Cohesive Interfaces by Interfacial Design (CoInS)

SPD deformation of pearlitic steels revealed exceptional strengthening, attaining with almost 7 GPa after wire drawing the highest strength of all structural materials to date. This is enabled by the high number of interfaces controlling plasticity and strength to a wide extent. For nanopearlitic steels and nanostructured materials in general, these interfaces are also the weakest spots when it comes to fracture. By targeted interface design we aim to increase cohesion of grain boundaries and interfaces to improve their crack resistance, what is specifically studied under fatigue loading conditions.

We therefore use high pressure torsion as a synthesis tool for nanostructured iron, iron-carbon and pearlitic steels. Although the strength is limited to 4 GPa with this synthesis route, it allows for flexible alloy design and interface structuring, what is in clear contrast to wire drawing. We choose a bottom-up process to sequentially study interface structuring on fracture toughness starting with pure iron, over Fe-C-alloys and eventually transferring the concept to nanopearlitic steels. We therefore use boron to deliberately dope grain boundaries, as atomistic calculations predict that boron increases grain boundary cohesion. This strategy is compared to the performance of nanolaminated iron nanostructures consisting predominately of low-energy boundaries. If successful, the boron-alloying concept is transferred to the Fe-C-system to study co-segregation of carbon and boron to the grain boundaries in iron. Finally, the doping concept should be transferred to nanopearlitic steels. Characterization of the interfacial structure will be done by HR-TEM and the boron excess will be assessed by APT measurements in collaboration with the Department of Materials Science at University of Leoben and The Groupe de Physique des Matériaux at the University of Rouen.