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

Development and Characterization of Mg-SiC Nanocomposite Powders Synthesized by Mechanical Milling

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Magnesium powder in micron scale and various volume fractions of SiC particles with an average diameter of 50 nm were co-milled by a high energy planetary ball mill for up to 25 h to produce Mg-SiC nanocomposite powders. The milled Mg-SiC nanocomposite powders were characterized by scanning electron microscopy (SEM) and laser particle size analysis (PSA) to study morphological evolutions. Furthermore, XRD, TEM, EDAX and SEM analyses were performed to investigate the microstructure of the magnesium matrix and distribution of SiC-reinforcement. It was shown that with addition of and increase in SiC nanoparticle content, finer particles with narrower size distribution are obtained after mechanical milling. The morphology of these particles also became more equiaxed at shorter milling times. The microstructural observation revealed that the milling process ensured uniform distribution of SiC nanoparticles in the magnesium matrix even with a high volume fraction, up to 10 vol%

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

Geração de uma excitação sísmica através do espectro de Kanai-Tajimi

O presente artigo apresenta os resultados da implementação de uma rotina computacional, no programa MATLAB R , para a geração de uma excitação sísmica e análise da resposta dinâmica de um edifício, com n graus de liberdade, sujeito a esse tipo de carregamento. Define-se a excitação sísmica como um processo estocástico estacionário unidimensional, passando por um processo de ruído branco Gaussiano através do filtro de Kanai-Tajimi, cuja função densidade espectral de potência permite a geração da aceleração do solo no domínio da frequência. A aceleração do solo é, então, passada para o domínio do tempo gerando um acelerograma sísmico artificial. De posse do acelerograma resolve-se, com a aplicação do método de Newmark, a equação de movimento dinâmico estrutural e encontram-se os deslocamentos, velocidades e acelerações referentes a cada andar do edifício. Por fim, uma análise dos valores máximos, em módulo, dos deslocamentos, acelerações e dos deslocamentos relativos entre os andares é ilustrada. Os resultados encontrados estão condizentes com a literatura confirmando, assim, a eficiência da rotina implementada.This paper presents the results of a computational routine implementation in the MATLAB R software for the generation of a seismic excitation and dynamic response analysis of a building, with n degrees of freedom, subject to this loading type. The seismic excitation is defined as an one-dimensional stochastic process, that is simulated undergoing by a Gaussian white noise process through a Kanai-Tajimi filter, which the power spectral density function generates the ground acceleration in the frequency domain. The ground acceleration is passed to the time domain generating an artificial seismic accelerogram. With the artificial seismic accelerogram and the application of the Newmark method, the structural dynamic motion equation is solved and the displacements, velocities and accelerations in each building floor are found. Finally, an analysis of the maximum values, in modulus, of the displacements, accelerations and drift is illustrated. The results are in agreement with the literature, confirming the efficiency of the implemented routine

Determinação da probabilidade de falha em um edifício submetido a uma excitação sísmica

This paper presents a methodology for the determination of the failure probability of buildings subjected to a seismic excitation. Uncertainties in the parameters of ground and of  structure are considered. The seismic excitation is defined as a one-dimensional stochastic process, which is simulated by passing a Gaussian white noise process through a Kanai-Tajimi filter, in which the frequency, damping and peak ground acceleration are random parameters. A 10-storey building is analyzed, in which the dynamic response is calculated taking into account the randomness in its mass, stiffness and damping ratio. For illustration purposes, 3,000 computational simulations, in MATLAB software, are carried, considering three types of soils in a region located in Peru. The mean maximum values, in modulus, of the displacements and accelerations, the inter-storey drift and the failure probability of the building are calculate.  A maximum value for the interstorey drift are considered as a limit state for damage. The results show that high failure probabilities are obtained in soils with frequencies located between the natural frequencies of the first two vibration modes of the structure.Este artigo apresenta uma metodologia para a determinação da probabilidade de falha em edifícios submetidos a uma excitação sísmica. Incertezas nos parâmetros do movimento do solo e da estrutura são consideradas nesse procedimento. A excitação sísmica é modelada como um processo estocástico estacionário unidimensional, utilizando a função densidade espectral de potência de Kanai-Tajimi, e são atribuídas aleatoriedades na frequência, amortecimento e pico de aceleração do solo. A estrutura analisada corresponde a um edificio, com 10 pavimentos, no qual a obtenção da resposta dinâmica é calculada levando-se em conta as aleatoriedades em sua massa, rigidez e razão de amortecimento. Para ilustrar a proposta, são realizadas 3.000 simulações computacionais, no programa MATLAB, considerando três tipos de solos, em uma região que poderia estar situada na zona sísmica 3 do Peru. Os valores máximos médios, em módulo, para o deslocamento e a aceleração em cada andar do edifício, o deslocamento relativo entre os andares e a probabilidade de falha são determinados. Considera-se como estado limite para dano estrutural um valor máximo para o deslocamento relativo entre os andares. Os resultados mostram que altas probabilidades de falhas são observadas em solos com frequências situadas entre as frequências naturais dos dois primeiros modos de vibrações da estrutura

