This research proposal aims at understanding the ROle of DIscoNnections in grain-boundary-based plasticity (RODIN). Grain boundaries (GBs) are the defects that separate two domains possessing different crystallographic orientations (commonly called "grains"). They are found in most of the metals, alloys, ceramic materials, and more generally in any crystalline solid. In studies about the deformation of polycrystalline solids, GBs are usually considered as static, obstacles for dislocations that are the main vector of plasticity. Recent years have seen the emergence of ultra-fine grained (grain size below 1 µm) or nanocrystalline (nc-, grain size below 100 nm) materials, where dislocation-mediated plasticity was reduced or even shut down. A considerable amount of studies have tried to understand the plastic deformation of these classes of materials. Remarkably, the plasticity of these materials seems to rely significantly on GB-based mechanisms. This generic term regroups several mechanisms (GB sliding, grain rotation, enhanced diffusion, etc…). Among them, the shear-coupling grain boundary migration drew a great interest in the last 10 years or so for its potential effectiveness.. By coupling molecular dynamic simulations and in situ electron microscopy, we recently showed that this mechanism is heavily dependent on GB-specific defects called "disconnections", or step-dislocations. These defects, specific to GBs, possess a Burgers vector (as a dislocation) and a step character. We also showed that many types of disconnections can be found in a single GB, and that most of the GB-based plasticity mechanisms may be explained using this single type of defect. Hence we have started to infer that a paradigm shift should be made: GBs are not simply defects of an hypothetical single crystal, but form networks of their own, which mechanical properties depend on their own defects (the disconnections). This concept extend to GBs the current understanding of mechanical properties of crystal dictated by their dislocations. Despite this potentially important implication in many fields of metallurgy and materials science, GB-based plasticity is very poorly known today. Indeed, GBs are complex objects with more degrees of freedom than crystals and they virtually contain an infinity of different disconnections. The gap between theoretical studies, often considering GBs as flat and perfect objects, and experiments that try to have statistical overviews of large ensemble of GBs is also considerable today. The goal of this project is to make some significant progress on the fundamental understanding of disconnection-based GB mechanisms, both at the atomic, nanometer and micrometer scales. We will work on defined Al bi-crystals where the shear-coupling mechanism involved in GB migration is both easier to study experimentally and to confront to molecular dynamics models that are also bi-crystalline for the moment. High Resolution Transmission Electron Microscopy (HRTEM) will be used to analyze the defects involved at the same scale than the simulations. On the same bicrystals, Scanning Tunneling/Atomic Force Microscopy (STM/AFM) tests will also be performed to apprehend the third dimension of plastic deformation that is often hard to measure in TEM and to complement TEM observations of events occurring in a truly bulk metal. We will then combine in situ TEM, SEM and STM/AFM experiments on nc-and UFG Ni and Al that have very different melting temperature. This will help us understand how disconnections are nucleated, interacting with regular lattice dislocations, and need or not diffusion (nucleation/absorption at triple junctions). Automatic crystalline orientation mapping (ACOM) will serve to complete a statistical view of various interactions on different GBs.

Next European regulations on CO2 emissions for automotive transportation (130 g/km by 2015) will lead to a reduction of car weight of about 20%. Car parts’ lightening has to be made for equivalent functions and similar safety requirements, and to be manufactured with a same productivity. Financial penalties, foreseen in case of the CO2 emission target is not fulfilled, make car makers ready to accept a slight increase in the material cost to obtain such substantial weight reductions. This allows new solutions to be proposed as the use of light alloys, the proposal of new designs… Regarding these new conditions, steel has still important assets for customers: large availability, moderate price, easy recyclability, formability and usability easy to manage, superior crash properties for safety design. A 1st generation of very high strength steels (HSS) gave rise to high mechanical properties resulting from design and control of the microstructure. These are essentially ferritic-bainitic matrix with a fraction of metastable carburized phases: Dual-Phase and TRIP steels are two of the typical grades. A 2nd HSS generation is under development based on the TWIP mechanism; however, the industrial manufacturing is not straightforward. The aimed properties (UTS above 1,000 MPa, uniform elongation larger than 15%, and, when possible, the density decreased by 5 to 10% with respect to standard carbon steel) require the development of a new steel generation. Their metallurgy is based on medium Mn, medium Al, steel grades (Mn between 5 and 8 wt%, Al lower than 8 wt%). A few laboratory trials show that fine duplex ferrite-austenite microstructures can be reached. Depending on the chemical composition and the thermomechanical processing, the volume fractions of each phase can be tailored, and the mechanical behaviour of the austenite is controlled over a wide range including the expected targets. Adding aluminium decreases the density and favours the hardening of the ferritic phase. It is already established that such steel grades can be produced on the present industrial lines. They are also easily recyclable in a common steel process. The MeMnAl Steels project is devoted to understanding the physical mechanisms involved in the development of these new steels, in particular those governing microstructural evolutions and deformation. Based on the expected results, these microstructural evolutions and the final properties will be modelled. Physical metallurgy and mechanical metallurgy approaches will be combined to map the final capabilities of these steel grades, to predict their ultimate behaviour, and to assist in defining the main stages of the industrial processing. The different teams involved in the project have complementary experimental skills and facilities, and modelling competencies: physical metallurgy, thermodynamics and kinetics, mechanical metallurgy and damage-fracture... These approaches are performed in a multi-scale and multi-physics framework. This 4-years project is divided in two strongly interacting domains: (i) modelling of microstructure genesis, and (ii) modelling of the relations between resulting microstructures and mechanical properties. i. For the first domain, thermodynamics and kinetics tools will be developed to predict the actual phases and their volume fraction. Ab initio models and CALPHAD approach will be used in this way. ii. The relations between the microstructures and the mechanical properties will be predicted using crystal plasticity modelling which links the macroscopic behaviour with the grain behaviour, accounting for interfaces between various constituents. The ultimate goal is to capitalize the whole knowledge acquired during the project to build a model supporting the developments of these new steel grades. It will help to define, in an easy and reliable way, the composition domains and the optimal processing schedules to reach the mechanical behaviour and, thus, to speed-up industrial developments.