Enhanced Strength and Ductility in Magnesium Matrix Composites Reinforced by a High Volume Fraction of Nano- and Submicron-Sized SiC Particles Produced by Mechanical Milling and Hot Extrusion

In the present study, Mg nanocomposites with a high volume fraction (10 vol %) of SiC particles were fabricated by two approaches: mechanical milling and mixing, followed by the powder consolidation steps, including isostatic cold pressing, sintering, and extrusion. A uniform distribution of the high content SiC particles in a fully dense Mg matrix with ultrafine microstructure was successfully achieved in the mechanically milled composites. The effect of nano- and submicron-sized SiC particles on the microstructure and mechanical properties of the nanocomposites was evaluated. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectrometer (EDS), and X-ray diffractometry (XRD) were used to characterize microstructures of the milled and mixed composites. Mechanical behavior of the Mg composites was studied under nanoindentation and compressive loading to understand the effects the microstructural modification on the strength and ductility of the Mg/SiC composites. The mechanical properties of the composites showed a significant difference regarding the size and distribution of SiC particles in the Mg matrix. The enhanced strength and superior ductility achieved in the mechanically milled Mg composites are mainly ascribed to the effective load transfer between matrix and SiC particles, grain refinement of the matrix, and strengthening effects of the nano- and submicron-sized SiC particles.DFG, 414044773, Open Access Publizieren 2019 - 2020 / Technische Universität Berli

Cyclic deformation behavior of Mg–SiC nanocomposites on the macroscale and nanoscale

Metal‐ceramic nanocomposites are promising candidates for applications necessitating light weight and excellent fatigue resistance. We produced Mg–SiC nanocomposites from mechanically milled powders, yielding a homogeneous nanocrystalline structure and excellent quasistatic strength values. Little is known, however, about the fatigue behavior of such composites. Here, we used load increase tests on the macroscale to yield estimation values of the fatigue endurance limit. Fatigue strength increased significantly for the materials processed by the powder metallurgical route. We further investigated the cyclic deformation behavior under stress‐controlled conditions on the macroscale and nanoscale. Cyclic nanoindentation showed that indentation depth and cyclic plastic deformation decreased with increasing reinforcement content, hinting to a higher cyclic strength and corroborating the results from the macroscopic load increase tests. Our results therefore show that cyclic nanoindentation reliably determines the plastic deformation behavior of Mg nanocomposites offering the possibility of fast material analysis.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659Peer Reviewe

Cyclic deformation behavior of Mg–SiC nanocomposites on the macroscale and nanoscale

Metal-ceramic nanocomposites are promising candidates for applications necessitating light weight and excellent fatigue resistance. We produced Mg–SiC nanocomposites from mechanically milled powders, yielding a homogeneous nanocrystalline structure and excellent quasistatic strength values. Little is known, however, about the fatigue behavior of such composites. Here, we used load increase tests on the macroscale to yield estimation values of the fatigue endurance limit. Fatigue strength increased significantly for the materials processed by the powder metallurgical route. We further investigated the cyclic deformation behavior under stress-controlled conditions on the macroscale and nanoscale. Cyclic nanoindentation showed that indentation depth and cyclic plastic deformation decreased with increasing reinforcement content, hinting to a higher cyclic strength and corroborating the results from the macroscopic load increase tests. Our results therefore show that cyclic nanoindentation reliably determines the plastic deformation behavior of Mg nanocomposites offering the possibility of fast material analysis

Von kugelgemahlenem Pulver zu Mg-SiC-Nanokompositen: Zusammenhänge zwischen Herstellung, Mikrostruktur und mechanischen Eigenschaften

