Scientific Events

The use of nanoindentation-based techniques to study high strain rate deformation behavior of materials is of immense scientific interest because it enables investigating the strain rate dependence of individual grains and small-scale structures. While nanoindentation impact tests, capable of reaching high strain rates, have been used for over two decades, they suffer from lack of indentation profile control and rapidly varying strain rate during impact. This makes extraction of reliable mechanical properties, for e.g. hardness, and determination of the representative strain rate rather difficult. It is only recently that advances in nanoindentation instrumentation have enabled reaching constant strain rates > 100 s^-1 in both micropillar compression [1] and indentation [2]. In this talk, I will present our progress in performing controlled, constant strain rate nanoindentation tests up to 10⁴ s^-1 for reliable extraction of mechanical properties and deformation activation parameters, particularly at high strain rates. Typically, high speeds and fast unloading rates excite the resonance of the nanoindenter, which affects the extracted hardness and modulus values. Novel experimental protocols and calibration procedures were developed to circumvent this issue, which will be discussed. Case studies of high strain rate nanoindentation testing on multiple material systems – single and ultrafine grained metals, amorphous glasses and polymers – will be presented. Deformation activation parameters, for e.g. activation volumes and strain rate sensitivity exponents, were successfully extracted at high strain rates to probe possible changes in the underlying deformation mechanism(s). It is hoped that this study will pave the way for routine high strain rate nanoindentation testing. [more]

Refractory metals and their alloys are considered a versatile group of high temperature resistant materials. Promising design approaches to extend their application range include the modification of both existing commercial materials, but also novel alloys that are still at earlier stages of development. Additionally, rapid solidification processes offer interesting possibilities in this respect. In this presentation, selected research activities in this combined area will be discussed and possible approaches for further research will be presented, while the focus is on nano-oxide composites and titanium-based alloys. [more]

Nanoscale metal-graphene nanolayered composites are known to have ultra-high strength due to the ability of graphene to effectively block dislocations from penetrating through the metal-graphene interface. The same graphene interface can deflect generated cracks, thereby serving as a toughening mechanism. In this talk, the role of graphene interfaces in strengthening and toughening the Cu-graphene nanolayered composite will be discussed. In-situ TEM tensile testing of Cu-graphene showed that the dislocation plasticity was strongly confined by the graphene interfaces and the grain boundaries. The weak interfacial bonding between Cu-graphene induced an interesting stress decoupling effect, which resulted in independent deformation of each Cu layer. MD simulations confirmed such independent deformation of each Cu layer and also showed that the graphene interfaces effectively block crack propagation as delamination occurs at the Cu- graphene interfaces to allow for elastic strain energy dissipation. Bending fatigue testing was also conducted on Cu-graphene nanolayered composites that indicated ~5 times enhancement in robustness against fatigue-induced damage in comparison to the conventional Cu only thin film. Such an enhancement in reliability under cyclic bending was found to be due to the ability of the graphene interface to stop fatigue-induced crack propagations through thickness of the thin film, which is contrary to how a metal only thin film fails under cyclic loadings. [more]

In the search for new energy sources, nuclear fusion of deuterium and tritium is one of the most promising options for human kind. However, thermonuclear fusion has set an enormous challenge to theory, experiment and technology due to the harsh environment that will take place in a future nuclear fusion reactor: 14 MeV neutron irradiation, helium accumulation and hydrogen isotope (HI) implantation, taking place at the same time and mutually influencing each other. Tritium self-sufficiency is one of the major prerequisites. For this reason, a macroscopic understanding of the phenomena involved is needed in order to predict transport and retention of fuel in future devices. I will give an overview on the knowledge gained so far for tungsten, the material chosen because of its good thermal conductivity, high melting point, low sputtering yield and low HI retention. I will show how laboratory studies addressing the synergism between displacement damage creation and presence of HIs [1-3] help in understanding the phenomena and to what extent they can be used to extrapolate to a future fusion reactor. I will also present new developments of ion beam techniques in order to study lattice disorder and position of trapped deuterium in the tungsten lattice. [more]

Development of innovative characterization tools is of paramount importance to advance the frontiers of science and technology in nearly all areas of research. In order to overcome the limitations of individual techniques, correlative microscopy has been recognized as a powerful approach to obtain complementary information about the investigated materials. High-resolution imaging techniques such as Transmission Electron Microscopy (TEM) or Helium Ion Microscopy (HIM) offer excellent spatial resolution. However, the analytical techniques associated with TEM such as Energy Dispersive X-ray spectroscopy (EDX) or Electron Energy-Loss Spectroscopy (EELS) are inadequate for the analysis of (i) isotopes, (ii) trace concentrations (< 0.1 at. % or < 1000 ppm) and (iii) light elements (H, Li, B). Secondary Ion Mass Spectrometry (SIMS), on the other hand, has several advantages such as the possibility to analyse elements and isotopes of all elements of the periodic table while also providing high-sensitivity to detect even trace concentrations. However, the main drawbacks of SIMS are (i) difficulty in quantification and (ii) lateral resolution of SIMS imaging is fundamentally limited by ion-solid interaction volume to ~10 nm. Owing to the complementary strengths of SIMS imaging, we developed new in-situ and operando instrumentations for correlative microscopy combining electron microscopy and SIMS imaging. In this presentation, we will discuss the instrumentation development aspects of correlative microscopy techniques based on SIMS imaging. With a range of examples from energy materials, we will show the powerful correlative microscopy possibilities that emerge due to these new in-situ and operando methods and compare with ex-situ correlation. Our recent work in the application of these methods in hydrogen containing materials and Li ion batteries will be reviewed. [more]

The mechanical behavior of most metals in engineering applications is dominated by the grain size. Physics-based models of the interaction between dislocations and the grain boundary are important to correctly predict the plastic deformation behavior of polycrystalline materials. Dislocation-grain boundary interaction is complex and a challenge to model. In this talk, I will present a short history, opportunities, and challenges for modeling grain boundaries at the mesoscale using discrete dislocation dynamics. This includes an effective model and a novel model for physical transmission of dislocations through grain boundaries with a residual grain boundary dislocation. In addition, I will provide an outlook how these models can and should be calibrated using micromechanical experiments on bicrystals. [more]

Our aim is to understand processes that lead to the emergence of catalytic function though direct observation using a combination of operando scanning and transmission electron microscopy. Starting with simple model catalysts, such as polycrystalline metal foils, we observe the propagation of chemical waves and reveal how catalytic activity depends on grain orientation, coupling mechanisms and reaction conditions. In the case of redox-reactions on non-noble metals, we find that the active catalyst is operating near a phase-boundary where metallic and oxidized phases co-exist. Real-time imaging reveals fascinating oscillatory redox dynamics that increase in complexity with increasing chemical potential of the gas-phase. When moving from simple model catalysts to industrially relevant metal nanoparticles supported on reducible oxide carriers, we apply in-situ transmission electron microscopy to study effects related to a strong metal-support interaction (SMSI) under reactive conditions. Using the archetypical titania supported platinum nanoparticles as a reference system, and hydrogen oxidation as model redox reaction, it will be shown that the well-described encapsulated state of platinum particles is lost as soon as the system is exposed to a redox-active environment. Structural incoherence at the platinum-titania interface lowers the barrier for redox processes, which give rise to dynamic reconstructions and particle migration. The particle orientation on the support determines the structure of the interface and the resulting particle dynamics, migration, and sintering behaviour. The aim of the presentation is to demonstrate that active catalysts are dynamically adapting to the reaction environment and that catalytic function is related to the catalysts ability to participate in the reaction through reversible changes in its structure and/or (local) composition. [more]