Grain Growth

Focusing on nanocrystalline (nc) pure face-centered cubic metals, where systematic experimental data are available, this paper presents a brief overview of the recent progress made in improving mechanical properties of nc materials, and in quantitatively and mechanistically understanding the underlying mechanisms. The mechanical properties reviewed include strength, ductility, strain rate and temperature dependence, fatigue and tribological properties. The highlighted examples include recent experimental studies in obtaining both high strength and considerable ductility, the compromise between enhanced fatigue limit and reduced crack growth resistance, the stress-assisted dynamic grain growth during deformation, and the relation between rate sensitivity and possible deformation mechanisms. The recent advances in obtaining quantitative and mechanics-based models, developed in line with the related transmission electron microscopy and relevant molecular dynamics observations, are discussed with particular attention to mechanistic models of partial/perfect-dislocation or deformation-twin-mediated deformation processes interacting with grain boundaries, constitutive modeling and simulations of grain size distribution and dynamic grain growth, and physically motivated crystal plasticity modeling of pure Cu with nanoscale growth twins. Sustained research efforts have established a group of nanocrystalline and nanostructured metals that exhibit a combination of high strength and considerable ductility in tension. Accompanying the gradually deepening understanding of the deformation mechanisms and their relative importance, quantitative and mechanisms-based constitutive models that can realistically capture experimentally measured and grain-size-dependent stress-strain behavior, strain-rate sensitivity and even ductility limit are becoming available. Some outstanding issues and future opportunities are listed and discussed.

The grain structure, dislocation density and second phase particles in various regions including the dynamically recrystallized zone (DXZ), thermo-mechanically affected zone (TMAZ), and heat affected zone (HAZ) of a friction stir weld aluminum alloy 7050-T651 were investigated and compared with the unaffected base metal. The various regions were studied in detail to better understand the microstructural evolution during friction stir welding (FSW). The microstructural development in each region was a strong function of the local thermo-mechanical cycle experienced during welding. Using the combination of structural characteristics observed in each weld region, a new dynamic recrystallization model has been proposed. The precipitation phenomena in different weld regions are also discussed.

A systematic study of various spark plasma sintering (SPS) parameters, namely temperature, holding time, heating rate, pressure, and pulse sequence, was conducted to investigate their effect on the densification, grain-growth kinetics, hardness, and fracture toughness of a commercially available submicrometer-sized Al2O3 powder. The obtained experimental data clearly show that the SPS process enhances both densification and grain growth. Thus, Al2O3 could be fully densified at a much lower temperature (1150°C), within a much shorter time (minutes), than in more conventional sintering processes. It is suggested that the densification is enhanced in the initial part of the sintering cycle by a local spark-discharge process in the vicinity of contacting particles, and that both grain-boundary diffusion and grain-boundary migration are enhanced by the electrical field originating from the pulsed direct current used for heating the sample. Both the diffusion and the migration that promote the grain growth were found to be strongly dependent on temperature, implying that it is possible to retain the original fine-grained structure in fully densified bodies by avoiding a too high sintering temperature. Hardness values in the range 21–22 GPa and fracture toughness values of 3.5 ± 0.5 MPa·m1/2 were found for the compacts containing submicrometer-sized Al2O3 grains.

Deformation experiments have been conducted to investigate the effect of melt fraction and grain size on the creep behavior of elivine aggregates in the diffusion creep regime. Both nominally melt-free and melt-added samples display stress exponents (n = 1.0 q-0.1) and grain size exponents (p =-3.0 q-0.5 for nominally melt-free, p =-3.2 q-1.2 for melt-added) indicative of grain boundary diffusion creep. The activation energy for creep of the nominally melt-free aggregates is 315 q-35 kJ/mol. An abrupt change in the theological behavior of the partially molten aggregates occurs at a melt fraction of 0.05. Below this melt fraction the influence of melt on strain rate is rather modest. For example, at a melt fraction of 0.04 the strain rate of melt-added samples is enhanced by a factor of-3 relative to that of melt-free aggregates. This result is consistent with previously published theoretical models for solution-precipitation enhanced grain boundary diffusion creep in which the melt phase is present along three-grain junction tubules and four-grain junction comers. A comparison with published diffusion data indicates that deformation is limited by transport of Si along melt-free grain boundaries under both melt-free and meltpresent conditions. At melt fractions above-0.05, the strain rate enhancement is significantly greater than that predicted by the theoretical models. For example, at a melt fraction of 0.07 the strain rate of melt-added samples is enhanced by a factor of-25 relative to that of melt-free aggregates. Microstructural observations of both hot-pressed and deformed aggregates with melt fractions greater than-0.05 demonstrate that a significant number of two-grain boundaries are "wetted" by melt. These boundaries provide rapid transport paths not accounted for in the theoretical models. The presence of wetted grain boundaries at melt fractions less than-0.19 indicates that the melt topology in the elivinebasalt system is affected by anisotropic interfacial energies. There is no difference in the strength of partially molten aggregates deformed with or without added water. This result is consistent with the observation that the solubility of water in basalt is-3 orders of magnitude greater than that in elivine and supports the conclusion that deformation is limited by transport along melt-free grain boundaries at all conditions tested. HIRTH AND KOHLSTEDT: RHEOLOGY OF PARTIALLY MOLTEN DUNITE spherical. In this case the grain size is obtained by multiplying the mean intercept length by a factor of 1.5. The grain size was measured on reflected light micrographs, such as the one shown in Figure 2, of polished specimens that had been etched for-•30 rain in phosphoric acid. Finally, the water content of deformed samples was analyzed using Fourier transform infrared spectroscopy (FTIR). To correct for background absorbance, a least squares fit to the data ß outside a wavenumber range of 260-380 mm 4 was calculated using a third-order polynomial. The resulting fit was then subtracted from the data to obtain an absorbance spectrum representative of the sample. Results of Isostatic Hot Pressing Optical and SEM analyses indicate that the hot-pressed specimens achieve 99 + 1% of the theoretical density, consistent with observations previously documented for samples prepared in the same manner [e.g., Riley and Kohlstedt, 1991; Beeman and Kohlstedt, 1993; Daines and Kohlstedt, 1993]. As illustrated in Figure 3a, the porosity is composed of spherical pores within the melt phase. The distribution of the melt phase in both olivine + MORB and olivine + syn-basalt specimens is also essentially the same as that documented for samples prepared using the 1,0 lam

