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Ab initio study of phase stability in metals
Káňa, Tomáš ; Hüger, E. ; Šob, Mojmír
We employ ab initio calculations to study the behavior of total energy along the transformation paths between the hexagonal close-packed (hcp) structure and face-centered cubic (fcc) structure in Al, Co, Ni, Cu and Pd. We found one peculiar transformation path wit h extremely low energy barrier of only 0.010 eV/atom for Al, 0.020 eV/atom for Pd, 0.030 eV/atom for Cu, 0.033 eV/atom for Ni and 0.050 eV/atom for Co.
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First principles study of polymorfy of Ni and Co thin layers
Zelený, Martin ; Šob, Mojmír
The total energy profiles of trigonal and hexagonal bulk Ni and Co are employed to advance our understanding of epitaxial growth of Ni and Co thin films on various fcc(111) substrates. For nickel films, we predict the fcc stacking in the neighbourhood of the ground state and for the distance of nearest neighbors larger than 5.18 a.u. the film should exhibit hcp layer stacking. Cobalt films behave differently. The hcp phases are energetically more favorable in whole region studied.
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Resistance of HDPE materials against crack propagation from two-parametric fracture mechanics point of view
Bareš, Pavel ; Fiedler, L.
The influence of geometry constraint on the resulting values of fracture characteristics of HDPE material is analysed. To this aim the crack growth resistance (J-R) curves and the initial fracture toughness values for HDPE material were experimentally determined for an arc-shaped test specimen with and without protective layer. Two parameter fracture mechanics is applied and transferability of the experimental data to the two-layer pipe used in praxis is discussed. Finite element calculations were used for the investigation. It is concluded that the estimation of the service life of pipes based on the approach is conservative.
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Material analysis of magnesium alloy AZ 91 with various amount of lithium
Man, O. ; Petrenec, Martin ; Ptáček, L.
he contribution deals with magnesium alloy of commercial grade AZ 91 with various amount of lithium addition (5.5, 8 and 12 wt.%). The alloy was observed in the as-cast state and after annealing. Various types of analysis were done, such as metallographic one using light microscopy, scanning electron microscopy and transmision electron microscopy, in order to gain knowledge about the phase composition of the alloy and its development. Several phases and morphological features were identified, but the rest was not, due to the properties of lithium-rich areas.
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Kalibrace potenciálové metody pro měření růstu únavové trhliny
Kotecký, Ondřej ; Degallaix, S. ; Polák, Jaroslav
The DC potential drop method widely used to follow long crack growth, was adapted to measure the growth and to estimate the shape of mechanically short fatigue cracks initiated on a small defect in an austenitic-ferritic stainless steel. The paper deals with problems of calibration of the method by the means of finite element analysis. A comparison between measured and actual crack length and crack front in notched specimens is presented.
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Mechanical properties evaluation by means of instrumented indentation test
Brumek, J. ; Strnadel, B. ; Dlouhý, Ivo
An instrumented indentation study focusing on the materials with non-linear stress-strain behavior is presented. Indentation tests were applied on steel R7T. The observed data were analyzed and mechanical properties, such as yield stress, strain hardening exponent and elastic modulus were assessed. Experimental results of load-indentation depth test were compared with FEM solution. FEM analysis was applied on two-dimensional model described by elastic-plastic elements. Presented approach to prediction of mechanical characteristics is useful for estimation of remanent lifre of steels and other materials by using indentation method.
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The role of microstructure in racture behaviour of TiAl based intermetalics
Krahula, Karel ; Prokešová, Zdenka ; Dlouhý, Ivo
The contribution is focused on the analysis of the role of microstructural parameters in fracture behaviour of TiAl intermetalics at room and increased temperatures. Flexural strength and fracture toughness have been evaluated for alloys Ti-40Al-2Cr-2Nb-1B and Ti-46Al-0,7Cr-0,1Si-7Nb-0,2Ni. Positive effect of Nb on fracture resistance has been found; fracture toughness of the high Nb alloy increased at contemporary increase of flexural strength. Fracture surfaces have been evaluated using scanning electron microscopy showing the key role of mechanical twinning in deformation of both alloys followed. As a governing mechanism of crack nucleation microcrack formation at boundaries between mechanical twins and gama grains has been confirmed.
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