Periodic Reporting for period 3 - TOUGHIT (Tough Interface Tailored Nanostructured Metals)
Período documentado: 2021-05-01 hasta 2022-10-31
The worldwide growing population in combination with a limited amount of resources requires us to utilize them as efficiently as possible. Translating this into materials science challenges us to create materials that are light-weight and simultaneously possibly strong in order to maximize load-bearing capacity, while concomitantly minimizing weight and material usage. At the same time, structures or components must be failure tolerant to prevent human harm in case of unexpected overload situations. This in fact poses a severe challenge to the field, as it has been established over decades that maximizing strength and ductility are exclusive targets. Extremely strong materials commonly lack ductility, and without local ductility, it becomes tremendously challenging to prevent a crack from propagation, thus catastrophic failure becomes a risk. Furthermore, increased ductility enhances the fatigue strength and leads to an extended lifetime of parts subjected to high cyclic loads. Satisfying all these contrary demands indeed requires a better material property balance in the use of essential and limited resources.
It is the overall objective of this project to overcome this trade-off between strength and ductility by tailoring novel metallic nanocomposites that uniquely combine high strength and high ductility, and at the same time avoid brittle fracture by dedicated grain boundary engineering strategies.
It is the overall objective of this project to overcome this trade-off between strength and ductility by tailoring novel metallic nanocomposites that uniquely combine high strength and high ductility, and at the same time avoid brittle fracture by dedicated grain boundary engineering strategies.
To date, we managed to synthesize metallic nanomaterials and nanocomposites starting from input powders that are competitive in their properties compared to those achieved from a refined commercial bulk composite, validating the feasibility of this additive approach chosen. Importantly, this powder-based material synthesis also enables us to deliberately add doping elements for grain boundary strengthening. This would be impossible utilizing standard processing strategies due to thermodynamic and kinetic limitations. Focusing on tungsten as the hard and brittle composite phase, we followed up theoretical ab-initio predictions to enhance grain boundary cohesion. Thereby, we were able to demonstrate increased ductility with no loss in strength for the grain boundary doped nanocrystalline materials. In addition, a subsequent mild thermal treatment allows to promote grain boundary segregation and thus a further increase in strength while retaining material ductility. This allowed us to create an adaptive material with outstanding characteristics. Similar achievements hold true for nanocomposites. Thereby, in line with the expectations, the addition of copper as a ductile phase to the nanostructured tungsten reduced the overall strength compared to pure tungsten. However, the ductility of the nanocomposite could be significantly improved. Here again striking results with respect to tremendously increased ductility were achieved by tailored grain boundary doping. To further advance the properties of the softer composite phase copper, we proceeded to create amorphous grain boundary layers and focused on synthesis of copper alloys with an increased twinning tendency. The powder based synthesis was again accomplished successfully, and the nanocrystalline copper with amorphous grain boundary films documented very high hardness, while the twinning tendency of the alloy beneficially enhanced uniform elongation. These impressive achievements would not have been possible without a detailed understanding of the governing mechanisms, which we establish by employing detailed micro- and nanomechanical deformation and fracture examinations to identify the fundamental deformation and fracture processes. Besides employing advanced nanoindentation techniques, this required us to develop elastic-plastic fracture mechanical testing strategies for micro- and nanoscale specimens conducted in scanning and transmission electron microscopes, respectively. Here it is particularly worth mentioning that we pioneered the unique capability to map the strain field at a crack tip with nanometer resolution and have the possibility to conduct these analyses even during in-situ testing in the transmission electron microscope, for example while a crack approaches or propagates at an interface. Accompanying analysis tools were also developed for a better understanding of local strain distribution. Concerning the in-situ experiments on micron-sized specimens conducted in the scanning electron microscope, novel digital vision analysis was designed to aid the continuous identification of the crack advancement process, tremendously aiding the analysis of the respective data. Furthermore, we also implemented micromechanical spectroscopy techniques for sampling grain boundary processes, as well as a transmission microscopy mechanical testing setup in the scanning electron microscope to connect the scanning and transmission worlds in terms of their respective beneficial capabilities. Lastly, we also expanded towards nanoporous but still nanocrystalline tungsten as another promising material to unite high levels of strength, ductility and toughness. We were successful in deriving a novel synthesis technique, establish a fundamental understanding of the formation process of the nanoporous topology, and could demonstrate that the received material again offers a very attractive combination of materials properties. Furthermore, the vast amount of interfaces and free surfaces renders it highly tolerant against radiation damage.
So far, we were successful in confirming our main hypothesis in most parts, already synthesized materials that outperform what is currently available on the market, and still see significant potential for further improvement of our achievements to date.
We are capable of executing and analyzing micro-/nanomechanical tests to address local deformation and fracture processes at a worldwide leading level, thereby providing unseen insights into the fundamental processes governing deformation and failure of the investigated materials.
The powder-based severe plastic deformation synthesis route as well as experimental micro /nanomechanical in-situ investigations are generally applicable and suited to a wide interdisciplinary palette of materials issues. As such, for the remainder of the project, we expect further improvement of the materials under investigation, but also the positive impact of our work in other fields of materials science. The former refers to possible co-doping in the nanocrystals, as well as improved twinning capability and amorphous grain boundary engineering in the nanocomposite case. An example of the latter is our very recent high impact publication on a biological auxetic nanocomposite to highlight transferability of these techniques to other fields.
We are capable of executing and analyzing micro-/nanomechanical tests to address local deformation and fracture processes at a worldwide leading level, thereby providing unseen insights into the fundamental processes governing deformation and failure of the investigated materials.
The powder-based severe plastic deformation synthesis route as well as experimental micro /nanomechanical in-situ investigations are generally applicable and suited to a wide interdisciplinary palette of materials issues. As such, for the remainder of the project, we expect further improvement of the materials under investigation, but also the positive impact of our work in other fields of materials science. The former refers to possible co-doping in the nanocrystals, as well as improved twinning capability and amorphous grain boundary engineering in the nanocomposite case. An example of the latter is our very recent high impact publication on a biological auxetic nanocomposite to highlight transferability of these techniques to other fields.