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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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Fatigue crack growth and delamination in fiber metal laminate (GLARE) during loading with positive mean stress
Chlupová, Alice ; Kozák, Vladislav
The aim of the paper is to present the results of a study on the damage of fiber metal laminate (GLARE) subjected to the low cycle fatigue loading with positive mean stress. The fatigue crack initiation and growth was observed on the surface of notched specimens and then the individual layers of fatigued specimens were removed by chemical etching and polishing to obtain data about cracks length and delamination shape and area. Mechanism of initiation and crack growth in this type of materials differs from homogeneous monolithic materials. The fatigue life in term of number of cycles to crack initiation depending on amplitude of local plastic deformation and local stress in the notch root was evaluated.
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Microstructure, tensile properties and fatigue behaviour of bulk nanoquasicrystalline Al alloy Al93Fe3Cr2Ti2
Chlupová, Alice ; Chlup, Zdeněk ; Kruml, Tomáš ; Kuběna, Ivo ; Roupcová, Pavla
The characterization of mechanical properties (i.e. stress-strain characteristics measured in tensile and fatigue tests) together with description of the microstructure of bulk nanocrystalline Al alloy is reported. Aluminium based material used in this work was prepared by hot extrusion of gas atomised powder of chemical composition Al93Fe3Cr2Ti2 which contains nanosized quasicrystalline phase. Changes in fracture mechanisms both at fatigue testing performed at room temperature and at tensile tests performed at different test temperatures were investigated. The quality of powder compaction after extrusion was proved as sufficient. Strong texture in longitudinal direction resulting from production route was detected. Presence of icosahedral phase in material was confirmed by TEM and XRD techniques. Fractographical analysis of fracture surface was performed and results were used for description of fracture mechanisms.
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Properties of bulk nanostructured Al-based alloy at cyclic loading
Chlupová, Alice ; Chlup, Zdeněk
Al-based alloy having a nominal chemical composition of Al93Fe3Cr2Ti2 (at.%) is highly promising structural material due to advantageous weight-strength ratio and in comparison with commercially used Al alloys also good thermal stability because of presence of reinforcing icosahedral quaiscrystals in the structure. In the present work the mechanical properties and especially fatigue behaviour of this type of nanoquazicrystalline material obtained in bulk form are described. For cyclic loading the cylindrical specimens were machined from two types of extrudates with different powder particle size (PPS). Fatigue properties were studied using servohydraulic MTS machine and cyclic loading was performed at constant stress amplitude with positive mean stress. Measured stress-strain data were evaluated and fatigue life in form of S-N curves was obtained. Fracture surface of broken testpieces was observed, fractographic analysis was performed and fracture mechanism was evaluated.
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