Experiments have consistently shown that metallic materials display strong size effects at the micron scale, where the peculiar phenomenon “smaller is stronger” appears. Considering the exponentially increasing trend towards miniaturization, correct modeling of such effects has become inescapable in various high-technology fields, including microbotics, microelectronics, micromedicine, etc. As a result, a strong scientific effort has been devoted to the subject and numerous size-dependent theories have been proposed in recent years. Among them, strain gradient plasticity (SGP) theories, which can be seen as an extension of conventional plasticity to small scales, can particularly be cited. Including internal length scale(s), these theories are capable of predicting plastic deformation gradients, which correlate with size effects as experimentally observed and numerically predicted using dislocation mechanics. Thanks to their capabilities in capturing size effects, SGP theories have become increasingly used for the prediction of the size-dependent behavior of materials at the micron scale. However, despite the significant progress made on these theories, they still suffer from some fundamental issues, so far preventing their application to real engineering problems. The present project is proposed to address one of the most challenging of these issues: the physical nature of elastic gaps which are numerically observed using major of such theories. Almost all existing thermodynamically-consistent SGP theories including higher-order dissipation predict elastic gaps (delay in plastic flow) under certain non-proportional loading conditions. Nevertheless, to date, there is no experimental evidence nor is there small-scale numerical confirmation proving the existence of such gaps in reality. This represents a major source of confusion and uncertainty, preventing the development of robust SGP models that can be applied in a real industrial context. In the absence of works studying these gaps from a physical point of view, the scientific investment on SGP theories has reached a bifurcation point making the scientific community divided into those who consider elastic gaps as “unknown” size effects (then possibly physical) and those who see no physical reasons for their occurrence in reality, at least within a single-crystal, as they reflect an instantaneous finite change in the higher-order stresses. This project aims at clearing-up this ambiguity and at providing the compelling answer to the question "are elastic gaps physical?". To achieve its goals, the project will create a multi-disciplinary synergy between original small-scale experiments, implying, for the first time, non-proportional loading conditions, and extensive discrete-dislocation-based simulations. Results of the experimental and numerical investigations will then be considered to review major existing SGP theories, with the aim of developing the first single- and poly-crystal SGP models that are robust enough for real engineering applications. As an application, the proposed models will finally be used to investigate one of the most challenging small-scale problems: impact of size effects on the formability of ultra-thin sheet metals, which show an ever-growing use in various engineering fields.

Hydrogen is a key issue for renewable energies. The solid state storage of hydrogen in the form of metallic hydrides offers numerous advantages, including safety, in comparison with cryogenic liquid and compressed gas possibilities. Before reaching a steady state reversible storage / de-storage under "reasonable" temperature and pressure conditions, a first step of "activation" is required. An effective way to activate metals is by refining the size of their overall microstructure via severe plastic deformation. Such "bulk" deformation is necessarily restricted to small parts. The aim of this project is to study the activation process of bulk metals (Mg and Ti) via the treatment of their surfaces by combining mechanical and chemical actions.

Despite many attempts to model effective behavior, there is currently no recognized approach that can capture the most important aspects of deformation of 3D textile reinforcements during their processing, and predict efficiently both the macroscopic response of the textile structure in the dry state or as pre-impregnated from the behavior of the fibers or yarns at the smaller scales. Recent developments in multi-scale simulation and 3D imaging techniques in materials science, particularly X-ray microtomography combined with appropriate image analysis techniques, make it possible to finely analyze the micro-mechanisms of deformation at the level of interactions between fibers and to enrich the interpretation of micro or meso-mechanical tests, which opens up new ways for the exploration and the understanding of the phenomena occurring at this level, in particular for elaborating and identifying models at intermediate scales, essential for an in-depth prediction of macroscopic behavior. The general objective of the project is the development of constitutive laws with an enriched kinematics for dry and pre-impregnated 3D technical textile reinforcements at different scales, which integrate the geometry of the constituents identified by X microtomography, the rheology of fibers and yarns, geometric and structural nonlinearities, singular and dissipative phenomena related to the presence of defects, irregularities in behavior (contact, friction, microcracking), as well as a statistical variability of the geometry and mechanical properties of the yarns or fibers within the armor. These aspects are scientific locks that define the innovative nature of the project compared to the literature works. The project is multidisciplinary since it concerns the science of fibrous materials, the mechanics of discrete and continuous media, multiscale higher order homogenization methods, rheology, stochastic methods, microtomography image analysis techniques, and numerical methods.

