Functionally Graded Materials VIII

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Authors: V.V. Kurbatkina, E.A. Levashov
Abstract: Composite carbide materials in Ti-C, Ti-Nb-C, Ti-Zr-C systems were produced by combustion synthesis using the force SHS-pressing technology and investigated. Composition and structure of the synthesis products before and after annealing at 850oC, 1 hour were studied using optical and SEM, X-ray diffraction and Auger Electron Spectroscopy. It was established that SHSproducts consist of two main phases: TiC0.6+β-Ti, (Ti,Nb)C1-х + β-(Ti,Nb), (Ti,Zr)C 1-х + β-(Ti,Zr). Structure of these composite materials presented by carbide phase with a grain size varying 4-8 µm, and surrounded intergranular thin layer of β-Ti solid solution with width less than 1.5 µm. Composition and structure of carbide grains and intergranular phase just after combustion process is not equilibrium: supersaturated solid solutions are formed because of high temperature gradient and combustion velocity. Annealing results in an equalization of the solid solution composition. Carbide grains content precipitations sized 20-200 nm based on solid solution (Me-Ti). Two possible schemes of concentration separation of supersaturated solid solution were discussed: (Ti, Nb)x+yC = (Ti, Nb)xC 1-х + y(Nb-Ti) and (Ti, Zr)x+y C = (Ti, Zr)xC1-х + y(Ti-Zr). Lattice parameter of the intraganular phase differs from the matrix ones. Graded distribution of Nb and Zr alloying elements inside the carbide grains was observed. The developed composite materials strengthened by nano- or microsized particles are attractive for high-temperature applications.
Authors: Alexandre Velhinho, Gerard Vignoles, P. Cloetens, X. Thibault, Elodie Boller, Francisco Manuel Braz Fernandes, L.A. Rocha, José D. Botas
Abstract: Reliability of functionally graded metal matrix composites (FGMMCs) for automotive components is still dependent on the detailed knowledge of the mechanisms of the microstructural build-up, for instance on the mechanisms leading to the distribution and relative positions of the reinforcing particles. In order to assess the influence of the SiC particle size on the 3-D inter-particle connectivity in functionally graded Al/SiCp composites produced by centrifugal casting, X-ray microtomography experiments were performed at the ID19 beamline in ESRF (European Synchrotron Radiation Facility). The FGMMCs consisted of an Al-10Si-2Mg alloy matrix, reinforced by an average SiC particle volume fraction of 0.10; two different average sizes were used: 37 µm and 12 µm. The holographic modification of the X-ray CMT (Computer Micro- Tomography) method allowed to obtain neatly contrasted images, as opposed to classical CMT.Good agreement was found between the particle size evaluated by CMT and by laser interferometry. Particle clustering has been evaluated in number and volume, showing that a lower mean particle size is related to more clustering. Such an adverse effect relies on the importance of particle/liquid alloy surface tension. Also, the mean particle size has been evaluated as a function of particle number within a cluster: as expected, the larger a cluster, the larger the particles inside it.
Authors: Amitava Ray
Abstract: The unique imaging and microanalytical capability of the electron-probe microanalyser (EPMA) has been exploited in conjunction with light microscopy for exploring the genesis of some defects, which are detrimental to steel product quality. Investigations of surface and internal defects have revealed that defects in the finished steel product are often inherited at upstream stages and accentuated in the later stages of steel processing. While the occurrence of surface defects such as “slivers” in hot-rolled sheets and “linear cracks” in hot-rolled rounds has been largely attributed to subsurface non-metallic inclusions and slag/casting powder entrapment, microprobe analysis of internal defects such as “centre-line cracking” of thick plates have revealed the deleterious role of C and Mn macrosegration inherited during the continuous casting of steel slabs. The paper elucidates some of these findings, as unravelled by both qualitative and quantitative microprobe analysis.
Authors: Gert Roebben, Uwe Wätjen, Andrée Lamberty
Abstract: Multifunctional materials are created by intimately integrating different materials. The assessment of the local properties and composition of the different components of such a multifunctional material necessarily relies on techniques with sufficient spatial resolution. The increased use of these microanalysis or microprobe techniques has created a demand for appropriate quality assurance tools, such as reference materials. In this paper a number of issues related to the certification of reference materials for multifunctional materials are discussed. Examples of reference materials are shown to illustrate the particular challenges the reference material producer is faced with such as micro-homogeneity, minimum sample intake and stability.
Authors: Guy Anné, Kim Vanmeensel, Jef Vleugels, Omer Van der Biest
Abstract: The residual stresses were measured on polished cross-sectioned Al2O3/ZrO2 FGM discs by means of Raman spectroscopy. The experiments revealed that it is possible to measure the residual stresses caused by the thermal expansion mismatch between the alumina and zirconia phase as a function of the position in the FGM. These experimental data are in excellent agreement with the thermal stresses calculated from the local composition using the model of Taya. However, the stresses due to the generated FGM profile could not be measured by means of Raman spectroscopy, due to stress relaxation upon cross sectioning of the FGM disc.
