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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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Použití lokálního přístupu při řešení transferability lomově mechanických parametrů
Dlouhý, Ivo ; Hadraba, Hynek ; Jurášek, L.
In some applications the pre-cracked Charpy specimens (PC) can be, after meeting the validity conditions applied for specification of fracture toughness characteristics corresponding to standard specimens. Typically this is the case of pearlitic steel R7T (used for railway wheelset production) with Lüders’ flow and higher strength properties. It has been found that there is a gap in fracture toughness data of PC specimens between the 60 and 90 MPam1/2 due to plastic hinge in net section of PC specimens. The distribution of maximum principal stress ahead of the crack tip in PC specimen loaded into fully plastic regime was found to be identical with that of reference stress-strain field in plane-strain SSY condition. 3D FEM simulations also shown that there is no constraint loss in PC specimens.
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Interaction between TiAl melt and coated oxide refractories
Dočekalová, Kateřina ; Dlouhý, Antonín ; Zemčík, L. ; Fiala, J.
The present study focuses on reactions between ceramic crucibles and molten TiAl intermetallics. Coating strategies are investigated that exhibit a potential in slowing down the decomposition of the crucible wall. Uncoated crucibles (system TiAl melt - Al2O3 crucible) and crucibles with the coat (system TiAl melt - CaO coat - Al2O3 crucible) were used in melting and casting experiments. The reference state of the coated crucible before the melting trial was also investigated. Results of the SEM, EDAX and X-ray diffraction analysis contributed to the identification of the reaction zone and its chemical composition. The melt contamination with oxide inclusions results from the erosion process in which individual components of the melt diffuse along grain boundaries of the refractory and activate chemical reactions that decompose the crucible wall. Assessment of thermodynamic parameters associated with the chemical reactions helped in identifying the nature of the erosion process.
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Initial stages of fatigue failure of polycrystalline materials
Man, Jiří ; Petrenec, Martin ; Obrtlík, Karel ; Polák, Jaroslav
Geometry of persistent slip markings (PSMs) formed during cyclic loading of polycrystalline stainless steels and superalloy was studied using atomic force microscopy (AFM) and high-resolution scanning electron microscopy (SEM-FEG). Internal dislocation structure was investigated using transmission electron microscopy (TEM). Persistent slip bands with dislocation structure corresponding to the cyclic strain localization were identified and correlated with PSMs observed on the specimen surface.
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Calculation of residual fatigue life of railway set
Náhlík, Luboš ; Hutař, Pavel ; Knésl, Zdeněk ; Matušek, P.
The calculation of residual fatigue life of railway set is described in the contribution. The initial defect is modeled as a semicircular crack, which propagates preferentially in normal mode of loading perpendicularly on the axis of the axle. The conservative approach based on linear elastic fracture mechanics was chosen for residual fatigue life prediction. The aim of calculation is to determine number of loading cycles, which permits safe run of the railway axle, i.e. the dependence of crack depth on number of loading cycles for given loading spectrum.
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The fracture behaviour of welded joints pearlitic and bainitic rail steel
Hadraba, Hynek ; Dlouhý, Ivo ; Holzmann, Miloslav ; Zbořil, J.
The contribution is focused on analysis of welded joint made from pearlitic rail steel of type UIC 900A and newly developed cast bainitic Lo8CrNiMo steel. The lowest dynamic fracture toughness and consequently the higher risk of brittle fracture possessed the pearlitic steel and its heat affected zone (HAZ). At application temperature range the fracture toughness of this steel was on lower shelf of ductile to brittle transition, the tempering in HAZ didn't affect the toughness substantially. The dynamic fracture toughness of cast bainitic steel in joint was two times highest comparing to pearlitic one and, in addition HAZ toughness increase was found. The weld joint itself was characterised by high scatter of toughness data, nevertheless, all the values were above the scatter band characterising the more brittle pearlitic steel.
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