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Low cycle fatigue behaviour and fatigue crack initiation in MAR-M247 at 700 °c
Šulák, Ivo; Obrtlík, Karel; Hrbáček, K.
2017 - English
The second generation nickel-based superalloy MAR-M247 offersa satisfying combination of fatigue and creep properties and oxidation and corrosion resistance that arerequired for application at elevated temperatures in hostile environments. The microstructure consists mainly oftheface centred cubic γ matrix and ordered γ ́ strengthening precipitates (L12crystal structure). The present work focuses on low cycle fatigue (LCF) behaviour of polycrystalline nickel-based superalloy MAR-M247 at high temperature. LCF tests were conducted on cylindrical specimens in symmetrical push-pull cycle under strain control with constant total strain amplitude and strain rate at 700 °C in ambientair. Cyclic stress-strain curvesand fatigue life curves in the representation ofplastic strain amplitude vs. stress amplitude andstress amplitude vs. the number of cycles to failure, respectively,were plotted and compared with data obtained on Inconel 713LC. Special attention waspaid to the investigation of crack initiation in MAR-M247 during low cycle fatigue. Crack initiation sites were studied by means of scanning electron microscopy (SEM) in dual beam microscope TESCAN LYRA 3 XMU FESEM equipped with focus ion beam (FIB). Specimens’ surface observations revealed the formation of pronounced surface relief indicating localisation of plastic deformation.Observations in transmission electron microscope (TEM)confirmed localisation of cyclic plastic deformation in persistent slip bands along {111} slip planes. Fractographic analysis revealed fatigue crack initiation sites. Fatigue crack propagation in stage I was typical of smooth facets up to 500 μm long.
Keywords:
Cyclic stress-strain curve; Fatigue crack initiation; Fatigue life
Available at various institutes of the ASCR
Low cycle fatigue behaviour and fatigue crack initiation in MAR-M247 at 700 °c
The second generation nickel-based superalloy MAR-M247 offersa satisfying combination of fatigue and creep properties and oxidation and corrosion resistance that arerequired for application ...
Magnetic properties of 42CrMo4 steel
Bulín, T.; Švábenská, Eva; Hapla, Miroslav; Roupcová, Pavla; Ondrušek, C.; Schneeweiss, Oldřich
2017 - English
Low alloyed high-grade chrome-molybdenum ferritic steel was investigated from the point of views of magnetic properties in dependence on heat and mechanical treatment. This steel can be used as components of magnetic circuits or some parts in electrical equipment. The basic information on structure and phase composition was obtained by optical and scanning electron microscopy, X-ray Powder Diffraction and Mössbauer Spectroscopy. The temperature stability of the material was proved by measurements of temperature dependences of magnetic moment. The magnetic parameters were obtained by measuring of magnetic hysteresis loops in dependence on saturation field and their frequencies. The results are discussed from the point of view of possible applications as a magnetic material in the very extremely environment, where high mechanical stresses and elevated temperatures can occur.
Keywords:
Magnetic materials; magnetic properties; steel
Available at various institutes of the ASCR
Magnetic properties of 42CrMo4 steel
Low alloyed high-grade chrome-molybdenum ferritic steel was investigated from the point of views of magnetic properties in dependence on heat and mechanical treatment. This steel can be used as ...
A numerical investigation of the stress intensity factor for a bent chevron notched specimen: Comparison of 2D and 3D solutions
Seitl, Stanislav; Miarka, Petr; Sobek, J.; Klusák, Jan
2017 - English
In the contribution, normalized stress intensity factors for three- and four-point bending specimens with a chevron notch is introduced by varying the chevron notch angle and length. The three- and two-dimensional models of bent chevron notched specimens in the software ANSYS were prepared by using possible symmetrical conditions. The 2D model was used with variable thicknesses of the layers representing the characteristic shape of the chevron notch (with the plane stress boundary condition). The numerically obtained results from the 2D and 3D solutions are compared with data from literature.
Keywords:
Fracture mechanics; Calibration curve; Stress intensity factors
Available at various institutes of the ASCR
A numerical investigation of the stress intensity factor for a bent chevron notched specimen: Comparison of 2D and 3D solutions
In the contribution, normalized stress intensity factors for three- and four-point bending specimens with a chevron notch is introduced by varying the chevron notch angle and length. The three- and ...
Fatigue damage prediction of short edge crack under various load: Direct Optimized Probabilistic Calculation
Krejsa, M.; Seitl, Stanislav; Brožovský, J.; Lehnert, P.
