Final Average Grain Research Articles

High coercivity in boron substituted Sm-Co melt-spun magnets

The structural and magnetic properties of nanocomposite melt-spun Sm(Co/sub 0.74-x/Fe/sub 0.1/Cu/sub 0.12/Zr/sub 0.04/B/sub x/)/sub 7.5/ magnets have been investigated as a function of boron content (x = 0.005 0.05), wheel speed and annealing conditions. The as-spun ribbons are nanocrystalline with fine microstructure and average grain size of 60-100 nm. X-ray diffraction indicates that the as-spun samples have the metastable hexagonal TbCu/sub 7/-type structure phase and fcc-Co as a secondary soft phase. Magnetization at nonsaturating 5 T field is 45-72 emu/g and the reduced remanence (Mr/Ms) is above 0.8. The loop shape exhibits a characteristic step due to the soft magnetic phase. At room temperature, Hc values of 20-28 kOe are obtained for as spun samples, with a record value of 38.5 kOe for x = 0.04. At 380/spl deg/C Hc values higher than 5 kOe are observed. Coercivity and loop shape are strongly dependent on annealing conditions.

Crystallization of Zr<SUB>55</SUB>Al<SUB>10</SUB>Ni<SUB>5</SUB>Cu<SUB>30</SUB> Bulk Metallic Glass Composites Containing ZrC Particles

Zr 55 Al 10 Ni 5 Cu 30 bulk metallic glass composite containing 10 vol% crystalline ZrC particles was prepared by an in-situ reaction between Zr-based melt and graphite powder being followed by an injection casting into a copper mould. The crystallization process of the composites during annealing at 723 K has been investigated by using DSC, TEM and microhardness test. It is found that an unknown crystalline phase appears first along the interface between ZrC particle and the matrix of the composite, and then a homogeneous nanocrystallization occurs in the matrix. The nanocrystalline phase precipitated in the matrix is determined to have orthorhombic structure by using TEM diffraction. It has a higher hardness than the hardness of the amorphous phase which remained in the matrix. Hardness of the matrix increases with increasing the volume fraction of the nanocrystalline phases. After annealed at 723 K for 40 min, most of the matrix has been transformed into crystalline phases with a final average grain size of about 120 nm.

Characterization of Cast Metals with Probability Distribution Functions

AbstractCharacterization of microstructure using a probability distribution function (PDF) provides a means for extracting useful information about material properties. In the extension of classical PDF methods developed in our research, material characteristics are evolved by propagating an initial PDF through time, using growth laws derived from consideration of heat flow and species diffusion, constrained by the Gibbs-Thomson law. A model is described here that allows for nucleation, followed by growth of nominally spherical grains according to a stable or unstable growth law. Results are presented for the final average grain size as a function of cooling rate for various nucleation parameters. In particular we show that the model describes linear variation of final grain size with the inverse cube root of cooling rate. Within a subset of casting parameters, the stable-to-unstable manifests itself as a bimodal distribution of final grain size. Calculations with the model are described for the liquid to epsilon phase transition in a plutonium I weight percent gallium alloy.

Influence of yttria content on the preparation of nanocrystalline yttria-doped zirconia

Nanocrystalline zirconia doped with 0–10 mol % Y2O3 has been prepared by a combustion synthesis process, followed by a rapid densification process. The concentration of Y2O3 in the as-reacted zirconia appeared to have a significant influence on the reduction of the crystallite size, in the combustion temperature range studied (450 °C–550 °C), as well as on the stabilization of the tetragonal and/or cubic phases. The green compacts were densified by a fast-firing process. During fast-firing, the dwell temperature significantly affected the final average grain size and the final density of the article. On the other hand, the ranges of heating rates and dwell times that were used in this study were shown to have a much less significant effect on the article's final density and final average grain size. The yttria content had the largest influence on the final density and final average grain size. The densification took place much more rapidly in the 4 mol % Y2O3-ZrO2 samples than in the 10 mol % Y2O3-ZrO2 samples. In particular, the difference in densification rates between the samples with different Y2O3 content was attributed to the influence and magnitude of the associated grain-growth process. It was determined, however, that a high final density (>99% ρth) and a very fine final average grain size (<200 nm) could be simultaneously achieved with each of three different heating rates for the 4 mol% Y2O3-ZrO2 articles.

Preparation of nanocrystalline yttria-stabilized zirconia

Nanocrystalline powder with an average crystallite size of 8–12 nm, which was produced by a combustion synthesis process, was used to prepare dense, nanocrystalline articles. Green compacts of high green density were prepared by dry pressing and densified by a fast-firing process. During fast-firing, the dwell temperature significantly affected the final grain size and final density. On the other hand, the ranges of heating rates and dwell times that were used had a much less significant effect on the final density and final grain size. It was determined, however, that a high final density (>99% ρth) and a very fine final average grain size (<200 nm) can be simultaneously achieved under three different firing conditions. The high densification rates are, in part, a result of the minimal coarsening that the particles undergo when the sample is taken rapidly through the temperature regime in which surface diffusion predominates to the temperature regime in which the densification mechanisms of grain boundary and lattice diffusion predominate.

