Multivariant experimental investigations and multiphysics microstructural modeling of the spark plasma sintering process of metallic powders have been performed up to a relative density of approximately 80%. In comparison, the effect of sintering temperature, pressure, and particle size on the interparticle contact area growth and axial shrinkage of cylindrical specimens of copper and nickel particles is measured in laboratory scaled tests. Herein, for the first time all relevant for sintering phenomena are considered simultaneously: the fully coupled thermo-electro-mechanical modeling of the spark plasma sintering processes, additionally taking into account for lattice, grain boundary, surface diffusion, electromigration, and thermomigration, has been carried out. The computational analysis of various physical phenomena allows to identify dominant and insignificant mechanisms. The two-level numerical simulation includes the modeling of the sintering setup at the macroscopic level and the neck formation process in particle chain systems at the microscopic level. The results of the numerical simulations show a very good agreement with the experimental data. Therefore, the impact of electrical and mechanical loads as well as of particle size on microscopic distribution of temperature, inelastic strain, and on densification has been studied by the finite element simulations.

Spark Plasma Sintering (SPS) is an innovative sintering technique, whereby many of the beneficial effects of this process on sintering are still elusive. To allow for the detailed investigations of the SPS process, a custom experimental set-up and a corresponding finite element (FE) model was developed. The miniaturised setup allows for very high current intensities, custom pulse patterns, a wide pressure range and dilatometric measurements. The FE model was employed to calculate the temperature field in the set-up and the sintering specimen itself. A very good correlation of the temperature, current and voltage over the entire process was observed. Our investigations show that the contact conductivities have a significant impact on the process temperature. Also, the imperfect contacts at the interfaces between the graphite foil and the real specimen may lead to a significant variance of the currents necessary to obtain the desired sintering temperature.

Multicomponent, high‐entropy alloys (HEAs) are promising candidates for replacing conventional alloys in high‐temperature applications. Herein, the high‐temperature corrosion of AlCrFeNiX0.5 (X = Co, Mo) is investigated. The samples are tested for their oxidation resistance at temperatures up to 1200 °C for 120 h and their behavior in NaCl/Na2SO4 at 900 °C for 96 h. They are benchmarked against commercial alloys such as FeCrAl. Despite the same contents of Al and Cr, the HEAs form different oxide layers showing very different oxidation resistance. The type of oxide is related to the multiphase microstructure. The samples exhibit different amounts of ordered and unordered body‐centered cubic (bcc) phase. The Co‐containing specimen shows an oxidation resistance that performs similarly well as FeCrAl. Its behavior is ascribed to the formation of an Al2O3 layer, which is very stable at high temperatures. The sample with X = Mo exhibits an additional Mo‐rich sigma phase, thus posing the risk of catastrophic oxidation. However, the Mo‐containing HEA is more resistant in the environment of molten salt. Preoxidation treatment at a lower oxygen partial pressure proves to prolong life span of the Mo‐containing HEA in hot air. Furthermore, a positive impact on oxidation resistance by addition of Y is affirmed.

Aluminium-copper alloys of the 2xxx type receive their excellent mechanical properties by the formation of copper-rich precipitates during hardening. Size, distribution and crystal structure of the formed precipitates determine the final strength of those alloys. Adding traces of certain elements, which bind to vacancies, significantly influences the decomposition behaviour, i.e. the diffusion of the copper atoms. For high-purity ternary alloys (Al-1.7 at.% Cu-X), we investigate the interaction of copper and trace element atoms (X=In, Sn, and Pb) with quenched-in vacancies by Positron Annihilation Lifetime Spectroscopy (PALS). By employing Vickers microhardness, Differential Scanning Calorimetry (DSC) and Small Angle X-Ray Scattering (SAXS) we obtain a comprehensive picture of the decomposition process: opposite to predicted binding energies to vacancies by ab-initio calculations we find during ageing at room and elevated temperature a more retarded clustering of copper in the presence of In rather than for Sn additions, while Pb, having the highest predicted binding to vacancies, shows nearly no retarding effect compared to pure Al-Cu. If the latter would be due to a limited solubility of lead, it had to be below 2 ppm. Transmission Electron Microscopy (TEM) as imaging method complements our findings. Annealing the quenched Al-1.7 at.% Cu-X-alloys containing 100 ppm In or Sn at $$150\,^\circ {\text {C}}$$ leads to finely distributed $$\theta \, '$$ -precipitates on the nanoscale, since due to the trace additions the formation temperature of $$\theta \, '$$ is lowered by more than $$100\,^\circ {\text {C}}$$ . According to TEM small agglomerates of trace elements (In, Sn) may support the early nucleation for the $$\theta \, '$$ -precipitates.

