Pokročilé kovové materiály a kompozity na bázi kovů
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- ItemStrength and Brittleness of Interfaces in Fe-Al Superalloy Nanocomposites under Multiaxial Loading: An ab initio and Atomistic Study(MDPI, 2018-10-24) Šesták, Petr; Friák, Martin; Holec, David; Všianská, Monika; Šob, MojmírWe present an ab initio and atomistic study of the stress-strain response and elastic stability of the ordered Fe3Al compound with the D03 structure and a disordered Fe-Al solid solution with 18.75 at.% Al as well as of a nanocomposite consisting of an equal molar amount of both phases under uniaxial loading along the [001] direction. The tensile tests were performed under complex conditions including the effect of the lateral stress on the tensile strength and temperature effect. By comparing the behavior of individual phases with that of the nanocomposite we find that the disordered Fe-Al phase represents the weakest point of the studied nanocomposite in terms of tensile loading. The cleavage plane of the whole nanocomposite is identical to that identified when loading is applied solely to the disordered Fe-Al phase. It also turns out that the mechanical stability is strongly affected by softening of elastic constants C and/or C66 and by corresponding elastic instabilities. Interestingly, we found that uniaxial straining of the ordered Fe3Al with the D03 structure leads almost to hydrostatic loading. Furthermore, increasing lateral stress linearly increases the tensile strength. This was also confirmed by molecular dynamics simulations employing Embedded Atom Method (EAM) potential. The molecular dynamics simulations also revealed that the thermal vibrations significantly decrease the tensile strength.
- ItemFatigue characterization of polyethylene under mixed mode I/III conditions(Elsevier, 2021-04-01) Gosch, Anja; Arbeiter, Florian; Berer, Michael; Vojtek, Tomáš; Hutař, Pavel; Pinter, GeraldMixed mode I/III fatigue fracture behaviour of bulk polymers is scarcely investigated. Thus, the aim of this work is to focus on the comparison of pure mode I and mixed mode I/III fatigue loading of polyethylene. During mixed mode I/III testing, both increases and decreases of lifetime were observed, depending on the levels of applied mode I and mode III loading. Additionally, the critical influence of friction and the accompanying increase of local temperature between the shear loaded crack flanks in mode III, as well as the dominant fracture mechanisms were discussed based on the fracture surface morphology.
- ItemMicrocrack interaction with circular inclusion and interfacial zone(Gruppo Italiano Frattura, 2019-01-04) Profant, Tomáš; Hrstka, Miroslav; Klusák, JanA geometrically simplified plane elasticity problem of a finitesmall crack emanating from a thin interfacial zone surrounding the circularinclusion situated in the finite bounded domain is investigated. The crack isarbitrarily oriented and modelled using the distribution dislocation technique.This model represents the inner solution of the studied problem. Thecorresponding fundamental solution is based on the application ofMuskhelishvili complex potentials in the form of the Laurent series. Thecoefficients of the series are evaluated from the compatibility conditionsalong the interfaces of the inclusion, the interfacial zone and the enclosingmatrix. The fundamental solution is also used in the solution of the boundaryintegral method approximating the stress and strain relations of the so-calledouter solution. The asymptotic analysis at the point of the crack initiationcombines the inner and the outer solution and results in the evaluation of thestress intensity factors of the crack tip, which lies in the matrix. Thetopological derivative is subsequently used to approximate the energy releaserate field associated with the perturbing crack in the matrix. The extremevalues of the energy release rate allow one to assess the crack path directionof the initiated microcrack.
- ItemImpact of Antiphase Boundaries on Structural, Magnetic and Vibrational Properties of Fe3Al(MDPI, 2020-11-01) Friák, Martin; Černý, Miroslav; Všianská, Monika; Šob, MojmírWe performed a quantum-mechanical study of the effect of antiphase boundaries (APBs) on structural, magnetic and vibrational properties of Fe3Al compound. The studied APBs have the {001} crystallographic orientation of their sharp interfaces and they are characterized by a 1/2 < 111 > shift of atomic planes. There are two types of APB interfaces formed by either two adjacent planes of Fe atoms or by two adjacent planes containing both Fe and Al atoms. The averaged APB interface energy is found to be 80 mJ/m(2) and we estimate the APB interface energy of each of the two types of interfaces to be within the range of 40-120 mJ/m(2). The studied APBs affect local magnetic moments of Fe atoms near the defects, increasing magnetic moments of FeII atoms by as much as 11.8% and reducing those of FeI atoms by up to 4%. When comparing phonons in the Fe3Al with and without APBs within the harmonic approximation, we find a very strong influence of APBs. In particular, we have found a significant reduction of gap in frequencies that separates phonon modes below 7.9 THz and above 9.2 THz in the defect-free Fe3Al. All the APBs-induced changes result in a higher free energy, lower entropy and partly also a lower harmonic phonon energy in Fe3Al with APBs when compared with those in the defect-free bulk Fe3Al.
- ItemInitiation of fatigue cracks in AZ91 Mg alloy processed by ECAP(IOP Publishing, 2014-08-08) Fintová, Stanislava; Kunz, LudvíkMechanism of fatigue crack initiation was investigated in ultrafine-grained (UFG) magnesium alloy AZ91 processed by equal channel angular pressing (ECAP). Fatigue behaviour of UFG material was compared to the behaviour of material in an initial as-cast state. Focused ion beam technique (FIB) was applied to reveal the surface relief and early fatigue cracks. Two substantially different mechanisms of crack initiation were observed in UFG structure, which can be characterized as bimodal even after 6 ECAP passes by route Bc. The bimodality consists in a coexistence of very fine grained areas with higher content of Mg17Al12 particles and areas exhibiting somewhat larger grains and lower density of particles. The fatigue cracks which initiate in areas of larger grains are related to the cyclic slip bands; this initiation mechanism is similar to that observed in cast alloy. The second initiation mechanism is related to the grain boundary cracking which takes place predominantly in the fine grained areas.