A 2xxx aluminum alloy crept at medium temperature: role of thermal activation on dislocation mechanisms

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Structure and chemical order in

The 300\un{K} structure of soft chemistry synthesized \chem{Co_{\it x} Rh_{1-{\it x}}} ($x = 0.25$, 0.50, 0.75) particles, 1.7 to 2.5\un{nm} large, has been determined by atomic-scale simulations using an interaction model fitted on both ab initio and experimental results. All the measured structural features, strongly different from the ones in the corresponding bulk alloys, have been consistently reproduced. Two major effects, i.e. a strong Co surface segregation and an expansion of the mean bond length, are demonstrated and both are consistent with the drastic enhancement of the magnetization observed in these clusters. Interestingly, the method also provides new insights into interpretation of the experimental results

Structural and electronic properties of the Au(001)/Fe(001) interface from density functional theory calculations

cited By 15International audienceA density functional theory (DFT) investigation of the structural and electronic properties of the Au(001)/Fe(001) interface, as a function of the number of Au layers deposited on the Fe substrate (from 1 to 11 Au monolayers), is presented. The elastic effects on the interface properties due to the lattice mismatch between Fe and Au, calculated by DFT using the generalized gradient approximation, are also evaluated. At the interface, the interlayer distances in the Fe substrate as well as in the Au slab expand. The Fe atoms of the interface exhibit an enhanced magnetic moment and the Au atoms of the interface bear a nonzero (but very low) magnetic moment. The calculated interface energy favors the formation of core-shell Fe@Au nanoparticles, where Au(001) is in epitaxy at 45 - on (001) facets of a Fe nanocube. Finally, the analysis of the electronic properties shows that the work of adhesion of the interface is maximum for a coating of Fe with 2 Au monolayers, which can be explained by a strong overlap between the electronic densities of the Fe interface atoms with those of the Au surface atoms. © 2012 American Physical Society

HREM and WAXS Study of the Structure of Metallic Nanoparticles

Different samples of fine metal colloids synthesized following an organometallic route were investigated by High Resolution Transmission Electron Microscopy (HREM) and Wide Angle X-rays Scattering (WAXS). Simulations based on models for crystalline and multiply twinned particles were used to interpret the observations. Deviations from the bulk structure were clearly observed in the smaller particles and attributed to an effect of dimensions. Combined HREM and WAXS experiments proved to be very efficient in the study of ultrafine particles. In particular, 55-atoms fcc Pt particles were identified.

A study of the hardening precipitation in a 2650 aluminium alloy for aeronautics

International audienceThis paper focus on the hardening precipitation in a 2650 Al(CuMg) aluminium alloy and on its interactions with the aluminium matrix. The study has been carried out using High Resolution Transmission Electron Microscopy (HRTEM) images analysis and simulated images. Two types of precipitation (an homogeneous one and an heterogeneous one) and two types of precipitates (fine needles and S-laths) can be distinguished. A tomographic atom probe analysis of the fine needles reveals a strong copper and magnesium atom enrichment. The lath shaped precipitates can be sorted into two types, each one presenting specific interface morphology and orientation relationship with respect to the matrix

A study of the hardening precipitation in a 2650 aluminium alloy for aeronautics

International audienceThis paper focus on the hardening precipitation in a 2650 Al(CuMg) aluminium alloy and on its interactions with the aluminium matrix. The study has been carried out using High Resolution Transmission Electron Microscopy (HRTEM) images analysis and simulated images. Two types of precipitation (an homogeneous one and an heterogeneous one) and two types of precipitates (fine needles and S-laths) can be distinguished. A tomographic atom probe analysis of the fine needles reveals a strong copper and magnesium atom enrichment. The lath shaped precipitates can be sorted into two types, each one presenting specific interface morphology and orientation relationship with respect to the matrix

High-resolution transmission electron microscopy and tomographic atom probe studies of the hardening precipitation in an Al-Cu-Mg alloy

International audienceThe hardening precipitation of an Al-Cu-Mg aluminium alloy designed for aeronautics was investigated using high-resolution transmission electron microscopy (HREM) and tomographic atom probe techniques. The observed precipitates clearly belong either to the Guinier-Preston-Bagaryatskii (GPB) zones type or to the so-called S-Al2CuMg precipitation. We analysed a large number of precipitates in order to obtain statistical information on the precipitation. We focused on the structural and/or chemical composition of the different precipitates. It was found, in particular, that the very numerous GPB zones do not present a single chemical composition. Evidence is also given for the presence of two different kinds of S-precipitate/matrix orientation relationships, strongly linked to the morphology of the precipitate. The structure of the S precipitates was confirmed by direct comparison with simulated HREM images. Particular attention was paid to the nature of the S-precipitate/matrix interfaces

Development of Bi-Metallic Fe-Bi Nanocomposites: Synthesis and Characterization

Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) is a widely used technique for assessing tissue physiology. Spoiled gradient echo (SPGR) pulse sequences are one of the most common methods for acquisition of DCE-MRI data, providing high temporal and spatial resolution with strong T(1)-weighting. Conversion of SPGR signal to concentration is briefly reviewed, and a new closed-form expression for concentration measurement uncertainty for finite signal-to-noise ratio (SNR) and baseline scan time is derived. This result is applicable to arbitrary concentration-dependent relaxation rate and is valid over the same domain as the theoretical SPGR signal equation. Expressions for the lower and upper bounds on measurable concentration are also derived. The existence of a concentration- and tissue-dependent optimal flip angle that minimizes concentration uncertainty is demonstrated and it is shown that, for clinically relevant pulse sequence parameters, this optimal flip angle is significantly larger than the corresponding Ernst angle. Analysis of three pulse sequences from the DCE-MRI literature shows that optimization of flip angle using the methods discussed here leads to potential improvements of 10-1166% in effective SNR over the 0.5-5.0 mM concentration range with minimal or no loss of measurement accuracy down to 0.1 mM. In vivo data from three study patients provide further support for our theoretical expression for concentration measurement uncertainty, with predicted and experimental estimates agreeing to within +/- 30%. Equations for concentration bias resulting from biases in flip angle and from pre-contrast relaxation time and contrast relaxivity (both longitudinal and transverse) are also derived in closed-form. The resulting equations show the potential for significant contributions to bias in concentration measurement arising from even relatively small mis-specification of flip angle and/or pre-contrast longitudinal relaxation time, particularly at high contrast concentrations