A continuum field theory approach is presented for modeling elastic and plastic deformation, free surfaces, and multiple crystal orientations in nonequilibrium processing phenomena. Many basic properties of the model are calculated analytically, and numerical simulations are presented for a number of important applications including, epitaxial growth, material hardness, grain growth, reconstructive phase transitions, and crack propagation.

We report new Spitzer Space Telescope observations from the IRAC and MIPS instruments of the young (~ 3 Myr) sigma Orionis cluster. We identify 336 stars as members of the cluster using optical and near-infrared color magnitude diagrams. Using the spectral energy distribution (SED) slopes in the IRAC spectral range, we place objects in several classes: non-excess stars, stars with optically thick disks(like classical T Tauri stars), class I (protostellar) candidates, and stars with ``evolved disks''; the last exhibit smaller IRAC excesses than optically thick disk systems. In general, this classification agrees with the location expected in IRAC-MIPS color-color diagrams for these objects. We find that the evolved disk systems are mostly a combination of objects with optically thick but non-flared disks, suggesting grain growth and/or settling, and transition disks, systems in which the inner disk is partially or fully cleared of small dust. In all, we identify 7 transition disk candidates and 3 possible debris disk systems. As in other young stellar populations, the fraction of disks depends on the stellar mass, ranging from ~10% for stars in the Herbig Ae/Be mass range (>2 msun) to ~35% in the T Tauri mass range (1-0.1 msun). We find that the disk fraction does not decrease significantly toward the brown dwarf candidates (<0.1 msun). The IRAC infrared excesses found in stellar clusters and associations with and without central high mass stars are similar, suggesting that external photoevaporation is not very important in many clusters. Finally, we find no correlation between the X-ray luminosity and the disk infrared excess, suggesting that the X-rays are not strongly affected by disk accretion.

The microstructure of commercially available nanocrystalline (nc) electroplated Ni foils is studied by means of X-ray diffraction and transmission electron microscopy. It is shown that the microstructure is inhomogeneous and batch-dependent. Tensile properties at strain rates between 10−5 and 103 s−1 are studied and compared with the results of coarse-grained Ni. Data on strength, strain-rate sensitivity and work hardening are presented. At the highest strain rates, shear banding with local grain growth is observed in the nc structure. It is also suggested that the differences found in nc Ni for 3 and 20 mm tensile specimens are the size effects related to the inhomogeneous microstructure.

Superplastic structural ceramics (Y-TZP, A1203, Si3N4, and their composites) that can withstand biaxial stretching to large strains have been developed recently. Microstructural design of these ceramics first requires an ultrafine grain size that is stable against coarsening during sintering and deformation. A low sintering temperature is a necessary, but not a sufficient, condition for achieving the required microstructure. In many cases, the selection of an appropriate phase, such as tetragonal phase in zirconia or a phase in silicon nitride, which is resistant to grain growth, is crucial. The use of sintering aids and grain-growth inhibitors, particularly those that segregate to the grain boundaries, can be beneficial. Second-phase particles are especially effective in suppressing static and dynamic grain growth. Another major concern is to maintain an adequate grain-boundary cohesive strength, relative to the flow stress, to mitigate cavitation or grain-boundary cracking during large strain deformation. Existing evidence suggests that a lower grainboundary energy is instrumental in achieving this objective. The selection of an appropriate phase and the tailoring of the grain boundary or liquid-phase composition can sometimes drastically alter the cavitation resistance. Related observations on forming methods, forming characteristics, and sheet formability are also reviewed. The basic deformation characteristics are similar to diffusional creep and are dominated by R. E. Newnham-contributing editor Manuscript No. 197477.

