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Cognition changes with age, and the amount and trajectory of change varies across individuals and functions. In this review, we argue that three general principles characterize adult life-span changes in brain and cognition. (1) Dimensionality: Many features of brain and cognition in aging and neurodegenerative disease represent quantitative differences along a continuum and are not unique to pathology. (2) Early influences – developmental origins of health and disease: Genetic dispositions and early environmental factors, likely even from fetal life, can have lasting impact on the brain and cognition. (3) Influences from a multitude of environmental factors: Current brain state and cognitive function will be determined by a combination of early factors and later environmental influences, often in interaction. These principles entail a model of age-associated cognitive decline and dementia based on dimensions rather than categories, life span rather than aging, and multidimensional systems-vulnerability rather than one major “biomarker.”
Snow appears as a granular material in most engineering applications. We examined the role of grain shape and cohesion in angle of repose experiments, which are a common means for the characterization of granular materials. The role of shape was examined by investigating diverse snow types with discernable shape and spherical ice beads. Two geometrical shape parameters were calculated from X-ray micro-computed-tomography images after a particle segmentation was performed with a watershed algorithm. Cohesion was examined by conducting experiments at six different temperatures between −40 and −2°C, assuming sintering as its cause, which accelerates with increasing temperature. As a cohesionless reference, experiments with glass beads were performed. We found that both shape and cohesion exerted about equally strong influence on the angle of repose. We utilized our results for an empirical model that describes the influence of shape and cohesion as additive corrections of the angle of repose of cohesionless spheres and explains all experiments with a correlation coefficient r2 = 0.95. With temperature and the chosen shape parameter as fitting variables, previous experiments with another snow type could be consistently included. The experiments highlight the relevance of these parameters in granular snow mechanics and can be used for model calibration.
The oxidation behavior of the selective laser melting (SLM)–fabricated Inconel 718 was investigated through isothermal oxidation testing at 650 °C for 500 h and compared with that of the as-cast and as-forged specimens at the same testing conditions. The effect of microstructure and surface roughness on the oxidation behavior of the SLM-fabricated, as-cast, and as-forged Inconel 718 specimens was examined. The result shows that Inconel 718 fabricated by SLM with the unique layer structure exhibited a better resistance to the 500 h oxidation at 650 °C compared with as-cast and as-forged 718 with coarse dendritic structure and uniform equiaxed grain microstructure, respectively. The influence of the surface roughness on the long-time oxidation resistance of SLM specimens is not pronounced compared with that of as-cast and as-forged specimens. The tiny dendrites instead of grain boundaries are a major influencing factor for the oxidation process of SLM specimens. The surface roughness has more evident influence on the oxidation resistance of as-forged specimens than that of the as-cast ones subjected to the 500 h oxidation at 650 °C.
The role of negative substrate bias voltage in influencing the microstructural evolution, along with the mechanical and scratch behavior of magnetron sputtered Ni–Zr alloyed thin films, has been investigated. The films have been deposited on a Si(100) substrate by direct current (DC) magnetron co-sputtering of high-purity elemental Ni and Zr targets, using an optimized target power in an argon atmosphere at room temperature by altering the negative substrate bias voltage (0 to −80 V). The increase in negative substrate bias voltages leads to an increase in Zr content of the investigated films. The characterization techniques such as grazing incidence X-ray diffraction and high-resolution transmission electron microscopy studies confirm that an increase in the negative substrate bias voltage leads to an increase in the volume fractions of amorphous phase and Ni3Zr, but a decrease in the deposition rate, surface roughness, and average grain sizes. Hardness and Young's modulus obtained by nanoindentation, along with the coefficient of friction obtained from nano-scratch experiments, appear to be related to the relative volume fractions of both nanocrystalline and the amorphous phase. Furthermore, increase in Ni3Zr volume fraction with decrease in grain size within the crystalline part of the film, with increase in substrate bias used during deposition may have contributed to both increase in both hardness and scratch resistance.
