Yann Le Bouar

Growth and structural properties of CuAg and CoPt bimetallic nanoparticles

Core/shell CuAg and alloyed CoPt have been synthesized using two vapor phase deposition techniques. For CuAg prepared by Thermal Evaporation (TE), the size and the morphology of the Cu cores are the key parameters to promote the formation of the core/shell arrangement. For CoPt synthesized by Pulsed Laser Deposition (PLD), the growth kinetics of nanoparticles, depending on the deposition rate, the substrate nature and the temperature, controls the nanoparticle morphology. The competition between the growth and the ordering kinetics governs the nanoparticle structure. By reducing the growth kinetics, as-grown L1(0) ordered nanoparticles are obtained according to the bulk phase diagram.

Correcting scanning instabilities from images of periodic structures

A method for measuring and correcting the row displacement errors in lattice images acquired using scanning based methods is presented. This type of distortion is apparent in lattice-resolved images acquired using scanning-based techniques such as scanning transmission electron microscopy (STEM) and translates to vertical streaks convolving every feature in Fourier space. This paper presents a method to measure and correct the distortion based on the phase analysis of the streaks in Fourier space. The validity and the precision of the method is demonstrated using a model image and two experimental STEM images of Si <110> thin film and a 5 nm CoPt disordered nanocrystal. The algorithm is implemented in a freely available Digital Micrograph™ script.

Flash laser annealing for controlling size and shape of magnetic alloy nanoparticles

Summary We propose an original route to prepare magnetic alloy nanoparticles with uniform size and shape by using nanosecond annealing under pulsed laser irradiation. As demonstrated here on CoPt nanoparticles, flash laser annealing gives an unprecedented opportunity to control the size and the shape of bimetallic nanoparticles without changing their composition. The mechanisms involved in the complete reshaping of the nanoparticle thin films are discussed and it is also shown that order-disorder phase transformations occur under laser irradiation. This technique is then very interesting for magnetic alloy nanoparticles studies and applications because it opens up a new way to fabricate size-controlled spherical nanoparticles with narrow size dispersion.

Density-based crystal plasticity: From the discrete to the continuum

Because of the enormous range of time and space scales involved in dislocation dynamics, plastic modeling at macroscale requires a continuous formulation. In this paper, we present a rigorous formulation of the transition between the discrete, where plastic flow is resolved at the scale of individual dislocations, and the continuum, where dislocations are represented by densities. First, we focus on the underlying coarse-graining procedure and show that the emerging correlation-induced stresses are scale-dependent. Each of these stresses can be expanded into the sum of two components. The first one depends on the local values of the dislocation densities and always opposes the sum of the applied stress and long-range mean field stress generated by the geometrically necessary dislocation (GND) density; this stress acts as a friction stress. The second component depends on the local gradients of the dislocation densities and is inherently associated to a translation of the elastic domain; therefore, it acts as a back-stress. We also show that these friction and back- stresses contain symmetry-breaking components that make the local stress experienced by dislocations to depend on the sign of their Burgers vector.

Growth of dendritic nanostructures by liquid-cell transmission electron microscopy: a reflection of the electron-irradiation history

Studying dynamical processes by transmission electron microscopy (TEM) requires considering the electron-irradiation history, including the instantaneous dose rate and the cumulative dose delivered to the sample. Here, we have exploited liquid-cell TEM to study the effects of the electron-irradiation history on the radiochemical growth of dendritic Au nanostructures. Besides the well-established direct link between the dose rate and the growth rate of the nanostructures, we demonstrate that the cumulative dose in the irradiated area can also induce important transitions in the growth mode of the nanostructures. By comparing in situ observations with an extended diffusion-limited aggregation model, we reveal how the shape of the nanostructures is severely affected by the local lack of metal precursors and the resulting restricted accessibility of gold atoms to the nanostructures. This study highlights the effects of electron irradiation on the solution chemistry in the irradiated area and in the whole liquid cell that are of primary importance to extract quantitative information on nanoscale processes.

Viscoplastic phase field simulation of microstructural evolutions under complex loadings in Ni-base superalloys

An elasto-viscoplastic phase field model is proposed to study the microstructural evolutions under mechanical loadings in a Ni-base superalloy single crystal. Elastic anisotropy and inhomogeneity, as well as the description of the long range order in the γ’ phase are included in the model. Plastic activity is introduced using a continuum crystal plasticity framework. The coupled model is used to study microstructural evolutions in superalloys under creep at high temperature. Two loading orientations are investigated, [001] and [011]. Moreover simulations under alternate fatigue loading in the [001] orientation are performed. The viscoplasticity simulations are compared to purely elastic phase field simulations and experiments in order to show the influence of plastic activity on microstructural evolutions.