Dynamic recrystallization (DRX) characteristics of a Mg Á/3Al Á/1Zn (AZ31) alloy sheet were investigated at temperatures ranging from 200Â/450 8C and constant strain rates of 1 )/10 (4 Â/2 )/10 (4 s (1 . The average grain size of the as-received alloy was 12 mm and can be refined to 6 mm via deformation at 250 8C, 1 )/10 (4 s (1 to a strain level of 60%. Grain refinement was less effective at higher temperatures due to rapid grain growth. The grain refinement was attributed to dynamic continuous recrystallization which involves progressive increase in grain boundary misorientation and conversion of low angle boundaries into high angle boundaries. During DRX, subgrains were developed through the conversion of dislocation cell walls into subgrain boundaries. The presence of precipitates was not essential for dynamic recrystallization in the magnesium alloy being investigated because of its limited slip systems, low stacking fault energy and high grain boundary diffusion rate. # : S 0 9 2 1 -5 0 9 3 ( 0 2 ) 0 0 0 9 6 -5

Microstructural characteristics of friction stir processed (FSP) 7075 Al samples in different regions behind the pin tool were investigated. Furthermore, the microstructure evolutions during friction stir welding (FSW)/FSP has been revealed. Multi-mechanisms, including dDRX, grain growth, dislocation introduction, dynamic recovery (DRV) and cDRX, were found to be inherent in the process at different stages. Because inhomogeneous plastic deformation was introduced by the process, individual grains in the final microstructure have undergone different evolution mechanisms. These are either from growth of initially recrystallized grains or are the result of cDRX of subgrains formed during DRV. Grains within the processed region exhibit different densities of dislocations and are in various degrees of recovery.

In this paper the relationship between the classical density functional theory of freezing and phase field modeling is examined. More specifically a connection is made between the correlation functions that enter density functional theory and the free energy functionals used in phase field crystal modeling and standard models of binary alloys (i.e., regular solution model). To demonstrate the properties of the phase field crystal formalism a simple model of binary alloy crystallization is derived and shown to simultaneously model solidification, phase segregation, grain growth, elastic and plastic deformations in anisotropic systems with multiple crystal orientations on diffusive time scales.

Molecular-dynamics (MD) simulations of fully three-dimensional (3D), model nanocrystalline face-centered cubic metal microstructures are used to study grain-boundary (GB) diffusion creep, one mechanism considered to contribute to the deformation of nanocrystalline materials. To overcome the well-known limitations associated with the relatively short time interval used in our MD simulation (typically Ͻ10 Ϫ8 s), our simulations are performed at elevated temperatures where the distinct effects of GB diffusion are clearly identifiable. In order to prevent grain growth and thus to enable steady-state diffusion creep to be observed, our input microstructures were tailored to (1) have a uniform grain shape and a uniform grain size of nm dimensions and (2) contain only high-energy GBs which are known to exhibit rather fast, liquid-like self-diffusion. Our simulations reveal that under relatively high tensile stresses these microstructures, indeed, exhibit steady-state diffusion creep that is homogeneous, with a strain rate that agrees quantitatively with that given by the Coble-creep formula. The grain-size scaling of the Coble creep is found to decrease from d Ϫ3 to d Ϫ2 when the grain diameter becomes of the order of the GB width. For the first time a direct observation of the grainboundary sliding as an accommodation mechanism for the Coble creep, known as Lifshitz sliding, is reported. 

It is widely believed that grain size reduction by dynamic recrystallization can lead to major rheological weakening and associated strain localization by bringing about a switch from grain size insensitive dislocation creep to grain size sensitive diffusion creep. Recently, however, we advanced the hypothesis that, rather than a switch, dynamic recrystallization leads to a balance between grain size reduction and grain growth processes set up in the neighborhood of the boundary between the dislocation creep field and the diffusion creep field. In this paper, we compare the predictions implied by our hypothesis with those of other models for dynamic recrystallization. We also evaluate the full range of models against experimental data on a variety of materials. We conclude that a temperature dependence of the relationship between recrystallized grain size and flow stress cannot be neglected a priori. This should be taken into account when estimating natural flow stresses using experimentally calibrated recrystallized grain size piezometers. We also demonstrate experimental support for the field boundary hypothesis. This support implies that significant weakening by grain size reduction in localized shear zones is possible only if caused by a process other than dynamic recrystallization (such as syntectonic reaction or cataclasis) or if grain growth is inhibited.

Carbon nanotubes-nano-WC-Co composites were prepared by spark plasma sintering process. The preparation steps involving purified carbon nanotubes were stirred into the nano-WC-Co particles in alcohol using ultrasonic shaker, then ball milled for 60 min. The densification and grain growth behavior, the mechanical properties of the nanocomposites were investigated. Experimental results show that with small amount of carbon nanotube additions, the density of the composites can be enhanced but the grain size has marginal increase; the sintered nanocomposites combine superior hardness to toughness relationships over that of the pure nanocrystalline WC-Co cermets fabricated under the same process. The reasons for the increments can be due to the extraordinary mechanical properties of carbon nanotubes, the toughing and bonding role of carbon nanotubes occurred in the nano-WC-Co matrix, as well as the spark plasma sintering technique used.

