Materials Structure & Micromechanics of Fracture

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Authors: Jakub Čížek, Ivan Procházka, Bohumil Smola, Ivana Stulíková, Radomír Kužel, Miroslav Cieslar, Z. Matěj, V. Cherkaska, Gerhard Brauer, W. Anwand, Rinat K. Islamgaliev, Olya B. Kulyasova
Abstract: In the present work, positron annihilation spectroscopy (PAS) is employed for microstructure investigations of various ultra fine grained (UFG) metals (Cu, Ni, Fe) prepared by severe plastic deformation (SPD), namely high-pressure torsion (HPT) and equal channel angular pressing (ECAP). Generally, UFG metals prepared using both the techniques exhibit two kinds of defects introduced by SPD: dislocations and small microvoids. The size of the microvoids is determined from the PAS data. Significantly larger microvoids are found in HPT deformed Fe and Ni compared to HPT deformed Cu. The microstructure of UFG Cu prepared by HPT and ECAP is compared and the spatial distribution of defects in UFG Cu samples is characterized. In addition, the microstructure of a pure UFG Cu prepared by HPT and HPT deformed Cu+Al2O3 nanocomposite (GlidCop) is compared.
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Authors: Klaus Unterweger, Otmar Kolednik
Abstract: Particle reinforced MMCs with a particle size of 100µm and an aluminium matrix (Al6061) are analyzed by the means of automatic local deformation analysis. Two different heat treatment conditions (under- and over-aged) are investigated. It is found that the local deformation behaviour is strongly determined by particle damage. While the under-aged material shows only particle fracture, the over-aged MMC shows also particle debonding as a relevant damage mechanism.
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Authors: I. Sabirov, Otmar Kolednik
Abstract: The relation between the local and global values of the fracture initiation toughness is investigated for a cast Al6061-based metal matrix composite with 10% of Al2O3 particles after different heat treatments. The global values of the fracture initiation toughness are determined by conventional fracture mechanics tests. The local values of the fracture initiation toughness are found by measuring the critical crack tip opening displacement (CODi) at different locations along the crack front. To do this, an automatic fracture surface analysis system is applied. Despite of the large scatter of the individual CODi-values, a good correlation to the global fracture initiation toughness values is found.
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Authors: Barbora Bártová, Jan Verner, Dalibor Vojtěch, A. Gemperle, M. Čerňanský
Abstract: The paper reports on a detailed investigation of microstructure and phase composition of rapidly solidified and annealed AlNi18.5 and AlNi17Zr1.8 ribbons. The ribbons were prepared by the melt spinning (planar flow casting) technique. The microstructure and phase composition have been studied by TEM and XRD. The specimens were annealed and subsequently subjected to microstructure investigations to asses their thermal stability. Rapidly solidified alloys are composed of a-Al and Al3Ni phase grains. No significant difference in the shape between Al and Al3Ni grains was found. The Al9Ni2 metastable phase was identified in the rapidly solidified AlNi17Zr1.8 alloy and the Al3Zr phase precipitates from the a-Al solid solution in the AlNi17Zr1.8 alloy after the high temperature annealing.
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Authors: Luboš Náhlík, Zdeněk Knésl, F. Kroupa
Abstract: Plasma-sprayed ceramic coatings contain a high density of intrasplat microcracks which are responsible for small Young’s moduli and low fracture toughness. The extension of an initial surface crack in the direction to the interface, where the crack is repelled by the metal substrate with higher Young’s modulus, is studied using the methods of fracture mechanics. It is shown that high tensile stresses induced by the crack in the interface can lead to a local decohesion along the interface so that the crack can deviate into the interface.
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Authors: Emilie Ferrié, Jean Yves Buffière, Wolfgang Ludwig
Abstract: In-situ fatigue tests monitored by Synchrotron Radiation X-ray microtomography were carried out in order to visualize the three dimensional (3D) shape and evolution of short cracks in the bulk of a cast Al alloy. After the in-situ fatigue test the sample has been infiltrated with liquid Gallium (Ga) in order to visualize the grain structure of the material. Irregularities of the crack advance along the crack front can clearly be correlated to the grain structure of the material.
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Authors: Suzanne Degallaix, Florence Jaupitre, Djimedo Kondo, Philippe Quaegebeur, Pierre Forget
Abstract: In order to model the mechanical behaviour of an austenitic-ferritic duplex stainless steel thanks to "composite" micromechanical non-linear models, its microstructure was analysed and the mechanical behaviour of each phase was characterised. The microstructural morphology of this steel was studied by selective dissolution of the austenitic phase. The microstructure consists of unconnected austenite islands dispersed in a ferritic matrix. Nano-indentation tests were carried out on each phase. These tests allowed to obtain the hardness and the Young modulus of each phase. A non-linear homogenization approach (secant and incremental formulations) was implemented and the results were compared to the monotonous macroscopic tensile tests carried out at constant strain rate. It allowed us to evaluate the relevance of the non-linear homogeneization models for the description of the elasto-plastic behaviour of the studied duplex stainless steel.
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Authors: A.A. Shaniavski
Abstract: Fatigue fracture features such as pseudo- and fatigue striations were investigated for the Al-alloys AL-Si-Mg and Acoustic Emission (AE) was used to analyze plastic deformation processes ahead of a crack tip. Dramatic change of fatigue striations spacing was demonstrated in accordance with AE-signals evolution. Specimens with fatigue crack have not macroscopic significance in plastic deformation process under tension up to the fast fracture when striation spacing is over 2.1x10-7 m. Mechanisms of material deformation and fracture around of a crack tipduring fatigue striations formation process are discussed on the bases of the synergistic approach.
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Authors: Ivan Saxl, Petr Ponížil, Katarína Sülleiová
Abstract: The estimators proportional to (NA(x,y)NA(x,z)NA(y,z))1/2 and (NL(x)NL(y)NL(z)) obtained by profile and intercept counts are recommended by ASTM Standards E 112 for 3D grain intensity l of anisotropic grain structures (NA(•) are estimated in three mutually perpendicular planes suitably oriented with respect to the prevailing anisotropy direction and NL(•) are intercept intensities along three suitably selected mutually perpendicular directions). However, l is also related to the profile and intercept intensities l', l', namely l= c'(l')3/2= c'(l')3. The induced intensities are estimated by arithmetic means NA = (NA(x,y)+NA(x,z)+NA(y,z))/3, NL = (NL(x)+NL(y)+NL(z))/3. A simple model isdeveloped, in which the role of geometric and arithmetic means of profile and intercept counts in the grain size estimation is elucidated. Its results are compared with computer simulated random tessellations with prevailing rod-like and plate-like cells and applied to anisotropic grain structures of pure Al produced by ECAP and to platinum based composite Pt - 0.5 Y2O3.
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Authors: Dalibor Vojtěch, Tomáš Kubatík, Hana Čížová
Abstract: The paper describes a positive influence of silicon on the high-temperature oxidation resistance of titanium. Since silicon additions can be realized both by bulk and by surface alloying, the surface siliconizing techniques, as well as structure of the Si-rich layers, are illustrated. Furthermore, the high-temperature cyclic oxidation resistance of the surface siliconized titanium and of the TiSi2 alloy are compared to that of pure Ti and TiAl6V4 alloy, and the oxidation mechanism is discussed.
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