THE RESEARCH UNIT

Created in 1988, the LEM is a mixed research unit, which depends on both CNRS (Centre National de la Recherche Scientifique) and ONERA (Office National d'Etudes et de Recherches Aérospatiales). It accommodates scientists from CNRS, ONERA and Universities.

The research activities focus on the understanding of the microstructures developed in materials, on their formation and evolution with time, as well as their impact on physical properties, notably plastic properties.
Beyond its many interactions with academic laboratories at international level, the laboratory is more specifically involved in fundamental research. Several investigated topics are related to the activities of the ONERA departments in charge of the understanding and optimisation of materials for aeronautics and aerospace.
The main goal is to maintain and develop a sound theoretical expertise on modelling in the field of materials physics, combined with an experimental expertise in transmission electron microscopy (TEM) imaging and spectroscopies, and in mechanical testing.

RESEARCH DOMAINS

The general approach consists in combining experiment, simulation and theory to study microstructures in relatively complex materials, especially multicomponent metallic alloys. Multiscale analyses are privileged, in order to understand the relationship between atomic structures and bulk physical properties.
At the atomic level, the experimental TEM observations and the simulations are used to model nanoscopic systems. At that scale, the predominant phenomena that arise at finite temperatures are mostly due to chemical atomic interactions with characteristic length scales of a few nanometers. At larger scales, up to the micrometer, elastic effects become important and govern the morphologies that develop in amaterial. The latter can be viewed as a more or less homogeneous quasi-continuum, which can be dealt with using phase field theories. At higher scales, the objective is to investigate and model the behaviour of the microstructures under external mechanical and thermal fields. This leads to a global description of the materials according to three scales (micro or atomic, meso or microstructural, macro or bulk material), thus involving the typical problems related to multiscale modelling, viz. the development and combination of models elaborated at different scales.

1. Atomic scale studies

crystallographic structure, defects and microstructure at equilibrium and under external fields.
Nanomaterials: structural studies of nanotubes, atomic characterization of carbon and BN nanotubes, analyses of growth processes.
Quasicrystals and complex intermetallics: Quasicrystallography and atomic structures of icosahedral quasicrystals, characterization and modes of motion of dislocations, plasticity of complex intermetallics.
Characterization studies of crystal defects by electron spectroscopy: dislocations, stacking faults, grain boundaries and precipitates; calculation of the influence of defects on electronic spectra.
Dynamics of phase transformations: chemical and elastic effects; formation and evolution of the microstructure in model alloys.
Dislocation dynamics in metals and alloys; dislocation motion and interactions, formation of structured dislocation networks at the mesoscopic scale.

 

2. Mesoscopic scale and multiscale analyses

Mesostructures of mixtures of different phases, domain and grain growth. Influence of temperature, influence of partial atomic order and elastic fields on growth, formation and spatial organization of precipitates.
Dislocation mesostructures: collective dislocation phenomena; materials with dislocation mobility governed by the atomic structure of the dislocation core; materials with dislocation mobility governed by dislocation-dislocation interactions and self-organization properties of the dislocation populations.
Connection between the physical discrete approach of plasticity (dislocations) and the continuum mechanical approaches.