The effect of composition, microstructure, and defect chemistry on sensing performance of gas sensors based on CuO-doped SnO is 2 Ž . investigated using sol-gel derived nano-sized powders about 20 nm . The particle size of copper oxide doped tin oxide is varied by annealing at different temperatures and a significant grain growth is observed at temperatures above 10008C due to the liquid phase sintering effect of copper oxide. The reduction of particle size to nanometers, or to the dimension comparable to the thickness of charge depletion layer, leads to a dramatic improvement in sensitivity and speed of response. It appears that the substitution of Sn by Cu in the cassiterite structure increases the concentration of oxygen vacancies and decreases the concentration of free electrons. In particular, the Ž q . 2q existence of cuprous ions Cu , due to partial reduction of Cu during sintering, plays an important role in enhancing the sensor Ž . response to nitric oxide NO and CO . q

The results of calculations are summarized as the followings; in the unmixed ejecta, a variety of grain species condense, reflecting the difference of the elemental composition at the formation site in the ejecta, otherwise only oxide grains condense in the uniformly mixed ejecta. The average size of newly formed grains spans the range of three orders of magnitude, depending on the grain species and the formation condition, and the maximum radius is limited to less than 1 $\mu$m, which does not depend on the progenitor mass. The size distribution function summed up over all grain species is approximated by a power--law formula whose index is -3.5 for the larger radius and -2.5 for the smaller one; the radius at the crossover point ranges from 0.004 to 0.1 $\mu$m, depending on the model of supernovae. The fraction of mass locked into dust grains increases with increasing the progenitor mass; 2--5 % of the progenitor mass for core collapse supernovae and 15--30 % for pair--instability supernovae whose progenitor mass ranges from 140 to 260 $M_{\odot}$. Thus, if the very massive stars populate the first generation stars, a large amount of dust grains would be produced in the early universe.

Accumulative roll bonding of two aluminium alloys, AA2219 and AA5086 was carried out up to 8 passes. During the course of ARB, the deformation inhomogeneity between the two alloy layers results in interfacial instability after the 4th pass, necking of the AA5086 layers after the 6th pass and fracture along the necked regions after the 7th and 8th pass. The EBSD analysis shows deformation bands along the interfaces after 8 passes of ARB. The ARBprocessed materials predominantly show characteristic deformation texture components. The weak texture after the 2nd pass results from the combination of a weakly-textured starting AA2219 layer and a strongly-textured starting AA5086 layer. A strong deformation texture forms due to the high imposed strain after a higher number of ARB passes. Subgrain formation and related shear banding induces copper/S components in the case of the small elongated grains, while planar slip leads to the formation of brass component in the large elongated grains.

We present self-consistent disk models for T Tauri stars which include a parameterized treatment of dust settling and grain growth, building on techniques developed in a series of papers by D'Alessio et al. . The models incorporate depleted distributions of dust in upper disk layers along with larger-sized particles near the disk midplane, as expected theoretically and as we suggested earlier is necessary to account for mm-wave emission, SEDs, scattered light images, and silicate emission features simultaneously. By comparing the models with recent mid-and near-IR observations, we find that the dust to gas mass ratio of small grains at the upper layers should be < 10 % of the standard value. The grains that have disappeared from the upper layers increase the dust to gas mass ratio of the disk interior; if those grains grow to maximum sizes of the order of mm during the settling process, then both the millimeter-wave fluxes and spectral slopes can be consistently explained. Depletion and growth of grains can also enhance the ionization of upper layers, enhancing the possibility of the magnetorotational instability for driving disk accretion.

Snow aging causes reflectance to vary significantly on timescales of days. This variability influences the strength of snow-albedo feedback, and can affect the timing of snowmelt. However, climate models have yet to incorporate important controls on snow aging and albedo evolution. We develop a physically-based model that predicts evolution of dry, pure-snow specific surface area, and apply aspherical ice particle theory to link these results with albedo evolution. This is the first theoretical study to quantify the relative roles of initial size distribution, vertical temperature gradient, and snow density in snow albedo evolution. Vapor diffusion caused by curvature differences causes rapid albedo decay in the first day following snowfall. Vertical temperature gradient generally dominates grain growth processes afterward, but is modulated by snow density, irregularity in particle spacing, and temperature. These processes operate as a coupled system, which we uniquely represent without abrupt transitions between regimes.

The physico-chemical properties of ceria (CeO 2 ) and rare earth modified ceria (with La, Pr, Sm, Y) catalysts are studied and correlated with the soot oxidation activity with using O 2 and O 2 +NO. CeO 2 modified with La and Pr shows superior soot oxidation activity with O 2 compared with the unmodified catalyst. The improved soot oxidation activity of rare earth doped CeO 2 catalysts can be correlated to the increased meso/micro pore volume and the stabilisation of the external surface area. On the other hand, unreducible ions decrease the intrinsic soot oxidation activity of rare earth modified ceria with both O 2 and NO+O 2 due to the decreased amount of redox surface sites. The catalyst bulk oxygen storage capacity is not a critical parameter in determining the soot oxidation activity. The modification with Pr shows the best soot oxidation with both O 2 and O 2 +NO compared with all other catalysts.

