Coarse-grained Polycrystalline Research Articles

A lower bound for the volume-averaged mean-square magnetostatic stray field

Based on a micromagnetics model, we develop a method through which quantitative information on the volume-averaged mean-square magnetostatic stray field �| H b| 2 � v due to non-zero divergences of the magnetization M within the bulk of a ferromagnetic body can be obtained by analysis of magnetic-field- dependent small-angle neutron scattering data. In the limit of high applied magnetic field Ha, when the direction of M deviates only sligthly from Ha, we have estimated a lower bound for �| H b| 2 � v as a function of the external field, and we have applied the method to bulk samples of nanocrystalline electrodeposited Ni and Co and coarse-grained polycrystalline cold-worked Ni. The root-mean-square magnetostatic stray field, which is inherent to a particular magnetic microstructure, shows a pronounced field dependence, with values ranging from about 5 to 50 mT. Even at applied fields as large as 1. 7T , the quantityµ0�| H b| 2 � 1/2 v of nanocrystalline Co is still 24 mT, which suggests that contributions to the total magnetostatic field originating from the bulk are significant in nanocrystalline ferromagnets; therefore, �| H b| 2 � v cannot be ignored in the interpretation of e.g. measurements of magnetization or spin-wave resonance. A comparison of �| H b| 2 � v with the volume-averaged mean-square anisotropy field reveals that both quantities are of comparable magnitude.

A comparison of torque-strengths in micromagnetics

In the approach-to-magnetic-saturation range, we present experimental data for the magnitude and applied magnetic-field dependence of the volume-averaged mean-square magnetostatic stray field 〈| H d b | 2 〉 v that results from non-zero divergences of the magnetization from within the volume of bulk samples of nanocrystalline electrodeposited Ni and Co and coarse-grained polycrystalline cold-worked Ni. The magnitude of 〈| H d b | 2 〉 v is compared to the volume-averaged mean-square anisotropy field 〈| H p | 2 〉 v , which arises from the combined effect of magnetocrystalline and magnetoelastic anisotropy.

高純度ｃＢＮ多結晶体の微細構造と特性

Microstructural features and some properties of high-purity polycrystalline cBN synthesized from hBN by the direct conversion method were investigated. A fine-grained (<0.5μm) and homogeneous high-purity polycrystalline cBN was obtained at 7.7GPa and 2200-2400°C. At lower pressure and lower temperature than these conditions, compressed hBN remained in the polycrystalline sintered body. Some cBN grains became larger rapidly at higher temperature above 2400°C, and inhomogeneous microstructure was formed. The fine-grained and homogeneous polycrystalline cBN had higher hardness and TRS, compared with the coarse-grained and inhomogeneous polycrystalline cBN. The thermal conductivity of the polycrystalline cBN increased with increasing grain size or synthesis temperature. The thermal conductivity of the fine-grained polycrystalline cBN, however, was 3 to 4 times higher than conventional cBN compacts containing binder materials. The fine-grained and homogeneous high-purity cBN is expected to have high potential in cutting tool applications because of its excellent mechanical and thermal properties.

Microindentation of Coarse-Grained Polycrystalline α-Brass

ABSTRACTEffects of the indent depth, the distance between the indent and the grain boundary, grain sizes, and crystallographic orientations on the microhardness of annealed coarse-grained polycrystalline α-brass were investigated using a Vickers indenter with indents much smaller than the α-brass grains (ranging from 80 μm to 550 μm). It is found that the microhardness of α-brass crystals shows a strong dependence on the indentation depth when it is smaller than 5 μm and a very weak dependence on both the distance between the indent and the grain boundary and the grain size when the indentation depth is about 5 μm and more. No significant dependence of the microhardness on the crystallographic orientations was observed in the fourteen grains of different orientations studied in this investigation.

Nanostructured palladium–iron membranes for hydrogen separation and membrane hydrogenation reactions

Hydrogen permselectivity and poisoning resistance of nanostructured palladium–iron alloy films were examined to evaluate these materials for potential membrane applications. At 200 °C, nanostructured palladium–iron films had hydrogen/helium selectivities of up to 35:1 and hydrogen fluxes of up to 10 sccm/cm 2. The rate-limiting step for hydrogen transport was diffusion through the bulk metal. Hydrogen flux in the nanostructured palladium-based membranes decreased with increasing iron content. The nanocrystalline membranes had higher hydrogen fluxes than coarse-grained polycrystalline systems of similar compositions. Nanostructured membranes also exhibited better resistance to hydrogen sulfide poisoning than polycrystalline membranes. When applied for the hydrogenation of ethene, the palladium–iron nanocrystalline membranes displayed stable activities over long time periods.

