In this study, we examined the beam-irradiation effect on the structural evolution of the grain boundary (GB) in a Cu bicrystal at room temperature using a C
-corrected, monochromated transmission ...electron microscope at an acceleration voltage of 300 keV. Faceting of the GB was observed at a low current density of the electron beam. With increasing current density, the GB became defaceted. The faceting-roughening transition was shown to be reversible, as the process was reversed upon decreasing the current density. The structural transition is explained by inelastic scattering effects by electron-beam irradiation.
Multiple applications of lithium‐ion batteries in energy storage systems and electric vehicles require highly stable electrode materials for long‐term battery operation. Among the various cathode ...materials, high‐Ni cathode materials enable a high energy density. However, cathode degradation accompanied by complex chemical and structural changes results in capacity and voltage fading in batteries. Cathode degradation remains poorly understood; the majority of studies have only explored the oxidation states of transition‐metal ions in localized areas and have rarely evaluated chemical degradation in complete particles after prolonged cycling. This study systematically investigates the degradation of a high‐Ni cathode by comparing the chemical, structural, and electrical changes in pristine and 500 times‐cycled cathodes. Electron probe micro‐analysis and X‐ray energy dispersive spectroscopy reveal changes in the Ni:O ratio from 1:2 to 1:1 over a large area inside the secondary particle. Electron energy loss spectroscopy analysis related to structural changes is performed for the entire primary particle area to visualize the oxidation state of transition‐metal ions in two dimensions. The results imply that the observed monotonic capacity fade without unusual changes is due to the continuous formation of the Ni2+ phase from the surface to the bulk through chemical and structural degradation.
The degradation of the high‐Ni layered oxide cathode is systematically investigated by comparing a pristine and 500‐cycled full‐cell through analyzing the microstructural visualization of the structural, chemical, and electrical changes from micrometer‐scale to the nanometer‐scale. This reveals that the monotonic capacity fade is due to the continuous formation of Ni2+ phase at the surface and bulk via chemical and structural degradation.
The growing popularity of Li-ion batteries in the field of energy storage necessitates the constant development of methods for improving their performance. To address this task, this study describes ...the effect of transition metal (TM=Al, Co, Fe) phosphate coatings on the electrochemical performance of Li1.0Ni0.8Co0.15Mn0.05O2 positive electrode materials, highlighting the importance of the TM/P mole ratio, and reveals that decreased amounts of Li residuals were observed for all coated samples. Considering Li residual removal as well as capacity and capacity retention upon cycling, the most effective surface modification was achieved in the case of a 1:1 Co/P coating, which increased initial capacity and capacity retention by 10mAhg−1 and 3%, respectively, as compared to the uncoated sample. Moreover, the beneficial effect of the above Co/P coating is also confirmed for the more Ni-rich Li1.0Ni0.91Co0.06Mn0.03O2.
This study derives a uniaxial tensile flow from spherical indentation data using an artificial neural network (ANN) combined with finite element (FE) analysis. The feasibility of the FE-based ...simulations is confirmed through experimental indentation for various steels. Parametric studies of the FE simulation are performed to generate an ANN training database. An encoding for feature extraction and a hyperparameter optimization is implemented to design the ANN with high predictive performance. The indentation load–depth curves are converted into hardening parameters through the trained ANN. The predictive performance of the FE–ANN model using real-life indentation data is investigated in-depth with thorough error evaluation, and verified by uniaxial tensile tests. The emphasis is made that the mean absolute percentage error between the experimental and simulated indentation data is required to be meticulously controlled below 1% to accurately predict the tensile properties. The validations demonstrate that the applied FE–ANN modeling approach is very robust and captures the tensile properties well. Furthermore, the Taguchi orthogonal array (OA) method that can achieve high efficiency and fidelity with less training data is discussed. The FE–ANN model is concisely designed using the Taguchi OA method and can predict elasticity as well as plasticity.
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•The finite element-Artificial neural network (FE-ANN) modeling approach is used to predict uniaxial tensile flow from real indentation data.•Encoding of input features and hyperparameter optimization are performed for the high performance of the ANN.•The FE-ANN model performance is outstanding when the error between measured and simulated indentation data is meticulously controlled.•Taguchi orthogonal array (OA) method provides high efficiency and accuracy in prediction with reduced training data pairs.•The concisely designed FE-ANN model using the Taguchi OA can cover broader parameter spaces in limited memories.
In this paper we discuss the transformation of a sheet of material into a wide range of desired shapes and patterns by introducing a set of simple cuts in a multilevel hierarchy with different ...motifs. Each choice of hierarchical cut motif and cut level allows the material to expand into a unique structure with a unique set of properties. We can reverse-engineer the desired expanded geometries to find the requisite cut pattern to produce it without changing the physical properties of the initial material. The concept was experimentally realized and applied to create an electrode that expands to >800% the original area with only very minor stretching of the underlying material. The generality of our approach greatly expands the design space for materials so that they can be tuned for diverse applications.