Porous hydroxyapatite (HAp) scaffolds were successfully prepared by using the HAp slurry based on the replication of polymer sponge substrate. The effect of HAp content in slurry on the pore morphology and size, and density, porosity, and mechanical strength of porous scaffolds was investigated. The scaffolds with average pore sizes ranging from 200 to 400 µm had an open, relatively uniform, and interconnected porous structure. As the HAp content increased, the porosity of scaffold decreased while the density increased. These phenomena were attributed to the fact that the pores became interconnected with more dense and thicker pore walls with increasing HAp content in slurry. The results suggest that the density, porosity, and compressive strength of the porous HAp scaffold were significantly affected by the content of the HAp powder in the slurry.

Two-and three-dimensional (3D) Cellular Automaton (CA) algorithms are proposed for modelling the growth of dendritic grains from the liquid phase. These CA growth algorithms are validated for simple thermal situations by comparing the predicted grain shapes with those deduced from analytical models. The insight obtained by the 3D approach is demonstrated by studying the extension of a single dendritic grain in a squared platform (i.e. at a section change of a casting mould) under various conditions. In particular, the effects of crystallographic orientation, thermal gradient, velocity of the isotherms and growth kinetics are shown. This 3D CA growth algorithm, coupled with finite element heat flow calculations, will become a major tool for the prediction of dendritic grain structures in solidification processes. 0 1997 Acta Metallurgica Inc

Water In Star-forming regions with Herschel (WISH) is a key program on the Herschel Space Observatory designed to probe the physical and chemical structures of young stellar objects using water and related molecules and to follow the water abundance from collapsing clouds to planet-forming disks. About 80 sources are targeted, covering a wide range of luminosities-from low ( < 1 L⊙ ) to high (> 105 L⊙ )-and a wide range of evolutionary stages-from cold prestellar cores to warm protostellar envelopes and outflows to disks around young stars. Both the HIFI and PACS instruments are used to observe a variety of lines of H2 O , H2 O 18 and chemically related species at the source position and in small maps around the protostars and selected outflow positions. In addition, high-frequency lines of CO, 13CO, and C O 18 are obtained with Herschel and are complemented by ground-based observations of dust continuum, HDO, CO and its isotopologs, and other molecules to ensure a self-consistent data set for analysis. An overview of the scientific motivation and observational strategy of the program is given, together with the modeling approach and analysis tools that have been developed. Initial science results are presented. These include a lack of water in cold gas at abundances that are lower than most predictions, strong water emission from shocks in protostellar environments, the importance of UV radiation in heating the gas along outflow walls across the full range of luminosities, and surprisingly widespread detection of the chemically related hydrides OH+ and H2 O+ in outflows and foreground gas. Quantitative estimates of the energy budget indicate that H2 O is generally not the dominant coolant in the warm dense gas associated with protostars. Very deep limits on the cold gaseous water reservoir in the outer regions of protoplanetary disks are obtained that have profound implications for our understanding of grain growth and mixing in disks.

We present the results of an infrared imaging survey of two clusters in the Cep OB2 Association, Tr 37 and NGC 7160, using the IRAC and MIPS instruments on board the Spitzer Space Telescope. Our observations cover the wavelength range from 3.6 to 24 µm, allowing us to detect disk emission over a typical range of radii ∼0.1 to ∼20 AU from the central star. In Tr 37, with an age of about 4 Myr, about 48% of the low-mass stars exhibit detectable disk emission in the IRAC bands. Roughly 10% of the stars with disks may be "transition" objects, with essentially photospheric fluxes at wavelengths ≤ 4.5µm but with excesses at longer wavelengths, indicating an optically thin inner disk. The median opticallythick disk emission in Tr 37 is lower than the corresponding median for stars in the younger Taurus region; the decrease in infrared excess is larger at 6 − 8µm than at 24µm, suggesting that grain growth and/or dust settling has proceeded faster at smaller disk radii, as expected on general theoretical grounds. Only about 4% of the low-mass stars in the 10 Myr-old cluster NGC 7160 show detectable infrared disk emission. We also find evidence for 24µm excesses around a few intermediate-mass stars, which may represent so-called "debris disk" systems. Our observations provide new constraints on disk evolution through an important age range.

We present the most sensitive 3 mm-survey to date of protoplanetary disks carried in the Taurus-Auriga star forming region (average rms of about 0.3 mJy), using the IRAM PdBI. With our high detection rate of 17/19, we provide the first detections at wavelengths longer than about 1 mm for 12 sources. This enables us to study statistically the mm SED slopes and dust properties of faint disks and compare them to brighter disks using a uniform analysis method. With these new data and literature measurements at sub-millimeter and millimeter wavelengths, we analyze the dust properties of a sample of 21 isolated disks around T Tauri stars in the Taurus-Auriga star forming region. Together with the information about the disks spatial extension from sub/mm-mm interferometric studies, we derive from the observed sub-mm/mm spectral energy distribution constraints on the dust opacity law at these wavelengths, using two-layer flared disk models and a self-consistent dust model that takes properly into account the variation of the dust opacity with grain growth. We find evidence for the presence in the disk midplane of dust particles that have grown to sizes as large as at least 1 millimeter in all the disks of our sample, confirming what was previously observed on smaller brighter objects. This indicates that the dust coagulation from ISM dust to mm-sized grains is a very fast process in protoplanetary disks, that appears to occur before a young stellar object enters the Class II evolutionary stage. Also, the amount of these large grains in the disk outer regions is stationary throughout all the Class II evolutionary stage, indicating that mechanisms slowing down the dust inward migration are playing an important role in the Taurus-Auriga protoplanetary disks. Another result is that the spectral index between 1 and 3 mm for the 6 faintest disks in our sample is on average smaller than for the brighter disks, indicating either that these fainter, yet unmapped, disks are spatially much less extended than the brighter spatially resolved disks, or that fainter disks have typically larger dust grains in their outer regions. Considering that these fainter disks are more representative of the bulk of the disk population than the brighter ones, this may have important consequences for the theories of planetesimal formation and disk formation and evolution. Finally, we investigate the relations between the derived dust properties, namely dust mass and grain growth, and the properties of the central star, like its mass, age and mass accretion rate.

