|
|
Microstructure, it´s Stability and Fatigue Properties of Ultra-Fine Grained Copper Prepared by ECAP Method
Navrátilová, Lucie ; Konečná, Radomila (referee) ; Obrtlík, Karel (referee) ; Kunz, Ludvík (advisor)
This work deals with fatigue properties and stability of microstructure of ultrafine-grained (UFG) copper prepared by severe plastic deformation by means of equal channel angular pressing (ECAP) method. The effect of different fatigue loading regimes and thermal exposition on microstructural changes was investigated and the fatigue lifetime curves were experimentally determined. The research attention was focussed on localization of cyclic plastic deformation and fatigue crack initiation in UFG structure. Experimental results indicate that after stress-controlled fatigue loading (both symmetrical and asymmetrical) the microstructure remains ultrafine; no grain coarsening was observed. Contrary to this, strain-controlled fatigue loading results in formation of bimodal structure. Grain coarsening was observed also after thermal exposition at 250 °C for 30 minutes. Annealing at lower temperatures does not result in grain coarsening or development of bimodal structure. Fatigue loading results in development of surface relief in form of cyclic slip markings. Their density, distribution and shape differ for particular fatigue loading regimes. Differences in crack initiation mechanism in low- and high-cycle fatigue region were found. Nevertheless, the characteristic feature for all loading regimes was stability of UFG microstructure in the region of cyclic slip bands and fatigue cracks.
|
|
|
Influence of Plasma-nitriding on Fatigue Life of High-strength Steels under Biaxial Loading
Major, Štěpán ; Kohout, Jan (referee) ; Růžička,, Milan (referee) ; Obrtlík, Karel (referee) ; Pokluda, Jaroslav (advisor)
This work deals with fatigue life and fractographycal analysis of plasma-nitrided and virgin specimen made of the low-alloy high-strength steel. Specimens were subjected to in-phase combined bending-torsion loading. The work is focused on three groups of problems. The first group of problems was associated with the fatigue life and its prediction. The objective was to examine a relationship between the presence of the nitrided layer and fatigue life. The application of surface layers has significantly improved the fatigue life in the high-cycle region. An extended comparison between classical and advanced multiaxial criteria was performed. Criterions proposed by Marin and Matake were found to be most precise. The second objective of this work was to examine a relationship between the loading ratio ( is the bending amplitude and is the torsion amplitude ) and the fracture surface topography in the highcycle fatigue. Properties of fracture surfaces were quantified by many roughness and fractal parameters. Many roughness parameters start to incrise rapidly above a critical value of the loading ratio zc = 0,5. The last part of this work deals with the fractographical analysis of fish-eye cracks that were formed in plasma-nitrided steel specimens under symetrical bending, symetrical torsion and biaxial in-phase bending-torsion combinations.
|
|
|
Short Crack Growth in Materials for High Temperature Applications
Mazánová, Veronika ; Obrtlík, Karel (referee) ; Krupp, habil Ulrich (referee) ; Polák, Jaroslav (advisor)
Pokročilá vysoce legovaná austenitická nerezová ocel Sanicro 25 s Fe-Ni-Cr matricí byla studována za podmínek nízkocyklové únavy za pokojové a vysoké teploty 700 °C. Široká škála moderních experimentálních technik byla použita ke studiu vzájemně souvisejících efektů chemického složení slitiny, mikrostrukturních změn a deformačních mechanismů, které určují odolnost materiálu vůči poškození. Hlavní úsilí bylo zaměřeno na studium iniciace únavových trhlin a růstu krátkých trhlin, tedy dvě stádia, která hrají zásadní roli ve výsledné celkové délce únavového života materiálu v provozu. • Vnitřní deformační mechanismy byly korelovány s vývojem povrchového reliéfu, který je pozorován ve formě persistentních skluzových stop na povrchu. Bylo zjištěno, že vysoce planární charakter dislokačního skluzu způsobuje vysokou lokalizaci cyklické plastické deformace do persistentních skluzových pásů, což v důsledku vede k nukleaci “Stage I” trhlin, která je spojena s přítomností persistentních skluzových stop na povrchu ve všech studovaných vzorcích. Bylo zjištěno, že praskání dvojčatových hraníc je taktéž spojeno s přítomností persistentních skluzových stop podél povrchové stopy dvojčatové roviny. • Interkrystalická iniciace únavové trhliny byla pozorována pouze zřídka, a to za podmínek zatěžování amplitudami vysoké deformace. Bylo zjištěno že interkrystalická iniciace je spojena s přítomností persistentních skluzových stop na hranicích zrn. Hranice zrn praskají za podmínek externího tahového zatížení zejména z důvodu vysokého počtu nekompatibilit na hranicích zrn, které jsou způsobené tvarem persistentních skluzových stop. • Mechanismy růstu přirozených krátkých trhlin byly studovány na vzorcích vystavených nízkocyklove únava s nízkou i vysokou deformací. Role mikrostruktury byla analyzována pomocí experimentálních technik a diskutována. • Rychlosti šíření nejdelších trhlin byly měřeny na vzorcích s mělkým vrubem. Výsledky byly analyzovány použitím přístupů lomové mechaniky založených na amplitudě KI a J-integrálu stejně jako na amplitudě plastické deformace. Všechny přístupy byly diskutovány v souvislosti s Mansonovými-Coffinovými křivkami únavové životnosti. Jednoduchý mocninový zákon růstu krátkých trhlin založený na amplitudě plastické deformace ukazuje velice dobrou korelaci se zákonem únavové životnosti. • Byla studována role oxidace v podmínkách cyklického zatěžování za vysokých teplot. Bylo zjištěno, že křehké praskání zoxidovaných hranic zrn hraje hlavní roli v počátečních stádiích nukleace trhlin. Později po iniciaci se dráha růstu trhliny mění preferenčně na transkrystalickou. Dráha šíření trhlin je velmi podobná dráze zjištěné při cyklování za pokojové teploty.
