jeudi 06 septembre 2018 à 14H00 (Thé à 15h45)

CINaM - Salle Raymond Kern

Christian Brandl

Institute for Applied Materials, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen, Germany

Advanced engineering materials with tailored functionality at the nanoscale: Insights from computer simulations & nanoscale experiments

In advanced engineering materials the state of microstructure determines the materials behaviour during synthesis and during operation in devices. Especially strength and deformation in metals depend on the state and evolution of the microstructure, which constitutes the spatial arangment of materials defects from the atomic-scale defect core structure to the mesoscale defect morphology. The focus of the talk will be on the emergence of novel thermomechanical behaviour by microstructure processes focusing on defect nucleation and defect migration at the nanoscale. The synergies of molecular dynamics (MD) simulations and experimental work are highlighted for the notch-insensitive deformation behaviour of gold nanowires, stress-driven grain boundary migration and self-healing gold nanoparticles.

The aspect of defect nucleation is discussed in context of flaw-insensitivity of Au nanowires. The predicted local deformation behaviour at the flaws and the underlying deformation mechanisms from MD simulations are validated by simulation-inspired nanomechanical testing complemented by (high-resolution) transmission electron microscopy. The combined approach allowed to deduce the origin of unique defect structures as a result of dislocation nucleation, grain boundary (GB) formation and accompanied stress-driven GB migration. The role of GB migration during plastic deformation is further evaluated in context of traditional GB migration at high temperature. The transition from a “diffusive” mobility regime at high temperatures to a pinning-depinning (“yield”) GB mobility regime is illustrated for different GBs. The underlying mechanisms are the nucleation and/or migration of disconnections in the interface. The simulations reveal, moreover, the same operating atomistic mechanisms, although the emerging kinetic signatures (mobility) change. The implications are analysed in context of failure suppression by GB motion in extreme environments, the associated structural ingredients for mobile GBs and a mesoscale GB mobility model for a multiscale modelling approach.

The role of surface steps (in analogy to the above interface disconnections) will be considered for the novel self-healing mechanisms of deformed gold nanoparticles. The detailed mechanisms, kinetic regimes and the structural prerequisites are deduced from the size- dependent mechanical behaviour and the kinetic mismatch of possible diffusion pathways on the nanoparticle surface.

I will conclude with the discussion on how computer simulations allow to deduce and predict guiding principles to tailor materials with defect-dominated properties.

Invitation : C. Varvenne - Entrée libre