Cellular structures or tessellations are ubiquitous in nature. Metals and ceramics commonly consist of space-filling arrays of single-crystal grains separated by a network of grain boundaries, and foams (froths) are networks of gas-filled bubbles separated by liquid walls. Cellular structures also occur in biological tissue, and in magnetic, ferroelectric and complex fluid contexts. In many situations, the cell/grain/bubble walls move under the influence of their surface tension (capillarity), with a velocity proportional to their mean curvature. As a result, the cells evolve and the structure coarsens. Over 50 years ago, von Neumann derived an exact formula for the growth rate of a cell in a two-dimensional cellular structure (using the relation between wall velocity and mean curvature, the fact that three domain walls meet at 120u and basic topology). This forms the basis of modern grain growth theory. Here we present an exact and much-sought extension of this result into three (and higher) dimensions. The present results may lead to the development of predictive models for capillarity-driven microstructure evolution in a wide range of industrial and commercial processing scenarios-such as the heat treatment of metals, or even controlling the 'head' on a pint of beer.

Although conventional Mg alloys develop strong crystallographic textures during deformation that persist during annealing, the addition of rare earth (RE) elements can induce comparably weaker textures. The texture weakening effect is explored using hot-rolled Mg-Y alloys of a single phase to focus on the possibility of solute effects. Of the studied compositions, the richer alloys (‡0.17 at. pct) show the weakening effect, whereas the most dilute alloy (£0.03 at. pct) does not. Electron backscattered diffraction (EBSD) analysis of intragranular misorientation axes (IGMA) indicate that the geometrically necessary dislocation (GND) content in dilute, hotrolled alloys contain primarily basal hai dislocations. At higher concentrations, the dislocations are predominantly prismatic hai type. This change in the GND content suggests a change in dynamic recrystallization (DRX) mode. For example, nonbasal cross slip has been associated with continuous DRX. Furthermore, nonbasal slip might also promote more homogenous shear banding/twinning. Both of these mechanisms have been shown previously to give rise to more randomly oriented nuclei during DRX. Energy dispersive X-ray spectroscopy performed through transmission electron microscopy shows that Mg-Y exhibits significant grain boundary solute segregation, consistent with recent observations of solute clustering. Slow grain growth may be explained by solute drag. It is hypothesized that limited grain boundary mobility suppresses conventional discontinuous DRX, which has been shown to retain the deformation texture. The promotion of nonbasal slip and suppression of grain boundary mobility are proposed as solid solution-based mechanisms responsible for the observed texture weakening phenomenon in Mg rare earth alloys.

21 31 The role of B O addition on the long phosphorescence of SrAl O :Eu , Dy has been investigated. B O is just not an inert high 2 3 2 4 2 3 temperature solvent (flux) to accelerate grain growth, according to SEM results. B O has a substitutional effect, even at low 4 21 charge density than Eu , prefers to occupy the Sr sites in the neighbourhood of BO , as the effective charge on borate is more negative 4 31 than that of AlO. Thus, Dy forms a substitutional defect complex with borate and acts as an acceptor-type defect center. These defects 4 21 trap the hole generated by the excitation of Eu ions and the subsequent thermal release of hole at room temperature followed by the recombination with electron resulting in the long persistent phosphorescence.

The mechanical properties of titanium nitride (TiN x ) thin films have been investigated using depth sensing nanoindentation tests. The effects of substrate temperature (T s ) and of substrate biasing (V b ) on the mechanical properties and the microstructure of the TiN x films were studied. T s and V b have strong effect on the film's microstructural characteristics such as density, grain size and orientation. It was found that deposition at high T s and V b promotes the growth of (002) oriented films with density close to the bulk density of stoichiometric TiN, indicating the absence of voids and the growth of stoichiometric TiN. The film hardness and elastic modulus were measured using the continuous stiffness measurements technique. It was found that there exists a direct correlation between the film's mechanical properties and microstructure. The films that exhibit the best mechanical performance are those grown along the (002) orientation and being denser and stoichiometric.

The crystal growth process in colloidal nanocrystal systems is usually associated with the Ostwald-ripening mechanism. Here, we report on experimental evidence indicating that another crystal growth process took place in a colloidal nanocrystal system at room temperature. This crystal growth process is based on grain rotation among neighboring grains, resulting in a coherent graingrain interface, which, by eliminating common boundaries, causes neighboring grains to coalesce, thereby forming a single larger nanocrystal. This phenomenon was observed in SnO 2 nanocrystals ͑particle size ranging from 10 to 30 Å͒.

