Dendritic Axis Research Articles

Processing of Bimetallic Inconel 625-16Mo3 Steel Tube via Supercritical Bend: Study of the Mechanical Properties and Structure.

Incineration is currently the standard way of disposing of municipal waste. It uses components protected by high-temperature-resistant layers of materials, such as Inconel alloys. Therefore, the objective of the current paper is to study the mechanical properties and structure of a bimetallic Inconel 625-16Mo3 steel tube. The Inconel 625 layer was 3.5 mm thick and was applied to the surface of the tube with a wall thickness of 7 mm via the cold metal transfer method. The bimetallic tube was bent using a supercritical bend (d ≤ 0.7D). This paper is focused on the investigation of the material changes in the Inconel 625 layer areas influenced by the maximum tensile and compressive stresses after the bend. The change in layer thickness after the bend was evaluated and compared to the non-deformed tube. In addition, the local mechanical properties (nanohardness, Young modulus) across the indicated interfacial areas using quasistatic nanoindentation were investigated. Subsequently, a thorough microstructure observation was carried out in areas with maximum tensile and compressive stresses to determine changes in the morphology and size of dendrites related to the effect of tensile or compressive stresses induced by bending. It was found that the grain featured a stretched secondary dendrite axis in the area of tensile stress, but compressive stress imparted a prolongation of the primary dendrite axis.

Effect of texture evolution and defect on the crack initiation of a Ni‐based superalloy electron beam welded joint

AbstractElectron beam welding (EBW), a high‐quality and precise joining process, is widely used in aerospace manufacturing. In this study, through in situ scanning electron microscope (SEM) fatigue experiments and X‐ray computed tomography (CT) scanning technology, we revealed the fatigue crack initiation and propagation mechanism in GH4169 superalloy EBW joints under different post‐welding heat treatment conditions. The welded pores have a decisive effect on the fatigue properties of direct aging treatment welded joints. Fatigue cracks initiate from pore defects on the surface or in the subsurface. Due to the heterogeneous microstructure in weld seam, fatigue cracks tend to grow along the primary dendrite axis and secondary dendrite arms. Besides, in homogenization and aging treatment specimen, intergranular fatigue cracks tend to nucleate in the slip zone of large grains and twin boundaries. Although homogenization treatment can eliminate segregation, it also results in significant grain growth in both weld seam and base material.

The effect of TIG welding on the structure and hardness of butt joints made of Inconel 718

The tests discussed in the article aimed to analyse the structure and hardness of the heat affected zone and that of the weld in thin butt joints (1.0 mm) made of Inconel 718 using the TIG method and variable welding linear energy restricted within the range of 45 J/mm to 80 J/mm. The test joints were subjected to visual tests, macro and microscopic metallographic tests, scanning electron microscopy-based structural observations and hardness measurements. The tests concerned with the effect of the TIG welding of butt joints made of 1.0 mm thick sheets (Inconel 718) in laboratory conditions revealed that the most favourable quality of the sheets was obtained when welding arc linear energy was restricted within the range of approximately 45 J/mm to 80 J/mm. An increase in linear energy within the above-presented range led to an increase in the width of the weld and that of the HAZ (observed in the joints subjected to macroscopic metallographic tests). In addition, an increase in linear energy restricted within the aforesaid range increased the grain size in matrix γ (in the HAZ) from approximately 10 μm–20 μm. The structure of the weld contained the zone of columnar grains oriented towards the fusion line as well as large groups of primary grains having the dendritic structure with clearly visible axes of primary dendrites of varied orientation. In addition, the weld structure also contained precipitates in the form of low-melting eutectics located in interdendritic spaces.

The Effect of AlTi5B1 Masteralloy on Al-Si Alloy Casts Grain Size

This article deals with the issue of silumin inoculation. Two alloys, AlSi7Mg0.3 and AlSi10.5Cu1.2Mn0.8Ni1.2, were selected. 4 pieces of castings were cast from each alloy. One casting was not inoculated and 3 castings were inoculated using AlTi5B1 prealloy in concentrations of 0.05 wt. %, 0.1 wt % and 0.2 wt. % Ti. The castings were cast into metal molds preheated to a temperature of 200°C ± 5°C. In total, 8 pieces of metallographic cuttings were prepared for each alloy. Four samples were taken from the bottom part of the casting and four samples from the middle part of the casting. Secondary axes of DAS dendrites were measured using a microscope. Subsequently, the microhardness of the solid solution α was measured according to Vickers.

