## X-ray tomography and elastic property prediction

### August 27, 2011

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.

## Inhomogeneous elasticity and diffusion induced recrystallization

### May 13, 2010

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.

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.

## Magnetic field induced strains

### April 25, 2010

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°.

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.

## Thin film stresses and phase boundaries

### June 20, 2009

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!

## High resolution electron microscopy of split patterns in Ni-base superalloys

### August 8, 2007

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

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.

## Phase separation in complex perovskitic oxides

### August 1, 2007

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

Authors: Beth S. Guiton and Peter K. Davies

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.

## Cubic to tetragonal transformation

### July 28, 2007

Title: The transformation sequences in the cubic → tetragonal decomposition

Authors: Y. Ni, Y.M. Jin and A.G. Khachaturyan

Abstract:

The decomposition of a generic supersaturated binary cubic solid solution into a mixture of cubic and tetragonal phases is investigated by phase field microelasticity modeling and simulations. It is shown that the decomposition in such a system is not necessarily developed by conventional nucleation and growth of the tetragonal phase. There are three temperature and composition ranges where the sequences of transient structures formed are different. The transformation pathways are predicted and the corresponding thermodynamic parameters are identified. In particular, the simulations reveal unusual transformation sequences occurring in the process of isostructural decomposition followed by cubic → tetragonal MT confined within one of the decomposed cubic phases. Mechanisms for the formation of the stress-accommodating multi-domain aggregates of the tetragonal phase and the checkerboard-like structures comprised of parallel rods of cubic and tetragonal phases are discussed.

## Effect of stress on pearlite formation

### July 28, 2007

Title: The influence of lattice strain on pearlite formation in Fe–C

Authors: I. Steinbach and M. Apel

Abstract:

The effect of stress and strain on the transformation kinetics of pearlite is investigated by phase-field simulation. Strain is considered in terms of expansion/contraction during transformation and due to concentration gradients in austenite. It is demonstrated that due to the concentration dependence of the eigenstrain, an inhomogeneous stress distribution ahead of the transformation front enhances diffusion in the austenitic phase and reduces chemical supersaturation in both austenite and ferrite. The main result of the investigation is that transformation strain inhibits the cooperative growth mode of cementite and ferrite, as considered by the Zener–Hillert model, and provokes the salient growth of cementite needles ahead of the ferrite front, which we call “staggered growth”. The predicted growth velocities give the right order of magnitude compared to the experiment and close the gap between theoretical models based on diffusion only, and experimental observations.

## Nanoparticle arrays as elastic membranes

### July 22, 2007

Title: Elastic membranes of close-packed nanoparticle arrays

Author: Klara E. Mueggenburg, Xiao-Min Lin, Rodney H. Goldsmith and Heinrich M. Jaeger

Abstract: Nanoparticle superlattices are hybrid materials composed of close-packed inorganic particles separated by short organic spacers. Most work so far has concentrated on the unique electronic, optical and magnetic behaviour of these systems. Here, we demonstrate that they also possess remarkable mechanical properties. We focus on two-dimensional arrays of close-packed nanoparticles and show that they can be stretched across micrometre-size holes. The resulting free-standing monolayer membranes extend over hundreds of particle diameters without crosslinking of the ligands or further embedding in polymer. To characterize the membranes we measured elastic properties with force microscopy and determined the array structure using transmission electron microscopy. For dodecanethiol-ligated 6-nm-diameter gold nanocrystal monolayers, we find a Young’s modulus of the order of several GPa. This remarkable strength is coupled with high flexibility, enabling the membranes to bend easily while draping over edges. The arrays remain intact and able to withstand tensile stresses up to temperatures around 370 K. The purely elastic response of these ultrathin membranes, coupled with exceptional robustness and resilience at high temperatures should make them excellent candidates for a wide range of sensor applications.

Notes: Via Scienceblog