|
|
|
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.
|
|
|
Thermomechanical and Isothermical Fatigue of Surface Treated Nickel Superalloys
Šulák, Ivo ; Pantělejev, Libor (referee) ; Polák, Jaroslav (referee) ; Obrtlík, Karel (advisor)
Yttria-stabilized zirconia-based thermal barrier coating systems are the most widely used commercial coatings in the industry, with practical applications in aircraft engines and land-based power turbines. The purpose of thermal barriers is primarily to protect the substrate from high temperatures and also to increase its oxidation resistance. Currently, concerning the relatively frequent volcanic eruptions and increasing air traffic intensity in desert areas, increased attention is being paid to the development of new thermal and environmental coatings that will withstand the so-called CMAS attack and still successfully meeting the strictest requirements of the aerospace industry. Two newly developed experimental coatings consisting of three successive layers have been developed for this work. The upper two layers are thermal insulating ceramic coatings, where two different uppermost coatings were deposited. The first uppermost layer of the coating is a mixture of mullite and hexacelsian in a ratio of 70/30 wt. %. The second upper most type of coating consists of Al6Si2O13 + MgAl2O4 + BaCO3 in a ratio of 6:3:1 wt. %. The interlayer is made of the commercially utilized yttria-stabilized zirconia. The metallic CoNiCrAlY coating, which is directly deposited on the nickel-based superalloy MAR-M247, fulfils a compensatory function between the mechanical properties of the nickel superalloy and the ceramic coating. The thermal and environmental barrier system was deposited using air plasma spraying (APS) technology. The main objective of this work was to evaluate the effect of the newly developed thermal and environmental barrier coating, which has a high potential for the protection of component surfaces in an aggressive environment, on isothermical and thermomechanical fatigue behaviour of nickel-based superalloy MAR-M247. Low cycle fatigue tests were performed in strain control mode with constant strain amplitude on both uncoated and TEBC coated superalloy. Fatigue hardening/softening curves, cyclic stress-strain curves and fatigue life curves in the representation of total strain amplitude, plastic strain amplitude and stress amplitude on the number of cycles to failure were obtained. Microstructural analysis of MAR-M247 superalloy and a newly developed experimental coating was performed in a scanning electron microscope. The fatigue crack initiation sites were identified and the process of fatigue crack propagation was described. The dislocation arrangement after fatigue loading of MAR-M247 was investigated in a transmission electron microscope. The findings of isothermical and thermomechanical low cycle fatigue behaviour of uncoated and TEBC coated MAR-M247 superalloy and identification of damage mechanisms presented in this dissertation will improve the estimation of safe-life that is particularly relevant to aircraft engines components.
|
|
|
Fatigue-creep interaction in Ni superalloys and TiAl alloys
Šmíd, Miroslav ; Věchet, Stanislav (referee) ; Kunz, Ludvík (referee) ; Obrtlík, Karel (advisor)
The present doctoral thesis is focused on the effect of dwells on the low cycle fatigue behaviour of advanced high temperature materials. 10 minutes strain holds are introduced into the cyclic straining of cast Ni-based superalloy IN792-5A and cast intermetallic alloy TiAl-7Nb which were chosen as experimental material. Fatigue experiments were conducted in strain control mode with constant total strain amplitude and strain rate. IN792-5A was subjected to continuous cyclic loading test as well as fatigue tests either with tensile peak strain holds or compressive peak strain holds at temperature 800 °C. TiAl-7Nb was experimentally examined by continuous cyclic loading tests and also by fatigue test with tensile peak strain hold times at temperature 750 °C. Cyclic hardening/softening curves, cyclic stress-strain curves, Manson-Coffin curves and Basquin curves were obtained. Stress relaxation data were measured during hold times. Mean stress evolution was observed and documented. Microstructure of both alloys was observed in as-received state and also after cyclic loading by means of SEM and TEM. Surface relief investigation revealed cyclic plastic strain localisation as well as fatigue crack initiation sites. Fatigue crack propagation was described using fracture surface and longitudinal gauge section observations. Dislocation structures developed in the course of fatigue tests were studied and documented.