Features of the Macro-, Micro-, and Fine Structure of the Nickel Superalloy Product Material Formed by the Method of Electron Beam Additive Manufacturing.

In the present work, the products in the form of vertical walls were made of heat-resistant nickel-based superalloy ZhS32 via the method of electron beam additive technology. Unidirectional printing strategy was applied. The effect of heat input and 3D printing strategy on the macrostructure, dimensions, and morphology of microstructure elements was established. It was shown that the additive product material has a directed macrostructure. The only exclusion was the final layer with a thickness of no more than 3.5 mm. The directed macrostructure consisted of dendrites oriented predominantly along the crystallographic direction {001} of the primary dendrite arms. The misorientation of the dendrite axes did not exceed 9 degrees. The angle between the predominant dendrite growth direction and the normal to the substrate was 23 degrees. The average primary dendrite arms' spacing increased monotonically from 16 µm at 5 mm from the substrate to 23 µm in the final layers of the product material (the overall height was 41 mm). It was found that the average size of γ' (Ni3Al)-phase precipitations in the form of nanoscale and submicrocrystalline cuboids varied in the range of 76 to 163 nm depending on the distance from the substrate. The size of γ'-phase precipitations reached a maximum at about 30 mm from the substrate, while in the final layers of the product material, the average cuboid size did not exceed 135 nm. Extreme dependence of the size of γ'-phase precipitations on the height of the product followed from a combination of a given monotonic decrease in heat input and heat accumulation in the product material as it formed, as did additional heat removal by means of radiation during formation of the final layer of the product without re-melting. Chemical elements of the austenitic steel substrate material were not detected in the product material more than 8 mm from the substrate. There were no macrodefects, such as voids, in the entire volume of the product material.

The Tip of Dendritic Crystal in an Inclined Viscous Flow

We study the flow around the tip of a dendritic crystal by an inclined stream of viscous incompressible liquid. The tip shape is chosen accordingly to recent theory [Phil. Trans. R. Soc. A 2020, 378, 20190243] confirmed by a number of experiments and computations [Phil. Trans. R. Soc. A 2021, 379, 20200326]. Our simulations have been carried out for a 0, 30, 60, and 90-degree flow slope to the dendrite axis. We show that the stream inclination has a significant effect on the hydrodynamic flow and shear stress. In particular, a transition from laminar to turbulent currents on the upstream side of the dendritic crystal may occur in an inclined hydrodynamic flow. This leads to the fact that the heat and mass transfer mechanisms on the upstream and downstream sides of a growing dendritic crystal may be different.

Reducing PDK1/Akt Activity: An Effective Therapeutic Target in the Treatment of Alzheimer's Disease.

Alzheimer’s disease (AD) is a common age-related neurodegenerative disease that leads to memory loss and cognitive function damage due to intracerebral neurofibrillary tangles (NFTs) and amyloid-β (Aβ) protein deposition. The phosphoinositide-dependent protein kinase (PDK1)/protein kinase B (Akt) signaling pathway plays a significant role in neuronal differentiation, synaptic plasticity, neuronal survival, and neurotransmission via the axon–dendrite axis. The phosphorylation of PDK1 and Akt rises in the brain, resulting in phosphorylation of the TNF-α-converting enzyme (TACE) at its cytoplasmic tail (the C-terminal end), changing its internalization as well as its trafficking. The current review aimed to explain the mechanisms of the PDK1/Akt/TACE signaling axis that exerts its modulatory effect on AD physiopathology. We provide an overview of the neuropathological features, genetics, Aβ aggregation, Tau protein hyperphosphorylation, neuroinflammation, and aging in the AD brain. Additionally, we summarized the phosphoinositide 3-kinase (PI3K)/PDK1/Akt pathway-related features and its molecular mechanism that is dependent on TACE in the pathogenesis of AD. This study reviewed the relationship between the PDK1/Akt signaling pathway and AD, and discussed the role of PDK1/Akt in resisting neuronal toxicity by suppressing TACE expression in the cell membrane. This work also provides a perspective for developing new therapeutics targeting PDK1/Akt and TACE for the treatment of AD.