We present an analysis of the optical spectra of 75 early-type emission-line stars, many of which have been classified previously as Herbig Ae/ Be (HAeBe) stars. Accurate spectral types were derived for 58 members of the sample; high continuum veiling, contamination by nonphotospheric absorption features, or a composite binary spectrum prevented accurate spectral typing for the rest. Approximately half of our sample exhibited [O i] k6300 forbidden-line emission down to our detection limit of 0.1 Å equivalent width; a third of the sample exhibited Fe ii emission (multiplet 42). A subset of 11 of the HAeBe sample showed abnormally strong Fe ii absorption; 75% of this subset are confirmed UX Ori objects. Combining our spectral typing results with photometry from the literature, we confirm previous findings of high values of total-to-selective extinction (R V $ 5) in our larger sample, suggesting significant grain growth in the environments of HAeBe stars. With this high value of R V , the vast majority of HAeBe stars appear younger than with the standard R V ¼ 3:1 extinction law and are more consistent with being pre-main-sequence objects.

We have investigated the structural, microstructural, interfacial, and surface morphological properties of Ba Sr TiO thin 0.60 0.40 3 films Mg doped from 0 to 20 mol%. A strong correlation was observed between the films materials properties and the prior determined dielectric and insulating characteristics as a function of Mg doping. Non-textured polycrystalline films with a dense microstructure and abrupt film-Pt electrode interface were obtained after annealing at 750ЊC for 30 min. Single phase solid solution films were achieved at Mg doping levels up to 5 mol%, while multiphased films were obtained for Mg doping levels of 20 mol%. Decreases in the films dielectric constant, dielectric loss, tunability and leakage current characteristics were paralleled by a reduction in grain size as a function of increasing Mg dopant concentration. Our results suggest that Mg doping serves to limit grain growth and is thereby responsible for lowering the dielectric constant from 450 to 205. It is suggested that Mg behaves as an acceptor-type dopant at the grain boundary and is responsible for the doped films low dielectric loss and good leakage current characteristics. Examination of the performance-property trade-offs advocates the 5-mol% Mg doped BST film to be an excellent choice for tunable microwave device applications. ᮊ

Textured silicon nitride (Si 3 N 4) has been intensively studied over the past 15 years because of its use for achieving its superthermal and mechanical properties. In this review we present the fundamental aspects of the processing and anisotropic properties of textured Si 3 N 4 , with emphasis on the anisotropic and abnormal grain growth of β-Si 3 N 4 , texture structure and texture analysis, processing methods and anisotropic properties. On the basis of the texturing mechanisms, the processing methods described in this article have been classified into two types: hot-working (HW) and templated grain growth (TGG). The HW method includes the hot-pressing, hot-forging and sinter-forging techniques, and the TGG method includes the cold-pressing, extrusion, tape-casting and strong magnetic field alignment techniques for β-Si 3 N 4 seed crystals. Each processing technique is thoroughly discussed in terms of theoretical models and experimental data, including the texturing mechanisms and the factors affecting texture development. Also, methods of synthesizing the rodlike β-Si 3 N 4 single crystals are presented. Various anisotropic properties of textured Si 3 N 4 and their origins are thoroughly described and discussed, such as hardness, elastic modulus, bending strength, fracture toughness, fracture energy, creep behavior, tribological and wear behavior, erosion behavior, contact damage behavior and thermal conductivity. Models are analyzed to determine the thermal anisotropy by considering the intrinsic thermal anisotropy, degree of orientation and various microstructure factors. Textured porous Si 3 N 4 with a unique microstructure composed of oriented elongated β-Si 3 N 4 and anisotropic pores is also described for the first time, with emphasis on its unique mechanical and thermal-mechanical properties. Moreover, as an important related material, textured α-Sialon is also reviewed, because the presence of elongated α-Sialon grains allows the production of textured α-Sialon using the same methods as those used for textured β-Si 3 N 4 and β-Sialon.

A new mechanistic model of N uptake and redistribution in wheat crops incorporating the effects of N shortages through variation in leaf area is described. The model is based on the assumption that the concentration of N in green tissue can be approximated as constant per unit leaf area and that excess N is stored in non-green tissue. Two hypotheses about how grain demand for N is set were also tested. These were that N-demand by the grain is set mostly in response to total N content in the crop (source determined), or that the N-demand is set according to the grain number per unit area (sink determined). The model was tested using data from widely separated, independent experiments. The model produced substantial improvement in the simulation of green area index (GAI) compared with an alternative older model, particularly in response to changes in N-supply, while retaining the ability to simulate biomass accumulation and grain growth accurately. Of the two hypotheses on the determi...

ZnO thin films were fabricated on glass substrate by sol-gel method. A stable and homogeneous solution was prepared by dissolving zinc acetate dehydrate as a starting material in a solution of 2-methoxyethanol and monoethanolamine (MEA). The precursor concentration was selected as the parameter to optimize the thin films quality. The molar ratio of zinc acetate dehydrate was kept at 1. The thin films were preheated at 300° C for 10 minutes after each coating. After depositing all layers, the thin films are annealed for 1 hour in order to obtain transparent ZnO thin films. It was found that the precursor concentration affect the IV characteristic, transmittance, and the thickness of the resultant ZnO thin films.

