| 1. |
- Andersson, Linnéa, et al.
(författare)
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Permeability, pore connectivity and critical pore throat control of expandable polymeric sphere templated macroporous alumina
- 2011
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 59:3, s. 1239-1248
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Tidskriftsartikel (refereegranskat)abstract
- We have regulated the permeability in macroporous alumina materials by manipulating the connectivity of the pore phase and the sizes of the smallest constrictions between connected pores. Templating with particle-coated expandable polymeric spheres (EPS) significantly increased the fraction of isolated pore clusters, and reduced both the sizes and the number of connections with neighboring pores, as determined by three-dimensional evaluation with X-ray micro-computed tomography. The stable particle coating, applied onto the EPS surfaces using polyelectrolyte multilayers, reduced the volume expansion and the coalescence of the EPS at elevated temperatures, which reduced the simulated permeability by as much as two orders of magnitude compared to templating with uncoated EPS in materials of similar porosities. We show that the Katz-Thompson model accurately predicts the permeability for the macroporous alumina materials with porosities of 46-76%. This suggests that the permeability to fluid flow in these materials is governed by the smallest constrictions between connected pores: the critical pore throat diameter.
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| 2. |
- Balakrishnan, A., et al.
(författare)
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Effect of particle size in aggregated and agglomerated ceramic powders
- 2010
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 58:3, s. 802-812
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Tidskriftsartikel (refereegranskat)abstract
- This work describes the compaction of agglomerated and aggregated ceramic powders with special emphasis on the role of primary particle size. Discrete element simulations are used to model weakly bonded agglomerates as well as strongly bonded aggregates. Crushing tests are carried out to obtain the characteristic strength of single agglomerate and aggregate. Microstructure evolution and stress-strain curves indicate that aggregates undergo a brittle to plastic-like transition as particle size decreases below 50 nm. It is shown that agglomerates made of nanoparticles exhibit much greater strength than those made of micron-sized particles, with an approximately inverse linear relationship with primary particle size. Simulation of the uniaxial compaction of a representative volume element of powder demonstrates that adhesive effects are responsible for the difficulty to compact nanopowders and for the heterogeneity of microstructure prior to sintering. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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| 3. |
- Borgh, Ida, et al.
(författare)
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On the three-dimensional structure of WC grains in cemented carbides
- 2013
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 61:13, s. 4726-4733
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Tidskriftsartikel (refereegranskat)abstract
- In the present work, the size distribution and shape of WC grains in cemented carbides (WC-Co), with different Co contents, have been investigated in three dimensions. Direct three-dimensional (3-D) measurements, using focused ion beam serial sectioning and electron backscattered diffraction (EBSD), were performed and a 3-D microstructure was reconstructed. These measurements were supplemented by two-dimensional (2-D) EBSD and scanning electron microscopy on extracted WC grains. The data from 2-D EBSD collected on planar sections were transformed to three dimensions using a recently developed statistical method based on an iterative inverse Saltykov procedure. This stereological analysis revealed that the assumed spherical shape of WC grains during the Saltykov method is reasonable and the estimated 3-D size distribution is qualitatively in good agreement with the actual distribution measured from 3-D EBSD. Although the spherical assumption is generally fair, the WC grains have both faceted and rounded surfaces. This is a consequence of the relatively low amount of liquid phase during sintering, which makes impingements significant. Furthermore, the observed terraced surface structure of some WC grains suggests that 2-D nucleation is the chief coarsening mechanism to consider.
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| 4. |
- Borgh, Ida, et al.
(författare)
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Synthesis and phase separation of (Ti,Zr)C
- 2014
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 66, s. 209-218
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Tidskriftsartikel (refereegranskat)abstract
- Synthesis and phase separation of (Ti,Zr)C were investigated in the present work. The (Ti,Zr)C phase was synthesized at 2200 C and subsequently aged at 1300 C for different times. The microstructure was investigated using X-ray diffraction and electron microscopy, and supplemented by first-principles calculations. The (Ti,Zr)C phase separates into a lamellar nanostructure with alternating Ti- and Zr-rich face-centered cubic domains as well as non-stoichiometric TiC and ZrC. The lamellar structure is a consequence of phase separation within the miscibility gap that is directionally constrained by high coherency stresses, as indicated by the first-principles calculations. Moreover, the increased hardness due to the phase separation suggests that the mixed carbide could be used as a strengthening constituent in, for example, cemented carbides.