Spatial regulation of endosomes in growing dendrites

Many membrane proteins are highly enriched in either dendrites or axons. This non-uniform distribution is a critical feature of neuronal polarity and underlies neuronal function. The molecular mechanisms responsible for polarized distribution of membrane proteins has been studied for some time and many answers have emerged. A less well studied feature of neurons is that organelles are also frequently non-uniformly distributed. For instance, EEA1-positive early endosomes are somatodendritic whereas synaptic vesicles are axonal. In addition, some organelles are present in both axons and dendrites, but not distributed uniformly along the processes. One well known example are lysosomes which are abundant in the soma and proximal dendrite, but sparse in the distal dendrite and the distal axon. The mechanisms that determine the spatial distribution of organelles along dendrites are only starting to be studied. In this review, we will discuss the cell biological mechanisms of how the distribution of diverse sets of endosomes along the proximal-distal axis of dendrites might be regulated. In particular, we will focus on the regulation of bulk homeostatic mechanisms as opposed to local regulation. We posit that immature dendrites regulate organelle motility differently from mature dendrites in order to spatially organize dendrite growth, branching and sculpting.

Super-Resolution Three-Dimensional Imaging of Actin Filaments in Cultured Cells and the Brain via Expansion Microscopy.

Actin is an essential protein in almost all life forms. It mediates diverse biological functions, ranging from controlling the shape of cells and cell movements to cargo transport and the formation of synaptic connections. Multiple diseases are closely related to the dysfunction of actin or actin-related proteins. Despite the biological importance of actin, super-resolution imaging of it in tissue is still challenging, as it forms very dense networks in almost all cells inside the tissue. In this work, we demonstrate multiplexed super-resolution volumetric imaging of actin in both cultured cells and mouse brain slices via expansion microscopy (ExM). By introducing a simple labeling process, which enables the anchoring of an actin probe, phalloidin, to a swellable hydrogel, the multiplexed ExM imaging of actin filaments was achieved. We first showed that this technique could visualize the nanoscale details of actin filament organizations in cultured cells. Then, we applied this technique to mouse brain slices and visualized diverse actin organizations, such as the parallel actin filaments along the long axis of dendrites and dense actin structures in postsynaptic spines. We examined the postsynaptic spines in the mouse brain and showed that the organizations of actin filaments are highly diverse. This technique, which enables the high-throughput 60 nm resolution imaging of actin filaments and other proteins in cultured cells and thick tissue slices, would be a useful tool to study the organization of actin filaments in diverse biological circumstances and how they change under pathological conditions.

Assessing the Impact of Ih Conductance on Cross-Frequency Coupling in Model Pyramidal Neurons.

Large cortical and hippocampal pyramidal neurons are elements of neuronal circuitry that have been implicated in cross-frequency coupling (CFC) during cognitive tasks. We investigate potential mechanisms for CFC within these neurons by examining the role that the hyperpolarization-activated mixed cation current (Ih) plays in modulating CFC characteristics in multicompartment neuronal models. We quantify CFC along the soma-apical dendrite axis and tuft of three models configured to have different spatial distributions of Ih conductance density: (1) exponential gradient along the soma-apical dendrite axis, (2) uniform distribution, and (3) no Ih conductance. We simulated two current injection scenarios: distal apical 4 Hz modulation and perisomatic 4 Hz modulation, each with perisomatic, mid-apical, and distal apical 40 Hz injections. We used two metrics to quantify CFC strength—modulation index and height ratio—and we analyzed CFC phase properties. For all models, CFC was strongest in distal apical regions when the 40 Hz injection occurred near the soma and the 4 Hz modulation occurred in distal apical dendrite. The strongest CFC values were observed in the model with uniformly distributed Ih conductance density, but when the exponential gradient in Ih conductance density was added, CFC strength decreased by almost 50%. When Ih was in the model, regions with much larger membrane potential fluctuations at 4 Hz than at 40 Hz had stronger CFC. Excluding the Ih conductance from the model resulted in CFC either reduced or comparable in strength relative to the model with the exponential gradient in Ih conductance. The Ih conductance also imposed order on the phase characteristics of CFC such that minimum (maximum) amplitude 40 Hz membrane potential oscillations occurred during Ih conductance deactivation (activation). On the other hand, when there was no Ih conductance, phase relationships between minimum and maximum 40 Hz oscillation often inverted and occurred much closer together. This analysis can help experimentalists discriminate between CFC that originates from different underlying physiological mechanisms and can help illuminate the reasons why there are differences between CFC strength observed in different regions of the brain and between different populations of neurons based on the configuration of the Ih conductance.