2017 - English
Fatigue crack propagation depends on a number and value of stress range cycles. This is a time factor in the course of reliability for the entire designed service life. Three sizes are important for the characteristics of the propagation of fatigue cracks - initial size, detectable size and acceptable size. The theoretical model of a fatigue crack progression can be based on a linear elastic fracture mechanics (uses Paris-Erdogan law). Depending on location of an initial crack, the crack may propagate in structural element (e.g. from the edge or from the surface under various load) that could be described by calibration functions. When determining the required degree of reliability, it is possible to specify the time of the first inspection of the construction which will focus on the fatigue damage. Using a conditional probability and Bayesian approach, times for subsequent inspections can be determined based on the results of the previous inspection. For probabilistic modelling of a fatigue crack progression was used the original and a new probabilistic method - the Direct Optimized Probabilistic Calculation ('DOProC'), which uses a purely numerical approach without any simulation techniques or approximation approach based on optimized numerical integration. Compared to conventional simulation techniques is characterized by greater accuracy and efficiency of the computation.
Keywords:
fatigue; crack propagation; calibration function
Available at various institutes of the ASCR
Fatigue damage prediction of short edge crack under various load: Direct Optimized Probabilistic Calculation
Fatigue crack propagation depends on a number and value of stress range cycles. This is a time factor in the course of reliability for the entire designed service life. Three sizes are important for ...
Effect of alloying and thermal processing on mechanical properties of tial alloys
Chlupová, Alice; Heczko, Milan; Obrtlík, Karel; Beran, Přemysl; Kruml, Tomáš
2017 - English
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.
Keywords:
Fatigue; Heat treatment; Lamellar microstructure; Mechanical properties; TiAl alloys
Available at various institutes of the ASCR
Effect of alloying and thermal processing on mechanical properties of tial alloys
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 ...
Theory-guided design of novel Fe-Al-based superalloys
Friák, Martin; Holec, D.; Jirásková, Yvonna; Palm, M.; Stein, F.; Janičkovič, D.; Pizúrová, Naděžda; Dymáček, Petr; Dobeš, Ferdinand; Šesták, Pavel; Fikar, Jan; Šremr, Jiří; Nechvátal, Luděk; Oweisová, S.; Homola, V.; Titov, Andrii; Slávik, Ondrej; Miháliková, Ivana; Pavlů, Jana; Buršíková, V.; Neugebauer, J.; Boutur, D.; Lapusta, Y.; Šob, Mojmír
2017 - English
Our modern industrialized society increasingly requires new structural materials\nfor high-temperature applications in automotive and energy-producing industrial\nsectors. Iron-aluminides are known to possess excellent oxidation and sulfidation\nresistance as well as sufficient strength at elevated temperatures. New Fe-Al-based\nmaterials will have to meet multiple casting, processing and operational criteria\nincluding high-temperature creep strength, oxidation resistance and room-temperature\nductility. Such desirable combination of materials properties can be achieved in multi-phase\nmulti-component superalloys with a specific type of microstructure (the matrix contains\ncoherent particles of a secondary phase - a superalloy microstructure). In order to design\nnew Fe-Al-based superalloys, we employ a state-ofthe-art theory-guided materials design\nconcept to identify suitable combinations of solutes.
Keywords:
superalloys; ab initio; materials design; Fe-Al
Available at various institutes of the ASCR
Theory-guided design of novel Fe-Al-based superalloys
Our modern industrialized society increasingly requires new structural materials\nfor high-temperature applications in automotive and energy-producing industrial\nsectors. Iron-aluminides are known to ...
Crack Resistance Characterization in TiAl Intermetallics with Enhanced Toughness
Dlouhý, Ivo; Stratil, Luděk; Fukutomi, H.; Hasegawa, M.
2017 - English
The paper is focused on the analysis of the role of lamellar microstructure in fracture performance of model TiAl intermetallic compound. Coarse lamellar colonies and, at the same time, fine lamellar morphology were prepared by compressive deformation at 1553 K (region of stable alpha phase in TiAl equilibrium diagram) followed by controlled cooling to 1473 K (region of alpha+gamma phase) with delay on this temperature and then cooling down. The fracture toughness was evaluated by means of chevron notch technique. In addition, because of enhanced toughness, crack resistance curves were obtained by load - unload technique of pre-racked beams, namely in two directions of crack propagation relative to lamellar structure. Extensive development of shear ligament toughening mechanism was observed in fracture surfaces leading to quite good fracture toughness thanks to the heat treatment applied.
