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Mudstones and shales are fine-grained sedimentary rocks that can serve as top seals of geological reservoirs in various geoenergy applications. Apart from traditional oil and gas exploration, the urgent need for underground storage of energy carriers (e.g., H2) and climate-relevant gases (e.g., CO2) facilitated extensive research on pore structural and mechanical parameters and their influence on the seal capacity of these rocks. The fracture behaviour of mudstone seal rocks controls the risk of seal failure due to microfracturing as a response to various geological processes (e.g., buoyancy pressure from the reservoir). In this contribution, the high-speed nanoindentation mapping approach was carried out for a proven mudstone top seal sample (~1629 m; quartz 31%, clay mineral 39%) from a Vienna Basin oil field. The nanoindentation results were then post-processed with machine learning-based tools to obtain representative mechanical parameters of the clay matrix. k-means clustering analysis was performed using three input features including hardness (H), reduced elastic modulus (Er), and the elastic-plastic deformation ratio based on the obtained load-displacement curves. In addition, broad ion beam-scanning electron microscopy (BIB-SEM) maps were taken before and after the nanoindentation to correlate the indentation results with direct imaging information and to verify the k-means clustering results. A total of 8 indentation map arrays (7 × 7 indents) were placed to test the sensitivity of different tips to indentation depth and load rate. The comparison of BIB-SEM image data and k-means clustering showed that decisions on phase assignment can be significantly improved and performed in a shorter time by k-means clustering analysis, still showing an overall good agreement with manual selections. For the studied mudstone sample, the resulting average Er and H values of the clay matrix range at 17.58 ± 6.89 GPa and 0.63 ± 0.76 GPa (n=30), respectively for the Berkovich tip and at 15.03 ± 4.79 GPa and 0.38 ± 0.23 GPa (n=62), respectively for the Cube Corner tip. The testing with both tips shows that despite the strongly heterogeneous microstructure of the indented clay matrix the obtained mechanical parameters are not sensitive to indentation depths and hence representative values can be determined from minimum volumes with statistical significance. Nevertheless, compared with the Berkovich tip, the Cube Corner tip sampled deeper depths (58-443 nm for the Berkovich tip and 412-1747 nm for the Cube Corner tip) and introduced more surface damage. By increasing the load rate from 1000 to 6000 μN s−1, the indentation testing tended to be unstable and the surface showed strong damage at the highest load rate. In conclusion, this contribution represents an important methodological step towards the implementation of combined high-speed nanoindentation mapping and machine learning data analysis as a feasible high throughput tool for the mechanical characterization of mudstones and similar fine-grained sedimentary rocks. The presented approach is planned to be applied to an extensive set of mudstone samples from the Vienna Basin with the purpose to link mechanical property changes to burial diagenesis.

We report a length-scale-controlled Brittle-Ductile Transition giving rise to significant toughening of a commonly brittle material. Using quantitative in-situ Transmission Electron Microscopy (TEM) fracture experiments at room temperature on single crystal Silicon, we find that large samples fracture concordant with the brittle bulk behavior at a stress intensity K I C ~ 1 M P a . m 1 / 2 . Below characteristic dimensions of about 250 nm, however, the fracture toughness strikingly increases inversely with size to at least triple. As evidenced from advanced in-situ TEM nanoscale strain mapping the stresses at the crack tip approach the theoretical strength. At the same time, below this critical transition length nucleation and propagation of dislocations was observed, shielding the crack tip and enabling the unprecedented rise in fracture toughness. These first time in-situ TEM observations in nanoscale Silicon at room temperature open new strategies to simultaneously strengthen and toughen indispensable yet brittle functional materials solely by geometrical miniaturization.

