Extraction of fibre network architecture by X-ray tomography and prediction of elastic properties using an affine analytical model

D. Tsarouchas and A.E. Markaki

This paper proposes a method for extracting reliable architectural characteristics from complex porous structures using micro-computed tomography (μCT) images. The work focuses on a highly porous material composed of a network of fibres bonded together. The segmentation process, allowing separation of the fibres from the remainder of the image, is the most critical step in constructing an accurate representation of the network architecture. Segmentation methods, based on local and global thresholding, were investigated and evaluated by a quantitative comparison of the architectural parameters they yielded, such as the fibre orientation and segment length (sections between joints) distributions and the number of inter-fibre crossings. To improve segmentation accuracy, a deconvolution algorithm was proposed to restore the original images. The efficacy of the proposed method was verified by comparing μCT network architectural characteristics with those obtained using high resolution CT scans (nanoCT). The results indicate that this approach resolves the architecture of these complex networks and produces results approaching the quality of nanoCT scans. The extracted architectural parameters were used in conjunction with an affine analytical model to predict the axial and transverse stiffnesses of the fibre network. Transverse stiffness predictions were compared with experimentally measured values obtained by vibration testing.

[1] Phase-field simulations with inhomogeneous elasticity: Comparison with an atomic-scale method and application to superalloys

G Boussinot et al

We present a 2D and 3D phase-field analysis of microstructure evolution in the presence of a lattice misfit and with inhomogeneous elastic constants. The method is first critically compared with a Monte Carlo modeling at the atomic scale. We then apply the phase-field model to the Ni–Al system under external load along a cubic axis. We find that the microstructure becomes anisotropic and that the situation qualitatively differs depending on the sign of the applied stress. The microstructure evolution operates mainly by shape changes and alignments of precipitates, but also by splitting of precipitates initially elongated along directions perpendicular to the stress-induced, elastically favorable directions. The final microstructure is finally qualitatively analyzed in terms of a mean field theory in which the elastic inhomogeneity is embedded into an effective eigenstrain. This analysis leads to a simple formulation which can be used to easily predict the coherent microstructural anisotropy induced by any external loading condition.

[2] The hidden link between diffusion-induced recrystallization and ideal strength of metals

G Schmitz et al

Diffusion-induced recrystallization (DIR) is a mechanism which destabilizes thin film multilayers. New grains formed are distinguished by preferred composition levels characteristic for the diffusion couple. By evaluating these concentrations for different material combinations, it is demonstrated that a break of coherency by spontaneous relaxation is the key to understand the DIR process. Based on this, a thermo-elastic model is derived to predict whether diffusion-induced recrystallization can be expected for a given multilayer and to calculate the characteristic concentration levels.

[1] Magnetically resettable 0.16% free strain in polycrystalline Ni-Mn-Ga plates

M Pötschke et al

We investigated the microstructure, mechanical training and free strain due to magnetically induced reorientation (MIR) of a polycrystalline Ni50Mn29Ga21 alloy prepared by directional solidification, which showed columnar grains with a strong A fibre texture. Two-side mechanical training greatly decreased the twinning stress level of the properly cut plates. Consequently, a free strain of 0.16% has been observed in the trained plates and this strain is magnetically resettable for more than 20 times by rotating the magnetic field by 90°.

[2] Large magnetic-field-induced strain in Co-Ni-Al single variant ferromagnetic shape memory alloy

H Morito et al

At room temperature, Co41Ni32Al27 single-variant ferromagnetic shape memory alloy with a uniaxial magnetic anisotropy constant of –2.0 × 106 erg/cm3 exhibits a large magnetic-field-induced strain of 3.3% under a static compressive stress of about 10 MPa applied along the hard magnetization direction of the c-axis (c/a > 1). Compared with the data for other alloys, it becomes clear that a small change of twinning stress in the stress-strain curve is necessary for a large strain.

Shifting of the morphotropic phase boundary and superior piezoelectric response in Nb-doped Pb(Zr, Ti)O3 epitaxial thin films

Z-X Zhu et al

A shift of the morphotropic phase boundary (MPB) and a superior piezoelectric response are observed in Nb-doped Pb(ZrxTi1−x)O3 (PNZT) thin films epitaxially grown on Nb-doped SrTiO3(1 0 0) (Nb:STO) substrates. X-ray diffraction and Raman spectra characterizations confirm that a phase transition from a tetragonal structure to a rhombohedral structure occurs when the Zr/Ti ratio varies from 20/80 to 80/20. The phenomenological theory and experimental analyses suggest that the MPB of epitaxial PNZT thin films is shifted to the higher Zr/Ti ratio (around 70/30) from the conventional ratio (52/48) due to the misfit compressive stress induced by the substrate. A maximum local effective longitudinal piezoelectric coefficient (d33) up to 307 pm V−1 is observed at a Zr/Ti ratio of 70/30 in the current compositional range, again confirming the shifting of MPB in epitaxial PNZT thin films. These findings offer a new insight for the fabrication of epitaxial PZT thin films at MPB with a superior piezoelectric response.

Wikidot-ing!

December 29, 2007

Remember the C-source code for calculating the elastic stress fields of a circular cavity in a square plate under uniaxial stress? I have found another neat way of shipping code with comments (and equations and figures, if need be) thanks to Abi: here is the wikidot page of code, schematic and equations. Have fun!

Title: Statistical analysis of \gamma^{\prime} quartet split patterns in \gamma-\gamma^{\prime} Ni alloys revealed by high resolution electron microscopy

Authors: H. A. Calderon; C. Kisielowski; T. Mori

Source: Philosophical Magazine Letters, Volume 87, Issue 1 January 2007 , pages 33 – 40

Abstract: The frequencies of adjacent \gamma^{\prime} pairs having in-phase and out-of-phase relationships in quartet configurations, determined by high resolution electron microscopy [Calderon et al., Phil. Mag. Lett. 85 51 (2005)], were examined in detail for \gamma^{\prime} particles belonging to the same translational order domain. Consequently, there were always at least two out-of-phase adjacent pairs in any quartet. It is concluded that the quartet split patterns in \gamma^{\prime} particles, but are produced by the migration of particles due to diffusion, these particles having been precipitated separately and independently prior to migration. An elasticity calculation is provided to show that two \gamma^{\prime} particles migrate to align along <100> when their initial position deviates from this direction.

Title: Nano-chessboard superlattices formed by spontaneous phase separation in oxides

Authors: Beth S. Guiton and Peter K. Davies

Source: Nature Materials 6, 586 – 591 (2007)

Abstract:

The use of bottom-up fabrication of nanostructures for nanotechnology inherently requires two-dimensional control of the nanostructures at a particular surface. This could in theory be achieved crystallographically with a structure whose three-dimensional unit cell has two or more—tuneable—dimensions on the nanometre scale. Here, we present what is to our knowledge the first example of a truly periodic two-dimensional nanometre-scale phase separation in any inorganic material, and demonstrate our ability to tune the unit-cell dimensions. As such, it represents great potential for the use of standard ceramic processing methods for nanotechnology. The phase separation occurs spontaneously in the homologous series of the perovskite-based Li-ion conductor, (Nd_{2/3-x}Li_{3x})TiO_3, to give two phases whose dimensions both extend into the nanometre scale. This unique feature could lead to its application as a template for the assembly of nanostructures or molecular monolayers.

Notes: A commentary on this paper by Patrick M Woodward is also available in the same issue of Nature Materials.

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