|
|
|
Fatigue behaviour of AZ31 and AZ61 magnesium alloys after corrosion degradation
Horynová, Miroslava ; Vojtěch, Dalibor (referee) ; Obrtlík, Karel (referee) ; Podrábský, Tomáš (advisor)
Dissertation thesis is focused on evaluation of influence of exposure in 5% salt fog on fatigue behavior of AZ31 and AZ61 magnesium alloy fabricated by squeeze casting method. Microstructure and mechanical properties of experimental materials have been evaluated. The AZ61 alloy was solution heat treated after prior optimization of the heat treatment process based on microstructural characteristics and mechanical properties. Depths of corrosion damage, corrosion rate and mechanism of corrosion of all three experimental alloys have been evaluated. Influence of prior corrosion exposure for 480 and 1000 hours on fatigue behavior of experimental materials was evaluated. Obtained data were compared with data obtained using smooth test specimens. Fractographic analysis was carried out on both smooth and precorroded specimens. Furthermore, influence of aluminium on corrosion and fatigue behavior of tested alloys was defined.
|
|
|
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.
|
|
|
Effect of casting conditions and heat treatment on high temperature low cycle fatigue performance of nickel superalloy Inconel 713LC
Šulák, Ivo ; Obrtlík, Karel ; Hrbáček, Karel
The present work is focused on the study of high temperature low cycle fatigue behaviour of Inconel 713LC produced by a vibratory investment casting (VIC) in as-cast conditions and in the condition after heat treatment (HT) consisting of hot isostatic pressing (HIP) followed by precipitation hardening. Low cycle fatigue tests were carried out on cylindrical specimens in symmetrical push-pull cycle under strain control with constant total strain amplitude and strain rate at 800 °C in air. Hardening/softening curves and fatigue life curves of both materials were assessed and compared with data of Inconel 713LC produced by a conventional investment casting (CIC). Cyclic hardening can be observed in the high amplitude domain while saturated stress response is apparent for low amplitude cycling for all material batches. Data presented in Basquin representation show an increase in fatigue life of both VIC batches compared to the CIC batch, however, no effect of HT on fatigue life of Inconel 713LC produced by VIC was observed. In contrast, the heat treated Inconel 713LC demonstrates slightly higher fatigue life in Coffin-Manson representation. The microstructure of both superalloys was studied by means of scanning electron microscopy (SEM). The microstructure of superalloy is characterized by dendritic grains with casting defects. It comprises the γ matrix, cubic γ´ precipitates, eutectics and carbides. The effect of the VIC and HT on fatigue performance and microstructure of Inconel 713LC is discussed.
|
|
|
Thermomechanical and Isothermical Fatigue of Surface Treated Nickel Superalloys
Šulák, Ivo ; Pantělejev, Libor (referee) ; Polák, Jaroslav (referee) ; Obrtlík, Karel (advisor)
Yttria-stabilized zirconia-based thermal barrier coating systems are the most widely used commercial coatings in the industry, with practical applications in aircraft engines and land-based power turbines. The purpose of thermal barriers is primarily to protect the substrate from high temperatures and also to increase its oxidation resistance. Currently, concerning the relatively frequent volcanic eruptions and increasing air traffic intensity in desert areas, increased attention is being paid to the development of new thermal and environmental coatings that will withstand the so-called CMAS attack and still successfully meeting the strictest requirements of the aerospace industry. Two newly developed experimental coatings consisting of three successive layers have been developed for this work. The upper two layers are thermal insulating ceramic coatings, where two different uppermost coatings were deposited. The first uppermost layer of the coating is a mixture of mullite and hexacelsian in a ratio of 70/30 wt. %. The second upper most type of coating consists of Al6Si2O13 + MgAl2O4 + BaCO3 in a ratio of 6:3:1 wt. %. The interlayer is made of the commercially utilized yttria-stabilized zirconia. The metallic CoNiCrAlY coating, which is directly deposited on the nickel-based superalloy MAR-M247, fulfils a compensatory function between the mechanical properties of the nickel superalloy and the ceramic coating. The thermal and environmental barrier system was deposited using air plasma spraying (APS) technology. The main objective of this work was to evaluate the effect of the newly developed thermal and environmental barrier coating, which has a high potential for the protection of component surfaces in an aggressive environment, on isothermical and thermomechanical fatigue behaviour of nickel-based superalloy MAR-M247. Low cycle fatigue tests were performed in strain control mode with constant strain amplitude on both uncoated and TEBC coated superalloy. Fatigue hardening/softening curves, cyclic stress-strain curves and fatigue life curves in the representation of total strain amplitude, plastic strain amplitude and stress amplitude on the number of cycles to failure were obtained. Microstructural analysis of MAR-M247 superalloy and a newly developed experimental coating was performed in a scanning electron microscope. The fatigue crack initiation sites were identified and the process of fatigue crack propagation was described. The dislocation arrangement after fatigue loading of MAR-M247 was investigated in a transmission electron microscope. The findings of isothermical and thermomechanical low cycle fatigue behaviour of uncoated and TEBC coated MAR-M247 superalloy and identification of damage mechanisms presented in this dissertation will improve the estimation of safe-life that is particularly relevant to aircraft engines components.
|
guest ::