Abstract Due to their outstanding properties, ultra-fine-grained tungsten and its alloys are promising candidates to be used in harsh environments, hence it is crucial to understand their high temperature behavior and underlying deformation mechanisms. Therefore, advanced nanoindentation techniques were applied to ultra-fine-grained tungsten–rhenium alloys up to 1073 K. A continuous hardness decrease up to 0.2 $$T_{\text{m}}$$ T m is rationalized by a still dominating effect of the Peierls stress. However, the absence of well-established effects of Rhenium alloying, resulting in a reduced temperature dependence of strength for coarse-grained microstructures, was interpreted as an indication for a diminishing role of kink-pair formation in ultra-fine-grained metals with sufficiently fine grain size. Despite slight grain growth in W, dislocation–grain boundary interaction was identified as the dominating deformation mechanism above 0.2 $$T_{\text{m}}$$ T m . Interaction and accommodation of lattice dislocations with grain boundaries was affected by a reduced boundary diffusivity through alloying with Re. Graphic abstract

Bulk nanoporous tungsten as an extremely strong and low density nanocrystalline material was for the first time created to satisfy the need for advanced high performance materials that can endure harsh environments. Synthesis of nanoporous tungsten was achieved by a unique procedure involving severe plastic deformation of a coarse-grained tungsten-copper composite followed by selective dissolution of the nobler copper phase. The used ammonium persulfate etching solution, in which the less noble tungsten is chemically stable, is proved to be effective in removing the nobler copper phase. A nanoporous microstructure characterized by a network of interconnected nanocrystalline tungsten ligaments and interconnected nanopores was obtained. Based on a high-resolution interface analysis, the underlined mechanisms for formation of the nanoporous tungsten structure were elucidated. Moreover, using nanoindentation we demonstrate that, due to the nanoscale microstructure, the created nanoporous tungsten possesses outstanding strength, making it an attractive material for applications in radiation shielding.

Here we employ the outcome of combining the abovementioned processes. Spark ablation process was used to decorate the printed 3D surface to yield specific surfaces with metal/metal oxide core shell nanoparticles. Broad characterization was applied using microscopy (SEM, AFM), mechanical testing (in situ SEM mechanical testing), diffraction analysis (XRD) and electrical characterization (J/V) before and after the assembly of complete solar cells. Namely such formations were found to be prosperous for electron transport layers in perovskite solar cells.

The lifetime and performance of any engineering component, from nanoscale sensors to macroscopic structures, are strongly influenced by fracture processes. Fracture itself is a highly localized event; originating at the atomic scale by bond breaking between individual atoms close to the crack tip. These processes, however, interact with defects such as dislocations or grain boundaries and influence phenomena on much larger length scales, ultimately giving rise to macroscopic behavior and engineering-scale fracture properties. This complex interplay is the fundamental reason why identifying the atomistic structural and energetic processes occurring at a crack tip remains a longstanding and still unsolved challenge. We develop a new analysis approach for combining quantitative in-situ observations of nanoscale deformation processes at a crack tip with three-dimensional reconstruction of the dislocation structure and advanced computational analysis to address plasticity and fracture initiation in a ductile metal. Our combinatorial approach reveals details of dislocation nucleation, their interaction process, and the local internal stress state, all of which were previously inaccessible to experiments. This enables us to describe fracture processes based on local crack driving forces on a dislocation level with a high fidelity that paves the way towards a better understanding and control of local failure processes in materials.

Scripta materialia 211, 114492 (2022). doi:10.1016/j.scriptamat.2021.114492, Owing to their combination of high hardness and increased fracture toughness, CrTaN coatings have re- cently gained increasing interest as suitable candidates for metal cutting applications. However, up to now, the detailed mechanisms underlying the oxidation of this promising coating system are not thor- oughly understood. Thus within this work, the evolution of microstructure and phase composition of a cathodic arc evaporated Cr$_{0.74}$Ta$_{0.26}$N coating were illuminated in ambient atmosphere up to 1400 ��C. In situ high-energy X-ray diffraction showed that powdered face-centered cubic (fcc) CrTaN displays an excellent oxidation resistance up to ���1050 ��C, where the formation of tetragonal (t) CrTaO$_4$ and rhom- bohedral (r) Cr$_2$O$_3$ sets in. The compact CrTaN deposited on sapphire subjected to 1225 ��C in ambient atmosphere exhibits intact fcc-CrTaN regions near the substrate, a porous intermediate layer of r-Cr$_2$O$_3$ and t-CrTaO$_4$ and a dense r-Cr$_2$O$_3$ oxide scale at the surface., Published by Elsevier Science, Amsterdam [u.a.]

