Transmission Electron Microscopy Study of Deformation Processes in Metallic Glasses

Amorphous metallic alloys known as metallic glasses exhibit remarkable mechanical strength, elasticity, and resistance to wear when compared to their crystalline counterparts. These intriguing attributes have generated considerable interest in utilizing them for engineering materials over the past decades. Nevertheless, a significant drawback of metallic glasses lies in their limited ductility, which causes them to undergo abrupt yielding when undergoing plastic deformation. This plastic deformation of metallic glasses primarily occurs through the creation of shear bands, brought by work-softening nature of glasses. As regions within the material experience plastic deformation, they become softer, promoting localized strain accumulation within a narrow band-like zone, so-called shear band. Unfortunately, the sudden emergence of shear bands contributes to the premature failure of metallic glasses and hinders their toughness. Hence, comprehending the mechanisms that give rise to shear band formation becomes pivotal in constructing the theory of glass deformation and enhancing the mechanical stability of metallic glasses. However, recent advancements in the deformation mechanisms of metallic glasses have predominantly leaned on simulations, as experimentally characterizing the amorphous phases and nanoscale volumes within shear bands comes with substantial challenges. The lack of experimental observations concerning the structures implicated in the deformation of metallic glasses has restricted research findings to a hypothetical level, stalling the progress in novel material development. This thesis focuses on an experimental investigation of deformed structures of metallic glasses using transmission electron microscopy (TEM) techniques, particularly four-dimensional (4D) scanning-TEM (STEM). The study incorporates methodological advancements, such as developing correlative mapping of nanoscale strain fields and atomic packing structure of glasses using 4D-STEM and Lorentz 4D-STEM, enabling the correlation of atomic structure and magnetic information. Machine learning analysis is applied to extract principal and correlated information from the 4D-STEM dataset. This development allows for direct experimental observations and detailed examination of the deformed structures in metallic glasses. The research outcomes establish an experimental foundation for understanding the formation of an individual shear band and the multiplication of shear bands. This is achieved through direct observations of strain concentrations, shear bands, shear band-affected zones (SBAZs), and local heterogeneity within a deformed glass matrix. Structure-property correlations in metallic glasses are discussed based on these microscopic observations. This new methodology is expected to open up extensive research possibilities for addressing questions in amorphous materials

Properties of Central Caustics in Planetary Microlensing

To maximize the number of planet detections, current microlensing follow-up observations are focusing on high-magnification events which have a higher chance of being perturbed by central caustics. In this paper, we investigate the properties of central caustics and the perturbations induced by them. We derive analytic expressions of the location, size, and shape of the central caustic as a function of the star-planet separation, $s$, and the planet/star mass ratio, $q$, under the planetary perturbative approximation and compare the results with those based on numerical computations. While it has been known that the size of the planetary caustic is \propto \sqrt{q}, we find from this work that the dependence of the size of the central caustic on $q$ is linear, i.e., \propto q, implying that the central caustic shrinks much more rapidly with the decrease of $q$ compared to the planetary caustic. The central-caustic size depends also on the star-planet separation. If the size of the caustic is defined as the separation between the two cusps on the star-planet axis (horizontal width), we find that the dependence of the central-caustic size on the separation is \propto (s+1/s). While the size of the central caustic depends both on $s$ and q, its shape defined as the vertical/horizontal width ratio, R_c, is solely dependent on the planetary separation and we derive an analytic relation between R_c and s. Due to the smaller size of the central caustic combined with much more rapid decrease of its size with the decrease of q, the effect of finite source size on the perturbation induced by the central caustic is much more severe than the effect on the perturbation induced by the planetary caustic. Abridged.Comment: 5 pages, 4 figures, ApJ accepte

Forest Cover Changes in North Korea Since the 1980s

North Korea used to have abundant forest stocks but underwent substantial deforestation and degradation of forest in recent decades. This study examined morphological changes of forest cover in North Korea between the 1980s and 2000s. Land cover data based on Landsat TM imagery were obtained as images from the Republic of Korea’s Ministry of Environment. The images were processed and used for the Morphological Spatial Pattern Analysis and network analysis. MSPA classified the forest cover into morphological classes such as core, islet, bridge, perforation, edge, loop, and branch. The network analysis identified individual networks of forest, each of which represents a patch of connected forest. The results are summarized as follows: (1) Forest cover sharply decreased between the 1990s and 2000s, particularly in western provinces; (2) Morphological classes indicating forest fragmentation such as islet, branch and edge consistently increased in their fraction to the total area between the 1980s and 2000s; (3) Islet, branch and edge also increased in number during the same period; (4) Forest networks shrank in size and increased in number. Overall, the results demonstrate that deforestation and fragmentation of forest occurred simultaneously in North Korea during the time

Mcredit2: Enhanced High-Performance Xen Scheduler via Dynamic Weight Allocation

Generally, operating only a single host on a single server results in hardware underutilization. Thus, hypervisors (e.g., Xen) have been developed to allow several hosts to operate on a single server. The Xen hypervisor provides processor schedulers (e.g., Credit and Credit2 schedulers) to assign processors to each host. The Credit2 scheduler provides work assurance to the domain relative to latency and it evenly assigns processors to each domain. In addition, the Credit2 scheduler can assign a weight value to each host. A greater host weight value allows processors to be assigned to a host for longer periods. However, the Credit2 scheduler shows poorer performance than the basic Credit scheduler, which utilizes idle processors. In this paper, we propose the Mcredit2 scheduler, which improves the Credit2 scheduler. The Credit2 scheduler takes no action when the load on a specific domain causes increased processor usage. The proposed Mcredit2 scheduler allows a domain to quickly process loads by temporarily assigning a greater weight value to a host with high processor usage. In addition, we introduce a processor monitoring tool that visualizes the processor usage

