Pokročilé kovové materiály a kompozity na bázi kovů

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Strength 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, Mojmír
We 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.
Near threshold fatigue crack growth in ultrafinegrained copper
(IOP Publishing, 2014-08-08) Arzaghi, Mandana; Fintová, Stanislava; Sarrazin-Baudoux, Christine; Kunz, Ludvík; Petit, Jean
The near threshold fatigue crack growth in ultrafine-grained (UFG) copper at room temperature was studied in comparison to conventional coarse-grained (CG) copper. The fatigue crack growth rates da/dN in UFG copper were enhanced at 7 MPa@m compared to the CG material. The crack closure shielding, as evaluated using the compliance variation technique, was shown to explain these differences. The effective stress intensity factor amplitude
An Ab Initio Study of Pressure-Induced Reversal of Elastically Stiff and Soft Directions in YN and ScN and Its Effect in Nanocomposites Containing These Nitrides
(MDPI, 2018-12-01) Friák, Martin; Kroupa, Pavel; Holec, David; Šob, Mojmír
Using quantum-mechanical calculations of second- and third-order elastic constants for YN and ScN with the rock-salt (B1) structure, we predict that these materials change the fundamental type of their elastic anisotropy by rather moderate hydrostatic pressures of a few GPa. In particular, YN with its zero-pressure elastic anisotropy characterized by the Zener anisotropy ratio A Z = 2 C 44 / ( C 11 C 12 ) = 1.046 becomes elastically isotropic at the hydrostatic pressure of 1.2 GPa. The lowest values of the Young’s modulus (so-called soft directions) change from h 100 i (in the zero-pressure state) to the h 111 i directions (for pressures above 1.2 GPa). It means that the crystallographic orientations of stiffest (also called hard) elastic response and those of the softest one are reversed when comparing the zero-pressure state with that for pressures above the critical level. Qualitatively, the same type of reversal is predicted for ScN with the zero-pressure value of the Zener anisotropy factor A Z = 1.117 and the critical pressure of about 6.5 GPa. Our predictions are based on both second-order and third-order elastic constants determined for the zero-pressure state but the anisotropy change is then verified by explicit calculations of the second-order elastic constants for compressed states. Both materials are semiconductors in the whole range of studied pressures. Our phonon calculations further reveal that the change in the type of the elastic anisotropy has only a minor impact on the vibrational properties. Our simulations of biaxially strained states of YN demonstrate that a similar change in the elastic anisotropy can be achieved also under stress conditions appearing, for example, in coherently co-existing nanocomposites such as superlattices. Finally, after selecting ScN and PdN (both in B1 rock-salt structure) as a pair of suitable candidate materials for such a superlattice (due to the similarity of their lattice parameters), our calculations of such a coherent nanocomposite results again in a reversed elastic anisotropy (compared with the zero-pressure state of ScN).
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.
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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