Scientific Events

The defactant concept allows to predict why at higher temperature the formation energy of vacancies, dislocations and surfaces is no longer decreased by hydrogen, because it is not trapped to these defects any more. Thus failure due to hydrogen embrittlement is not present at high temperatures. At low temperatures the diffusion of hydrogen to defect generated by deformation will be reduced and, therefore, the decrease of defect formation energy by segregated hydrogen will not occur. Based on these scenarios equations for crack growth or strain to failure are derived and compared with experimental result for power law creep, stress-strain tests and fatigue. [more]

Solid-state batteries (SSBs) promise to meet the increasing demand for safe, high-power, and high-capacity energy storage. SSBs with solid electrolytes (SEs) offer potential advantages over conventional lithium-ion batteries with liquid electrolytes. Their performance, however, strongly depends on the structure and composition of the various interfaces contained in the different materials, which also change upon electrochemical cycling. We use Scanning transmission electron microscopy (STEM), to quantify properties of interfaces in battery materials. When compared to image simulations, the information on the sample structure and composition derived from STEM data can be quantitative. Combining STEM with a fast, pixelated detector allows for the acquisition of a full diffraction pattern at each scan point. From this, four-dimensional STEM (4D-STEM) datasets are available, which can be used to generate different data, e.g. annular dark field (ADF) as well as (annular) bright field ((A)BF) images, angular resolved STEM (ARSTEM) or differential phase contrast (DPC) data. With the example of cathode, anode and different SE materials for battery applications (e.g., NCM, Si, LLZO, LATP), we track the formation of different phases of and defects within the materials in dependence on synthesis as well as cycling conditions of the material and derive ABF as well as BF images from 4D datasets. These are used to also obtain difference images (ABF-BF). It will be shown that the composition of the materials and especially the Lithium content can be derived from the contrast of the different atomic columns in the structure. This is possible by comparing the experimental data sets to state of the art multi-slice simulations. This contribution will summarize the material science aspects of the energy materials investigated but also elucidate the potential of quantitative 4D-STEM to investigate materials. [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 order to clarify the deformation and fracture mechanism in oxides such as Al2O3 and STO, TEM in situ nanoindentation experiments were conducted for their single crystals and bicrystals. We successfully observed the dynamic behavior of twin formation, twin-GB interaction, pile-up dislocation, jog and kink formation and jog drag dynamics and so on. The mechanism of each dynamic behavior will be discussed in detail in this presentation. GB migration plays an important role in considering the high temperature mechanical properties. Recently, we have found that GB migration behavior in Al2O3 can be precisely controlled by the aid of the high-energy electron beam irradiation. This technique was applied to directly visualize the atomistic GB migration. It was revealed that the GB migration is processed by a cooperative shuffling of atoms in GB ledges along specific routes. References [1] S. Kondo, T. Mitsuma, N. Shibata, Y. Ikuhara, Sci. Adv., 2[11], e1501926(2016). [2] S. Kondo, A. Ishihara, E. Tochigi, N. Shibata, and Y. Ikuhara, Nat. Commun., 10, 2112 (2019). [3] J.Wei, B.Feng, R.Ishikawa, T.Yokoi, K.Matsunaga, N.Shibata and Y.Ikuhara, Nat. Mater.,20 (7), 951 [4] J.Wei, B.Feng, E.Tochigi, N.Shibata and Y.Ikuhara, Nat. Commun., 13(1), 1455, (2022) [more]

Polycrystalline Ni-based superalloys are vital materials for disks in the hot section of aerospace and land-based turbine engines due to their exceptional microstructural stability and strength at high temperatures. In order to increase operating temperatures and hold times in these engines, hence increasing engine efficiency and reduction of carbon emissions, creep properties of these alloys becomes increasingly important. Microtwinning and stacking fault shearing through the strengthening g’ precipitates are important operative mechanisms in the critical 600-800°C temperature range. Atomic-scale chemical and structural analyses indicate that local phase transformations (LPT) occur commonly during creep of superalloys. Furthermore, the important deformation modes can be modulated by LPT formation, enabling a new path for improving high temperature properties. [more]

The persistent demand for green, strong and ductile advanced high strength steels, with a reduced climate footprint, calls for novel and improved multi-phase microstructures. The development of these new steels requires an in-depth understanding of the governing plasticity mechanisms at the micron scale. In order to address this challenge, novel numerical-experimental methods are called for that account for the discreteness, statistics and the intrinsic role of interfaces. This lecture sheds light on recent and innovative developments unravelling metal plasticity at the micron scale. Multi-phase through-thickness samples allow for a full characterization of the underlying microstructure. Using computational crystallographic insights, a slip system based local identification method has been developed, which provides full-field crystallographic slip system activity maps. The resulting deformation maps are directly used to assess the model predictions. Heterogeneous spatial variations are introduced by sampling the slip system properties of individual atomic slip planes from a probability density function. This allows to recover naturally localized slip patterns with a high resolution. It is demonstrated that this discrete slip plane model adequately replicates the diversity of active slip systems in the corresponding experiment, which cannot be achieved with standard crystal plasticity models. Recent experimental observations on dual-phase steels demonstrate substructure boundary sliding parallel to the habit plane in lath martensite, for which a habit-plane slip enriched laminate model is developed. This model adequately captures the role of the substructure boundary sliding on the deformation of the martensite aggregate. [more]

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