Influence of Boron Modifications and Cooling Conditions during the Structural and Phase State Solidification of Heat- and Wear-Resistant White Cast Iron

The paper examines the effect of boron modification and temperature conditions for metal cooling in a mold with the phase composition, morphology and chemical composition of structural components found in a heat- and wear-resistant white cast iron Fe–C–Cr–Mn–Ni–Ti–Al–Nb system. The phase composition of the metallic base changed from the dualphase (α- and γ-phases) to the completely single-phase (γ-phase). Boron modification influenced on the secondary carbide type, while secondary hardens in a mold, occurs through extraction of dispersed niobium carbides (without boron but with chromium carbides). The modified cast iron structure is presented by the primary complex carbides (Ti, Nb, Cr, Fe)C, as well as by solid solution dendrites, eutectics and secondary carbides MeC. Boron addition changes the chemical composition of primary carbides with a decrease of niobium content from 44 to 2% and an increase of titanium content from 24 to 65%; the eutectic carbide content rises as well. Regarding hypereutectic carbides, they are characterized by a ferrum content increase and chrome content decrease. Parameters 893 of the primary phases (MeC carbides and solid solution dendrites) were investigated using quantitative metallography methods. The special technique of Thixomet PRO image analyzer was used for the evaluation of the F form factor which is the criterion of primary phase compatibility. The following parameters were used in this work as dendrite-structure parameters: dispersity of the dendrite structure (δ), volumetric dendrite parts (V), distance between the axes of second-order dendrites (λ2), form factor (F), average dimensions of dendrites—square (S), length (l) and width (β). All the suggested characteristics provided a quantitative evaluation of the dendrite structure, as well as determined the modification degree as the relative variation (in %) of each modified cast iron criterion in comparison with the non-modified iron. The quantitative relation between the modification degree and crystallization conditions were established as well.

Mechanism of Grain Selection in the Channel of a Crystal Selector to Obtain Single Crystal Castings of Nickel-Base Heat-Resistant Alloys

Using numerical simulation in the CAFE module of the ProCast commercial program, a study was performed of the competitive growth of grains in the seeding unit of the setup to obtain single crystal castings of nickel-base heat-resistant alloy in the process of directional crystallization using the Bridgman method. Using the example of a flat crystal selector, the mechanism of selection of a single grain from several grains with a close axial and arbitrary azimuthal orientation has been considered. It has been found that a significant role in the grain selection is played by the position of the grain in the input section of the crystal selector system. The initial growth advantage comes to those grains that lie in the zone of the crystallization front, from which the way for an unimpeded growth along the crystallization channel is opened. The position of this zone changes as the front moves in the curvilinear channel of the crystal selector. The main tool for selecting a single grain from a variety of closely oriented grains is a change in the position of the zone that is favorable for unimpeded growth. The continuous cyclic change of the zone of favorable growth, which occurs while the grain grows in the helicoid crystal selector, leads to a rapid selection of one grain. In a flat zigzag-shaped crystal selector, the zone of the favorable growth changes only a limited number of times, so this crystal selector is less efficient. The described mechanism of grain selection is applicable in the case of crystal selectors with an angle between the channel axis and the vertical axis Z of no more than 45°, i.e., when the primary axes of dendrites are formed in the crystallographic direction [001].

