arXiv subjects

Christoph Kirchlechner

Publications and source records attributed to Christoph Kirchlechner.

13 recordsLinked to original sources

arXiv2026

Influence of dislocation density on the tribological response in oxides: case study on SrTiO3

Most ceramics suffer from brittle surface damage and cracking when small particles slide across the surface under low load. Microscratching tests are a useful technique to mimic this loading scenario while retaining the material's deformation history; however, the impact of pre-seeded dislocations, which can significantly facilitate plastic deformation, has not been explored in ceramics. Here, the influence of mechanically seeded dislocations on the microscratching response of oxides is investigated using a model perovskite, SrTiO3. First, various dislocation densities over four orders of magnitude are introduced via room-temperature cyclic Brinell indenter scratching. Subsequently, load-ramped microscratching tests are performed within the pristine and dislocation-seeded regions using a nominally 30 μm spherical diamond tip. On the reference pristine surface, we observe a clear transition from elastic to elasto-plastic deformation, followed by median/radial cracking at higher loads. In contrast, pre-seeded dislocations, accompanied by residual compressive stresses, suppress elastic deformation and lead to median/radial crack shortening and subsequent transition to a partial cone crack. The subsurface cracks are characterized by 3D Nano-CT. The changes in crack geometry, with and without dislocations, were described using the Lawn-Evans-Marshall and Lawn-Wiederhorn-Roberts models. These findings provide direct evidence of dislocation-regulated near-surface damage tolerance, with general applicability to other plastically deformable oxides.

cond-mat.mtrl-sci↗

arXiv2026

Mircomechanical insights into unconstrained grain boundary sliding

Grain boundary sliding (GBS) is a key deformation mechanism at high homologous temperatures in polycrystalline materials, however, its intrinsic behavior is often obscured by additional strain accommodation processes. In this study, dislocation-mediated unconstrained GBS was investigated using Ni bicrystal micropillars containing a single high-angle grain boundary. Micropillar compression tests were conducted over a temperature range from room temperature to $600\,^{\circ}\mathrm{C}$ and strain rates between $5\times10^{-4}$ and $10^{-1}\,\mathrm{s}^{-1}$. By comparing bicrystal and single-crystal responses, the intrinsic contribution of GBS was isolated. The strain-rate sensitivity remained low (SRS $\approx 0.034 \pm 0.017$), comparable to room temperature values, indicating the absence of diffusion-controlled accommodation mechanisms. The activation energy for GBS was determined to be $234\,\mathrm{kJ\,mol^{-1}}$, consistent with grain boundary diffusion-assisted glide of grain boundary dislocations. These results demonstrate that the high strain-rate sensitivity commonly associated with GBS in polycrystals originates primarily from accommodation processes rather than the intrinsic sliding mechanism.

cond-mat.mtrl-sci↗

arXiv2026

Tritium as an Unambiguous Isotopic Tracer for Nanoscale Hydrogen Analysis by Atom Probe Tomography

Accurate nanoscale detection of hydrogen is essential for understanding hydrogen-related phenomena in materials, yet conventional deuterium tracing is often complicated by residual background hydrogen. This study evaluates tritium as an unambiguous isotopic marker for nanoscale hydrogen analysis in metals using atom probe tomography (APT). Titanium was selected for its ability to incorporate hydrogen isotopes, providing a suitable platform for tritium detection. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and electron backscatter diffraction (EBSD) were performed prior to tritium charging to characterize the initial composition and microstructure. APT analysis in laser-mode before and after tritium charging, at three post-charging intervals, enables tracking of tritium incorporation over time. Thermal desorption analysis (TDA) confirmed the presence of tritium and complemented the SIMS measurements, highlighting the role of the surface oxide layer in modulating tritium release. This work serves as a fundamental benchmarking study for leveraging tritium and APT as a combined tool for understanding the nanoscale location of hydrogen in materials, being relevant for interpreting local processes related to e.g., hydrogen embrittlement.

cond-mat.mtrl-sci↗

arXiv2026

Predicting New Research Directions in Materials Science using Large Language Models and Concept Graphs

Due to an exponential increase in published research articles, it is impossible for individual scientists to read all publications, even within their own research field. In this work, we investigate the use of large language models (LLMs) for the purpose of extracting the main concepts and semantic information from scientific abstracts in the domain of materials science to find links that were not noticed by humans and thus to suggest inspiring near/mid-term future research directions. We show that LLMs can extract concepts more efficiently than automated keyword extraction methods to build a concept graph as an abstraction of the scientific literature. A machine learning model is trained to predict emerging combinations of concepts, i.e. new research ideas, based on historical data. We demonstrate that integrating semantic concept information leads to an increased prediction performance. The applicability of our model is demonstrated in qualitative interviews with domain experts based on individualized model suggestions. We show that the model can inspire materials scientists in their creative thinking process by predicting innovative combinations of topics that have not yet been investigated.

