Y. Le Bouar

Size and shape effects on the order -> disorder phase transition in CoPt nanoparticles

Chemically ordered bimetallic nanoparticles are promising candidates for magnetic-storage applications. However, the use of sub-10 nm nanomagnets requires further study of possible size effects on their physical properties. Here, the effects of size and morphology on the order-disorder phase transition temperature of CoPt nanoparticles (T(C)(NP)) have been investigated experimentally, using transmission electron microscopy, and theoretically, with canonical Monte Carlo simulations. For 2.4-3-nm particles, T(C)(NP) is found to be 325-175 degrees C lower than the bulk material transition temperature, consistent with our Monte Carlo simulations. Furthermore, we establish that T(C)(NP) is also sensitive to the shape of the nanoparticles, because only one dimension of the particle (that is, in-plane size or thickness) smaller than 3 nm is sufficient to induce a considerable depression of T(C)(NP). This work emphasizes the necessity of taking into account the three-dimensional morphology of nano-objects to understand and control their structural properties.

Phase field methods and dislocations

Abstract We present a general formalism for incorporating dislocations in Phase Field methods (PFM) based on the elastic equivalence between a dislocation loop and a platelet inclusion of specific stress-free strain. Dislocations may be elastically and dynamically coupled to any other field such as a concentration field. Special attention is paid to the treatment of dislocation cores after the discretization of real and reciprocal space required by the computer implementation of any PFM. In particular, we propose a method based on two length scales to account for dislocation cores much smaller than the grid spacing. The method is illustrated through the simulation of the motion of a dislocation loop in a microstructure representative of a late-stage γ/γ′ microstructure.

Coupling phase field and viscoplasticity to study rafting in Ni-based superalloys

An elasto-viscoplastic model is developed to study the microstructural evolution during creep loading in a model AM1 superalloy. Elastic anisotropy and inhomogeneity, as well as the description of long-range order in the γ ′ phase, are included in the model. Plastic activity is introduced using a continuum crystal plasticity framework at the mesoscale. Special attention is paid to the corresponding parameter identification from experiments. Two-dimensional simulations of creep in the [100] direction are performed, and the results are compared to the predictions of an elastic phase field model, in order to characterize the influence of plastic activity on the microstructural evolution. In particular, our simulations show that plastic activity in the γ channels significantly increases the rafting kinetics and allows misalignments of rafts with respect to cubic directions. The simulation results are critically discussed and improvements to the model are proposed.

A TEM in situ experiment as a guideline for the synthesis of as-grown ordered CoPt nanoparticles

CoxPt1−x nanoparticles on amorphous alumina are synthesized by a finely tuned pulsed laser deposition (PDL) technique. By means of an alternate deposition experiment using pure Co and Pt targets, we have shown that the composition of the nanoparticles can be controlled with a precision of 2%. This composition varies linearly with the lattice parameter of the nanoparticles following the well-known Vegard law. During the synthesis, the substrate temperature is a key parameter to control the structure and the morphology of the nanoparticles. We have performed an in situ heating experiment in an electron microscope using disordered Co55Pt45 nanoparticles to study their thermodynamic behaviour. We have observed both ordering within the nanoparticles and coalescence between the nanoparticles. The temperature regimes evidenced by the in situ experiment are used to determine the temperature of the substrate to synthesize as-grown L10 ordered nanoparticles with better control on their size and composition.

Atomistic study of the coherency loss during the B.C.C.–9R transformation of small copper precipitates in ferritic steels

Abstract Using 3D atomistic static simulations, we first investigate the atomic coherency between a small twinned 9R embryo (less than 6 nm in diameter) and the b.c.c. Fe matrix. We show that the (11 4 ) 9R planes of the precipitate are aligned with the (1 1 0) b.c.c. planes of the matrix, and that the coherency loss of the precipitate/matrix interface is due to the formation of dislocation loops inside the precipitate. Then, we use 3D atomistic simulations to study the structure changes of an initially b.c.c. copper precipitate and show that the structure evolves towards a 9R structure. Finally, we use the above results to propose a simple geometric mechanism which may be relevant for the b.c.c./9R transformation at low temperature and when the influence of vacancies can be neglected.