Magnesium nanocomposites are promising for use as lightweight components in the automotive and aviation industries, since reinforcing pure magnesium with ceramic nanoparticles not only increases strength, but also improves ductility. SiC nanoparticles are particularly suitable as a reinforcement phase due to their high specific fracture toughness and their stable behaviour towards magnesium. However, the production of magnesium nanocomposites has some challenges, such as the achievement of a homogenous distribution of the nanoparticles and an ultra-fine structure in the completely dense material. These requirements were met via a new powder metallurgical production route, which includes mechanical milling, cold isostatic pressing, sintering and indirect hot extrusion. Initially, powder composites with a uniform SiC distribution and a nano- to sub-microcrystalline structure were produced by mechanical milling. The composite powder was then pressed into green compacts by cold isostatic pressing followed by sintering. The density increased up to 93 %, but hot extrusion was required to completely densify the magnesium nanocomposites. By achieving a sub-microcrystalline microstructure by hot extrusion, all challenges were overcome by the new processing route. However, extrusion also leads to a textured material, which is the reason why hot isostatic pressing was carried out as alternative last consolidation step. The achieved microstructure did not meet the requirements. Therefore, the mechanical properties were only investigated on the extruded Mg-SiC nanocomposites. An increase in hardness was observed with an increasing reinforcement content. Compression and tensile tests of the extruded nanocomposites showed a compression up to 41.5 %, however, the influence f the texture on the tensile elongation behaviour resulted in only minimal elongation of at most 2.5 %. Fatigue tests on macro and nano scale suggest that conclusions on plastic deformability of pure Mg can be drawn from nano fatigue to macro fatigue. Furthermore, nanocomposites showed less plastic strain than pure Mg produced in the same way. Using the new powder metallurgical production route, significantly higher fatigue endurance limits were achieved for milled Mg and its nanocomposites than for non-milled Mg.Der Einsatz von Magnesium-Nanokompositen als Leichtbaukomponenten ist in der Automobil- und Flugindustrie vielversprechend, da aufgrund der Verstärkung des reinen Magnesiums nicht nur die Festigkeit erhöht, sondern auch die Duktilität verbessert werden kann. SiC-Nanopartikel sind relativ inert gegenüber Magnesium und eignen sich als Verstärkungsphase besonders aufgrund ihrer hohen spezifischen Bruchzähigkeit. Die Herstellung der Magnesium-Nanokomposite birgt allerdings einige Herausforderungen, wie zum Beispiel die gleichmäßige Verteilung der Nanopartikel und ein ultrafeines Gefüge im vollständig dichten Material. Über eine neue pulvermetallurgische Herstellungsroute konnten diese Anforderungen durch hochenergetisches Kugelmahlen, kalt-isostatisches Pressen, Sintern und indirektes Warm-Strangpressen erfüllt werden. Zunächst konnten Pulverkomposite mit gleichmäßiger SiC-Verteilung und nano- bis submikroskristallinem Gefüge hergestellt werden. Das Kompositpulver wurde über kalt-isostatisches Pressen zu Grünlingen gepresst und anschließend gesintert, wodurch die Dichte auf 93% erhöht werden konnte. Um die Nanokomposite vollständig zu verdichten, wurde indirektes Warm-Strangpressen genutzt. Da Strangpressen allerdings auch zu einem texturiertem Material führt, wurden die Nanokomposite zudem über einen alternativen Verfahrensschritt, dem heiß-isostatischen Pressen, verdichtet. Allerdings entsprach die Mikrostruktur nicht den oben genannten Anforderungen, sodass die mechanischen Eigenschaften nur an den stranggepressten Mg-SiC-Nanokompositen untersucht wurden. Mittels Nanoindentation konnte zusätzlich die Härte der Kompositpulverpartikel nach dem Kugelmahlen ermittelt werden, die mit steigendem SiC-Gehalt stieg. In den Druck- und Zugversuchen konnten die stranggepressten Nanokomposite Stauchungen von bis zu 41.5 %, jedoch nur geringe Dehnungen von maximal 2.5 % erreichen, was auf den Einfluss der Textur zurückzuführen ist. Die Ermüdungsversuche legen nahe, dass Rückschlüsse zur plastischen Verformbarkeit von der Nano- zur Makroermüdung gezogen werden können. Unter Verwendung des neuen pulvermetallurgischen Produktionsweges wurden für Mg und seine Nanokomposite signifikant höhere Ermüdungsfestigkeiten erreicht als für reines, nicht kugelgemahlenes Mg.DFG, 280646214, Herstellung, Gefügebildung und mechanischen Eigenschaften (unter quasi-statischer und zyklischer Belastung) von Mg-SiC-Nanokomposite