Knowledge about the adhesion of protective hard coatings on tool materials is of great importance to understand their failure mechanisms in metalworking. Until now, common techniques such as scratch and indentation tests are used to establish a qualitative ranking of a coating’s adhesion on various substrate materials. Nevertheless, there is a lack of quantitative measures to describe the strength of the interfaces between individual microstructural constituents of substrate-coating composites. The current work investigates the interfacial strength and thus the adhesion of TiN deposited as a hard coating on an MC-type carbide, an M6C-type carbide and on martensite being constituents of high speed steels. Tensile stresses were introduced at the interface between TiN and the individual microstructural constituents of a high speed steel via micromechanical testing of a novel MSC specimen within a scanning electron microscope. The tested MSC specimens were subsequently investigated in detail by scanning electron microscopy. Evaluation of the interface stress at fracture via finite element analysis yielded a ranking in interface strength and therefore coating adhesion in a sequence from high to low strength values from MC/TiN over M6C/TiN to martensite/TiN.

In order to quantify the fracture toughness and reveal the failure mechanism of crystalline/amorphous nanolaminates (C/ANLs), in-situ micro-cantilever bending tests were performed on Ag/Cu–Zr and Mo/Cu–Zr C/ANLs in a scanning electron microscope over a wide range of cantilever widths from several microns to the submicron scale. The results demonstrate that the fracture behavior was strongly influenced by sample size and constituent phases, respectively. The Ag/Cu–Zr micro-cantilevers failed in a ductile manner, with fracture toughnesses higher than the Mo/Cu–Zr samples that exhibited brittle failure. Both materials also displayed different cantilever width-dependences of fracture toughness. The Ag/Cu–Zr beams showed a fracture toughness that increases with the cantilever width, mainly due to a size-dependent constraining effect on the deformation of the crystalline phase. For the Mo/Cu–Zr beams, the fracture toughness decreased gradually to a low plateau as the cantilever width exceeded ∼1500 nm, which can be rationalized by a transition in stress condition. The underlying fracture mechanism of the Ag/Cu–Zr micro-cantilevers was identified as the interconnection of microcracks initiated in the amorphous Cu–Zr layers, compared to a catastrophically penetrating crack propagation in the Mo/Cu–Zr samples. The discrepancy in size-dependent fracture behavior between the two material systems is discussed in terms of plastic energy dissipation of ductile phases, crack tip blunting, crack bridging and the effect of strain gradient in the plastic zone on crack propagation.

Nanocrystalline materials exhibit various properties or phenomena not common in the conventional grain size regime, including enhanced strain rate sensitivity for FCC metals or strength increase during recovery annealing. These peculiarities are associated with the enhanced confinement of plasticity. Accordingly, the interaction of dislocations with the numerous grain boundaries, the boundary state as well as its local chemistry are of importance for a complete understanding. Due to the various influencing factors, determination of the dominant and rate controlling processes remains challenging. Here, we present a study on selected nanostructured FCC materials where dislocation-grain boundary interactions have been studied earlier for their coarse grained counterparts. High temperature nanoindentation revealed for all materials a pronounced increase of strain rate sensitivity when exceeding a certain temperature, peaking prior to the occurrence of significant grain growth. Static annealing of samples close to these peak temperatures leads, for sufficiently small grain sizes, to a maximum hardness increase. Interestingly, despite the nanocrystalline grain size, these temperatures perfectly agree with those obtained earlier for annihilation of lattice dislocations at grain boundaries in coarse-grained samples. This suggests that at elevated temperatures the dominant mechanism controlling the enhanced rate sensitivities in nanocrystalline metals is the thermally activated annihilation of lattice dislocations at grain boundaries. Measurements of activation energies being close to reported values for grain boundary diffusion further support this concept.

