0.5 % of C with nearly lamellar microstructure without b phase. 2Mo alloy exhibited reasonably good ductility while 0.5C alloy reached the highest tensile strength. Also low cycle fatigue behaviour of these two materials was the best of all five materials under investigation. Fatigue deformation characteristics were better in the case of 2Mo alloy while 0.5C alloy exhibited higher cyclic stresses. Fracture mechanisms were determined using fractographic analysis. The major fracture mode of all alloys was trans-lamellar.
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Thermo-mechanical and isothermal fatigue behavior of austenitic stainless steel AISI 316L
Škorík, Viktor ; Šulák, Ivo ; Obrtlík, Karel ; Polák, Jaroslav
Many structural components of nuclear power plant systems are made of austenitic stainless steels. These structures undergo degradation by thermo-mechanical fatigue (TMF) caused by simultaneous cyclic straining and temperature cycling, particularly during start-up, shut-down and transient operations. The present work reports the cyclic deformation behavior and fatigue damage of austenitic stainless steel AISI 316L during TMF and isothermal fatigue (IF) testing in air. Total strain controlled in-phase TMF loading in the temperature range 200 - 600 °C and isothermal fatigue (IF) at 600 °C were performed. Hardening/softening curves, cyclic stress-strain response and fatigue life diagrams were obtained both for TMF and IF tests. Fatigue damage was documented using surface relief and fracture surface observations. Mean stress evolution and fatigue degradation data are employed to discuss the fatigue behavior of 316L steel both in TMF and IF regimes.
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Effect of grit blasting surface treatment on high temperature low cycle fatigue life of Inconel 713LC
Šulák, Ivo ; Obrtlík, Karel ; Čelko, L. ; Gejdoš, P.
This paper presents the results of high temperature low cycle fatigue tests carried out on Inconel 713LC. This polycrystalline nickel based superalloys is mainly used for high temperature applications such as disks, blades and vanes of gas turbine engines. These machine components are manufactured by investment casting and undergo a series of postcast processes to remove remnants of shells, oxides and other contaminants. Grit blasting is one of the most widely used and cheapest technology ensuring adequate cleaning of the surface and at the same time is used to treat the substrate surface prior to application of metalic bond coat and various protective coatings (like diffusion, overlay or thermall barier coatings). Low cycle fatigue tests were conducted on as-received and grit blasted material in fully reversed pull-push cycle in strain control mode with constant total strain amplitude and strain rate at 900 °C in air. Experimental data on fatigue life were evaluated and parameters of Baquin and Coffin-Manson curves were obtained for both tested materials. Specimen surface and cross sections of both treated and untreated materials were analysed prior to fatigue tests and also after testing by means of optical microscopy (OM), as well as scaning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) and Vickers microhardness was measured. Fatigue degradation mechanisms was documented in both materials and negligible effect of grit blasting on low cycle parameters was discussed.
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IN-SITU HIGH TEMPERATURE LOW CYCLE FATIGUE STUDY
Petrenec, M. ; Polák, Jaroslav ; Šamořil, T. ; Dluhoš, J. ; Obrtlík, Karel
In-situ Low Cycle Fatigue test (LCF) at temperature 635 °C have been performed in Scanning Electron Microscope (SEM) equipped with Electron Backscatter Diffraction analysis (EBSD) on a small dog-bone-shaped specimen of cast Inconel 713LC superalloy. The aim of the work was to study early stage fatigue damage at high temperature by the observations of the characteristic surface relief evolution and crystallographic characterization changes by EBSD. The detail of slip bands shape was checked by FIB and AFM microscopes. The LCF test was conducted on GATAN stage with pre tilted position and constant stress amplitude of total cycle number of 20. The relief produced in the first cycle determines the other locations of the localized cyclic slip to the primary slip planes (111). The relief was modified in the next cycles but without forming additionally new slip traces in the primary system. Based on EBSD analysis before and after LCF, the orientation of two grains was changed which caused activation of second slip system. The damage mechanism evolution is closely connected with the cyclic strain localization to the persistent slip bands where the fatigue cracks were initiated.
