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METALLURGY AND PROPERTIES OF ADVANCED NiAl-Mo EUTECTICS
Barták, Tomáš ; Kuchařová, Květa ; Záležák, Tomáš ; Dlouhý, Antonín
A NiAl-Mo eutectic alloy was melt from 99,99% purity components and cast by the drop casting technique. The drop-cast ternary alloy (nominal composition of Ni-45Al-9Mo at. %), was re-melted and directionally solidified using a high temperature optical floating zone furnace. A resulting in-situ composite consists of Ni-45,2Al matrix and Mo-10Al-4Ni fibers, all in at. %. The volume fraction of 14% Mo-fibers stems from the eutectic composition. Backscatter diffraction shows that the NiAl-matrix and the Mo-fibers are both < 001 >-oriented with respect to the axis of the cylindrical rods. Preliminary creep experiments confirmed an immense improvement of high temperature strength due to the fine distribution of Mo-fibres. The amount of strengthening in terms of minimum creep rate can be as high as 7 orders of magnitude. Post-mortem transmission electron microscopy experiments provided evidence that creep in the temperature range of 800 - 900 degrees C results in an extensive formation of subgrain boundaries. The strengthening effect is very likely associated with the reactions between subgrain boundaries and fine Mo-fibres.
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METALIC NANOPARTICLES AND NANOSTRUCTURES
Nohavica, Dušan
A brief overview of the field of metallic nanoparticles and nanocrystalline materials preparation and their properties is presented. Dependence of the chemical potential on surface curvature is important for particles solubility in the melt, vapour pressure of liquids as a function of droplet radius, Ostwald ripening and sintering of the individual particles. Melting point and lattice constant depends on the radius of nanoparticles as well. The major processing for nanocrystalline materials production includes chemical reaction, vapour condensation, electrodeposition, ball grinding, SPD etc.
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Local approach in mechanical properties prediction
Brumek, J. ; Strnadel, B. ; Dlouhý, Ivo
Indentation technique was focused on the prediction of the strain hardening behaviour of carbide steels. An improved technique to determine the plastic properties of material from the load-displacement curve from a ball indentation test was proposed. The time severity for the search for an optimal solution for a non-linear constitutive model is dependent on a number of design variables. Common methods like gradient methods or linear programming can fail due the fact that they drop to the local minimum. The advantage of a genetic algorithm does not require knowledge of the target function. Proposed method was applied to the data from the instrumented indentation technique. Results were found to be in good agreement with the data from conventional, standard tests, and in less time.
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Influence of lattice defects on grain boundary self-diffusion in nickel
Rothová, Věra ; Buršík, Jiří ; Svoboda, Milan
An extremely wide spectrum of experimental results on the grain boundary (GB) self-diffusivities in nickel was published in the literature. Currently, our previously published results show a pronounced dependence of diffusivities on both purity and thermo-mechanical history. A possible reason for the large scatter of published experimental data could be a contribution arising from some crystal lattice defects other than GBs. Thus, in the present work, the impact of the selected lattice defects on diffusion processes, grain growth and twin production has been investigated. Hence, the cast and cold-rolled nickel specimens of purity grades from 99.5% to over 99.99% were selected to study the influence of the material purity and fabrication on microstructure and its evolution during thermal treatment and thereby on the mass transport properties.
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Corrosion of chosen Ni-based material exposed to LiF-NaF molten salts
Král, Lubomír ; Čermák, Jiří ; Matal, O. ; Šimo, T. ; Nesvadba, L.
This work is devoted to study of changes of structure and chemical composition in the surface layer of chosen candidate alloy Ni – 6 W – 9 Mo – 1.7 Ti – 7 Cr (in wt. %) for components of a LS-VHTR. The material was studied after its exposure to molten mixture 60 mol. % LiF + 40 mol. % NaF at temperature 993 and 1013K. The exposition was carried out in a quasidynamic regime, which simulates coolant (the mixture of salts) natural circulation conditions in components. The structure of the alloy was studied by SEM. Severe corrosion damage was observed in the surface layer, which thickness was about 300 μm.