Miniaturized materials, in general, exhibit higher strength compared to their bulk counterparts. As a consequence, their resistance to fracture is often compromised. However, the effect of material inhomogeneities can be used to significantly improve the fracture toughness of thin film components. In this work, the material inhomogeneity effect on the crack driving force, caused by material property and residual stress variations in thin tungsten and copper stacks, is numerically investigated. To this purpose, a finite element analysis is performed using the concept of configurational forces. In this way, we are able to distinguish between the various inhomogeneity effects and draw conclusions about the effective crack driving force. It is demonstrated that the material inhomogeneity effect is not solely determined by the material property variations at the interfaces, since an important contribution emerges due to a smooth residual stress gradient within the layers. The possibility to separate the different effects represents an opportunity for cost efficient design of future reliable thin film microelectronic components.

Manipulating the twin morphology to achieve the reinforcement of strength is a great challenge in Al with high stacking fault energy. In this work, the influence of Zr addition on the twin morphology and strengthening response of nanostructured Al films was symmetrically studied. The results showed that, for low Zr addition (≤ 4.0 at.%), the Zr atoms were homogeneously distributed within the matrix, while Zr segregation at grain boundaries was evident at higher Zr addition (> 4.0 at.%). Twins were substantially observed in all the films, and the twin morphology was highly dependent on the Zr addition. In the pure Al film, only twins with a single coherent twin boundary were detected. In comparison, nanotwins with coplanar twin boundaries (C-nanotwins) and 9R phase were predominant in the Al-Zr films with Zr addition ≤ 4.0 at.%. Further raising the Zr content, multiple nanotwins (M-nanotwins) coexisted with the C-nanotwins and 9R phase. In particular, a zero-strain twinning mechanism was applied to account for the C-nanotwins and 9R phase formation, and a zig-zag grain boundary feature induced by Zr segregation was responsible for the M-nanotwin formation. The hardness also exhibited a strong Zr dependence that increased monotonically with the Zr addition. The Al-13.4 at.% Zr film displayed a hardness of ~4.3 GPa, about 11 times greater than the pure Al film. Strengthening mechanisms were quantitatively evaluated, and the highly-promoted hardness was mainly ascribed to the 9R phase and solid solution strengthening.

ZnO is a semiconductor having large exciton binding energy, high electron mobility [1] and recently ability to prepare and describe its films in low-dimensional configurations. Desirable 1D configuration greatly facilitate boosting some of the materials physical properties such as efficiency of the charge transfer. For synthesis purpose, physical deposition techniques are proven to be successful but many of them require demanding conditions. On the contrary, chemical processing such as chemical bath route enables the large-scale fabrication of wellaligned ZnO nanorods at mild temperatures [2]. For characterization purpose, conventional methods fail to provide a full understanding of the investigated material, thereof it was elucidated that more detailed insight in mechanical characteristics (hardness and elastic modulus) is highly desirable, particularly in the nanoscale regime, where nanoindentation method was shown to be a flexible and useful tool [3]. Here we prepared 1D nanostructured zincite thin- films and combine investigation of their hardness and elastic modulus as well as fracture toughness with advanced imaging techniques. For characterization we focus on the in-situ micromechanical experiments with advanced nanoindentation techniques (at elevated temperatures) in the scanning and transmission electron microscopy (SEM/TEM). The course of characterization was assisted by focused ion beam (FIB) based material structuring and digital image correlation techniques to observe, describe and understand occurrence of local deformations and generally microstructural evolution. We were able to conduct a miniaturized fracture testing within the SEM and TEM apparatus. Results were utilized to qualify and quantify the materials resistance to mechanical failure such as undesirable cracking events. The mitigation of such events should enable major development role of the nanoscale optoelectronic devices. [1] A. Janotti, C. G. Van De Walle, Reports Prog. Phys. 72 (2009) 126501. [2] L. Schmidt-Mende, J. L. MacManus-Driscoll, Materials today 10 (2007) 40. [3] X.Y. Tao, X.D. Li, Nano Lett. 8 (2008) 505.