The effect of titanium addition beyond the solubility limit was investigated in order to proof the precipitation hardening possibility of the Cantor alloy CoCrFeMnNi. As high entropy alloys are currently in the focus of interest, this method is a highly suitable possibility to further improve their remarkably properties. After appropriate heat treatment consisting of homogenisation, solution and precipitation annealing a significant increase in hardness was detected. The high hardness was caused by nanometer sized precipitates, which were identified by TEM-analysis.

In the present research, a numerical modeling approach of the initial stage of consolidation during spark plasma sintering on the microscopic scale is presented. The solution of a fully coupled thermo-electro-mechanical problem also accounting for grain boundary and surface diffusion is found by using a staggered way. The finite-element method is applied for solving the thermo-electro-mechanical problem while the finite-difference method is applied for the diffusion problem. A Lagrange-based non-linear formulation is used to deal with the detailed description of plastic and creep strain accumulation. The numerical model is developed for simulating the structural evolution of the involved particles during sintering of powder compacts taking into account both the free surface diffusion of the particles and the grain boundary diffusion at interparticle contact areas. The numerical results obtained by using the two-particle model—as a representative volume element of the powder—are compared with experimental results for the densification of a copper powder compact. The numerical and experimental results are in excellent agreement.

The mechanisms of densification in spark plasma sintering (SPS) were investigated both analytically and numerically for a model system of two spherical metallic powder particles. From the microscopic temperature distribution, the possibility of a micro-local overheating of the particle-particle contacts was analysed for different particle sizes, contact geometries, materials, and electrical loads. It is shown that, for particles below the size of one millimetre, local overheating is below one Kelvin. Subsequently, the material transport by thermomigration, electromigration, and diffusion driven by surface curvature and external pressure was derived from microscopic field distributions obtained from analytical calculations and finite-element simulations. The results show that, while the mechanical pressure accelerates material transport by orders of magnitude, the electrical current and the temperature gradients do not. It is also shown that pulsing the current has no significant influence on the densification rate.

Bone replacement and osteosynthesis require materials which can at least temporarily bear high mechanical loads. Ideally, these materials would eventually degrade and would be replaced by bone deposited from the host organism. To date several metals, notably iron and iron-based alloys have been identified as suitable materials because they combine high strength at medium corrosion rates. However, currently, these materials do not degrade within an appropriate amount of time. Therefore, the aim of the present study is the development of an iron-based degradable sponge-like (i.e. cellular) implant for bone replacement with biomechanically tailored properties. We used a metal powder sintering approach to manufacture a cylindrical cellular implant which in addition contains phosphor as an alloying element. No corrosion inhibiting effects of phosphorus have been found, the degradation rate was not altered. Implant prototypes were tested in an animal model. Bone reaction was investigated at the bone-implant-interface and inside the cellular spaces of the implant. Newly formed bone was growing into the cellular spaces of the implant after 12 months. Signs of implant degradation were detected but after 12 months, no complete degradation could be observed. In conclusion, iron-based open-porous cellular biomaterials seem promising candidates for the development of self-degrading and high load bearing bone replacement materials.

Molybdenum as a potentially new biodegradable material was investigated. Degradation behavior of commercially high purity molybdenum was observed in simulated physiological salt solutions (Kokubo's SBF with/without TRIS-HCl, Cu2+ addition and 0.9 % NaCl solution). Potentiodynamic polarization, immersion mass loss and ion concentration measurements paired with REM/EDX analysis reveal gradual dissolution of molybdenum in the proper order of magnitude for stent application, associated with formation of thin, non-passivating corrosion products. The underlying corrosion mechanism is discussed as well as a comparison to literature data. However, formation of calcium phosphates (CaP) in SBF significantly decreases corrosion rates. In-situ polarization was found to be a potential way for overcoming this problem and simultaneously enhancing corrosion above the benchmark for a degradable stent material. A first in-vitro cytotoxicity assessment for HFIB fibroblasts in direct contact to corroding molybdenum did not show any signs for decreased cell viability. Thus, molybdenum appears to be a promising novel biodegradable metal for structural applications.

The mechanisms of densification during spark plasma sintering (SPS) of spherical copper particles are investigated both experimentally and analytically. Experimentally measured densification rates are compared to expected contributions to densification coming from Coble creep, Nabarro-Herring creep, power-law creep, and pressure-assisted sintering described by the two-particle model. The results indicate densification by Coble creep or a low-temperature version of power-law creep in the investigated range of temperature and pressure at a relative density of 70%. The findings are supported by (i) activation energies obtained for various conditions using the Dorn-method as well as (ii) the pressure dependence of the densification rate.