cs.LG↗

arXiv2025

Hydrogen response to high-density dislocations in bulk perovskite oxide SrTiO3

Hydrogen plays an increasingly important role in green energy technologies. For instance, proton-conducting oxides with high performance for fuel cell components or electrolysers need to be developed. However, this requires a fundamental understanding of hydrogen-defects interactions. While point defects and grain boundaries in oxides have been extensively studied, the role of dislocations as line defects remains less understood, primarily due to the challenge for effective dislocation engineering in brittle oxides. In this work, we demonstrate the impact of dislocations in bulk single-crystal perovskite oxide SrTiO3 on hydrogen uptake and diffusion using deuterium as tracer. Dislocations with a high density up to ~10 to the power of 14 per square meter were mechanically introduced at room temperature. Exposing this dislocation-rich and the reference regions (with a dislocation density of ~10 to the power of 10 per square meter) to deuterium at 400 °C for 1h, followed by secondary ion mass spectrometry measurements, we observed a ~100 times increase in deuterium incorporation in the dislocation-rich region. The result suggests that dislocations in oxides can act as an effective reservoir for deuterium. This proof-of-concept brings new insights into the emerging hydrogen-dislocation interactions in functional oxides.

cond-mat.mtrl-sci↗

arXiv2025

Drastic softening of Pd nanoparticles induced by hydrogen cycling

Single crystalline faceted Pd nanoparticles attached to a sapphire substrate were fabricated employing the solid state dewetting method. The as-dewetted nanoparticles tested in compression exhibited all features of dislocation nucleation-controlled plasticity, including the size effect on strength and ultrahigh compressive strength reaching up to 11 GPa. Hydrogen cycling of as-dewetted Pd nanoparticles resulted in their drastic softening and in change of the deformation mode. This softening effect was correlated with the high density of glissile dislocations observed in the cycled particles. This work demonstrates that the nanomechanical behavior of hydride-forming metals can be manipulated by hydrogen cycling.

cond-mat.mtrl-sci↗

arXiv2024

Columnar grain boundaries are the weakest link in hard coatings: Insights from micro-cantilever testing with bridge notches

The effect of columnar grain boundaries on the fracture toughness was investigated using micro-cantilever fracture testing with a bridge notch, and a unique hard coating consisting of two distinct microstructures: one with columnar grains and another with an epitaxial layer. The bridge-failure sequence qualitatively demonstrated the lower fracture toughness at the columnar-grained structure. Quantitatively, the load drops measured at bridge-failure also revealed a significant decrease in fracture toughness due to grain boundaries. Specifically, the fracture toughness decreased by around 30%, from 4.1 +/- 0.4 MPa m1/2 for epitaxial microstructure to 3.0 +/- 0.3 MPa m1/2 for columnar-grained structure. The fracture toughness of columnar-grained structure is 3.0 +/- 0.2 MPa m1/2 perpendicular to the growth direction higher than 2.7 +/- 0.1 MPa m1/2 along it. These findings suggest that future optimization of hard coatings should focus on grain boundary toughening, and the present toolbox proposes suitable techniques for such microstructure optimization.

physics.app-ph↗

arXiv2024

Redundant Cross-Correlation for Drift Correction in SEM Nanoparticle Imaging

Scanning Electron Microscopy (SEM) is a widely used tool for nanoparticle characterization, but long-term directional drift can compromise image quality. We present a novel algorithm for post-imaging drift correction in SEM nanoparticle imaging. Our approach combines multiple rapidly acquired, noisy images to produce a single high-quality overlay through redundant cross-correlation, preventing drift-induced distortions. The preservation of critical geometrical properties and accurate imaging of surface features were verified using Atomic Force Microscopy. On platinum nanoparticles with diameters of 300 to 1000 nm, significant improvements in the mean-based signal-to-noise ratio (SNR) were achieved, increasing from 4.4 dB in single images to 11.3 dB when overlaying five images. This method offers a valuable tool for enhancing SEM image quality in nanoparticle research and metrology, particularly in settings without specialized hardware-based drift correction.

