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MICROSTRUCTURE EVOLUTION DURING FATIGUE LOADING OF BULK NANOSTRUCTURED ALLOY AL(93)FE(3)CR(2)TI(2)
Chlupová, Alice ; Chlup, Zdeněk ; Kuběna, Ivo
The results of observation of the microstructure evolution during fatigue loading of the bulk nanostructured quasicrystalline Al alloy are presented. Feedstock powder with nominal composition of Al93Fe3Cr2Ti1 produced by gas atomization was hot extruded for powder compaction. The enhanced strengthening of the quasicrystalline nanostructured Al alloy is attributed to the microstructure refinement, formation of supersaturated solid solution, presence of reinforcing icosahedral quasicrystalline phase and work hardening caused by the deformation during extrusion. Good thermal stability is supported by presence of spherical quasicrystalline particles with different interior structure and sizes of up to 250 nm in the diameter. Low cycle fatigue stress-strain characteristics and microstructural evolution of the material were studied. For the microstructure observation the samples of material before and after fatigue loading the TEM and SEM microscopy was employed. The microstructure evolution caused by cyclic loading with positive mean stress was evaluated.
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Interferometrical system for bulge test thin film characterization
Pikálek, Tomáš ; Holzer, Jakub ; Tinoco, H.A. ; Buchta, Zdeněk ; Lazar, Josef ; Chlupová, Alice ; Náhlík, Luboš ; Sobota, Jaroslav ; Fořt, Tomáš ; Kruml, Tomáš
Behavior of thin film materials undergoing stress and deformation differs from bulk materials. A common method for the mechanical characterization of thin films is nanoindentation based on indenting a small tip into the material. A different approach is a bulge test technique. In this method, a differential pressure is applied on a free-standing membrane and the mechanical properties (Young’s modulus and residual stress) are calculated from the shape of the bulged membrane. In our experiments, we developed an interferometrical system for the membrane shape measurement during the bulge test.
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Fatigue crack growth in 316L under uniaxial and torsional loading
Karol, Michal ; Chlupová, Alice ; Mazánová, Veronika ; Kruml, Tomáš
A study of fatigue crack initiation and growth in 316L austenitic stainless steel is reported. Fatigue experiments were performed at room temperature on full cylindrical specimens cycled axially (tension-compression) and on hollow cylindrical specimens tested in reversed torsion. Microstructure and damage evolution (crack initiation and growth) on the surface of mechanically and electrolytically polished specimens were observed using light and electron microscopy. An analysis of the orientation of microcracks and macrocracks which led to failure was made. Axially loaded specimens exhibited presence of several microcracks which resulted in macrocrack propagating perpendicularly to specimen axis. In the case of torsional loading, orientation of macrocrack propagation was dependent on applied load and presence of notches. High amount of short cracks initiated parallel to specimen axis. Long cracks exhibited a tendency to bifurcate with crack branches oriented at approximately 45° to the specimen axis.
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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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Microstructural Investigation and Mechanical Testing of an Ultrafine-grained Austenitic Stainless Steel
Chlupová, Alice ; Man, Jiří ; Polák, Jaroslav ; Karjalainen, L. P.
Special thermomechanical treatment based on high degree deformation followed by reversion annealing was applied to 301LN austenitic stainless steel to achieve ultrafine-grained (UFG) structure with considerably enhanced mechanical properties. Two different conditions of the thermomechanical treatment were adopted and resulting microstructures with different grain sizes were characterised by optical and high resolution scanning electron microscopy (SEM-FEG). Hardness measurements and tensile tests were performed to characterize mechanical properties. To reveal structural changes induced during thermomechanical treatment and during tensile tests a magnetic induction method was additionally applied. Experimental study validated the ability of the above special treatment to produce austenitic stainless steel with grain size about 1.4 m which exhibits tensile strength about 1000MPa while ductility remains on level about 60 %. The results obtained for both thermomechanical conditions are compared and the relationship between microstructure refinement, phase content and mechanical properties is discussed.
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Analysis of cyclic plasticity of fatigued nickel based superalloys at elevated temperatures
Petrenec, Martin ; Tobiáš, Jiří ; Polák, Jaroslav ; Šmíd, Miroslav ; Chlupová, Alice ; Petráš, Roman
Cyclic strain controlled multiple step tests have been performed on cylindrical specimens of cast Inconel 738LC superalloy at 23 and 800 °C and Inconel 792-5A superalloy at 800 °C in laboratory atmosphere. The effect of temperature on the cyclic stress-strain response characterized by internal and effective cyclic stress components and on the dislocation arrangement has been studied. The evolution of the effective and internal stress components and effective elastic moduli were derived from the hysteresis loops analyzed according to the statistical theory of the hysteresis loop. Cyclic hardening/softening curves and shortened cyclic stress-strain curves were obtained at all temperatures in both superalloys. The shortened cyclic stress-strain curves (CSSCs) can be fitted by power law at all temperatures and for both materials. They are shifted to lower stresses with increasing temperature. The CSSC of Inconel 792-5A is shifted to higher stress amplitude in comparison with that of Inconel 738LC due to different number and type of precipitates. Observation of dislocation structures by TEM revealed cyclic strain localization into persistent slip bands. Cyclic stress-strain response is compared at all temperatures in both superalloys and discussed in relation to the changes of internal and effective stress components and microstructural parameters of the material. Higher stress response of the Inconel 792-5A than of the Inconel 738LC superalloy at 800°C is due to higher effective stresses of respective γ´ phases.
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Low Cycle Fatigue and Analysis of the Cyclic Stress-strain Response in Superalloy Inconel 738LC
Tobiáš, Jiří ; Chlupová, Alice ; Petrenec, Martin ; Polák, Jaroslav
The paper describes the results of fatigue behavior study on cast polycrystalline nickel based superalloy tested at 23 and 800 °C. Cylindrical specimens of Inconel 738LC were cyclically strained under total strain control to fracture and multiple step tests were performed to study the effect of temperature on the internal and effective cyclic stress components. Fatigue life curves were approximated by the Manson- Coffin and Basquin laws. The resulting curves were shifted to lower fatigue lives with increasing temperature. The evolution of the effective and internal stress components and effective elastic modules were derived from the hysteresis loops which were analyzed according to the statistical theory of hysteresis loop. Cyclic stress-strain response at both temperatures and the changes of internal and effective stress components were discussed in relation to microstructural parameters of the superalloy.
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Microstructure Characterization and Faigue Behavior of Beta Phase Containing Gama-TiAl Alloy
Petrenec, Martin ; Beran, Přemysl ; Šmíd, Miroslav ; Chlupová, Alice ; Kuběnová, Monika
The microstructure and fatigue properties of Ti–44Al–7.8Nb – 0.2Ni (at.%) alloy after graded cooling heat treatment were investigated. Different techniques as SEM, TEM and FIB were used for characterization of microstructure. Quantification of phase volume fraction was measured by neutron diffraction method. After graded cooling heat treatment the structure consists of fully lamellar uniform grains which contain lamellar phases /2 and cubic phases (ordered B2 and disordered phases) at grain boundary and interlamellar spaces. The foils for TEM were prepared using FIB technique and observed in TEM in order to identify B2 phase by selected electron diffraction. Low cycle fatigue behaviour at 750 °C shows stable fatigue crack propagation due the presence of plastic beta and B2 phases.
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