Méthodes en Ingénierie de Microstructures multi-échelle. Propriétés mécaniques et Modélisation micromécanique. – MIMIC

The general principles underlying the processing of high strength, and ductile, metallic materials have been quite well understood since the beginning of the development of dislocation theory. If ductility requires a significant mobility of dislocations, strengthening is related to the building of obstacles that restrict their propagation. One of the methods to introduce such obstacles is the reduction of the mean grain size. However, the mechanical properties of aggregates do not only depend on the mean grain size. They are also sensitive to the grain size dispersion and to their spatial arrangement. Within this framework, the present project focuses on the effect of grain size reduction and grain size distribution on improving the strength and ductility of polycrystals. To this end, due to their versatility, powder metallurgy routes such as Hot Isostatic compaction (HIP), Spark Plasma Sintering (SPS) and Soft Hydrostatic Extrusion (SHE) will be used to process a wide range of Nickel based microstructures (from ultrafine-grained regime to a well controlled distribution of different volume fraction of a coarse grained Ni phase within the ultrafine-grained matrix), whose effects on the macroscopic behavior will be investigated through a wide range of (monotonic and cyclic) mechanical tests at room temperature. The underlying deformation mechanisms (intergranular and/or intragranular, damage') will be tackled, post mortem and in situ, via TEM and XRD investigations. In parallel, since the grain size alone neither defines the microstructure nor characterizes the mechanical behavior of metallic polycrystals, a micromechanical model that is based on a generalized multi-scale self-consistent approach will be developed such as to specifically shed light on the effect of microstructural parameters such as the grain size and its statistical distribution on the macroscopic behavior.