Showing papers on "Grain size published in 2009"

Tuning the Grain Size and Particle Size of Superparamagnetic Fe3O4 Microparticles

Abstract: Secondary structural, superparamagnetic Fe3O4 microparticles with an average diameter of 280 nm have been successfully synthesized by using a one-step hydrothermal method. The size of the primary nanograins has been controlled from 5.9 to 21.5 nm by varying the sodium acrylate/sodium acetate weight ratios. The magnetic properties of the Fe3O4 microparticles have been characterized at room temperature, whereas the saturation magnetization values of the Fe3O4 microparticles increase with increasing grain sizes. Magnetic resonance imaging reveals that Fe3O4 microparticle with larger grain size yields higher molar T2 relaxation rate. A plausible growth mechanism of the particles is proposed, and the role of sodium acrylate and sodium acetate for tuning the grain size of the particles has been discussed. Additionally, the size of the secondary structural Fe3O4 particles can also be continuously controlled from 6 to 170 nm by varying the volume ratio of ethylene glycol/diethylene glycol in a bisolvent system. T...

Giant magnetic-field-induced strains in polycrystalline Ni–Mn–Ga foams

Abstract: The magnetic shape-memory alloy Ni-Mn-Ga shows, in monocrystalline form, a reversible magnetic-field-induced strain (MFIS) up to 10%. This strain, which is produced by twin boundaries moving solely by internal stresses generated by magnetic anisotropy energy, can be used in actuators, sensors and energy-harvesting devices. Compared with monocrystalline Ni-Mn-Ga, fine-grained Ni-Mn-Ga is much easier to process but shows near-zero MFIS because twin boundary motion is inhibited by constraints imposed by grain boundaries. Recently, we showed that partial removal of these constraints, by introducing pores with sizes similar to grains, resulted in MFIS values of 0.12% in polycrystalline Ni-Mn-Ga foams, close to those of the best commercial magnetostrictive materials. Here, we demonstrate that introducing pores smaller than the grain size further reduces constraints and markedly increases MFIS to 2.0-8.7%. These strains, which remain stable over >200,000 cycles, are much larger than those of any polycrystalline, active material.

Grain boundary segregation and thermodynamically stable binary nanocrystalline alloys

Abstract: A free-energy function for binary polycrystalline solid solutions is developed based on pairwise nearest-neighbor interactions. The model permits intergranular regions to exhibit unique energetics and compositions from grain interiors, under the assumption of random site occupation in each region. For a given composition, there is an equilibrium grain size, and the alloy configuration in equilibrium generally involves solute segregation. The present approach reduces to a standard model of grain boundary segregation in the limit of infinite grain size, but substantially generalizes prior thermodynamic models for nanoscale alloy systems. In particular, the present model allows consideration of weakly segregating systems, systems away from the dilute limit, and is derived for structures of arbitrary dimensionality. A series of solutions for the equilibrium alloy configuration and grain size are also presented as a function of simple input parameters, including temperature, alloy interaction energies, and component grain boundary energies.

Microstructural evolution in high purity aluminum processed by ECAP

Abstract: High purity (9999%) aluminum was processed by equal-channel angular pressing (ECAP) through 1–12 passes and examined using orientation imaging microscopy The results reveal two distinct processing regimes: from 1 to 4 passes the microstructure evolves from elongated subgrains to an essentially equiaxed array of ultrafine grains and from 4 to 12 passes there is no measurable change in the average grain size and grain aspect ratio The boundary misorientation angle and the fraction of high-angle boundaries increase rapidly up to 4 passes and at a slower rate from 4 to 12 passes Anomalously large grains were visible in the central region of the billet pressed through 12 passes due to dynamic recovery and grain growth The results suggest optimum processing is achieved by pressing through 4–8 passes

Grain boundary segregation and thermodynamically stable binary nanocrystalline alloys

Abstract: A free-energy function for binary polycrystalline solid solutions is developed based on pairwise nearestneighbor interactions. The model permits intergranular regions to exhibit unique energetics and compositions from grain interiors, under the assumption of random site occupation in each region. For a given composition, there is an equilibrium grain size, and the alloy configuration in equilibrium generally involves solute segregation. The present approach reduces to a standard model of grain boundary segregation in the limit of infinite grain size, but substantially generalizes prior thermodynamic models for nanoscale alloy systems. In particular, the present model allows consideration of weakly segregating systems, systems away from the dilute limit, and is derived for structures of arbitrary dimensionality. A series of solutions for the equilibrium alloy configuration and grain size are also presented as a function of simple input parameters, including temperature, alloy interaction energies, and component grain boundary energies.