Effects of dendrite axis and fusion boundary on stress corrosion cracking of ER 308 L/SS 304L welds in a high-temperature water environment

This study investigated the effects of dendrite axis and fusion boundary (FB) on the stress corrosion cracking (SCC) of 308 L/304 L welds by using an alternating current potential drop (ACPD) technique to measure the SCC growth rates in a simulated boiling water reactor environment. The results show that the ferrite/austenite interfaces are more susceptible to SCC in the SS 308 L welds. Furthermore, the angle between the applied load and the dendrite axis affects the SCC growth rate, and as this angle approached 90°, the SCC crack growth rate increases. The SCC cracks in the heat affected zone (HAZ) are mainly intergranular and some cracks propagate transgranularly. Furthermore, the primary crack in the HAZ propagates along the FB; when reaching FB, some secondary cracks are arrested at the FB. The residual strain caused by weld shrinkage is the crucial factor for the increase in SCC susceptibility in HAZ.

Synaptic crosstalk conferred by a zone of differentially regulated Ca2+ signaling in the dendritic shaft adjoining a potentiated spine

Patterns of postsynaptic activity that induce long-term potentiation of fast excitatory transmission at glutamatergic synapses between hippocampal neurons cause enlargement of the dendritic spine and promote growth in spine endoplasmic reticulum (ER) content. Such postsynaptic activity patterns also impact Ca2+ signaling in the adjoining dendritic shaft, in a zone centered on the spine-shaft junction and extending ∼10-20 µm in either direction along the shaft. Comparing this specialized zone in the shaft with the dendrite in general, plasticity-inducing stimulation of a single spine causes more profound depletion of Ca2+ stores in the ER, a greater degree of interaction between stromal interaction molecule 1 (STIM1) and L-type Ca2+ channels, and thus stronger STIM1 inhibition of these channels. Here we show that the length of this zone along the dendritic axis can be approximately doubled through the neuromodulatory action of β-adrenergic receptors (βARs). The mechanism of βAR enlargement of the zone arises from protein kinase A-mediated enhancement of L-type Ca2+ current, which in turn lowers [Ca2+]ER through ryanodine receptor-dependent Ca2+-induced Ca2+ release and activates STIM1 feedback inhibition of L-type Ca2+ channels. An important function of this dendritic zone is to support crosstalk between spines along its length such that spines neighboring a strongly stimulated spine are enabled to undergo structural plasticity in response to stimulation that would otherwise be subthreshold for spine structural plasticity. This form of crosstalk requires L-type Ca2+ channel current to activate STIM1, and βAR activity extends the range along the shaft over which such spine-to-spine communication can occur.

Influence of Atomizing Gas and Cooling Rate on Solidification Characterization of Nickel-based Superalloy Powders

Nickel-based superalloy powders have been produced by high pressure argon gas and nitrogen gas atomization, separately. The microstructural characterization of nickel-based alloy powders has been performed by a scanning electron microscope equipped with an EDS microanalysis unit. Based on a Newtonian cooling model, the flight speed and the cooling rate of two kinds of alloy droplets were calculated. The results show that the droplet cooling rate, which depends on atomization medium and droplet size, has an effect on the solidification microstructure. For argon-gas atomized powders, the developed dendrite structure is predominant at a lower cooling rate and a mixed microstructure composed of dendrite structure and cellular structure is observed at a higher cooling rate. For nitrogen-gas atomized powders, the dendrite structure is predominant at a lower cooling rate and a full cellular structure can be observed at a higher cooling rate. According to calculation, the cooling rate of argon-gas atomized droplets is in a range from 1.0×105 K·s−1 to 4.24×106 K·s−1, while the cooling rate of nitrogen-gas atomized droplets is from 1.0×105 K·s−1 to 4.8×106 K·s−1. The cooling rate increases with decreasing of droplets diameter. Two kinds of atomizing gases have a slight influence on the cooling rate of droplets. The dendrite axis is rich in elements such as Cr, Co, W, Ni and Al while the inter-dendrite region is rich in Ti element.