Authors: Valeria Cannillo, Tiziano Manfredini, M. Montorsi, Cristina Siligardi, A. Sola
Abstract: Functionally graded materials are a new and attractive class of materials incorporating an engineered spatial variation in composition and/or microstructure: this idea has immediately revealed successful since it allows to reach peculiar mechanical properties such as resistance to wear and contact damage. As a matter of fact, the final behaviour of a Functionally Graded Material is mainly influenced by its graded composition and/or microstructure. Therefore a good fabrication technique should provide a high control and reproducibility of the spatial variation in composition and/or microstructure; on the other hand, a reliable model should take into account the gradient in order to accurately predict the final behaviour of a Functionally Graded Material. The present study is focused on glass-alumina FGMs: the compositional variation, which occurs along only one direction, has been realized through percolation of a molten glass into a bulk polycrystalline alumina. The resulting Functionally Graded Coatings have been carefully characterized through Scanning Electron Microscopy, X-ray diffraction, classical mechanical tests and analysis. Moreover, their behaviour has been modeled by means of a microstructure-based FEM method. A great attention has been paid to the validation of the computational model on the basis of the experimental data. Furthermore, the experimental and the computational approaches have been combined in order to define the correlation between fabrication parameters, such as time and temperature, and resulting gradients in composition and microstructure as well as related performances. Since changes in material properties can be easily evaluated, the resulting model may be useful to simulate the material response to a given thermo-mechanical loading and to tailor the gradient as a function of the specific application.
Authors: T. Lauwagie, K. Lambrinou, Iulian Mircea, Marion Bartsch, W. Heylen, Omer Van der Biest
Abstract: Cylindrical specimens made of the Ni-based super-alloy Inconel 625 (IN 625) were coated with (a) NiCoCrAlY, or (b) NiCoCrAlY and yttria-stabilised zirconia (YSZ: in this case, zirconia with 7-8 wt% yttria), using the electron beam - physical vapor deposition (EB-PVD) technique. In the bi-layer coatings, the YSZ layer is the thermal barrier coating (TBC) and the NiCoCrAlY layer is the metallic bond coat (BC). The BC improves the bonding between the substrate and the ceramic TBC, while the low thermal conductivity of the TBC o ers high-temperature protection to the substrate. This paper focuses on the determination of the elastic moduli of the substrate and the coating layers of the test samples. The elastic moduli of the three di erent materials (IN 625, NiCoCrAlY and YSZ) were determined by means of a mixed numerical - experimental technique (MNET). The employed MNET was based on the comparison of the experimentally measured resonant frequencies of the rst bending mode of the test samples to the numerically calculated ones. The unknown elastic properties were determined by ne-tuning the elastic material parameters of the numerical models so as to enable the reproduction of the experimentally measured resonant frequencies.
Authors: Won Jun Lee, Youn Seoung Lee, Sa Kyun Rha, Kwang Yong Lim, Hee Soo Lee, Chung Nam Whang
Abstract: The surface modification of polyimide (PI) by the irradiation of 3 keV He+, Ne+ and Ar+ ions was studied using XPS and AFM at various ion dose ranging from 1􀂯1014 to 1􀂯1017 ions/cm2. The change of surface roughness of PI by 3 keV inert ions was closely connected to the change of surface composition. At low doses below 5􀂯1015 ions/cm2, the surface of polyimide was carbonized by ion irradiations, showing rough surfaces. At higher ion doses, the surface composition was saturated due to the accumulation of damages, and the surface morphology became smoother as compared with the low dose case.
Authors: A. Ozieblo, Tomasz Wejrzanowski, K. Konopka, Mikolaj Szafran, Krzysztof Jan Kurzydlowski
Abstract: This paper describes the technology and microstructure of Al2O3-Fe functionally graded composites, FGM, obtained by slip-casting under magnetic field. Alumina a-Al2O3, provided by Alcoa (symbol A16SG), with average grain size of 0.5 µm, and iron powder, (symbol Distaloy AB) from Hoganas, with average grain size of 35 µm, were used to produce a series of specimens which differed in contents of Fe particles in Al2O3. As a source of magnetic force a permanent magnet was used. Preforms were sintered in a vacuum at temp. 1470oC. The microstructures of the specimens were quantitatively described via stereological methods. Sections, parallel to the magnetic field lines were analyzed using special image analysis software. Stereological methods presented in this work have been used to determine gradient in the volume fraction of the Fe particles and variation in their size and dispersion. These parameters are essential for controlling the technological process of interest and to design microstructure for needed properties (fracture toughness).
Authors: Hyun Jun Kim, Kevin P. Trumble, Keith J. Bowman
Abstract: Melt infiltration of preforms prepared by sequential centrifugal casting and by tape casting and lamination has been accomplished using a short-time infiltration process that significantly suppresses reaction product formation. For layered materials produced via infiltration of laminated ceramic tapes, of particular interest is the effect that a large change in microstructure has on infiltration, phase formation and mechanical properties.

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