Pokročilé kovové materiály a kompozity na bázi kovů

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    Entropy-Driven Grain Boundary Segregation: Prediction of the Phenomenon
    (MDPI, 2021-08-01) Lejček, Pavel; Hofmann, Siegfried
    The question is formulated as to whether entropy-driven grain boundary segregation can exist. Such a phenomenon would be based on the assumption that a solute can segregate at the grain boundary sites that exhibit positive segregation energy (enthalpy) if the product of segregation entropy and temperature is larger than this energy (enthalpy). The possibility of entropy-driven grain boundary segregation is discussed for several model examples in iron-based systems, which can serve as indirect evidence of the phenomenon. It is shown that entropy-driven grain boundary segregation would be a further step beyond the recently proposed entropy-dominated grain boundary segregation as it represents solute segregation at "anti-segregation" sites.
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    Stress coupling effect on ideal shear strength: tungsten as a case study
    (Hindawi, 2016-12-05) Černý, Miroslav; Šesták, Petr; Pokluda, Jaroslav
    Mechanical response of a perfect bcc tungsten crystal to a multiaxial loading was investigated from first principles. The multiaxial stress state consisted of the shear stress and a superimposed compressive triaxial stress with various levels of differential stresses. The studied shear system was 111{110}. Results obtained within a relatively wide range of the compressive stresses showed that increasing hydrostatic triaxial stress (with zero differential stresses) increased the shear strength almost linearly. On the other hand, triaxial stresses with greater portion of the differential components did not have such a simple effect on the shear strength: We found a certain optimum value of the superimposed triaxial stress yielding the maximum shear strength. Any change (both increase and decrease) in the triaxial stress then reduced the ideal shear strength value.
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    Initiation of Fatigue Cracks in Ultrafine-grained Materials in High-cycle Fatigue Region
    (Elsevier, 2014-06-25) Kunz, Ludvík; Fintová, Stanislava
    Initiation of fatigue cracks in materials with conventional grain (CG) size was investigated very thoroughly in the past. There is an extensive knowledge on the localization of cyclic plasticity and early crack development; however, it cannot be straightforwardly applied to the ultrafine-grained (UFG) structures with the grain size below 1 micrometer, because the crack initiation mechanisms are related to dislocation structures, which cannot develop in UFG materials simply from the size reasons. The paper brings results of an experimental investigation of the cyclic strain localization and crack initiation by means of focused ion beam technique (FIB). Two substantially different materials as regards the crystallographic structure, namely UFG Cu and magnesium alloy AZ91 processed by equal channel angular pressing (ECAP) were investigated and the observed characteristic features of crack initiation were discussed. The observations bring evidence that in the high-cycle fatigue (HCF) region point defects generated by dislocation activity do play very important role in the fatigue crack initiation process in UFG Cu. Fatigue cracks initiate in slip bands which form in areas of near-by oriented grains and are characteristic by surface relief, consisting of extrusions and intrusions. Point defects and formation of cavities and voids along the active slip planes governs the HCF crack initiation. No grain coarsening and development of specific dislocation structure was observed in the regions of crack initiation in UFG Cu. The mechanism of the crack initiation in AZ91 alloy processed by ECAP was found to be similar to that known from CG alloy. The cracks initiate in cyclic slip bands which forms in individual grains due to their relatively large grain size. The initiated cracks propagate along the slip planes in a crystallographic way which corresponds to the quasicleavage mechanism often reported for CG Mg alloys.
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    Microcrack interaction with circular inclusion and interfacial zone
    (Gruppo Italiano Frattura, 2019-01-04) Profant, Tomáš; Hrstka, Miroslav; Klusák, Jan
    A 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.
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    Effect of Underload Cycles on Oxide-Induced Crack Closure Development in Cr-Mo Low-Alloy Steel
    (MDPI, 2021-05-01) Pokorný, Pavel; Vojtek, Tomáš; Jambor, Michal; Náhlík, Luboš; Hutař, Pavel
    Underload cycles with small load amplitudes below the fatigue crack growth threshold are dominantly considered as insignificant cycles without any influence on fatigue lifespan of engineering structural components. However, this paper shows that in some cases these underload cycles can retard the consequent crack propagation quite significantly. This phenomenon is a consequence of oxide-induced crack closure development during cyclic loading below the threshold. The experimentally described effect of fatigue crack growth retardation was supported by measurement of the width and the thickness of the oxide debris layer using the EDS technique and localized FIB cuts, respectively. Both the retardation effect and the amount of oxide debris were larger for higher number and larger amplitudes of the applied underload cycles. Crack closure measurement revealed a gradual increase of the closure level during underload cycling. Specimens tested in low air humidity, as well as specimens left with the crack open for the same time as that needed for application of the underload cycles, revealed no retardation effect. The results can improve our understanding of environmental effects on fatigue crack propagation and understanding the differences between the results of laboratory testing and the fatigue lives of components in service.