Fracture at interfaces
The focus lies on the investigation of the fracture properties of materials down to the individual microstructural length scale, with special attention on grain/phase boundaries or material interfaces, while considering the role of crystallography, chemistry and non-ambient conditions such as high temperatures and cryogenic conditions on fracture.
Whether macroscale components for structural applications such as bridges, ships, aircraft, or microscale systems such as thin-films or interconnects in microelectronic devices, fracture is largely governed by interactions between growing crack tips and microstructural interfaces. A prime example is intergranular stress corrosion cracking (IGSCC), where cracks preferentially follow chemomechanically affected grain boundaries. The primary goal of this research theme is to investigate the fracture properties of materials down to the individual microstructural length scale, focusing on grain/phase boundaries or material interfaces, while considering the role of crystallography, chemistry and non-ambient conditions such as high temperatures and cryogenic conditions on fracture. In our group, we probe the site-specific fracture properties through in situ small-scale testing methods inside a scanning electron microscope to glean novel insights into material failure.
Chemical effects on interfacial fracture: The segregation of chemical species to low-energy sites, such as grain boundaries (GBs) can have a pronounced effect on fracture along these interfaces. In collaboration with the FZ Jülich, together with molecular dynamics (MD) support from Prof. Erik Bitzek, we have studied in detail the embrittlement of recrystallised Tungsten using site-specific microcantilever tests where the sharp notch used to initiate fracture was localised directly at the intersection of GBs with the surface. This work was funded by the ERC projects of both Prof. Bitzek and Prof. Dehm. Using our novel micromechanical tests coupled with atom probe tomography, we were able to demonstrate that during recrystallisation and grain growth, Phosphorous segregates to GBs and leads to a clear embrittlement of the W (Acta Materialia,259, (2023)119256 ). Leveraging similar concepts, and in collaboration with Dr. Xufei Fang from TU Darmstadt, we have also investigated the high-temperature oxidation of a Fe-Cr alloy where significant oxygen accumulates at GBs using novel in situ approaches to study local fracture at these interfaces.
Fracture of intermetallic materials, including at high-temperatures: In research led by Dr. Anwesha Kanjilal, and supported by the DFG through SFB1394, we investigate the role of crystallography and composition on the fracture of intermetallic materials. Many metallic alloys have microscale intermetallic precipitates dispersed in their matrix, which may improve the strength of the alloy but their brittle nature also makes them susceptible to cracking and fracture. Through micro- and nano-mechanical testing we investigate the fracture toughness of Laves phases in the Mg-Al-Ca system, while additionally highlighting the role of temperature and cleavage planes, which are then used to understand the interface strength between the Laves phase and Mg. Cubic C15 CaAl2 and hexagonal C14 CaMg2 Laves phases are studied as single-phase specimens, which are commonly found in Mg-alloys. Microscale fracture testing based on microcantilever bending and micropillar splitting are used to determine the toughness, while cleavage planes are initially identified through spherical indentation assisted by MD simulations in collaboration with the group of Prof. Erik Bitzek. We employ high-temperature nanoindentation to monitor changes in slip activity and cracking and screen the transition temperature from brittle to ductile behaviour for the Laves phases, where results are put into context of high-temperature plasticity measurements from collaborators at the RWTH Aachen. The effect of temperature on fracture toughness is evaluated using microscale fracture testing techniques, while the composition is additionally varied using combinatorial thin films of the Laves phases, allowing us to obtain fundamental insights into the role of crystallography and chemistry on fracture properties of these materials.
(adapted from preprint article SSRN ).
Ternary Fe-Al-X alloys are an attractive intermetallic material for structural applications and the addition of B is known to promote toughening of the alloys at low temperatures. Macroscale 3-point bending tests have shown that Fe-Al-X-B alloys have a lower brittle to ductile transition temperature, attributed to boride precipitates at grain boundaries (GBs). However, the underlying reason for the improved toughness of the chemically decorated GB in Fe-Al-X alloys is unknown. The role of GB chemistry on the toughening of Fe-Al-X-(B) is here studied using site-specific micromechanical testing to probe FeAl GBs containing boride precipitates. Different GB types, both in presence and absence of the precipitates are chosen and microcantilevers are prepared with the GB positioned close to the fixed end of the cantilevers. The deformation at the GB is monitored and compared to develop a mechanistic understanding of the influence of B addition on toughening in these alloys.
Low-temperature interfacial fracture: Fracture under cryogenic or low-temperature conditions has been observed in cases such as the failure of Liberty ships, the Challenger space shuttle disaster, sinking of the Titanic due to the hard glacier ice, or even of glaciers. It is essential to understand low-temperature deformation and failure to avoid such catastrophic events, as material brittleness generally increases at lower temperatures. In addition to collaborations with Prof. Vera Popovich of TU Delft investigating low-temperature fracture of high strength steel containing high intermetallic fractions, we also focus on developing experimental methodologies to investigate the nanomechanics and fracture of frozen liquids, such as water ice (see PLoS ONE 18(2): e0281703). This work on the in situ mechanical testing ice is led by PhD student Taulant Sinani, co-supervised by Prof. Baptiste Gault and supported the Leibniz prize of Prof. Gault. Understanding ice fracture is critical for glaciology and helps us better predict the movement of glaciers in nature. The fracture of water ice is highly affected by chemical impurities, like salts or carbon-based pollution, which strongly segregate to GBs. However, intergranular failure and chemical heterogeneities are not considered in current glacial models. In our work, ice samples with different cooling rates, microstructures and impurities are being prepared to perform micromechanical testing to unravel the influence of chemically-decorated GBs on the mechanical response.