Selective laser melting (SLM) is a state-of-the-art technology in the additive manufacturing field. This study focuses on the influence of scanning speed on the fabrication of Ti6Al4V samples produced by SLM. This article contributes to the effect of SLM scanning speed parameters on micropores, surface morphology, and roughness. The detailed characterizations for the parts produced by the SLM process are evaluated. An SLM scanning speed of 695, 775, or 853 mm/s was selected. The findings show that a high quality of surface morphology and microstructure is obtained at a scanning speed of 775 mm/s. In addition, the maximum surface roughness values for both upper and side surfaces are approximately 0.460 µm and 0.592 µm, respectively. Furthermore, surface defect characteristics regarding the speed mechanism parameter for the SLM system are also discussed, and the challenges to the part quality, and potential for numerous industries (e.g., aerospace, automotive, and biomedical), creating microstructures, are observed.
Most solid-to-solid phase transformations are much more interesting than just the growth of a small, homogeneous particle of the new phase. For reasons of both kinetics and thermodynamics, the new particles evolve in crystal structure, chemical composition, interface structures, defects, elastic energies, and shapes. Chapter 14 gives an overview of processes that occur during the nucleation and growth of a new phase from a parent phase. It covers essential features of precipitation in a solid, with a few traditional examples from steels, such as the pearlite transformation, and examples of precipitation sequences in aluminum alloys. Much of the content is central to physical metallurgy. The Kolmogorov-Johnson-Mehl-Avrami model of the rates of nucleation and growth transformations is presented. The late-stage coarsening process is also discussed in terms of the self-similarity of the microstructure.
This chapter introduces key concepts that are developed in this textbook. It describes the concept of microstructure and other features of materials that undergo interesting changes with temperature or pressure. These changes are motivated by the thermodynamic free energy, but require a kinetic mechanism for atoms to move. Chemical unmixing and ordering on a crystal lattice are described, and the kinetics of diffusion by vacancies is explained. The free energy is used to explain melting. A summary of essential aspects of thermodynamics and kinetics is given at the end of the chapter, including basic ideas of statistical mechanics and the kinetic master equation.
We here design and fabricate a new kind of copper matrix composites, where titanium carbide nanoparticles are in situ incorporated into and embedded within the copper matrix, by virtue of laser powder-bed-fusion (L-PBF) process. We made a multiscale examination on the microstructures of the additively manufactured samples, unraveling that there are many unusual microstructural features, including grain refinement, the existence of high-density dislocations, and supersaturation of titanium solute atoms in the as-printed metal matrix composites. These unique microstructural features are mainly interpreted by the intense thermal history and the rapid solidification nature of the L-PBF process. The resultant composites then integrate the most important four strengthening mechanisms in metals: grain boundary strengthening, dislocation strengthening, solid solution strengthening, and second-phase strengthening, rendering this new kind of copper matrix composites a remarkably high yield strength (∼490 MPa) and large uniform elongation (∼12%), surpassing many high-performance copper matrix composites and copper alloys.
Tungsten (W) alloy is of difficulty in processing for conventional way because of its high melting point. Here, W alloy sample with the addition of 3 wt% Ta was prepared by selective laser melting. The influence of volumetric energy density (VED) on the surface morphology and the relative density was discussed, and microstructure, phase composition, and microhardness were investigated. The results show that a smooth surface and high relative density (95.79%) can be obtained under optimal VED. The W–Ta substitutional solid solution formed because of the replacement of Ta atom. There are strip and block fine grains in the W–3Ta sample with no significant texture. In addition, subgrain structure with a size of around 1 μm formed inside the strip grain, owing to the large thermal gradient and extremely fast cooling rate. Finally, the W–3Ta alloy shows higher microhardness than that obtained by traditional methods.
Twin–twin interactions (TTIs) take place when multiple twinning modes and/or twin variants are activated and interact with each other. Twin–twin junctions (TTJs) form and affect subsequent twinning/detwinning and dislocation slip, which is particularly important in determining mechanical behavior of hexagonal metals because twinning is one major deformation mode. Atomic-level study, including crystallographic analysis, transmission electronic microscopy (TEM), and molecular dynamics (MD) simulations, can provide insights into understanding the process of TTIs and structural characters associated with TTJs. Crystallographic analysis enables the classification of TTIs and the prediction of possible interfaces of twin–twin boundaries (TTBs), characters of boundary dislocations, and possible reactions of twinning dislocations and lattice dislocations at TTBs. MD simulations can explore the process of TTIs, microstructures of TTJs, atomic structures of TTBs, and stress fields associated with TTJs. The predictions based on crystallographic analysis and the findings from MD can be partially verified by TEM. More importantly, these results provide explanation for microstructural characters of TTJs and guidance for further TEM characterizations.