Recrystallization mechanism of as-cast AZ91 magnesium alloy during hot compressive deformation

Abstract: Dynamic recrystallization (DRX) behavior of as-cast AZ91 magnesium alloy during hot compression at 300 °C and the strain rate of 0.2 s−1 was systematically investigated by electron backscattering diffraction (EBSD) analysis. Twin DRX and continuous DRX (CDRX) are observed in grains and near grain boundaries, respectively. Original coarse grains are firstly divided by primary { 1 0 1 ¯ 2 } tensile twins and { 1 0 1 ¯ 1 } compression twins, and then { 1 0 1 ¯ 1 }–{ 1 0 1 ¯ 2 } double twins are rapidly propagated within these primary compression twins with increasing compressive strain. Some twin-walled grains are formed by the mutual crossing of twins or by the formation of the { 1 0 1 ¯ 1 }–{ 1 0 1 ¯ 2 } double twins and furthermore, subgrains divided by low-grain boundaries in the double twins are also formed. Finally, DRXed grains are formed by the in situ evolution of the subgrains with the growth of low-angle boundaries to high-angle grain boundaries in twins. CDRX around the eutectic Mg17Al12 phases at grain boundaries occurs together with the precipitation of discontinuous Mg17Al12 phase and the fragmentation of the precipitates during compression. The discontinuous fragmented precipitates distribute at the newly formed CDRXed grain boundaries and have remarkable pinning effect on the CDRXed grain growth, resulting in the average grain size of about 1.5 μm.

Fabrication of high temperature oxides dispersion strengthened tungsten composites by spark plasma sintering process

Abstract: The paper describes the fabrication process of high temperature oxides, such as Y 2 O 3 , HfO 2 and La 2 O 3 , dispersed tungsten composites by spark plasma sintering. The oxide contents varied from 0 to 5 wt% and sintering was conducted for 3 min at 1700 °C. Among three kinds of oxides, Y 2 O 3 is the most efficient element to consolidate W powder. As dispersed up to 5 wt% Y 2 O 3 into the matrix, the relative density of the W composite is increased up to nearly 100% of theoretical value. In order to analyze the effect of Y 2 O 3 particles on the densification of W powders, the microstructure of W–Y 2 O 3 composite is observed using the transmission electron microscopy. By this experiment, it is found that dark phases, which had been known as Y 2 O 3 phase, are composed of W, Y and O. Therefore, during sintering, W atoms move through Y 2 O 3 phases as well as W grain boundaries, thereby W and Y 2 O 3 are soluble, and so sinterability of W is enhanced. The hardness of the composite is increased from 350 to 510 kg/mm 2 with increasing Y 2 O 3 contents since the relative density is increased and the grain size is reduced from 20 to 4 μm. However, in case of HfO 2 and La 2 O 3 , the hardness of the composites is decreased even though the grain size is reduced because of their lower relative densities.

Effects of process variables and size-scale on solidification microstructure in beam-based fabrication of bulky 3D structures

Abstract: A number of laser and electron beam-based fabrication processes are under consideration for aerospace components, where the ability to obtain a consistent and desirable microstructure and resulting mechanical properties is of critical concern. To this end, this work employs a combination of analytical and numerical modeling approaches to investigate the effects of process variables and size-scale on solidification microstructure (grain size and morphology) in beam-based fabrication of bulky 3D structures. Thermal process maps are developed for predicting solidification microstructure in any material system, and results are plotted on solidification maps to investigate trends in grain size and morphology in Ti–6Al–4V. The results of this work suggest that changes in process variables (beam power and velocity) can result in a grading of the microstructure throughout the depth of the deposit, with a transition from columnar to mixed or equiaxed microstructure at higher powers.

New opportunities for 3D materials science of polycrystalline materials at the micrometre lengthscale by combined use of X-ray diffraction and X-ray imaging

Abstract: Non-destructive, three-dimensional (3D) characterization of the grain structure in mono-phase polycrystalline materials is an open challenge in material science. Recent advances in synchrotron based X-ray imaging and diffraction techniques offer interesting possibilities for mapping 3D grain shapes and crystallographic orientations for certain categories of polycrystalline materials. Direct visualisation of the three-dimensional grain boundary network or of two-phase (duplex) grain structures by means of absorption and/or phase contrast techniques may be possible, but is restricted to specific material systems. A recent extension of this methodology, termed X-ray diffraction contrast tomography (DCT), combines the principles of X-ray diffraction imaging, three-dimensional X-ray diffraction microscopy (3DXRD) and image reconstruction from projections. DCT provides simultaneous access to 3D grain shape, crystallographic orientation and local attenuation coefficient distribution. The technique applies to the larger range of plastically undeformed, polycrystalline mono-phase materials, provided some conditions on grain size and texture are fulfilled. The straightforward combination with high-resolution microtomography opens interesting new possibilities for the observation of microstructure related damage and deformation mechanisms in these materials.

Effects of heating rate on microstructure and transparency of spark-plasma-sintered alumina

Abstract: Commercial alumina powder was densified by spark plasma sintering (SPS) at 1150 °C. During SPS processing, the effects of the heating rate were examined on microstructure and transparency. With decreasing heating rate, the grain size and the residual porosity decreased, while the transparency increased. At a heating rate of 2 °C/min, the grain size was 0.29 μm, and the in-line transmission was 46% for a wavelength of 640 nm. The mechanisms for the fine microstructure and low porosity at slow heating, which are conflicting with some existing results, were explained by considering the role of defect concentration and grain-boundary diffusion during densification.