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

PERFIL DO COMPORTAMENTO GERENCIAL: UM ESTUDO DE CASO DOS DIRETORES DOS CENTROS DE ENSINO DA UFSM

Com o aumento das pesquisas acerca de como deve ser um líder capaz de levar sua organização para níveis mais altos de eficiência, o presente trabalho objetiva identificar o perfil de comportamento gerencial, de acordo com o modelo proposto por Yukl (1998), dos Diretores de Centro da Universidade Federal de Santa Maria (UFSM). Utilizando a metodologia qualitativa, de caráter descritivo, a pesquisa foi realizada através de entrevistas semi-estruturadas, aplicadas a cada um dos oito Diretores dos Centros de Ensino. A análise dos dados foi realizada pela análise de conteúdo. Os resultados demonstram um perfil gerencial desses Diretores muito ligado ao gerenciamento de tarefas, devido à estrutura burocrática da própria instituição. Assim, atividades como planejar, solucionar problemas, administrar conflitos e esclarecer papéis e objetivos, atendidos pela exigência da universidade, e atividades relacionadas a formas de administrar relações, como monitorar desempenho, acaba negligenciado

Quantitation of progenitor cell populations and growth factors after bone marrow aspirate concentration

Background: The number of Mesenchymal Stem/Stromal Cells (MSCs) in the human bone marrow (BM) is small compared to other cell types. BM aspirate concentration (BMAC) may be used to increase numbers of MSCs, but the composition of MSC subpopulations and growth factors after processing are unknown. The purpose of this study was to assess the enrichment of stem/progenitor cells and growth factors in BM aspirate by two different commercial concentration devices versus standard BM aspiration. Methods: 120 mL of BM was aspirated from the iliac crest of 10 male donors. Each sample was processed simultaneously by either Emcyte GenesisCS® (Emcyte) or Harvest SmartPReP2 BMAC (Harvest) devices and compared to untreated BM aspirate. Samples were analyzed with multicolor flow cytometry for cellular viability and expression of stem/progenitor cells markers. Stem/progenitor cell content was verified by quantification of colony forming unit-fibroblasts (CFU-F). Platelet, red blood cell and total nucleated cell (TNC) content were determined using an automated hematology analyzer. Growth factors contents were analyzed with protein quantification assays. Statistical analyses were performed by ANOVA analysis of variance followed by Tukey’s multiple comparison test or Wilcoxon matched-pairs signed rank test with p < 0.05 for significance. Results: Cell viability after processing was approximately 90% in all groups. Compared to control, both devices significantly enriched TNCs and platelets, as well as the CD45−CD73+ and CD45−CD73+CD90+ cell populations. Further, Harvest significantly concentrated CD45−CD10+, CD45−CD29+, CD45−CD90+, CD45−CD105+, CD45−CD119+ cells, and CD45dimCD90+CD271+ MSCs, whereas Emcyte significantly enriched CD45dimCD44+CD271+ MSCs. BM concentration also increased the numbers of CFU-F, platelet-derived growth factor, vascular endothelial growth factor, macrophage colony-stimulating factor, interleukin-1b, VCAM-1 and total protein. Neither system concentrated red blood cells, hematopoietic stem cells or bone morphogenetic proteins. Conclusion: This data could contribute to the development of BMAC quality control assays as both BMAC systems concentrated platelets, growth factors and non-hematopoietic stem cell subpopulations with distinct phenotypes without loss of cell viability when compared to unprocessed BM

Gibberellin-mediated RGA-LIKE1 degradation regulates embryo sac development in Arabidopsis

[EN] Ovule development is essential for plant survival, as it allows correct embryo and seed development upon fertilization. The female gametophyte is formed in the central area of the nucellus during ovule development, in a complex developmental programme that involves key regulatory genes and the plant hormones auxins and brassinosteroids. Here we provide novel evidence of the role of gibberellins (GAs) in the control of megagametogenesis and embryo sac development, via the GA-dependent degradation of RGA-LIKE1 (RGL1) in the ovule primordia. YPet-rgl1.17 plants, which express a dominant version of RGL1, showed reduced fertility, mainly due to altered embryo sac formation that varied from partial to total ablation. YPet-rgl1.17 ovules followed normal development of the megaspore mother cell, meiosis, and formation of the functional megaspore, but YPet-rgl1.17 plants had impaired mitotic divisions of the functional megaspore. This phenotype is RGL1-specific, as it is not observed in any other dominant mutants of the DELLA proteins. Expression analysis of YPet-rgl1.17 coupled to in situ localization of bioactive GAs in ovule primordia led us to propose a mechanism of GA-mediated RGL1 degradation that allows proper embryo sac development. Taken together, our data unravel a novel specific role of GAs in the control of female gametophyte development.We wish to thank the IBMCP microscopy facility, and Ms J. Yun for technical assistance. We also thank Jennifer Nemhauser (University of Washington, USA) for the HACR sensor. Cambridge proofreading (https://proofreading.org/order/) provided proofreading and editing of this manuscript. This work was supported by grants from the Spanish Ministry for Science and Innovation-FEDER [BIO2017-83138R] to MAP-A and National Science Foundation [MCB-0923727] to JMA. 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