We describe experimental approaches to real time examination of the microstructural evolution of Ti 6%Al 4%V upon cooling from above the beta transus (∼995 °C) while imaging in the scanning electron microscope. Ti 6%Al 4%V is a two phase, α+β titanium alloy with high strength and corrosion resistance. The β →α transformation on cooling can give rise to different microstructures and properties through various thermal treatments. Fully lamellar microstructures, bi-modal microstructures, and equiaxed microstructures can each be obtained by accessing different cooling rates upon the final treatment above the beta temperature, each resulting in uniquely enhanced material properties.
Utilizing the capabilities of a heating/ tensile stage developed by Kammrath & Weiss Inc., are able to apply real-time imaging techniques in the scanning electron microscope to monitor the development of the microstructure. Annealing temperatures up to 1100 °C are attainable, with cooling rates ranging from 0.1 ° C per second to 3.3 °C per second. This has allowed us to directly observe the formation of lamellae at different annealing temperature/ cooling rate combinations to determine the lamellar microstructure width, separation, and colony size.
A novel approach is utilized to investigate the deformation mechanisms at the microstructural level in 3D-printed alloys. The complex formation methods leave a unique and complicated microstructure in the as-built 3D-printed alloys. The microstructure is three leveled, composed of meltpools, grains, and cells. Deformation mechanisms in this microstructure are still highly unexplored due to the complexities of analysis at this scale. To understand these, we establish an image processing framework that converts scanning electron microscope (SEM) images directly into models that are scaled up and 3D printed with representative stiff and soft materials for the proposed material types within the body. These bodies are loaded in uniaxial tension with digital image correlation to study the strain gradient and stress delocalization as a result of the microstructure. The same models were tested through Finite Element Analysis (FEA) with materials similar to reality. Our testing shows the hierarchical material distribution leads to an increased damage tolerance.
Tempering cooling rate plays a significant role in the impact toughness of 2CrMoV weld metal. Three different tempering cooling rate experiments were carried out; it is found that the impact toughness of weld metal improved from 44.61 to 117.49 J as the cooling rate increased from 5 to 40 °C/h. Microstructure characterization revealed that the large blocky M–A constituents and cluster precipitation were considered to act as stress concentration sources and cleavage fracture initiators at a cooling rate of 5 °C/h. Under the cooling rate of 20 °C/h, the decrease of blocky M–A constituents as well as homogeneous distribution of precipitation induced the transition from cleavage to interfacial decohesion. The chance of crack propagation in intragranular ferrite matrix was increased, which needed to absorb more energy and improve impact toughness. When the tempering cooling rate reached at 40 °C/h, the cracks mainly propagated in the ferrite matrix; meanwhile, fine and homogeneous distribution of precipitation greatly inhibited crack propagation and led to higher impact toughness.
The breakdown of the columnar grains and lamellar α + β colony microstructure in two-phase Ti alloys during conversion of ingot to billet is critical to the development of desired combination of mechanical properties. Colony breakdown occurs during a series of thermomechanical processing steps in the α + β phase field. However, fundamental knowledge of the microstructural dependence of this transformation is limited, particularly its dependence on the initial orientation of the α + β colony relative to the imposed strain-path. In this study, the viscoplastic self-consistent polycrystal plasticity model is used to examine deformation behavior as a function of crystal loading direction. Criteria were developed to predict relative globularization rates; it was found that both slip system activities in the α phase and relative crystal rotations of each phase must be considered. Predictions are demonstrated to be consistent with literature and suggest that further experimental investigation of relative globularization rates is necessary.