Aluminium alloys play an important role in the transportation industry. In the aircraft or the car industry, the demand for high strength alloy to design low weight structures is nowadays strongly driven by energy savings and low CO2 emissions. Thus developing, optimizing and producing new alloys with enhanced properties is one of the key factors for the competitiveness of this industry, including metal processing as well as forming. The present proposal deals both with fundamental research to develop the initial concepts and develop an “alloy by design” approach and with more applied research seeking to demonstrate the possibility for industrialization. We will develop and optimize an up-scaled severe plastic deformation (SPD) process named sheet-ECAP able to process at a large scale ultrafine grained aluminium alloys with an alloy composition specifically designed for this process, providing a better thermal stability of the fine grain structure together with a combination of nano-scaled precipitates. The target is to obtain high strength aluminium alloys (up to 800 MPa) with a uniform elongation ranging to 10%. From a fundamental point of view, special emphasis will be given to precipitate hardening mechanisms in nano-scaled grains. To achieve these goals, it is proposed to combine an experimental approach using up-to-date characterization techniques (TEM, EBSD, APT, SAXS) with modelling (grain refinement, structure/properties relationship). One of the innovative aspects of this project is that we propose to adapt the chemical composition of an aluminium alloy to optimize the stability of the UFG structure and the precipitation of nano-scaled precipitates within grains. The project is based on the idea to design alloys with a relatively high concentration of Fe leading to the formation of a significant volume fraction of intermetallic particles, and take advantage of the SPD process to break them into small particles, efficient to pin the ultra fine grain structure during the precipitation treatments. We will also explore the possibility to create nano-scaled precipitates through the formation of super saturated solid solutions induced by SPD and thus creating a structure that cannot be achieved by classical metallurgical processes. Thus, our methodology will provide a unique opportunity to transform low purity recycled Aluminium enriched in iron into high strength aluminium. The project is organized in four work packages, from a very fundamental approach on a model Al-Fe alloy with a progressive move to commercial alloys (AA2050, AA7449) with optimized Fe contents and last the process optimization (sheet-ECAP). This project is also based on the complementary approach and expertise of scientists from three different French laboratory (GPM, SIMAP and LEM3). This is multidisciplinary consortium, ranging from mechanics to physics and chemistry of materials. It gathers experts in light alloy characterization using various experimental techniques, mechanics of materials, ultrafine grain structures, precipitation in Al alloys and processing. One of the fundamental aspects of the present proposal will be also treated in collaboration with the research team of Prof. Zenji Horita (Kyushu University, Japan). Last, this project is also supported by Constellium (aluminium industry) that will supply the industrial material for the studies.

Among the strategic sectors in French industry, aeronautics takes a special place both in terms of image and innovation and the number of jobs involved despite a still economic context unfavorable. To preserve this force and dynamic facing the increased competition from emerging countries, it is essential to maintain the competitiveness of our industries and the capacity for innovation. This requires the continuous development of new production processes and alloys with high added value. In the case of titanium alloys, machining is identified by industry in this sector as a critical operation despite significant technological progress over the last decade. Indeed, many finished parts are machined integrally in the mass causing a significant financial cost due to poor machinability compared to a large number of other alloys such as aluminum alloys. To improve the machinability of titanium alloys, the traditional approach is to optimize the machining process by focusing, for example, on tool geometry or cutting forces based on turning or milling models. Our approach focuses on the role of the microstructural parameters of the material and their interaction with the cutting tool on the integrity of surfaces. The research project is devoted to the understanding and the quantification of the role of the microstructure of titanium alloys on the machinability and wear of cutting tool. The originality of our approach is based on the design of a set "model" microstructures to understand the basic physical, chemical and mechanical mechanisms. This study will include a fine analysis of the material/tool chemical reactivity and expertize of the machining chips according to the starting microstructure as well as a crystalline scale modeling. This project is challenging because it is at the limits of the state of art, and because it is based on a multi-scale and quantitative approach taking into account fundamental physics, metallurgy, mechanical and modeling, and finally because it deals with high added value materials that are used in a competitive international business. To achieve the goal will allow establishing the best link between the microstructure and the machinability properties and contributing through this approach to the design of "ideal" microstructures as a function of requirement specification and to provide the required material data for the simulation of the machining process. The expected progress will also help tool manufacturers to develop new coatings and even new tools to improve surface quality and reduce wear, leading to reduced cutting tool consumption.