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.
Micromechanical fatigue loading is chosen as primary testing procedure for assessing the interfacial resistance to crack initiation and growth. This setup allows to in-situ track the crack path, e.g. intercrystalline versus transcrystalline propagation, and thereby to elucidate failure mechanisms. Similar experiments will be conducted under hydrogen charging conditions to assess the concept under harsh environments. These experiments are conducted at the Department of Materials Science and Engineering at Saarland University. If successful, this concept allows for versatile design strategies not only for ultra-strong pearlitic steels but for nanostructured materials in general.
SPD deformation of pearlitic, bainitic and martensitic steels
M.W. Kapp, A. Hohenwarter, A. Bachmaier, T. Müller, R. Pippan
Special Issue on ‘Superfunctional Nanomaterials by Severe Plastic Deformation’ in Materials Transaction 0.2320/matertrans.mt-mf2022027
This project has received funding from the Österreichischer Wissenschaftsfonds (FWF) under the Hertha-Firnberg programme (T 1347-N).