Structure and thermal evolution of57Fe implanted chromium

57Fe ion implanted chromium foils have been analyzed by conversion electron Mossbauer spectroscopy. After annealing at temperatures up to 600 °C, the CEM spectra show two magnetic components which are attributed to iron atoms in the core and the outer layer of grains. By means of a simple model using the spectral data, the final average iron grain has been evaluated to be about 5 nm.

A general theory of secondary recrystallization in grain oriented Fe-Si: Metallurgical parameters controlling the microstructural evolution

The secondary recrystallization (SR) process and the related effects on the magnetic properties of silicon-iron are discussed in the framework of the general statistical theory of grain growth including texture and Zener drag influence. It has been shown that SR in silicon-iron is related to a selective growth process experienced by Goss grains due to specific grain boundary conditions generated by the surrounding grains (texture). In this paper some examples showing the specific role of various microstructural parameters influencing the selectivity of SR (such as Zener drag intensity and its drop rate, initial grain size, etc.), are discussed and related to the magnetic properties, which are mainly controlled by the final average grain size and orientation.

Rock magnetic characteristics of surficial marine sediments from the California continental borderland

The rock magnetic variability of surficial marine sediments from the California continental borderland adjacent to southern California is described. The data were derived from measurement of the natural (NRM), anhysteretic (ARM), and isothermal (IRM) remanent magnetization and magnetic susceptibility of 385 samples from 230 ☐ cores. Magnetic mineralogy studies carried out on selected samples indicate that detrital magnetite predominates throughout the borderland, but there are also locally important contributions from detrital and authigenic hematite, altered glauconite, and biogenic magnetite. Maps of the distributions of various rock magnetic parameters show that: (1) the proportion of magnetic material decreases outward from the coastline through dilution by organic material and carbonate, (2) the magnetic grain size distribution shifts to finer average grain size outward from the coastline, (3) important local contributions to the sediment magnetic material can come from erosion of submerged bank tops or ridges, (4) glauconite-rich zones on bank tops can be altered to remanent magnetic material that is anomalously strong in intensity and coercivity, and (5) that diagenetic processes occur ubiquitously within the deep basins causing large scale dissolution of the magnetic material. Comparisons of ARM, χ , and SIRM (saturation IRM) indicate that ARMχ is very sensitive to grain-size changes and that both the quantity (ARM 2 + χ 2) 0.5 and SIRM are good indicators of volume of remanent magnetic material in the borderland.

Oxidation of copper and reduction of Cu2O in an environmental scanning electron microscope at 800�C

The present study reports on the in situ oxidation of copper to Cu2O and subsequent reduction to metallic copper in an environmental scanning electron microscope. The oxidation was carried out at approximately 125 μm oxygen pressure while the reduction was done at the same pressure using H2 gas. The first visible signs of oxidation occurred in about 2 min: submicron size nuclei of Cu2O formed randomly on the metal surface. No preferred nucleation along grain boundaries could be observed. The surface was completely covered with Cu2O in about 30 min of oxidation time, the final average grain size of Cu2O being approximately 3 μm. The reduction kinetics of Cu2O were slower than the oxidation kinetics, the first visible copper nuclei appearing only after about 6 min of reduction. A reduction mechanism is suggested where the diffusion of copper vacancies from the copper particle-Cu2O interface to the Cu2O-H2 interface limits the overall kinetics. Based on this assumption, a copper supersaturation corresponding to a copper activity of 1.0005 in Cu2O has been calculated.

Computer simulation of grain growth-III. Influence of a particle dispersion

A Monte Carlo computer simulation technique has been developed which models grain growth in the presence of a particle dispersion. The simulation allows for the monitoring of an evolving microstructure as a function of time. The model predicts normal grain growth, i.e. R ̄ = Ct n , where R ̄ is the average grain size and n is the grain growth exponent, followed by an abrupt transition to a pinned state. Both the exponent n and the grain size distribution are found to be in close agreement with that observed for grain growth in the absence of particles. The grain size distribution and kinetics are independent of particle concentration. The final average grain area and the time required for the microstructure to pin are both approximately proportional to the inverse of the particle concentration. The results are quantitatively accounted for in terms of a simple topological theory.

Temperature ratios for measuring efficiency and for the control of driers

Various ratios based on the dry and wet bulb temperatures of the drying and exhaust air of a crop drier have been considered by previous workers as measures of performance. These ratios were found to be unsatisfactory as they take no account of the sensible heat in the ambient air. A ratio has been proposed which considers the sensible heat in the drying air as being the effective heat for drying. The relationship between this ratio and the three drying parameters, air flow rate, drying air temperature and the final average grain moisture content, is established experimentally and a chart showing these relationships for a given moisture content developed. From this chart, knowing the air flow rate and the drying air temperature, it is possible to determine the value of the ratio at which the drying operation should be terminated. Alternatively air flow rates and drying air temperatures to give a high drying efficiency may be selected.