Abstract
Steel is the global backbone material of industrialized societies, with more than 1.8 billion tons produced per year. However, steel-containing structures decay due to corrosion, destroying ...annually 3.4% (2.5 trillion US$) of the global gross domestic product. Besides this huge loss in value, a solution to the corrosion problem at minimum environmental impact would also leverage enhanced product longevity, providing an immense contribution to sustainability. Here, we report a leap forward toward this aim through the development of a new family of low-density stainless steels with ultra-high strength (> 1 GPa) and high ductility (> 35%). The alloys are based on the Fe–(20–30)Mn–(11.5–12.0)Al–1.5C–5Cr (wt%) system and are strengthened by dispersions of nano-sized Fe
3
AlC-type κ-carbide. The alloying with Cr enhances the ductility without sacrificing strength, by suppressing the precipitation of κ-carbide and thus stabilizing the austenite matrix. The formation of a protective Al-rich oxide film on the surface lends the alloys outstanding resistance to pitting corrosion similar to ferritic stainless steels. The new alloy class has thus the potential to replace commercial stainless steels as it has much higher strength at similar formability, 17% lower mass density and lower environmental impact, qualifying it for demanding lightweight, corrosion resistant, high-strength structural parts.
Strain rate sensitivity (SRS) of a face-centered cubic (fcc) CoCrFeNi high-entropy alloy (HEA) with grain sizes ranging from 57 μm to 45 nm was investigated using nanoindentation, and was compared ...with those reported for conventional fcc metals. Experimental results show pronounced grain boundary strengthening in the HEA. Estimated values of the SRS parameter, m and activation volume, V*, indicate similar plastic deformation mechanisms in HEA and Ni in nanocrystalline regime that are grain boundary mediated. In coarse-grained regime, the high lattice friction stress in the HEA results in much higher m and smaller V* as compared to coarse grained Ni.
A series of Fe-30 wt%Mn-10.5 wt%Al-1.1 wt%C steels with Mo addition from 0 to 5 wt% were prepared to investigate the effect of Mo on the microstructure and tensile deformation behavior of austenitic ...lightweight steel. When the Mo content was below 4 wt%, the microstructure of solution-treated samples consisted of austenite and κ–carbide, while Mo-enriched M6C and M23C6 carbides were additionally precipitated in samples containing Mo more than 4 wt% during a solution treatment at 1050 °C. These carbides inhibited austenite grain growth during the solution treatment, resulting in significant grain refinement in the samples containing more that 4 wt% of Mo. Tensile test results showed that the yield strength gradually decreased with an increase in the Mo content up to 3 wt% due to the suppression of κ–carbide precipitation, whereas it significantly increased when the Mo content exceeded 4 wt% due to grain refinement and precipitation strengthening caused by Mo-enriched carbides. During the tensile deformation, the strain hardening rates of all alloys increased and then the deformation mode subsequently changed with an increase in the Mo content from shearband-induced plasticity (SIP) to microband-induced plasticity (MBIP). Finally, the change in the κ-carbide precipitation behavior upon the addition of Mo and its effect on the deformation behavior were carefully analyzed and discussed through nanoindentation experiments, first-principles calculations and atom probe tomography analyses.
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High strain rate tensile tests were conducted for three advanced high strength steels: DP780, DP980 and TRIP780. A high strain rate tensile test machine was used for applying the strain rate ranging ...from 0.1/s to 500/s. Details of the measured stress–strain responses were comparatively analyzed for the DP780 and TRIP780 steels which show similar microstructural feature and ultimate tensile strength, but different strengthening mechanisms. The experimental observations included: usual strain rate dependent plastic flow stress behavior in terms of the yield stress (YS), the ultimate tensile strength (UTS), the uniform elongation (UE) and the total elongation (TE) which were observed for the three materials. But, higher strain hardening rate at early plastic strain under quasi-static condition than that of some increased strain rates was featured for TRIP780 steel, which might result from more active transformation during deformation with lower velocity. The uniform elongation that explains the onset of instability and the total elongation were larger in case of TRIP steel than the DP steel for the whole strain rate range, but interestingly the fracture strain measured by the reduction of area (RA) method showed that the TRIP steel has lower values than DP steel. The fractographs using scanning electron microscopy (SEM) at the fractured surfaces were analyzed to relate measured fracture strain and the microstructural difference of the two materials during the process of fracture under various strain rates. Finally, constitutive modeling for the plastic flow stresses under various strain rates was provided in this study. The proposed constitutive law could represent both Hollomon-like and Voce-like hardening laws and the ratio between the two hardening types was efficiently controlled as a function of strain rate. The new strength model was validated successfully under various strain rates for several grades of steels such as mild steels, DP780, TRIP780, DP980 steels.
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