Cyclic deformation behavior of Mg–SiC nanocomposites on the macroscale and nanoscale

Metal‐ceramic nanocomposites are promising candidates for applications necessitating light weight and excellent fatigue resistance. We produced Mg–SiC nanocomposites from mechanically milled powders, yielding a homogeneous nanocrystalline structure and excellent quasistatic strength values. Little is known, however, about the fatigue behavior of such composites. Here, we used load increase tests on the macroscale to yield estimation values of the fatigue endurance limit. Fatigue strength increased significantly for the materials processed by the powder metallurgical route. We further investigated the cyclic deformation behavior under stress‐controlled conditions on the macroscale and nanoscale. Cyclic nanoindentation showed that indentation depth and cyclic plastic deformation decreased with increasing reinforcement content, hinting to a higher cyclic strength and corroborating the results from the macroscopic load increase tests. Our results therefore show that cyclic nanoindentation reliably determines the plastic deformation behavior of Mg nanocomposites offering the possibility of fast material analysis

Effect of hot isostatic pressing on densification, microstructure and nanoindentation behaviour of Mg–SiC nanocomposites

The production of fully dense nanocomposites with a homogeneous distribution of nanoparticles through powder metallurgy (PM) techniques is challenging. Additionally to mechanical milling, pressing and sintering, a final consolidation process is needed to fully densify the nanocomposite. Hot isostatic pressing (HIP) is a promising alternative method to other hot forming processes to eliminate porosity in these PM parts. In contrast to hot extrusion, for instance, isotropic properties are achieved, and textures, as they are usually observed in Mg after uniaxial deformation, are avoided. Here, we evaluate the effect of HIP on the densification, microstructure and (nano)hardness of Mg-SiC nanocomposites. Even though density increased indeed, we observed no increase in the mechanical properties, due to significant heterogeneity in the microstructure. SiC-free regions with a higher grain size developed. Local nanohardness measurements of the HIPed Mg nanocomposite revealed that these regions had a significantly lower nanohardness than the SiC-containing regions. Under consideration of mechanisms reported to be active in Mg in the pressure and temperature regime we used, we conclude that grain growth is the most likely mechanism leading to the microstructure observed after HIP. This is driven by the thermodynamic pressure to decrease the grain boundary energy and facilitated by a slightly inhomogeneous distribution of SiC nanoparticles in the sintered nanocomposite

Real-world evidence study on tolerance and growth in infants fed an infant formula with two human milk oligosaccharides vs mixed fed and exclusively breastfed infants

Abstract Introduction Human milk oligosaccharides (HMOs) are important components of human milk having diverse functions in the development of infants. Randomized controlled trials (RCTs) have demonstrated that infant formulas with the HMOs 2′-fucosyllactose (2′FL) and lacto-N-neotetraose (LNnT) are safe, well-tolerated, and support normal growth. This study aimed to generate real-world evidence (RWE) on growth and gastrointestinal (GI) tolerance in infants consuming a formula with 1 g/L 2′FL and 0.5 g/L LNnT, including a mixed feeding group not studied before in RCTs. Participants and methods This 8-week open-label prospective multicenter study was conducted in Germany and Austria, and included groups of healthy, exclusively breastfed infants (BF), exclusively formula-fed infants (FF) who received the HMO-formula, and infants mixed fed with both HMO formula and human milk (MF). Co-primary outcomes were anthropometry and gastrointestinal tolerance via validated Infant Gastrointestinal Symptom Questionnaire (IGSQ). Secondary outcomes included formula satisfaction and adverse events (AEs). Results One-hundred six infants completed the study (46 FF, 22 MF, and 38 BF). Mean anthropometric z-scores were comparable between groups and generally within ± 0.5 of WHO medians at week 8. IGSQ composite scores demonstrated good GI tolerance in all groups with no significant group differences at week 4 or 8. IGSQ composite scores in FF improved during the course of the study and parents provided high satisfaction ratings for the HMO-formula. Four potentially product-related AEs were reported in FF (no in MF). Conclusions In this RWE study examining an infant formula with HMOs, growth and GI tolerance outcomes were confirming the good tolerance and safety of this early feeding option previously reported in RCTs