METAFORES

During the last decades, the principal strategy for improving multi-functional structural materials (MFSM) consisted in increasing complexity and reducing size of their microstructure. The METAFORES project is focusing on one class of MFSM, copper/niobium (Cu/Nb) high-strength and high-conductivity nanostructured metals which choice is motivated by their application in high field magnets. LNCMI and Pprime participants have shown how their deformation mechanisms are modified both by the reduction of microstructure size and the modification of the geometry. However, the exact origin of this observation is not clear and only advanced experimental tools coupled with simulation can help understanding the exact role of architecture (versus size effect) in the macroscopic mechanical response. Hence the project relies on three axes: 1) fine experimental microstructure characterization, 2) micromechanical modelling and 3) advanced strain distribution measurements. The first axis aims at providing accurate and statistically relevant data on grain geometry, texture and misorientation to ensure realistic representation of the microstructure in the finite element modeling computations and perform realistic and robust micromechanical and electrical modeling (PIMM and MINES participants). From this, correlations and scaling laws between stress, hardening and dislocation densities will be introduced in the crystal plasticity constitutive equations used in the second axis of the project. However these laws are not sufficient for describing the size-dependence of plastic strain fields: sophisticated continuum models are needed for full-field computations and comparison with strain field measurements. Therefore, computational homogenization methods recently developed at MINES will be implemented to compute the full-fields and the effective response of multiphase crystalline microstructures, including size-dependent crystal plasticity models: such novel approach will be applied to investigate microstructure and architecture effects. In parallel, a less time-consuming approach will be developed: mean-field homogenization methods such as the self-consistent scheme is especially well suited for polycrystals. But contrarily to a full-field approach, this scheme usually considers uniform stress inside grains; this drawback will be overcome by using the Ponte-Castaneda & Willis approach where microstructure features can be taken into account (grain shape and geometrical arrangement). This second original approach should allow obtaining additional insight into the effect of the alignment of Cu and Nb grains with the nanocomposites. The third axis deals with advanced experimental in-situ deformation experiments to monitor the non-uniform distribution of strain within the composites upon loading. Since diffraction offers a unique non-destructive tool for the measurement of internal strains into individual structural phases of the nanocomposites, in-situ loading strain measurements will be performed using a small tensile machine mounted on the 6T1 thermal neutrons diffractometer (LLB participant). In this way, strain pole figures will be obtained helping to characterize the mechanical responses of Nb and of the various scales of Cu and to compare the experimental strain distribution to the different simulations. Finally, all the experimental and simulation results will be combined to assess the roles of microstructure versus architecture in order to define design criteria for tailored MFSM not only from the choice of materials but also from their geometry (microstructure and shape).