Recently, layered double hydroxides (LDHs) have attracted intensive research interest as the next-generation supercapacitor electrodes due to their unique two-dimensional (2D) hydrotalcite-like structure. However, the inevitable agglomeration significantly decreases the accessible surface areas and blocks the pseudocapacitive sites, thus severely hinders their electrochemical applications. Herein, we develop a facile one-step growth approach to fabricate porous agglomerate of NiCo-LDH nanosheets and reduced graphene oxide (rGO) nanoflakes. By adjusting feeding molar ratios, the obtained NiCo-LDH/rGO electrode delivers a high specific capacity of 879.5 C/g at a current density of 0.5 A/g and still remains 485 C/g at 20 A/g. Furthermore, the fabricated asymmetric supercapacitor (ASC) has demonstrated a superior energy density of 48.7 W h/kg at a power density of 401 W/kg. After 2000 cycles, the assembled ASC exhibits an improved capacity retention of 81% within a potential window of 1.6 V at 2 A/g.
Uranium–35 wt.% zirconium (U–35 wt.% Zr) alloy was annealed for 1 h and 24 h at 650 °C and characterized to understand the early-stage microstructure evolution. Dendritic microstructure with fine (∼300 nm in length) α-U precipitates clustered between dendrite branches were observed in the 1-h annealed sample. After 24-h annealing at 650 °C, the α-U precipitates coarsened, and the dendritic microstructure disappeared because of microstructure homogenization. Furthermore, microchemical homogenization observed with energy-dispersive X-ray spectroscopy analysis suggests that α-U precipitates are approaching thermodynamic equilibrium in the 24-h annealed sample. The findings from this study have potential impacts on the manufacturing and computer modeling of metallic nuclear fuel.
In this study, precipitate phase transformation behavior, microstructure, and properties of the Cu–1Cr–1Co–0.4Si (wt%) alloy were investigated. Precipitate phase transformation kinetic equations of the alloy under room temperature rolling (RTR) 90% deformation and aging at different temperatures (440–520 °C) were established. The alloy yielded excellent mechanical and electrical properties under RTR 90% deformation and aging at 440 °C for 1 h, and the corresponding hardness, yield strength (YS), ultimate tensile strength (UTS), elongation, and electrical conductivity were 181.6 HV, 573.6 MPa, 653.7 MPa, 7.3%, and 51.6% International Annealed Copper Standard, respectively. The precipitate phase transformation behavior determined the size and volume fraction of the precipitate phase fv, which played a key role in improving the YS. Impurity scattering caused by surplus Si atoms was mainly responsible for decreasing the electrical conductivity. Therefore, these results can provide a reliable theoretical guidance to prepare Cu–Cr–based alloys with high strength and high electrical conductivity.
We divide the corrosion products on ancient bronzes into two categories, i.e., "inward growth" and “outward growth” corrosions. Several selected Chinese ancient bronzes with the "inward growth” corrosion are studied; and their chemical compositions, microstructures and morphologies are characterized systematically. According to the results, it is found that the “inward growth” corrosion can be further divided into three types, i.e., "noble patina", "noble-like patina" and "lamellar peeling patina". We propose that the growth mechanism of the “inward growth” corrosion is that the corrosion initiates at and develops along α-Cu phase. Furthermore, the effect of alloy Sn content on the “inward growth” corrosion is also studied.
Hot deformation behavior of a new tailored cobalt-based superalloy for turbine discs was investigated in the temperature range of 1050–1200 °C and the strain rate range of 0.01–10 s−1. The results show that the flow stress is closely related to the deformation temperature and strain rate, and the flow stress curve of the new tailored alloy belongs to a typical dynamic recrystallization (DRX) type. Microstructure observation reveals that the dominant nucleation mechanism of DRX for the new tailored alloy belongs to discontinuous DRX, while continuous DRX only acts as an assistant nucleation mechanism. The optimum processing parameters of hot working are obtained in the temperature range of 1155–1200 °C and the strain rate range of 0.01–0.1 s−1. The activation energy for the new tailored alloy is determined to be 833.0 kJ/mol, and the relationship between grain size and processing parameters is established by appropriate constitutive equations.
Melting, solidification and solid-state transformation of the intermetallic Ni3Sn compound were investigated in situ using synchrotron high-energy X-ray diffraction. It was observed that the compound undergoes a hexagonal to cubic transition before melting. In solidification, a disordered cubic phase crystallizes from the liquid at a large undercooling but it is reordered prior to bulk solidification. In melting and solidification, forced or natural flows are active bringing about significant changes of crystal orientations. These in situ observations provided insights into phase transformations of Ni3Sn at elevated temperatures and their roles in formation of metastable microstructure consisting of coarse grains and subgrains.