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Low-temperature diffusion of carbon in iron measured by SIMS technique
Čermák, Jiří ; Král, Lubomír
Depth profiling mode of MiniSIMS device was applied to diffusion measurement. As an example, carbon diffusion in BCC iron was studied. There are sparse experimental diffusion data at temperatures below the eutectic horizontal (996 K). This is caused by the fact that direct measurement of carbon diffusion coefficient in ferrite at low temperatures is complicated by very low solubility of carbon. For practical purposes, the extrapolation of high-temperature data to low temperature region is usually used for estimation of diffusioncontrolled kinetics in ferrite alloys. Present work has shown that the SIMS technique can be applied to the study of carbon diffusivity in iron. Special technique was developed, which avoids radio-tracer measurements with C-14. Obtained results are lower than the extrapolated values, which may be due to limited solubility of carbon in iron.
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Fatigue properties of nickel-base superalloy inconel 792-5A at 800°C
Šmíd, Miroslav ; Obrtlík, Karel ; Petrenec, Martin ; Polák, Jaroslav ; Hrbáček, K.
Smooth specimens were cyclically strained under strain control with constant strain amplitude and constant strain rate. Low cycle fatigue tests were conducted in servo-hydraulic pulsator MTS equipped with a three zone resistance furnace at temperature 800°C in air. Fracture surface was studied in SEM after fatigue test termination. Selected specimens were used to prepare foils for the transmission electron microscope (TEM) observation of microstructure and dislocation arrangement. They were used to obtain cyclic hardening/softening curves, cyclic stress-strain curve and fatigue life curves in the representation of stress amplitude, total strain amplitude and plastic strain amplitude versus number of cycles to fracture. Experimental points can be approximated with the Manson-Coffin law and the Basquin law. Fracture surface examinations revealed fatigue crack initiation sites.
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Effect of heat treatments with a pointed cooling on the mechanical compression properties of TiAl intermetalic
Petrenec, Martin ; Kruml, Tomáš ; Zemanová, Adéla ; Krahula, Karel
The influence of the graded cooling heat treatments on a change in the structure and the room temperature mechanical compression properties of cast Ti-44Al-8Nb intermetallic were studied. The coolings were the air cooling and the graded cooling which was composed of combination of furnace cooling and air cooling for selected two temperatures 1315 °C and 1200 °C. These selected temperatures were used from the measuring of the differential thermal analysis (DTA). All heat treatments used remold the cast nearly lamellar structure ?/?2 to the fully lamellar uniform structure and decreased the average grain size to the half. The air cooling and the graded cooling with temperature 1315 °C were produced in higher yield stress and lower plasticity due the very thin lamellar thickness and planar grain boundary. While higher plasticity and lower yield stress are obtained using the graded cooling with temperature 1200 °C. It is caused by the very thick lamellar spacing and locked grain boundaries.
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Microstructure and mechanical properties of powder aluminium prepared by severe plastic deformation
Dvořák, Jiří ; Král, Petr ; Balog, M. ; Simančík, F. ; Sklenička, Václav
This paper describes properties of Al 99.7% produced by PM. Powder green were prepared via CIP. The prepared powder about size less than 400 µm was compacted via conventional direct hot-extrusion (DE) at temperature of 450°C. Some extruded material was additionally processed by ECAP. The microstructural investigations were performed using OM and SEM equipped with EBSD unit. Examination of extruded powder compacts showed that during DE initial powder particles deformed into grains about size of 5 m. This material was further subjected to ECAP passes. The mechanical properties of this material were investigated using hardness measurements. In addition, creep tests in tension were conducted at temperature of 473 K and at the different applied stresses. The basic aim of this work is microstructure characterization and description of creep behaviour of powder aluminium prepared by DE. The further aim of this work is the evaluation of ECAP technique effect on mechanical properties of powder Al.
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