Nanoindentation is a valuable tool, which enables insights into the material properties of natural, highly inhomogeneous composite materials such as shales and organic matter-rich rocks. However, the inherent complexity of these rocks and its constituents complicates the extraction of representative material parameters such as the reduced elastic modulus (Er) and hardness (H) for organic matter (OM) via nanoindentation. The present study aims to extract the representative H and Er values for OM within an over-mature sample set (1.33-2.23%Rr) from the Chinese Songliao Basin and evaluate influencing factors of the resulting parameters. This was realized by means of high-speed nanoindentation mapping in combination with comprehensive optical and high resolution imaging methods. The average Er and H values for the different particles range from 3.86 +/- 0.17 to 7.52 +/- 3.80 GPa and from 0.36 +/- 0.02 to 0.64 +/- 0.09 GPa, respectively. The results were subsequently processed by the unsupervised machine learning algorithm k-means clustering in order to evaluate representative Er and H results. The post-processing suggests that inherent heterogeneity of OM is responsible for considerable data scattering. In fact, surrounding, underlying and inherent mineral matter lead to confinement effects and enhanced Er values, whereas cracks and pores are responsible for a lowered stiffness. Adjusted for these influencing factors, a declining trend with increasing maturity (up to 1.96%Rr) could be observed for Er, with average values calculated from representative clusters ranging from 5.88 +/- 0.37 down to 4.07 +/- 0.32 GPa. Er slightly increases again between 2.00 and 2.23%Rr (up to 4.85 +/- 0.35 GPa). No clear relationship of H with thermal maturity was observed. The enhanced accuracy archived by a large data set facilitated machine learning approach not only improves further modelling attempts but also allows insights of impacting geological processes on the material parameter and general understanding of mechanical behavior of OM in rock formations. Thus, the presented multimethod approach promotes a fast and reliable assessment of representative material parameters from organic rock constituents.

The aim of the current research work is to understand how individual local microstructure features such as grain size, grain orientation, grain boundary characteristics, as well as the global parameters of the microstructure (texture, grain size distribution, twin boundary fraction) influence the process of fatigue damage initiation in thin films. Cyclic strain with an amplitude of 1% was applied to polymer-supported ultrafine grained gold and copper films allowing for observation of numerous independent localized plasticity events within a single macrosample. Detailed electron backscatter diffraction analysis of the microstructure in the vicinity of these plasticity events was performed to gain statistical information and elucidate robust correlations. On the local scale, the damage was initiated in large grains (>1 µm) in the vicinity of coherent twin boundaries specifically oriented with respect to the loading direction. When appropriate large grains were not present in the initial microstructure, mechanically induced grain coarsening preceded fatigue damage initiation. If the initial microstructure did not contain properly oriented coherent twin boundaries, then the local grain size of dynamically coarsened grains was the sole parameter controlling plastic slip localization. On the global scale, the major parameters of the microstructure influencing the fatigue damage initiation process are the fraction of twin boundaries, texture, and the width of the grain size distribution. Based on the presented results a clear strategy for improving the fatigue life of thin films is proposed.

Dislocation-grain boundary interaction is widely accepted as the rate-controlling process for ultra-fine grained bcc metals in their high temperature deformation regime above the critical temperature. However, the influence of different types of grain boundaries remains widely unexplored so far. To this end we present here an advanced high temperature nanoindentation study on Ta2.5W specimens consisting of two distinctively different grain boundary types, but with similar submicron average spacing. While one set of samples consisted of a predominant fraction of high-angle boundaries, the second set contained mainly low-angle boundaries. Fully recrystallized samples served as a coarse grained reference batch. Using advanced nanoindentation at elevated temperatures up to 823 K, we find a temperature invariant hardness in the case of the low- and a strongly pronounced temperature dependence for the high-angle grain boundary samples. This underlines the importance of grain boundary diffusivity for the predominant process of interfacial stress relaxation. Pronounced interaction of dislocations with oxygen impurity atoms was observed from 473 to 773 K for the coarse grained microstructure, yielding serrated flow as an indicator for a Portevin-Le Chatelier effect up to 573 K. Both grain boundary types showed a significant influence to the dislocation-impurity interaction, whereby the high-angle grain boundaries suppress discrete flow characteristics.

