Nanofracture – Origin of fracture in high strength materials

The fracture behavior of nanostructured materials is the Achilles heel of modern materials science. Efforts to increase the strength via different approaches of hardening lead to distinct high-strength materials, however, at the expense of ductility and toughness. Common approaches to improve microstructural morphology are grain boundaries, lamellar structures or precipitates in the nanometer scale as well as sophisticated alloying concepts. In the latter case, the alloying elements may segregate at grain boundaries or lamellar interfaces, threby modifying their configuration. Hypothetically, tailored interface segregation should enhance fracture toughness, enabling delayed crack onset and propagation. In this study, the insufficiently understood fracture behavior of application- relevant nanocrystalline Cr-Cu/Ni and nano-lamellar TiAl alloys will be investigated. The sophisticated fabrication and testing of micro- and nano-bending beams will allow to illuminate the influence of alloying elements on interface segregation and therefore their fracture behavior. To unveil the crack propagation, assess fracture progress zones and visualize atomic details, continuous stiffness methods and digital image correlation will be performed. A scanning electron microscope for microscaled and a transmission electron microscope for nanoscaled in-situ experiments will be employed to tackle these tasks. The observed influence of grain and phase boundary types, segregation levels and dislocations on the crack propagation will be related to results obtained from molecular dynamic (MD) and density functional theory (DFT) simulations. Additionally, in-situ nano-beam diffraction allows the evaluation of local strains of nano specimens during loading. Thereby, the simulated strains to emit dislocations in front of the crack tip will be experimentally validated. The combination of advanced in-situ investigations and cross-scale simulations constitutes an innovative approach to evaluate the fracture behavior of nanoscaled materials, which goes well beyond any currently available approach. Understanding fundamental mechanisms promoting crack growth in nanocrystalline and nanolamellar materials will finally enable to inhibit these mechanisms by knowledge-based microstructural adjustments and alloying. This will enable us to improve nanoscaled materials with regard to their currently limited fracture properties.