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| 5. |
- Bugnet, M, et al.
(författare)
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Contribution of core-loss fine structures to the characterization of ion irradiation damages in the nanolaminated ceramic Ti3AlC2
- 2013
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Ingår i: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 61:19, s. 7348-7363
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Tidskriftsartikel (refereegranskat)abstract
- The effect of low-energy ion irradiation on the nanolaminated Ti3AlC2 is investigated by means of X-ray diffraction, transmission electron microscopy, electron energy loss and X-ray absorption spectroscopy. The chemical sensitivity and local order probing from core-loss edges provide new insights into the structural modifications induced under irradiation. From the analysis of the C K energy loss near-edge structure and Al K X-ray absorption near-edge structure by ab initio calculations, the influence of the layered structure of this compound on the irradiation damage is demonstrated, and damage is found to be preferentially localized in the aluminum planes of the structure. On the basis of comparisons between calculations and experimental spectra, a structural model is proposed for the irradiated state. This study emphasizes the utility of core-loss fine structure analysis to enhance understanding of ion irradiation-induced damage in complex crystalline materials.
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| 6. |
- Cacucci, Arnaud, et al.
(författare)
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The interdependence of structural and electrical properties in TiO2/TiO/Ti periodic multilayers
- 2013
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 61:11, s. 4215-4225
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Tidskriftsartikel (refereegranskat)abstract
- Multilayered structures with 14-50 nm periods composed of titanium and two different titanium oxides, TiO and TiO2, were accurately produced by DC magnetron sputtering using the reactive gas pulsing process. The structure and composition of these periodic TiO2/TiO/Ti stacks were investigated by X-ray diffraction and transmission electronic microscopy techniques. Two crystalline phases, hexagonal close packed Ti and face centred cubic TiO, were identified in the metallic-rich sub-layers, whereas the oxygen-rich ones comprised a mixture of amorphous TiO2 and rutile phase. DC electrical resistivity rho measured for temperatures ranging from 300 to 500 K exhibited a metallic-like behaviour (rho(473K) = 1.05 x 10(-5) to 1.45 x 10(-6) Omega m) with a temperature coefficient of resistance ranging from 1.20 x 10(-3) K-1 for the highest period (Lambda = 50.0 nm) down to negative values close to -4.97 x 10(-4) K-1 for the smallest one (Lambda = 14.0 nm). A relationship between the dimensions of periodic layers and their collective electrical resistivity is proposed where the resistivity does not solely depend on the total thickness of the film, but also depends on the chemical composition and thickness of each sub-layer. Charge carrier mobility and concentration measured by the Hall effect were both influenced by the dimension of TiO2/TiO/Ti periods and the density of ionized scattering centres connected to the titanium concentration in the metallic sub-layers.
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| 7. |
- Chen, Hao, et al.
(författare)
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Application of interrupted cooling experiments to study the mechanism of bainitic ferrite formation in steels
- 2013
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Ingår i: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 61:12, s. 4512-4523
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Tidskriftsartikel (refereegranskat)abstract
- New interrupted cooling experiments have been designed to study the kinetics of bainitic ferrite formation starting from a mixture of austenite and bainitic ferrite. It is found that the kinetics of bainitic ferrite formation during the cooling stage is determined by the isothermal holding time. The formation rate of bainitic ferrite at the beginning of the cooling decreases with increasing prior isothermal holding time. An unexpected stagnant stage during the cooling stage appears when the isothermal holding time increases to a critical point. There are two reasons for the occurrence of the stagnant stage: (i) a solute spike in front of the interface; and (ii) kinetic transition. A so-called Gibbs energy balance approach, in which the dissipation of Gibbs energy due to diffusion inside the interface and interface friction is assumed to be equal to the available chemical driving force, is applied to theoretically explain the stagnant stage. A kinetics transition from a fast growth mode without diffusion of Mn and Si inside the austenite-bainitic ferrite interfaces to a slow growth mode with diffusion inside the interface is predicted. The stagnant stage is caused by the transition to a slow growth mode. The Gibbs energy balance approach describes the experimental observations very well.