Keywords:
Titanium aluminides; Intermetallics; Fracture micromechanisms; Shear ligament toughening; Fracture toughness; Crack resistance
Available at various institutes of the ASCR
Crack Resistance Characterization in TiAl Intermetallics with Enhanced Toughness
The paper is focused on the analysis of the role of lamellar microstructure in fracture performance of model TiAl intermetallic compound. Coarse lamellar colonies and, at the same time, fine lamellar ...
Effect of residual stresses on the fatigue lifetime of railway axle
Hutař, Pavel; Pokorný, Pavel; Poduška, Jan; Fajkoš, R.; Náhlík, Luboš
2017 - English
The operation of railway axles should fulfill at least two main demands: safety and low operation costs. A significant part of operation costs is given by the length of regular inspection intervals which should reveal potential fatigue cracks in railway axle. The detection of cracks is of a probabilistic nature, therefore their detection is not ensured in all cases. For the safe operation of trains, an existence of potential initial crack should be considered on the axle surface and residual fatigue lifetime should be conservatively determined for this case. Reliable procedure of residual fatigue lifetime estimation should take into account real axle geometry, material characteristics and loading of the railway axle. This paper shows methodology for determination of residual fatigue lifetime (RFL) based on the fracture mechanics approach, taking into account real spectrum of the loading cycles, existence of press-fitted wheels and surface residual stresses given by the thermo-mechanical surface treatment of the railway axle. It is demonstrated that the effect of the residual stresses is significant and should not be neglected in the numerical estimation of residual fatigue lifetime of the axle.
Keywords:
Railway axle; residual fatigue lifetime; residual stresses; inspection intervals; numerical simulation
Available at various institutes of the ASCR
Effect of residual stresses on the fatigue lifetime of railway axle
The operation of railway axles should fulfill at least two main demands: safety and low operation costs. A significant part of operation costs is given by the length of regular inspection intervals ...
Method of Threshold Stress Determination for a Local Approach to Cleavage Fracture
Kotrechko, S.; Gryshchenko, V.; Kozák, Vladislav; Dlouhý, Ivo
2017 - English
The contribution is focused on a new methodology description for determination of threshold stress sigma th, as the third parameter in Beremin local approach to cleavage fracture that is using three-parameter Weibull statistics. Nature of the methodology lies in tensile testing of rounded notched specimens at liquid nitrogen temperature and corresponding calculations. Reactor pressure vessel steel was chosen as an example for the illustration.
Keywords:
Cleavage; Fracture; Local approach; Threshold stress
Available at various institutes of the ASCR
Method of Threshold Stress Determination for a Local Approach to Cleavage Fracture
The contribution is focused on a new methodology description for determination of threshold stress sigma th, as the third parameter in Beremin local approach to cleavage fracture that is using ...
HYDROGENATION OF CHOSEN MgMXN INTERMETALLICS (X=Al, Ga, In, Si, Sn)
Čermák, Jiří; Král, Lubomír; Roupcová, Pavla
2017 - English
Hydrogen sorption in chosen intermetallics MgmXn (Mg17Al12 , Mg2Ga, mixture of beta-Mg-In phases, Mg2Si and Mg2Sn) formed by magnesium and elements from the 13 th (X: Al, Ga, In) and 14 th (X: Si, Sn) groups was studied between the room temperature and T = 350 °C. Hydrogen pressure p varied from 10-3 MPa to 6 MPa.\nHydrogen solubility in alpha phase (solid solution of hydrogen in MgmXn before the hydride phase was nucleated) was close to hydrogen solubility in pure Mg. Concentration cH of hydrogen solved in Mg m X n depended linearly on pressure p , dependence of cH on temperature T was relatively weak. MgH2 was the main hydrogen storage phase in all the compounds MgmXn . Nucleation of MgH2 at 350 °C was observed at pressure above approximately 1 MPa in Mg17Al12 and above 2 Ma in other compounds. Maximum values of c H in hydride-containing compounds MgmXn detected at T = 350 °C depended on the stability of MgmXn .
Keywords:
Hydrogen; hydrogen storage; Mg alloys; hydrogen solubility; MgH2
Available at various institutes of the ASCR
HYDROGENATION OF CHOSEN MgMXN INTERMETALLICS (X=Al, Ga, In, Si, Sn)
Hydrogen sorption in chosen intermetallics MgmXn (Mg17Al12 , Mg2Ga, mixture of beta-Mg-In phases, Mg2Si and Mg2Sn) formed by magnesium and elements from the 13 th (X: Al, Ga, In) and 14 th (X: Si, Sn) ...
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