Characteristics of Deforestation in the Democratic People’s Republic of Korea (North Korea) between the 1980s and 2000s

There has been a significant lack of land cover change studies in relation to deforestation in the Democratic People’s Republic of Korea (North Korea). The purpose of this study is to characterise deforestation in North Korea through land cover change trajectory and spatial analysis. We used three 30-m gridded land cover data sets for North Korea representing the conditions of the late 1980s, 1990s, and 2000s, respectively, as well as a digital elevation model. We examined the land cover trajectories during the two decades, i.e. which land cover became which at the pixel level. In addition, we calculated topographic characteristics of deforested pixels. Major findings from the study are summarised as follows: (1) net forest loss in North Korea slowed since the 1990s, whereas land cover changes were active; (2) as a result of deforestation, forest land cover became mostly agricultural and grassland; (3) expansion of agricultural land cover continued during the time; and (4) elevation and slope of deforested areas decreased slightly in the latter decade. The key contribution of the study is that it has demonstrated which land cover became which at the 30-m pixel level, complementing existing studies that examined overall forest stock in North Korea

Design process of the nanofluid injection mechanism in nuclear power plants

Nanofluids, which are engineered suspensions of nanoparticles in a solvent such as water, have been found to show enhanced coolant properties such as higher critical heat flux and surface wettability at modest concentrations, which is a useful characteristic in nuclear power plants (NPPs). This study attempted to provide an example of engineering applications in NPPs using nanofluid technology. From these motivations, the conceptual designs of the emergency core cooling systems (ECCSs) assisted by nanofluid injection mechanism were proposed after following a design framework to develop complex engineering systems. We focused on the analysis of functional requirements for integrating the conventional ECCSs and nanofluid injection mechanism without loss of performance and reliability. Three candidates of nanofluid-engineered ECCS proposed in previous researches were investigated by applying axiomatic design (AD) in the manner of reverse engineering and it enabled to identify the compatibility of functional requirements and potential design vulnerabilities. The methods to enhance such vulnerabilities were referred from TRIZ and concretized for the ECCS of the Korean nuclear power plant. The results show a method to decouple the ECCS designs with the installation of a separate nanofluids injection tank adjacent to the safety injection tanks such that a low pH environment for nanofluids can be maintained at atmospheric pressure which is favorable for their injection in passive manner

Learning to Place Unseen Objects Stably using a Large-scale Simulation

Object placement is a fundamental task for robots, yet it remains challenging for partially observed objects. Existing methods for object placement have limitations, such as the requirement for a complete 3D model of the object or the inability to handle complex shapes and novel objects that restrict the applicability of robots in the real world. Herein, we focus on addressing the Unseen Object Placement (UOP}=) problem. We tackled the UOP problem using two methods: (1) UOP-Sim, a large-scale dataset to accommodate various shapes and novel objects, and (2) UOP-Net, a point cloud segmentation-based approach that directly detects the most stable plane from partial point clouds. Our UOP approach enables robots to place objects stably, even when the object's shape and properties are not fully known, thus providing a promising solution for object placement in various environments. We verify our approach through simulation and real-world robot experiments, demonstrating state-of-the-art performance for placing single-view and partial objects. Robot demos, codes, and dataset are available at https://gistailab.github.io/uop/Comment: 8 pages (main

Direct Observation of Quadrupolar Strain Fields forming a Shear Band in Metallic Glasses

For decades, scanning/transmission electron microscopy (S/TEM) techniques have been employed to analyze shear bands in metallic glasses and understand their formation in order to improve the mechanical properties of metallic glasses. However, due to a lack of direct information in reciprocal space, conventional S/TEM cannot characterize the local strain and atomic structure of amorphous materials, which are key to describe the deformation of glasses. For this work, 4-dimensional-STEM (4D-STEM) is applied to map and directly correlate the local strain and the atomic structure at the nanometer scale in deformed metallic glasses. Residual strain fields are observed with quadrupolar symmetry concentrated at dilated Eshelby inclusions. The strain fields percolate in a vortex-like manner building up the shear band. This provides a new understanding of the formation of shear bands in metallic glass

Simultaneous mapping of magnetic and atomic structure for direct visualization of nanoscale magnetoelastic coupling

Achieving a correlative measurement of both magnetic and atomic structures at the nanoscale is imperative to understand the fundamental magnetism of matters and for fostering the development of new magnetic nanomaterials. Conventional microscopy methods fall short in providing the two information simultaneously. Here, we develop a new approach to simultaneously map the magnetic field and atomic structure at the nanoscale using Lorentz 4-dimensional scanning transmission electron microscopy (Ltz-4D-STEM). This method enables precise measurement of the characteristic atomic and magnetic structures across an extensive field of view, a critical aspect for investigating real-world ferromagnetic materials. It offers a comprehensive visualization and statistical evaluation of the different structural information at a pixel-by-pixel correlation. The new method allows to directly visualize the magnetoelastic coupling and the resulting complex magnetization arrangement as well as the competition between magnetoelastic and magnetostatic energy. This approach opens new avenues for in-depth studying the structure-property correlation of nanoscale magnetic materials.Comment: 28 pages, 14 figure