Porous electrodes for alkaline water electrolysis were prepared by spark plasma sintering, a short-time sintering technique, in combination with a space holder method. After removal of the space holders, highly porous layers of polycrystalline Ni and of a nanocrystalline Ni-Fe alloy were obtained on a metallic substrate. Both porosity and thickness of the electrocatalytic layers can be controlled by the space holder volume content and the sintering process conditions, for example, the applied pressure and temperature. The active surface of the electrode can be increased significantly by a roughness factor of up to 1,120 determined by double layer charging. The porous layers are efficient towards oxygen evolution reaction (OER), whereas activity is greatly influenced by the chemical composition. The porous Ni-Fe electrodes exhibit an extremely low OER-overpotential of 230 mV at 0.3 A cm(-2) in highly concentrated KOH (29.9 wt.-%) at 333 K. Beside the high surface area, the efficiency of the porous Ni-Fe layer is characterized by a high intrinsic activity resulting in a low Tafel slope of around 23 mV dec(-1) at low and 50 mV dec(-1) at high current densities as well as a high turnover frequency (TOF) of approximately 3.4 s(-1) at 0.3 V. The porous Ni electrodes have a lower intrinsic activity with higher Tafel slopes and lower TOF. Moreover, an excellent stability and activity under realistic operating conditions of intermittent electrolysis (up to 1 A cm(-2)) for 100 h was proven for the porous Ni-Fe electrode. (C) 2019 Elsevier Ltd. All rights reserved.

Additive manufacturing is a powerful tool for rapid prototyping and fabricating metal articles having a complicated geometry. This method is known to be used almost solely for the manufacture of articles consisting of pure metals and alloys. In the present work the possibility of obtaining dense carbide articles by a single-step process of additive manufacturing based on selective electron beam melting was evaluated. A new technology for fabricating cemented carbide granules suitable for selective electron beam melting was developed. It includes conventional granulating WC-Co powders followed by solid-state pre-sintering and preliminary screening of the granules. After that their liquid-phase sintering and final screening are carried out to obtain a desired fraction needed for the additive manufacturing process. Results of experiments on selective electron beam melting at different scan rates and current values indicated that it was possible to obtain non-porous carbide articles of complex geometry from WC-Co granules initially containing 13 wt% Co. The selective electron beam melting process led to the evaporation of some liquid Co and very intense local WC grain growth resulting in peculiar microstructures of the cemented carbide articles comprising layers with medium-coarse and abnormally large WC grains. A near-surface layer of the cemented carbide articles obtained by additive manufacturing is characterized by a high roughness comparable with the mean size of the original WC-Co granules.

In this study different types of multi-walled carbon nanotubes (MWCNT) were produced by the fixed bed and aerosol chemical vapor deposition (CVD) method. Nanocomposite materials were prepared by incorporation of different MWCNTs in copper matrix using powder metallurgy methods. By using hot pressing in combination with hot extrusion, the orientation of the carbon reinforcement was tuned from 3D to 1D alignment. After a selective etching process the carbon reinforcement is partially free-standing at the composite surface, but still embedded in the metal matrix. The engineered surface acts almost like a black body. The spectral evaluation of the surface functionalization will be shown for wavelengths from 200 nm to 20 µm. These results are compared to bulk copper. The free-standing MWCNT also behave like fins/pins in heat exchanger structures or surface enhancement in pool boiling. The experimental setup will be explained and the measurement described for pure copper. The theoretical heat transfer coefficient of the engineered surface is calculated depending on diameter and length of the free-standing MWCNTs. The results are compared to bulk copper.

No AccessRapid.Tech + FabCon 3.D International Hub for Additive Manufacturing: Exhibition + Conference + NetworkingJul 2019Additive Manufacturing of wear-resistant materialsM. Jurisch, P. Kluge, J. Boes, B. Klöden, T. Weißgärber, B. KiebackM. Jurisch, P. Kluge, J. Boes, B. Klöden, T. Weißgärber, B. KiebackFraunhofer IFAM, Institutsteil Dresdenhttps://doi.org/10.3139/9783446462441.024SectionsAboutPDF ToolsAdd to FavoritesDownload CitationTrack CitationsCopy LTI LinkPDF key 'share (en)' returned an object instead of string.FacebookTwitterEmailLinkedIn previous chapternext chapter FiguresReferencesRelatedDetails 2019Pages: 335-348Print ISBN: 978-3-446-46223-6eISBN: 978-3-446-46244-1 Copyright & Permissions© 2019 Carl Hanser Verlag GmbH & Co. KGPDF downloadLoading ...