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EFFECT OF HIP ON LOW CYCLE FATIGUE OF MAR-M247 AT 900°C
Šulák, Ivo ; Obrtlík, Karel ; Škorík, Viktor ; Hrbáček, K.
Polycrystalline nickel-base superalloy MAR-M247 is used for high temperature applications requiring excellent combination of fatigue properties, creep resistance and surface stability. These superior high temperature characteristics derive from the microstructure which habitually consists of face centred cubic matrix γ and precipitate γ´ (L12 type ordered structure). In the present work, the high temperature low cycle fatigue behaviour of cast nickel-base superalloy MAR-M247 in as received condition and in hot isostatically pressed (HIP) condition was studied. The microstructure of the materials is characterized by dendritic grains, carbides and casting defects. Distribution and size of defects in both materials were studied. Isothermal low cycle fatigue (LCF) tests were performed on cylindrical specimens under total strain control at 900°C in air. Cyclic stress–strain response and fatigue life of both materials were assessed. Beneficial effects of HIP process on cyclic stress-strain and fatigue life curves are discussed.
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Effect of plasma sprayed AlSi coating on low cycle fatigue properties of cast superalloy Inconel 738LC
Obrtlík, Karel ; Pospíšilová, S. ; Juliš, M. ; Čelko, L. ; Podrábský, T.
Air plasma spraying was applied to obtain AlSi protective surface coating on cylindrical specimens of cast polycrystalline superalloy Inconel 738LC. Chemical composition of the surface treated layer was studied and the hardness depth profile was measured. Surface treated and untreated specimens were cyclically strained under total strain control at 800 °C in air. Cyclic stress-strain response and fatigue life of both materials were obtained. The coating results in a slight decrease of cyclic stress-strain curve. A detrimental effect of the surface treatment on derived Wöhler curve is documented while Manson-Coffin curves of both materials are almost identical. Specimen section observations and fracture surface examinations help to discuss fatigue behaviour of both materials.
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High Temperature Fatigue Behaviour of Third Generation of TiAl Alloys
Obrtlík, Karel ; Kruml, Tomáš
Low cycle fatigue properties of lamellar TiAl with 7.8 at.% Nb were studied at four temperatures: room temperature, 700, 750 and 800 °C. Up to 750 °C, stable cyclic behaviour is observed while cyclic softening is characteristic for 800°C. The strength of the alloy is still high even at 800 °C. The TEM observation did not reveal any substantial changes in the microstructure due to the cycling at RT. At 750 °C, the lamellar structure was in some places destroyed by cyclic plasticity and pure g-phase islands with high density of dislocation debris were formed. At 800 °C, the domains without lamellar structure cover about 10% of inspected volume and are almost dislocation free. The results show that the diffusion processes leading to vanishing of small prizmatic dislocation loops start to operate between 750 and 800 °C. The destruction of lamellar microstructure and annealing of dislocation debris is the reason for cyclic softening at 800 °C.
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Fatigue properties of nickel-base superalloy inconel 792-5A at 800°C
Šmíd, Miroslav ; Obrtlík, Karel ; Petrenec, Martin ; Polák, Jaroslav ; Hrbáček, K.
Smooth specimens were cyclically strained under strain control with constant strain amplitude and constant strain rate. Low cycle fatigue tests were conducted in servo-hydraulic pulsator MTS equipped with a three zone resistance furnace at temperature 800°C in air. Fracture surface was studied in SEM after fatigue test termination. Selected specimens were used to prepare foils for the transmission electron microscope (TEM) observation of microstructure and dislocation arrangement. They were used to obtain cyclic hardening/softening curves, cyclic stress-strain curve and fatigue life curves in the representation of stress amplitude, total strain amplitude and plastic strain amplitude versus number of cycles to fracture. Experimental points can be approximated with the Manson-Coffin law and the Basquin law. Fracture surface examinations revealed fatigue crack initiation sites.
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