Scientific Events

Host: on invitation of Prof. Gerhard Dehm

Mesoscale Modeling of Crystalline Materials: Decoding Microstructure Evolution

Comprehensive investigations of material properties in crystalline systems require information spanning atomistic to continuum scales. Mesoscale models play a crucial role in this context. They enable the study of large systems and long timescales while retaining microscopic details relevant to the targeted applications. This presentation illustrates recent results on key aspects of microstructure evolution in crystalline systems obtained via newly developed mesoscale frameworks for defects and interfaces. Using a bicrystallography-respecting continuum model that incorporates parameters obtained from atomistic methods, we first discuss phenomena associated with disconnection-mediated grain boundary (GB) motion, such as GB faceting and grain rotation. Then, through a phase-field formulation of this model, we demonstrate that internal stresses generated by disconnection flow (shear coupling) induce significant deviations from classical curvature-driven grain growth in microstructures. This provides a compelling explanation for the lack of correlation between GB velocity and curvature observed in recent experiments, identifying its primary cause. In the final section, an overview of other mesoscale frameworks that build upon the phase-field crystal model and its coarse-graining is provided. Representative results are presented, including the effects of temperature cycling on GB motion, as well as an emerging general framework for analyzing defect dynamics in systems with microscopic order, which extends beyond conventional crystals. [more]

Visualizing Atomic Vibrations: A New Frontier in Electron Microscopy

Recent groundbreaking developments in aberration-corrected scanning transmission electron microscopy (STEM) combined with advanced vibrational electron energy-loss spectroscopy (EELS) techniques have fundamentally transformed the way atomic-scale lattice dynamics and phonon behaviors are studied. In this seminar, I will highlight our seminal work in developing and applying state-of-the-art, spatially and momentum-resolved vibrational EELS methodologies to directly visualize phonon modes at atomic resolution. Our approach enables the unprecedented observation of localized phonon phenomena at individual defects, interfaces, and nanostructures, profoundly advancing our understanding of phonon-defect interactions, thermal boundary conductance, and electron–phonon coupling in materials. I will present key examples from our recent studies, including the direct imaging of defect-localized vibrational modes, nanoscale mapping of interfacial phonons, and quantification of phonon momentum distributions in quantum dots and phonon-electron coupling at superconducting interfaces. These insights provide critical foundations for addressing fundamental challenges in thermal management, quantum materials engineering, and solid-state ionic devices. Ultimately, our innovations offer powerful tools to elucidate and engineer the atomic-scale behaviors that dictate the performance of next-generation functional materials and systems. [more]

Role of Preferred Interfacial Structures on Phase Transformation Crystallograph

Microstructures in many engineering alloys are predominantly influenced by solid-state phase transformations that occur during industrial processing; these transformations almost always proceed by nucleation and growth. Quantitative modelling of the process often requires detailed knowledge of the interfaces, notably the interfacial energies that determine nucleation barriers and the interfacial mobilities that control growth kinetics—both of which depend sensitively on the the interfacial structures. Beyond their role in transformation kinetics, interfacial structures and the accompanying orientation relationships (ORs) and interface orientations (IOs) are microstructural features in their own right and directly influence bulk properties. Based on extensive studies of diverse alloy systems, we have formulated a unified framework that rationalises the preferred interfaces and their reproducible ORs produced by phase transformations, by employing preferred interfacial structures of two hierarchical levels. At the fine (atomic) level, the interface adopts a low-energy, periodically matched configuration that minimises the nucleation barrier. Such matching is possible only for specific intrinsic ORs and IOs, thereby imposing the geometric constraints. The structures of the coarse level are characterized by singular interfacial defects. Their development, preferred under given phase transformation conditions, allows the OR and IO to deviate within certain limits from the intrinsic values. This talk will present general methods for correlating ORs and IOs with interfacial structures at both levels and will illustrate the approach with examples from several material systems [more]

Computational Multiscale Modelling of Material Interfaces

The importance of different length scales in materials science is well-recognized and subject of intense interdisciplinary research efforts. In these developments, multiscale modelling approaches take a key role as these enable the prediction of the effective material response based on detailed microstructure representations. Against this background, we focus on a scale-bridging understanding of macroscale material interfaces and on the influence of microscale interfaces on the effective properties of continua. We make use of classic energy-based homogenisation approaches, extend these to material interfaces, and demonstrate the usefulness of the proposed generalised multiscale formulations by comparison with experimental data. [more]

Temperature dependence of hydrogen embrittlement

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]

Insights in Battery Materials by Electron Microscopy

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]

Hydrogen effects on the deformation and fracture of alloys

MPI SusMat Colloquium
The increasing demand on lightweight structures requires high-strength materials. However, with increasing strength many materials show an increasing susceptibility to hydrogen embrittlement. Hence, it is of vital interest to understand the mechanisms of hydrogen embrittlement. [more]
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