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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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Study of advanced high strength magnesium alloys by in situ techniques
Fekete, Klaudia ; Dobroň, Patrik (vedoucí práce) ; Lejček, Pavel (oponent) ; Mayama, Tsuyoshi (oponent)
Hlavním cílem disertační práce bylo stanovení aktivních deformačních mechanismů v moderních hořčíkových slitinách s vysokou pevností s využitím pokročilých in-situ metod umožňující vysoké rozlišení v čase a v prostoru. Zkoumalo se deformační chování dvou extrudovaných Mg-LPSO slitin s různým objemovým podílem LPSO (long-period stacking order) fáze v průběhu deformace v tahu při pokojové teplotě a v tlaku při pokojové teplotě a při teplotách 200 řC, 300 řC a 350 řC. Výsledky získané pomocí in-situ metod akustické emise a difrakce synchrotronového záření byly ověřeny pomocí transmisní a rastrovací elektronové mikroskopie, zejména zobrazováním zpětně odražených elektronů a difrakcí zpětně odražených elektronů. Z dosažených výsledků vyplývá, že jak teplota, tak i obsah LPSO fáze významně ovlivňují plasticitu hořčíkové matrice, zejména aktivaci tahových dvojčat a nebazálního skluzu. Navíc mají velký vliv na tvorbu deformačních vybočení (kinking) v LPSO fázi. Klíčová slova: Mg-LPSO slitiny, deformační mechanismy, akustická emise, difrakce synchrotronového záření, in-situ metody.
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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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ELASTIC-PLASTIC BEHAVIOUR OF ADVANCED ADI STUDIED BY IN-SITU SEM TENSILE TEST
Petrenec, M. ; Tesařová, H. ; Krahula, Karel ; Polák, Jaroslav
In-situ SEM tensile tests at room temperature have been performed on flat specimen of advanced austempered ductile cast iron (ADI) alloyed with nickel with the aim to study elastic-plastic behavior together with the nucleation and growth of cracks. During tensile loading the systematic observation of selected locations was studied. The in-situ observation was used to elucidate the tensile curves in agreement with the deformation mechanisms. In the early stage of loading the stress-strain response is elastic. After exceeding elastic limit decohesion of graphite nodules from the matrix was observed. It can be related to the departure of the tensile curve from the elastic behavior. The cracks initiated preferably from interface of graphite nodule and the ausferrite matrix. Elastic-plastic region is connected with the plastic deformation of the matrix and growth and linking of short cracks.
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