Nanocrystalline Nickel-based alloys were investigated as catalysts for the oxygen evolution reaction(OER) at industrial operation conditions for alkaline water electrolysis. Different alloys were prepared by rapid solidification and subsequent high-energy milling. Regarding OER activity, the best efficiency was obtained for a nanocrystalline Ni-Fe alloy in 29.9 wt.% KOH at 298 K. However, at elevated temperature (333 K), comparable activities were determined in short-term experiments for nanocrystalline Ni-Fe and Ni alloys as well as for polycrystalline Ni. This initially incomprehensible outcome can be explained by the incorporation of Fe, which is present as impurity in the reagent grade KOH solution, into the NiOOH anode surface layer. However, after a long-term operation, the nanocrystalline Ni-Fe alloy shows a significantly better activity, in particular, at altering current density of up to 1 A cm(-2). As a result, the nanocrystalline Ni-Fe alloy exhibits a very high efficiency and excellent long-term activity (375 mV overpotential at 0.3 A cm(-2)) after 95 h of operation at different loads. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

In this study, the evolution of the electrical resistivity of metal powders during densification and the resulting current flow through punch, powder compact, and die is investigated. The evaluation of the accompanying Joule heating identifies the graphite punches as main heating element providing more than 90% of the heat. The high electrical resistance of the punches and the low resistance of the graphite die as parallel electrical load to the specimen determine the current flow in the tool. For powder particles with intact oxide layers, virtually no current flows through the compact. On the other hand, for a powder resistivity below omega cm about 50% of the current flows through the compact. This fraction is constant despite further decreasing resistivity of the compact during densification. A constant current through the specimen has important implications for the microscopic temperature distribution and the understanding of the so-called 'spark plasma effects'.

Abstract For the assessment of the precipitation state in aluminum-based alloys by transmission electron microscopy (TEM), the sample must have an orientation in the [100] direction of the matrix. TEMs equipped with objective lenses that are optimized for high resolution are mostly characterized by limited sample tilt angles. The required crystal orientation must thus be taken into account during sampling. A favorably oriented grain was chosen in a coarse-grained material using electron backscatter diffraction and prepared in a suitable direction applying the in-situ lift-out method in the focused ion beam device. After that, the final correction of the orientation in the TEM amounts to less than 12°.

Contact growth and temperature behavior in time during a single high-current pulse representing the initial stage of field-assisted/spark plasma sintering (FAST/SPS) have been studied experimentally and numerically. The measured evolution of the electrical resistance and of the neck formation process in two-particle systems is compared to the results obtained from the fully coupled thermo-electro-mechanical finite element simulations. The results of simulations with various models of inelastic deformation show that the viscoelastic/viscoplastic material model provides a realistic contact growth in initial stage of FAST/SPS. The impact of electrical and mechanical loads, material parameters and particle size on temperature, on inelastic strain distribution and on densification has been studied by finite element simulations for copper, stainless steel and nickel particles.

A powder metallurgy route is described as a promising route to produce highly active Raney-Ni electrodes. An expanded Ni mesh was used as metallic substrate on the surface of which Raney-Ni phases were produced via a heat-treatment step using Al powder at different loads. The overpotential at -300 mA/cm(2) as well as the active surface area were determined to evaluate the electrodes. The results reveal that a high Al loading is necessary to achieve a stable electrode and a high activity for the hydrogen evolution reaction. (c) The Author(s) 2019. Published by ECS.

Although the view that nm-sized oxide particles modify and essentially improve the irradiation resistance of Fe-Cr-based alloys is widely accepted, the correctness of this view has only been demonstrated in singular cases. An extension of the field of considered microstructures, irradiation conditions, and measures of irradiation resistance is required. The present study is focused on nanostructured ferritic Fe-14%Cr-based alloys, with and without the addition of 0.6 wt% Y2O3, produced via mechanical alloying and consolidation by spark plasma sintering. The materials were exposed to single-beam (Fe) and dual-beam (Fe and He) ion irradiations at room temperature. The initial microstructures were characterized, bimodal grain size distributions were observed and nanoindentation was applied to measure irradiation hardening for fine-grained and coarse-grained areas separately. We have found that grain size governs irradiation hardening for single-beam irradiation, while oxide nanoparticles play a dominant role for dual-beam irradiations. This sheds a light on the role of particle-matrix interfaces on helium management. (C) 2019 Elsevier B.V. All rights reserved.