We present observations of six isolated, pre-main-sequence, intermediate mass stars selected for shallow spectra at submillimeter wavelengths at 1.3, 2.6, 7.0, and 36 millimeters from the IRAM PdBI and the VLA. We analyze the new observations of these stars (HD 34282, HD 35187, HD 142666, HD 143006, HD 150193, HD 163296) together with similar observations of three additional stars from the literature (CQ Tau, UX Ori, TW Hya), in the context of self-consistent irradiated disk models. Our aim is to constrain the wavelength dependence of the dust opacity and the total dust mass in the disks. The shallow wavelength dependence of the opacity is confirmed and for a few stars extended to significantly longer wavelengths. For any plausible dust properties, this requires grain growth from interstellar sizes to maximum sizes of at least a few millimeters, and very likely to several centimeters or more. For four of the stars (HD 34282, HD 163296, CQ Tau, TW Hya), the millimeter emission has been spatially resolved, and the large disk radii (>100 AU) rule out that high optical depths play a role. The mass of dust that has been processed into large grains is substantial, and in some cases implies a disk mass comparable to the mass of the central star.

A good choice of model formulation and model parameters is one of the most important and difficult aspects in mesoscale modeling and requires a systematic and quantitative analysis. In this paper, it is studied how the model parameters of a generalized phase field model affect the landscape of the free-energy density functional, the phase field profiles at the grain boundaries, and the corresponding trajectory along the free-energy landscape. The analysis results in quantitative relations between the model parameters, on one hand, and grain boundary energy and mobility, on the other hand. Based on these findings, a procedure is derived that generates a suitable set of model parameters that reproduces accurately a material's grain boundary energy and mobility for arbitrary misorientation and inclination dependence. The misorientation and inclination dependence are formulated so that the diffuse interface width is constant, resulting in uniform stability and accuracy conditions for the numerical solution. The proposed model formulation and parameter choice allow us to perform quantitative simulations with excellent controllability of the numerical accuracy and therefore of the material behavior.

Single crystals of Ba͑Zr x Ti 1Ϫx )O 3 were grown by templated grain growth ͑TGG͒. Millimeter size single crystals of Ba͑Zr x Ti 1Ϫx )O 3 were produced by heating a BaTiO 3 crystal in contact with a sintered polycrystalline matrix of 4.5, 5.0, or 8.5 mol % Zr-doped barium titanate for 30 h at 1350°C. To facilitate boundary migration, the ceramic compact was made 3 mol % TiO 2 excess. The 4.5 and 5.0 mol % Zr-doped crystals were orthorhombic at room temperature, and for a pseudocubic ͑001͒ orientation, they showed remanent polarizations of 13 C/cm 2 and a high field d 33 of 340-355 pC/N. The 8.5 mol % Zr-doped crystal ͓again oriented along the pseudocubic ͑001͔͒ was rhombohedral at room temperature with a remanent polarization of 10 C/cm 2 . A k 33 value of 0.74 from resonance measurements was observed for the 4.5 mol % Zr-doped crystal.

We present 7-mm continuum observations of 14 low-mass pre-main-sequence stars in the Taurus-Auriga star-forming region obtained with the Very Large Array with ∼1. ′′ 5 resolution and ∼0.3 mJy rms sensitivity. For 10 objects, the circumstellar emission has been spatially resolved. The large outer disk radii derived suggest that the emission at this wavelength is mostly optically thin. The millimetre spectral energy distributions are characterised by spectral indices αmm = 2.3 to 3.2. After accounting for contributions from free-free emission and corrections for optical depth, we determine dust opacity indices β in the range 0.5 to 1.6, which suggest that millimetre-sized dust aggregates are present in the circumstellar disks. Four of the sources with β > 1 may be consistent with submicron-sized dust as found in the interstellar medium. Our findings indicate that dust grain growth to millimetre-sized particles is completed within less than 1 Myr for the majority of circumstellar disks.

The microstructure of the eutectic alloy Fe 30 Ni 20 Mn 35 Al 15 (in at.%) was modified by cooling at different rates from 1623 K, i.e., above the eutectic temperature. The lamellar spacing decreased with increasing cooling rate, and in water-quenched specimens lamellae widths of *100 nm were obtained. The orientation relationship between the fcc and B2 lamellae was found to be sensitive to the cooling rate. In a drop-cast alloy the Kurdjumov-Sachs orientation relationship dominated, whereas the orientation relationship in an arc-melted alloy with a faster cooling rate was fcc " 112 ð Þ==B2 011 ð Þ; fcc 1 " 11 ½ ==B2 1 " 11 ½ and fcc 0 " 11 ð Þ==B2 001 ð Þ; fcc 011 ½ ==B2 " 1 " 10 ½ . The hardness increased with microstructural refinement, obeying a Hall-Petch-type relationship. The strength of the alloy decreased significantly above 600 K due to softening of the B2 phase.