Random vs realistic amorphous carbon models for high resolution microscopy and electron diffraction

Amorphous carbon and amorphous materials in general are of particular importance for high resolution electron microscopy, either for bulk materials, generally covered with an amorphous layer when prepared by ion milling techniques, or for nanoscale objects deposited on amorphous substrates. In order to quantify the information of the high resolution images at the atomic scale, a structural modeling of the sample is necessary prior to the calculation of the electron wave function propagation. It is thus essential to be able to reproduce the carbon structure as close as possible to the real one. The approach we propose here is to simulate a realistic carbon from an energetic model based on the tight-binding approximation in order to reproduce the important structural properties of amorphous carbon. At first, we compare this carbon with the carbon obtained by randomly generating the carbon atom positions. In both cases, we discuss the limit thickness of the phase object approximation. In a second step, we show the influence of both carbons models on (i) the contrast of Cu, Ag, and Au single atoms deposited on carbon and (ii) the determination of the long-range order parameter in CoPt bimetallic nanoalloys.

Strain-induced microstructure development due to a wetting phenomenon in the Co-Pt system

Abstract In the Co–Pt system, a cooling experiment can drive a sample ordered in the cubic L1 2 structure inside the two-phase region involving L1 2 and the tetragonal L1 0 structure. We had shown previously ( Phys. Rev. B . 61(5) (2000) 3317) that the first step of the microstructural development is the decoration of the antiphase boundaries (APBs) in the L1 2 structure by a thin L1 0 layer. Using transmission electron microscopy observations, we present here the later stages of the microstructural evolution, i.e. when the cooling procedure induces a larger amount of L1 0 structure in the sample. Depending on the aging temperature, the final microstructure consists of large L1 2 domains separated by either thick platelets of the L1 0 structure, or by a saw-tooth like morphology. We explain these microstructural developments by a theoretical analysis of the accommodation of the elastic stresses generated by the difference in symmetry and lattice parameters between the L1 2 and L1 0 structures.

Influence of atomic size mismatch on binary alloy phase diagrams

Abstract The aim of this paper is to investigate the consequences of atomic size mismatch on the thermodynamics and the topology of binary phase diagrams of face centred cubic alloys. Simple pairwise interatomic potentials with few controlling parameters are used to identify general tendencies. Thermodynamic states are computed by Monte Carlo simulations on a non-rigid lattice. A special attention has been paid to the comparison between calculations in the canonical ensemble, where composition–temperature phase diagrams are determined through van der Waals loops, and in the grand canonical ensemble, where phase diagrams are computed using an interface migration technique. It is shown that these two procedures lead essentially to the same incoherent phase diagram. In the case of phase separating systems, we argue that the introduction of a size mismatch leads to a shrinkage of the solid solution domain and that the asymmetry of the miscibility gap is essentially controlled by the anharmonicity of the heteroatomic potential. Finally, in the case of ordering systems, we show that the asymmetry of the phase diagram may be due to the anharmonicity of the pair potentials or to the differences between their curvatures, the former effect being dominant if the atomic size mismatch is large.

Taking advantage of the concept of driven alloys to study the wear of swift train wheels

Abstract The rolling strip of swift train wheels undergoes cyclic loading, which triggers phase transformations in the steel. We define an intensity of forcing, comparable to the milling intensity of the high energy ball milling process, which accounts for the observed evolution of the wear process with distance.

A Vacancy Model of Pore Annihilation During Hot Isostatic Pressing of Single Crystals of Nickel-Base Superalloys

An improved diffusion model of pore annihilation during hot isostatic pressing of single crystals of nickel-base superalloys is proposed. The model considers dissolution of pores by emission of vacancies and their diffusion sink to low-angle boundaries. The calculation, which takes into account pore size distribution, predicts the kinetics of pore annihilation similar to experimental one.