ВЛИЯНИЕ ХАРАКТЕРА И СТЕПЕНИ ЛЕГИРОВАНИЯ НА СКЛОННОСТЬ К ПРОЯВЛЕНИЮ ЛИКВАЦИИ ЛИТЫХ ЗАГОТОВОК ДЕФОРМИРУЕМЫХ АЛЮМИНИЕВЫХ СПЛАВОВ

Production of high-quality semi-finished product from aluminum wrought alloys for subsequent deformation (rolling, stamping or pressing) is provides by obtaining the initial defect-free cast billet. The level of mechanical properties of alloys in the cast and heat-treated state is determined mainly by the internal structure of the grains, therefore one of the main parameters affecting the quality of the cast billet for subsequent plastic deformation is the manifestation of chemical heterogeneity of grains in the content of alloying elements or segregation. In this work, the alloys Al–Mn, Al–Mg and duralumin are investigated according to the method previously proposed by the same authors. These alloys can be conventionally chosen as model ones, to study the influence of the alloying character on the tendency of wrought aluminum alloys to manifest segregation effects in the cast state. Comparing the local content of alloying elements on the axes of dendrites with the brand composition of the alloys, a significant discrepancy was noted. This discrepancy is a measure of the inhomogeneity of the alloy. Taking the degree of deviation of the local composition from the brand as the value characterizing the degree of inhomogeneity of cast alloys Al–Mn, Al–Mg and duralumin, the values of the average homogeneity coefficient of these alloys are calculated. The homogeneity criterion, determined for the aluminum base of alloys, shows the dependence of the degree of homogeneity on the change in the nature of alloying and the content of alloying elements. Significant differences between the local content of alloying elements in the axes of the dendrites and the grade composition of the alloys were revealed.

АНАЛИТИЧЕСКИЕ ВОЗМОЖНОСТИ ОЦЕНКИ СТЕПЕНИ ГОМОГЕННОСТИ ЛИТЫХ АЛЮМИНИЕВЫХ СПЛАВОВ

Homogenization of castings from cast aluminum alloys is a priority for improving their technological parameters. The mode of homogenizing annealing is regulated by two parameters – temperature and holding time. Traditionally, one or two-stage annealing modes are used. To assess the degree of homogeneity of the alloy after such heat treatments at the enterprises, as a rule, various methods of mechanical testing are used. The paper proposes a new method for calculating the criterion for estimating the homogeneity of the structure of cast alloys using the aluminum alloy B95 as an example. The criterion of homogeneity (γ) can be the degree of approximation of compositions of local volumes of cast alloys to branded composition. Experience shows that it is more convenient to measure the content of alloying elements along the axes of dendrites. Dendrites, as a rule, contain a smaller number of basic alloying elements, which are usually localized in interdendritic volumes. Taking the value characterizing the degree of inhomogeneity of the cast alloy, the deviation of the composition from the branded (Δ), the homogeneity criterion can be represented as: γi = 1 – Δ. For a perfect homogeneous alloy, the value of the homogeneity criterion should approach 1. The average homogeneity coefficient for the cast alloy B95 calculated according to this formula is equal to γΣ = 0.503. Comparing the content of alloying elements in the center of grains and along the boundaries of homogenized grains, it should be noted that the local composition of grains, both in the center and at the boundaries, became close to the brand composition of alloy B95. This is indicated by the conditional homogeneity coefficient γ that is close to 1, introduced earlier. Indeed, the average homogeneity coefficient became equal to γΣ = 0.94. The proposed method is recommended as an analytical method for evaluating the degree of homogeneity of aluminum cast alloys, and as a criterion of homogeneity (γ), a generalized calculated index based on the analysis of local compositions along the dendrites and interdendritic regions can be proposed. Homogenization of castings from cast aluminum alloys is a priority for improving their technological parameters. The mode of homogenizing annealing is regulated by two parameters – temperature and holding time. Traditionally, one or two-stage annealing modes are used. To assess the degree of homogeneity of the melt after such heat treatments at the enterprises, as a rule, various methods of mechanical testing are used. The paper proposes a new method for calculating the criterion for estimating the homogeneity of a structure using the aluminum alloy B95 as an example. The proposed method consists in comparing the chemical composition in the dendritic and interdendritic areas and calculating on the basis of the obtained values of the homogeneity criterion (γ).