Hydrogen embrittlement (HE) is one of the main limitations in the use of advanced high-strength steels in the automotive industry. To have a better understanding of the interaction between hydrogen (H) and a complex phase steel, an in-situ method with plasma charging was applied in order to provide continuous H supply during mechanical testing in order to avoid H outgassing. For such fast-H diffusion materials, only direct observation during in-situ charging allows for addressing H effects on materials. Different plasma charging conditions were analysed, yet there was not a pronounced effect on the mechanical properties. The H concentration was calculated while using a simple analytical model as well as a simulation approach, resulting in consistent low H values, below the critical concentration to produce embrittlement. However, the dimple size decreased in the presence of H and, with increasing charging time, the crack propagation rate increased. The rate dependence of flow properties of the material was also investigated, proving that the material has no strain rate sensitivity, which confirmed that the crack propagation rate increased due to H effects. Even though the H concentration was low in the experiments that are presented here, different technological alternatives can be implemented in order to increase the maximum solute concentration.

Bulk-metallic glasses (BMGs) are a class of structural materials with many attractive processing features such as the ability to be processed into parts with fine features, dimensional precision, and repeatability; however, their fracture behavior is complex and size-dependent. Previous work has shown that BMGs can display strong size effects on toughness, where multiple mechanisms on different length-scales, e.g., crack bridging and bifurication, shear band spacing and length, can significantly affect the properies. This length-scale dependence on the fracture toughness has importance not only for advancing the understanding of fracture processes in these materials, but also for the potential future applications of BMGs, such as for microdevices. Here, using in situ scanning electron microscopy (SEM), we report on notched micro-cantilever bending experiments to address the lack of data regarding fracture properties of BMGs at the microscale. Sudden catastrophic propagation of shear bands resulted in failure for these specimens at stress intensities much lower than the bulk material, which may be due to a lack of extrinsic toughening mechanisms at these dimensions. This is explored further with post mortem SEM and transmission electron microscopy (TEM) analysis of the fractured beams while the fracture toughness results are verified using finite element modeling. The excellent agreement between model and micro cantilever beam bending experiments suggests that the intrinsic fracture toughness of Vitreloy 105, 9.03±0.59 MPa.m½, is being reported for the first time.

An in situ hydrogen (H) plasma charging and in situ observation method was developed to continuously charge materials, while tensile testing them inside a scanning electron microscope (SEM). The present work will introduce and validate the setup and showcase an application allowing high-resolution observation of H-material interactions in a Ni-based alloy, Alloy 718. The effect of charging time and pre-straining was investigated. Fracture surface observation showed the expected ductile microvoid coalescence behavior in the uncharged samples, while the charged ones displayed brittle intergranular and quasi-cleavage failure. With the in situ images, it was possible to monitor the sample deformation and correlate the different crack propagation rates with the load-elongation curves. H-charging reduced the material ductility, while increasing pre-strain decreased hydrogen embrittlement susceptibility due to the possible suppression of mechanical twinning during the tensile test and, therefore, a reduction in H concentration at grain and twin boundaries. All the presented results demonstrated the validity of the method and the possibility of in situ continuously charging of materials with H without presenting any technical risk for the SEM.

The study of grain boundaries (GBs) in polycrystalline materials is a field of major interest, as many fundamental properties are influenced by their actual structure. One of the main challenges in investigating GBs is that electron microscopy techniques capable to resolve their atomistic arrangement require very small sample volumes of a few tens of nanometers and might therefore change the natural state of the GB via removal of occurring material constraint. To counteract this influence one could apply indirect measurements such as internal friction to probe for changes of the GB structure. However, with the drive towards smaller volumes most of these techniques are at their limit. The current work proposes a new approach based on mechanical spectroscopy of micron sized specimens applied in-situ in a scanning electron microscope. Applying this novel concept to ultra-fine grained Ta we investigate changes in the defect structure between the as-deformed condition and after a heat treatment. In fact, we detect a decrease in internal friction of about 50% upon annealing. This pronounced change occurs in conjunction with an increase in flow stress and hardness and is related to thermally induced GB relaxation.