cond-mat.mtrl-sci↗

arXiv2024

Data-efficient 4D-STEM in SEM: Beyond 2D Materials to Metallic Materials

Four-dimensional scanning transmission electron microscopy (4D-STEM) is a powerful tool that allows for the simultaneous acquisition of spatial and diffraction information, driven by recent advancements in direct electron detector technology. Although 4D-STEM has been predominantly developed for and used in conventional TEM and STEM, efforts are being made to implement the technique in scanning electron microscopy (SEM). In this paper, we push the boundaries of 4D-STEM in SEM and extend its capabilities in three key aspects: (1) faster acquisition rate with reduced data size, (2) higher angular resolution, and (3) application to various materials including conventional alloys and focused ion beam (FIB) lamella. Specifically, operating the MiniPIX Timepix3 detector in the event-driven mode significantly improves the acquisition rate by a factor of a few tenths compared to conventional frame-based mode, thereby opening up possibilities for integrating 4D-STEM into various in situ SEM testing. Furthermore, with a novel stage-detector geometry, a camera length of 160 mm is achieved which improves the angular resolution amplifying its utility, for example, magnetic or electric field imaging. Lastly, we successfully imaged a nanostructured platinum-copper thin film with a grain size of 16 nm and a thickness of 20 nm, and identified annealing twins in FIB-prepared polycrystalline copper using virtual darkfield imaging and orientation mapping. This work demonstrates the potential of synergetic combination of 4D-STEM with in situ experiments, and broadening its applications across a wide range of materials.

cond-mat.mtrl-sci↗

arXiv2023

Hydrogen embrittlement of twinning-induced plasticity steels: contribution of segregation to twin boundaries

Metallic materials, especially steel, underpin transportation technologies. High-manganese twinning induced plasticity (TWIP) austenitic steels exhibit exceptional strength and ductility from twins, low-energy microstructural defects that form during plastic loading. Their high-strength could help light-weighting vehicles, and hence cut carbon emissions. TWIP steels are however very sensitive to hydrogen embrittlement that causes dramatic losses of ductility and toughness leading to catastrophic failure of engineering parts. Here, we examine the atomic-scale chemistry and interaction of hydrogen with twin boundaries in a model TWIP steel by using isotope-labelled atom probe tomography, using tritium to avoid overlap with residual hydrogen. We reveal co-segregation of tritium and, unexpectedly, oxygen to coherent twin boundaries, and discuss their combined role in the embrittlement of these promising steels.

cond-mat.mtrl-sci↗

arXiv2022

Variable-wavelength quick scanning nano-focused X-ray microscopy for in situ strain and tilt mapping

Compression of micro-pillars is followed in situ by a quick nano-focused X-ray scanning microscopy technique combined with three-dimensional reciprocal space mapping. Compared to other attempts using 2 X-ray nanobeams, it avoids any motion or vibration that would lead to a destruction of the sample. The technique consists of scanning both the energy of the incident nano-focused X-ray beam and the in-plane translations of the focusing optics along the X-ray beam. Here, we demonstrate the approach by imaging the strain and lattice orientation of Si micro-pillars and their pedestals during in situ compression. Varying the energy of the incident beam instead of rocking the sample and mapping the focusing optics instead of moving the sample supplies a vibration-free measurement of the reciprocal space maps without removal of the mechanical load. The maps of strain and lattice orientation are in good agreement with the ones recorded by ordinary rocking-curve scans. Variable-wavelength quick scanning X-ray microscopy opens the route for in situ strain and tilt mapping towards more diverse and complex materials environments, especially where sample manipulation is difficult.

cond-mat.mtrl-sci↗

arXiv2021

Dissecting functional degradation in NiTi shape-memory-alloys containing amorphous regions via atomistic simulations

Molecular dynamics simulations are performed to provide a detailed understanding of the functional degradation of shape memory alloys at small scale. The origin of the experimentally reported accumulation of plastic deformation and the anomalous sudden increase of the residual strain under cyclic mechanical loading are explained by detailed insights into the relevant atomic scale processes. Our work reveals that the mechanical response of shape-memory-alloy pillars under cyclic compression is significantly influenced by the presence of an amorphous-like surface region as experimentally induced by focused ion beam milling. The main factor responsible for the observed degradation of superelasticity under cyclic loading is the accumulated plastic deformation and the resultant retained martensite originating from a synergetic contribution of the amorphous and crystalline shape-memory-alloy regions. We show that the reported sudden diminishment of the stress plateaus and hysteresis under cyclic loading is caused by the increased stability of the martensite phase due to the presence of the amorphous phase. Based on the identified mechanism responsible for the degradation, we validate reported methods of recovering the superelasticity and propose a new method to prohibit the synergetic contribution of the amorphous and crystalline regions, such as to achieve a sustainable operation of shape memory alloys at small scale.

cond-mat.mtrl-sci↗

arXiv2020

Faceting diagram for Ag segregation induced nanofaceting at an asymmetric Cu tilt grain boundary

In this work, we experimentally establish the isothermal nanofacet evolution at an asymmetric tilt grain boundary in the Cu-Ag system using a diffusion couple approach. We investigate the nanofacet formation along the grain boundary in dependence of the Ag solute excess concentration. The initial grain boundary dissociates into asymmetric Ag-lean segments and Ag-rich symmetric (210) segments. Increasing Ag excess leads to an increase in Ag-rich facet segment length, while the length of the asymmetric facets remains constant. From this, we construct a grain boundary nanofaceting diagram deduced from our experiments relating local atomic structure, overall inclination and Ag solute excess.

cond-mat.mtrl-sci↗