Development of an Integral Production Process for Manufacturing Machinery Billets Based on Computer Simulation

Examples are presented of using computer simulation for evaluating the effect of production parameters on billets' quality properties and development based upon this of rational production regimes. Criteria evaluated during simulation for solidification and forging criteria are described. The possibility is considered of using results obtained during simulation of solidification as starting data in modeling forging processes. One method of reducing the manufacturing cost of metal products, improving the level of mechanical properties of structural and functional materials, and also a way to reduce the scatter of these properties is prior simulation of production process stages and revelation of the optimum values of production parameters. In each stage of a production cycle, billet qual- ity is described by its own collection of criteria, whose values mainly govern features of the further conversion technology. In view of this, development of production processes (including the use of computer modeling) for each subsequent stage should be performed taking account of the maximum amount of billet-semiproduct quality characteristics achieved in the course of preceding stages. Questions of developing integral production processes based on computer modeling have been the subject of many publications, the last of which is work in (1-2). Modeling solidification. The first stage of the cycle for producing billets for engineering, whose technology may be developed on the basis of results of computer modeling, is ingot pouring and solidification. In order to model processes occur- ring in this stage, there is a considerable number of special program products, i.e., Procast, Magma, LVM-Flow, Thercast, and the domestic product Polion. At the Central Research Institute of Machine Building Technology (TsNIITMASh), a computer program of in-house development, called "Large Ingot" (4), and the ProCast commercial program are used for this purpose. The fundamental approach to developing technology for obtaining an ingot involves planning an orthogonal two- factor experiment with a study of the effect on ingot quality test characteristic, mainly parameters governing ingot solidifi- cation features to a considerable extent, i.e., conicity and ratio of ingot height to its central diameter (H/D). As variation parameters it is also possible to use the proportion of sinkhead and/or kumpel part of an ingot, thermophysical properties of the heat insulation materials used, mold wall thickness, etc. The dependence of a test ingot quality characteristic on these parameters is evaluated as a rule by a second order regression equation. The test characteristics considered are pore volume in the axial porosity zone, dimensions of this zone, concentra- tion of liquates over an ingot cross section (primarily, carbon, sulfur, and phosphorus), and distance between dendrite axes.

Brain intracellular metabolites are freely diffusing along cell fibers in grey and white matter, as measured by diffusion-weighted MR spectroscopy in the human brain at 7 T.

Due to the specific compartmentation of brain metabolites, diffusion-weighted magnetic resonance spectroscopy opens unique insight into neuronal and astrocytic microstructures. The apparent diffusion coefficient (ADC) of brain metabolites depends on various intracellular parameters including cytosol viscosity and molecular crowding. When diffusion time (t d) is long enough, the size and geometry of the compartment in which the metabolites diffuse strongly influence metabolites ADC. In a previous study, performed in the macaque brain, we measured neuronal and astrocytic metabolites ADC at long t d (from 86 to 1,011 ms) in a large voxel enclosing an equal proportion of white and grey matter. We showed that metabolites apparently diffuse freely along the axis of dendrites, axons and astrocytic processes. To assess potential differences between these two tissue types, here we measured for the first time in the Human brain the t d-dependency of metabolites trace/3 ADC at 7 teslas using a localized diffusion-weighted STEAM sequence, in parietal and occipital voxels, respectively, containing mainly white and grey matter. We show that, in both tissues and over the observed timescale (t d varying from 92 to 712 ms) metabolite ADC reaches a non-zero plateau, suggesting that metabolites are not confined inside subcellular regions such as cell bodies, or inside subcellular compartments such as organelles, but are rather free to diffuse in the whole fiber-like structure of neurons and astrocytes. Beyond the fundamental insights into intracellular compartmentation of metabolites, this work also provides a new framework for interpreting results of neuroimaging techniques based on molecular diffusion, such as diffusion-weighted magnetic resonance spectroscopy and imaging.

Evidence of High‐Temperature Strain Heterogeneities in a Nickel‐Based Single‐Crystal Superalloy

The creep behavior of a single crystal superalloy is investigated at the dendritic scale. The chemical segregation in the dendritic structure is correlated to strain heterogeneities after creep experiments on ultrathin specimens. It is shown that primary dendrite axes have a strengthening effect.