Materials Structure & Micromechanics of Fracture V

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Authors: Jenő Gubicza, Sergey V. Dobatkin, Z. Bakai, Nguyen Q. Chinh, Terence G. Langdon
Abstract: The correlation between the microstructure and the mechanical behavior of ultrafinegrained face centered cubic (f.c.c.) metals processed by equal-channel angular pressing (ECAP) was studied. It was found that the maximum value of the yield strength obtained at high strains is determined by the shear modulus and the saturation value of the dislocation density according to the Taylor equation. It was also revealed that the value of the parameter α in this equation decreases with decreasing stacking fault energy, indicating the effect of different geometrical arrangements of dislocations in the grain boundaries. In addition, it was shown that for ECAP processed Cu, the ductility decreases with increasing strain but at extremely high strains the ductility is partially restored due to a recovery of the grain boundary structure.
Authors: Miroslav Piska
Abstract: Modern trends in metal cutting, high speed/feed machining, dry cutting and hard cutting set more demanding characteristics for cutting tool materials. The exposed parts of the cutting edges must be protected against the severe loading conditions and wear. The most significant coatings methods for cutting tools are PVD and CVD/MTCVD today. The choice of the right substrate or the right protective coating in the specific machining operation can have serious impact on machining productivity and economy. In many cases the deposition of the cutting tool with a hard coating increases considerably its cutting performance and tool life. The coating protects the tool against abrasion, adhesion, diffusion, formation of comb cracks and other wear phenomena.
Authors: Zdeněk Drozd, Zuzanka Trojanová
Abstract: The Mg micropowder was mixed and ball milled with ceramic nanoparticles. The material was compacted and then hot extruded. Samples have been deformed in tensile as well as compression tests at temperatures between 20 and 300 °C at a constant initial strain rate. The flow stress is significantly influenced by temperature. The yield stress and maximum stress decrease with increasing temperature. Stress strain curves obtained at lower temperatures in tensile tests substantially differ from ones estimated in compression. Stress relaxation tests were conducted in order to find the internal stress as well as to identify possible thermally activated process(-es).
Authors: Ivan Saxl, L. Ilucová, Milan Svoboda, Vàclav Sklenička, Vladimir I. Betekhtin, Andrey G. Kadomtsev, Petr Král
Abstract: The grain structure of pure aluminium processed by equal channel angular pressing (ECAP) was examined electron backscattered diffraction (EBSD) in the as pressed state and after heating preceding the creep deformation using various stereological methods. Area intensities of grain and subgrain boundaries, length intensities of triple grain and subgrain junctions, structural homogeneity and its thermal stability are strictly dependent on the number of passes.
Authors: Dalibor Vojtěch, Alena Michalcová, Jan Verner, Jan Šerák, František Šimančík, Martin Balog, Juraj Nagy
Abstract: In the presented paper, properties of Al-Cr-Fe-Ti alloy produced by powder metallurgy (PM) are described. Rapidly solidified powder alloy was prepared by the pressure nitrogen melt atomization. The granulometric powder fraction of less than 45 μm was then hot-extruded. Microstructure of the as-extruded material comprised recrystallized α-Al grains and spheroids of intermetallic phases. Tensile strength of the investigated material was similar to that of a conventional casting Al-Si alloy commonly used in elevated temperature applications. Excellent thermal stability of the PM Al-Cr based material, which much exceeded the elevated temperature casting alloy, was proved by room temperature tensile tests after long-term annealing at elevated temperature. Reasons for the observed thermal stability of the investigated PM alloy are discussed.
Authors: Boris Monchev, P. Petkov, V. Boev, T. Petkova
Abstract: The novel bulk glasses from the chalcohalide Ge-S-AgI system have been synthesized. From the as-prepared samples amorphous films have been deposited by vacuum thermal evaporation. The amorphous nature of the studied bulk and layered materials has been proved by Xray diffraction. The composition of the synthesized bulk chalcohalide glasses and corresponding amorphous thin films has been ascertained by means of Auger electron spectroscopy. The morphology and uniformity of the deposited layers have been investigated using scanning electron microscopy. The basic optical properties of the studied glassy films have been defined. Variations in the optical behaviors as a function of the composition have been derived. Experiments related to optical recording in the investigated Ge-S-AgI layers has been implemented. The diffraction efficiency as a function of various recording beam intensities has been obtained.
Authors: Petra Cihlářová, Jiří Švejcar, Vàclav Sklenička
Abstract: There are two types of experimental material used – pure nickel and its nano-composite reinforced with nano-sized SiO2. These materials were produced by TU Clausthal, Germany. The results have shown that the creep resistance of the nickel nano-composite reinforced with nanosized SiO2 particles is higher in comparison with non-reinforced nickel. The mechanism responsible for creep behaviour is the dislocation creep at 293 K and at elevated temperatures the dislocation creep is controlled by grain boundary sliding.
Authors: D. Holec, Colin J. Humphreys
Abstract: We investigate critical thicknesses of InGaN epilayers grown on GaN substrates with the growth-plane not being the c-plane. In particular, we focus on non-polar orientations with growth planes being the m- and a-planes. We have taken into account the proper hexagonal symmetry of wurtzite GaN. We have found that there is only a small difference in the critical thickness for the cplane and the a-plane material; however, in the case of the m-plane material, we predict a quite different behaviour along the (in-plane) c-axis and the perpendicular (in-plane) a-direction.
Authors: Jaroslava Vaníčková, Jiří Děd, P. Bartuška, Pavel Lejček
Abstract: Silver objects obtained from archaeological excavations often exhibit an extended embrittlement. The causes of this embrittlement were studied on silver artefacts buried for more than one thousand years in the soil using light and electron microscopy techniques and EDX microanalysis. Our investigation revealed presence of chlorine, sulphur and oxygen along the grain boundaries suggesting that most likely, the catastrophic failure of the grave objects made of Ag–Cu alloys is caused by an intercrystalline corrosion attack. The role of possible grain boundary segregation and/or precipitation of copper as a path of easy corrosion is discussed.
Authors: Pavel Novák, Dalibor Vojtěch, Filip Průša, Jan Šerák, Thomáš Fabián
Abstract: Hydrogen is the promising pollutant-free fuel of the near future. For various hydrogen applications, suitable storage systems have to be developed. One of the safe ways is the reversible storage of hydrogen in the form of light metal (lithium or magnesium) hydrides. MgH2 magnesium hydride shows very high storage capacity (approx. 7 wt. %), but its problem is high thermodynamic stability. Therefore, high temperature (over 400°C) is necessary for MgH2 to decompose producing hydrogen. The solution of this problem can be the utilization of the complex magnesium hydrides containing nickel, copper or other transition metals. In this work, the microstructure and hydrogen storage properties of the various magnesium alloys (Mg-Ni, Mg-Zn, Mg-Cu and Mg-Cu-Al) are described. The aim was to find suitable hydrogen storage system with good storage capacity and sufficient rate of formation and decomposition of hydrides. Microstructure, chemical and phase composition of the alloys were determined by the light and scanning electron microscopy, EDS and XRD. Hydrogen saturation was carried out by cathodic polarization in the alkaline solution. Hydrogen content in the material was estimated by XRD from the shift of the diffraction lines of present phases.

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