Effect of mixed grain sizes on thermoelectric performance of Bi2Te3 compound

Abstract: Coarse (about 1 μm) and fine (about 100 nm) Bi2Te3 particles were synthesized by mechanical alloying for different times, respectively. Their mixtures at different ratios were consolidated by spark plasma sintering to produce nano-/microstructured composites with the same chemical compositions. The Seebeck coefficient was significantly enhanced by addition of fine particles through the potential barrier scattering. The electrical resistivity increased rapidly when the weight fraction of fine particles exceeded about 60%, probably due to an effect similar to percolation phenomenon. However, the thermal conductivity was reduced almost linearly with increasing fraction of fine particles. Because of the combined effects of the potential barrier scattering, nonlinear change in electrical property and linear change in thermal transport property, a peak value of dimensionless figure of merit (ZT) was achieved for the sample containing 60% fine particles. This study demonstrated the possibility to enhance ZT valu...

Increase of Mn solubility with decreasing grain size in ZnO

Abstract: Nanograined (grain size 20 nm) ZnO films with various Mn content (from 0 to 47 at%) were synthesized by the novel wet chemistry method. The solubility limit for Mn was determined at 550 ◦ C. The lattice parameter c of the ZnO-based solid solution with wurzite structure ceases to grow at 30 at% Mn. The peaks of the second phase (Mn3O4 with cubic lattice) become visible in the X-rays diffraction spectra at 30 at% Mn. The same second phase appears in the bulk ZnO already at 12 at% Mn. The recently published papers on the structure and magnetic behaviour of Mn-doped ZnO allowed us to obtain the size-dependence of Mn solubility in ZnO for the polycrystals and small single-crystalline particles. The overall Mn solubility drastically increases with decreasing grain size. The quantitative estimation leads to the conclusion that, close to the bulk solubility limit, the thickness of an Mn-enriched layer is several monolayers in GBs and at least two monolayers in the free surfaces. © 2009 Elsevier Ltd. All rights reserved.

Densification of nanocrystalline Y2O3 ceramic powder by spark plasma sintering

Abstract: Nanocrystalline Y 2 O 3 powders with 18 nm crystallite size were sintered using spark plasma sintering (SPS) at different conditions between 1100 and 1600 °C. Dense specimens were fabricated at 100 MPa and 1400 °C for 5 min duration. A maximum in density was observed at 1400 °C. The grain size continuously increased with the SPS temperature into the micrometer size range. The maximum in density arises from competition between densification and grain growth. Retarded densification above 1400 °C is associated with enhanced grain growth that resulted in residual pores within the grains. Analysis of the grain growth kinetics resulted in activation energy of 150 kJ mol −1 and associated diffusion coefficients higher by 10 3 than expected for Y 3+ grain boundary diffusion. The enhanced diffusion may be explained by combined surface diffusion and particle coarsening during the heating up with grain boundary diffusion at the SPS temperature.

Grain Size Effects in Sensor Response of Nanostructured SnO2- and In2O3-Based Conductometric Thin Film Gas Sensor

Abstract: Based on the experimental results, obtained by studying both structural and gas-sensing properties of the SnO2 and In2O3 films deposited by the spray pyrolysis method, we analyzed the influence of crystallite size on the parameters of the SnO2- and In2O3-based thin film solid-state gas sensors. For comparison, the behavior of ceramic-type gas sensors was considered as well. In particular, we examined the correlation between the grain size and parameters of conductometric-type gas sensors such as the magnitude of sensor signal, the rate of sensor response, thermal stability, and the sensitivity of sensor signal to air humidity. Findings confirmed that that grain size is one of the most important parameters of metal oxides, controlling almost all operating characteristics of the solid state gas sensors fabricated using both the ceramic and thin film technologies. However, it was shown that there is no single universal requirement for the grain size, because changes in grain size could either improve, or wor...

New Fe-metalloids based nanocrystalline alloys with high Bs of 1.9 T and excellent magnetic softness

Abstract: The melt-spun Fe83.3–84.3Si4B8P3–4Cu0.7 alloys have a heterogeneous amorphous structure including a large amount of α‐Fe-like clusters. Compared with the FeSiB alloys, the particle size rapidly decreases from several hundred nanometers to 2–3nm due to the proper amounts of simultaneous P and Cu additions. By controlling the crystallization process, a homogeneous nanocrystalline structure composed by small α‐Fe grains with size of 10–17nm in diameter can be realized from the heterogeneous amorphous alloys. The nanocrystallized Fe83.3–84.3Si4B8P3–4Cu0.7 alloys show the extremely high saturation magnetic flux density of 1.88–1.94T sufficiently near to 1.97T of Fe‐3.5mass%Si crystalline soft magnetic alloys, and exhibit low coercivity of less than 10A∕m and higher effective permeability of 16 000–25 000 at 1kHz due to the simultaneous realization of the homogeneous nanocrystalline structure and the small magnetostriction of (2–3)×10−6. In addition, the nanocrystalline alloys exhibit the superior core loss to ...