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| 8. |
- Endrino, J L, et al.
(författare)
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Spectral evidence of spinodal decomposition, phase transformation and molecular nitrogen formation in supersaturated TiAlN films upon annealing
- 2011
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 59:16, s. 6287-6296
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Tidskriftsartikel (refereegranskat)abstract
- Thermal treatment of supersaturated Ti(1-x)Al(x)N films (x approximate to 0.67) with a dominant ternary cubic-phase were performed in the 700-1000 degrees C range. Grazing incidence X-ray diffraction (GIXRD) shows that, for annealing temperatures up to 800 degrees C, the film structure undergoes the formation of coherent cubic AlN (c-AlN) and TiN (c-TiN) nanocrystallites via spinodal decomposition and, at higher temperatures (>= 900 degrees C), GIXRD shows that the c-AlN phase transforms into the thermodynamically more stable hexagonal AIN (h-AlN). X-ray absorption near-edge structure (XANES) at the Ti K-edge is consistent with spinodal decomposition taking place at 800 degrees C, while Al K-edge and N K-edge XANES and X-ray emission data show the nucleation of the h-AlN phase at temperatures >800 degrees C, in agreement with the two-step decomposition process for rock-salt structured TiAlN, which was also supported by X-ray diffraction patterns and first-principle calculations. Further, the resonant inelastic X-ray scattering technique near the N K-edge revealed that N(2) is formed as a consequence of the phase transformation process.
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| 9. |
- Furlan, Andrej, et al.
(författare)
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Control of crystallinity in sputtered Cr–Ti–C films
- 2013
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Ingår i: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 61:17, s. 6352-6361
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Tidskriftsartikel (refereegranskat)abstract
- The influence of Ti content on crystallinity and bonding of Cr–Ti–C thin films deposited by magnetron sputtering have been studied by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and Raman spectroscopy. Our results show that binary Cr–C films without Ti exhibit an amorphous structure with two non-crystalline components; amorphous CrCx and amorphous C (a-C). The addition of 10–20 at.% Ti leads to the crystallization of the amorphous CrCx and the formation of a metastable cubic (Cr1−xTix)Cy phase. The observation was explained based on the tendency of the 3d transition metals to form crystalline carbide films. The mechanical properties of the films determined by nanoindentation and microindentation were found to be strongly dependent on the film composition in terms of hardness, elasticity modulus, hardness/elasticity ratio and crack development.
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| 10. |
- Gebhardt, Thomas, et al.
(författare)
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Elastic properties of fcc Fe-Mn-X (X = Al, Si) alloys studied by theory and experiment
- 2011
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 59:8, s. 3145-3155
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the influence of Al and Si additions on the elastic properties of face-centered cubic (fcc) Fe-Mn random alloys with Fe/Mn ratios of 4.00 and 2.33 using ab initio calculations. When Al is added up to 8 at.% the shearing elastic constants (C-11-C-12)/2 and C-44 decrease, resulting in a drop of similar to 20% in shear and similar to 19% in Young's modulus. In fcc Fe-Mn-Si alloys, the trends in the elastic constants are similar, but less drastic, with a similar to 7% shear and similar to 6% Young's modulus decrease when Si is added up to 8 at.%. The Fe/Mn ratio exhibits a minor influence on the shear and Young's modulus values at constant Al and Si contents. To assess the quality of the ab initio data Fe-Mn-Al and Fe-Mn-Si thin films with an fcc structure were combinatorially synthesized and the elastic properties measured using nanoindentation. For both systems the measured and calculated lattice parameters are in good agreement. Although the measured Young's modulus data showed significant scatter due to the high surface roughness, they are in good agreement with the predicted values. For the Fe-Mn-Al system the calculations generally underestimate the experimental data by similar to 15%. For the Fe-Mn-Si system the calculated data are in general lower by similar to 10% than the experimentally determined values. The presented results are of relevance for multicomponent alloy design, since the effect of Si and Al addition on the elastic properties of Fe-Mn alloys can be predicted based on ab initio data.
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