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High Temperature Deformation Mechanisms
Heczko, Milan ; Gröger,, Roman (oponent) ; Spätig, Philipp (oponent) ; Kruml, Tomáš (vedoucí práce)
Two advanced highly-alloyed austenitic steels based on the Fe-Ni-Cr matrix were studied in conditions of low cycle fatigue both at room and elevated temperature. Extensive set of experimental and characterization tools was used for the investigation of inter-related effects of alloys composition, microstructure, deformation mechanisms and overall material response under load. Key mechanisms and factors determining mechanical properties and performance in the service were analysed and discussed in the relation to the materials design. • Standard fatigue experiments were performed at room temperature and at 700°C. Cyclic hardening/softening curves, cyclic deformation stress-strain curves, Coffin-Manson and Wöhler fatigue life curves were determined. • Various characterization techniques of electron microscopy were used to study changes of the microstructural state of the alloys due to the cyclic loading at room and elevated temperatures. • Fatigue behaviour, strength and cyclic plastic response of studied materials were explained in relation to the microstructure and microstructural aspects of deformation mechanisms both at room and elevated temperatures. • It was found that Sanicro 25 exhibits the highest high temperature strength of all alloys from the same class. Its extraordinary properties are related to the two nanoparticle populations, Cu-rich coherent precipitates and dispersoid-like MX nanoparticles, which play fundamental role in the determination of strength and overall cyclic response. As a result of pinning effects and associated obstacles, dislocation motion in this alloy is significantly retarded preventing formation of substructures with lower stored internal energy. With recovery heavily suppressed, forest dislocation strengthening supported by precipitation and solid solution hardening, leads to the remarkable increase of cyclic strength at elevated temperatures.
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High Temperature Deformation Mechanisms
Heczko, Milan ; Gröger,, Roman (oponent) ; Spätig, Philipp (oponent) ; Kruml, Tomáš (vedoucí práce)
Two advanced highly-alloyed austenitic steels based on the Fe-Ni-Cr matrix were studied in conditions of low cycle fatigue both at room and elevated temperature. Extensive set of experimental and characterization tools was used for the investigation of inter-related effects of alloys composition, microstructure, deformation mechanisms and overall material response under load. Key mechanisms and factors determining mechanical properties and performance in the service were analysed and discussed in the relation to the materials design. • Standard fatigue experiments were performed at room temperature and at 700°C. Cyclic hardening/softening curves, cyclic deformation stress-strain curves, Coffin-Manson and Wöhler fatigue life curves were determined. • Various characterization techniques of electron microscopy were used to study changes of the microstructural state of the alloys due to the cyclic loading at room and elevated temperatures. • Fatigue behaviour, strength and cyclic plastic response of studied materials were explained in relation to the microstructure and microstructural aspects of deformation mechanisms both at room and elevated temperatures. • It was found that Sanicro 25 exhibits the highest high temperature strength of all alloys from the same class. Its extraordinary properties are related to the two nanoparticle populations, Cu-rich coherent precipitates and dispersoid-like MX nanoparticles, which play fundamental role in the determination of strength and overall cyclic response. As a result of pinning effects and associated obstacles, dislocation motion in this alloy is significantly retarded preventing formation of substructures with lower stored internal energy. With recovery heavily suppressed, forest dislocation strengthening supported by precipitation and solid solution hardening, leads to the remarkable increase of cyclic strength at elevated temperatures.
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Effect of alloying and thermal processing on mechanical properties of tial alloys
Chlupová, Alice ; Heczko, Milan ; Obrtlík, Karel ; Beran, Přemysl ; Kruml, Tomáš
Two -based TiAl alloys with 7 at.% of Nb, alloyed with 2 at.% Mo and 0.5 at.% C, were studied. A heat treatment leading to very fine lamellar microstructure was applied on both alloys. Microstructure after the heat treatment was described and mechanical properties including fatigue behaviour were measured. The as-received material alloyed with C possesses high strength and very limited ductility, especially at RT. After application of selected heat treatment it becomes even more brittle; therefore, this process could be considered as not appropriate for this alloy. On the contrary, in the case of Mo alloyed material, both strength and ductility are improved by the heat treatment at RT and usual working temperature (~750 °C). Presence of the phase is responsible for this effect. The selected heat treatment thus can be an alternative for this alloy to other thermomechanical treatments as high temperature forging.
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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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NANOSTRUCTURE CHARACTERIZATION OF IN738LC SUPERALLOY FATIGUED AT HIGH TEMPERATURE
Petrenec, M. ; Strunz, Pavel ; Gasser, U. ; Heczko, Milan ; Zálešák, J. ; Polák, Jaroslav
The nanostructure of Inconel 738LC Ni-superalloy strengthened by trimodal Y precipitates distribution was investigated after Low Cycle Fatigue (LFC) loading at temperature 700°C. Different microscopic techniques as Scanning Electron Microscope (SEM) equipped with STEM detectro, transmission Kikuchi diffraction in the SEM, transmission electron microscope (TEM) in the bright field mode and high resolution transmission electron mciroscopes (HRTEM) in STEM mode were used for the characterization and quantification of superalloy nanostructure. The characteristic morphology of y precipitates was examined by ex-situ and in-situ Small Angle Neutron Scattering (SANS) at high temperatures. All adopted microscopic techniques indicate that the morphology of y precipitates distributed in the y matrix as recived state corresponds to two types, i.e. large cuboid-like precipitates with the size around 670 nm, and the spherical precipitates with the diameter 52nm. After eh LCF tests at temperature 700°C, the ex-situ SANS measurement yielded addititonal scattering intensities coming from another small y precipitetes with estimated size up to 10nm.
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NANOSTRUCTURE CHARACTERIZATION OF IN738LC SUPERALLOY FATIGUED AT HIGH TEMPERATURE
Petrenec, M. ; Strunz, Pavel ; Gasser, U. ; Heczko, Milan ; Zálešák, J. ; Polák, Jaroslav
The nanostructure of Inconel 738LC Ni-superalloy strengthened by trimodal γ’ precipitates distribution was investigated after Low Cycle Fatigue (LCF) loading at temperature 700°C. Different microscopic techniques as Scanning Electron Microscope (SEM) equipped with STEM detector, transmission Kikuchi diffraction in the SEM, transmission electron microscope (TEM) in the bright field mode and high resolution transmission electron microscopes in STEM mode were used for the characterization of nanostructure. The characteristic morphology of γ’ precipitates was examined by ex-situ and in-situ Small Angle Neutron Scattering (SANS) at high temperatures. All microscopic techniques indicate that the morphology of γ’ precipitates distributed in the γ matrix as received state corresponds to two types, i.e. large cuboid-like precipitates with the size around 670 nm, and the spherical precipitates with the diameter 52 nm. After the LCF tests at temperature 700°C, the ex-situ SANS measurement yielded additional scattering intensities coming from another small γ’ precipitates with estimated size up to 10 nm.
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