Synthetic Semiconductor Diamond Electrodes: Comparison of Electrochemical Impedance at the Growth and Nucleation Surfaces of a Coarse-Grained Polycrystalline Film

A comparative study of the impedance of the growth and nucleation surfaces of a free-standing film of polycrystalline diamond deposited from RF-plasma and moderately doped with boron is performed. Using Mott–Schottky plots, the acceptor concentration is determined. It is shown that in the diamond bulk adjacent to the film growth side, which has more perfect crystal structure, this concentration is much lower than that near the nucleation side, where the film consists of submicron-sized grains.

High-tensile ductility in nanocrystalline copper

In this work we report a high-tensile ductility in a fully dense bulk nanocrystalline (nc) pure copper sample prepared by electrodeposition. A tensile ductility with an elongation to fracture of 30% was obtained in the nc Cu specimen with an average grain size of 27 nm, which is comparable to that for the coarse-grained polycrystalline Cu. An enhanced yield stress (119 MPa) and a depressed strain hardening exponent (0.22) were observed in the nc Cu sample with respect to the conventional polycrystalline Cu. The high-tensile ductility was attributed to the minimized artifacts in the nc sample, and the grain-boundary sliding deformation mechanism resulted from the numerous amount small-angle grain boundaries and the low microstrain (dislocation density).

Structure transition comparison between the amorphous nanosize particles and coarse-grained polycrystalline of cobalt

Amorphous cobalt nanoparticles were synthesized by laser-inductive vaporization condensation technique. The nanoparticles are in fine size dispersion and the average particle size is 20 nm. When heating the amorphous cobalt nanoparticles in O 2 ambient condition there is a sharp exothermic reaction in temperature range from 2072°C to 2972°C with a peak temperature of 260°C and exothermic enthalpy of 100.08 kJ/mol. When heating in Ar ambient condition, the amorphous cobalt nanoparticles may begin to transform from the amorphous solid to a supercooled liquid state at about 167°C and keep the supercooled liquid state in a wide temperature span from 167°C to 277°C, followed by crystallization at 277°C. The crystallization takes place in the temperature span from 277°C to 477°C. The corresponding exothermic heat is equal to 23.2 kJ/mol, which is larger than the heat of fusion of bulk cobalt (14.4 kJ/mol).

Mechanical behaviour of nanocrystalline iron and nickel in the quasi-static and low frequency anelastic regime

In this research, we made use of mechanical spectroscopy to study the anelastic behaviour of nanocrystalline Fe and Ni in quasi-static, low-frequency (0.01–10 Hz) regime. The elastic energy dissipation coefficient (Q −1) and the stress relaxation have been measured as a function of frequency and temperature, in a range of temperatures where appreciable grain growth is not expected to occur. The use of such low frequency probes puts into evidence a very strong change in the material response, induced by low temperature annealing (T < 200 °C). In spite of the fact that the grain size was nearly the same after the annealing treatment, the as-prepared samples displayed much higher Q −1 values and faster rates of stress relaxation than the annealed samples. The anelastic spectrum of annealed specimens has been analysed by a combination of quasi-static and dynamic measurements. The results of this study are discussed in terms of typical activation energies reported for lattice and grain boundary diffusion in coarse-grained polycrystalline and nanocrystalline metals.

Magnetic Properties Study Of Nanocrystalline Cobalt and Cobalt-Based Alloys

N.E. Fenineche1, O. El Kedim2, E.Gaffet2 1LERMPS/UTBM - F90010 - BELFORT CEDEX – France E-mail: Nour-eddine.fenineche@utbm.fr 2CNRS UMR 5060,UTBM, F90010 - BELFORT CEDEX - FRANCE Keywords : Ball milling, magnetic properties, nanostructures, coercivity, squareness ratio Abstract The effect of Ball milling parameters on the microstructure and consequently on the magnetic properties has been studied. The combination of low values of the plateau rotation and the high vial velocities w can enhance the cubic phase formation. Coercivity and crystallite size exhibit both a regular and similar diminution when plateau rotation velocity and mechanical alloying time are increasing. Introduction Ball milling (BM) is one of the possibilities to prepare materials with exceptional mechanical and magnetic properties. The objective of milling includes particle size reduction, mixing or blending, and particle shape change [1]. Because of the very fine grain sizes, nanocrystalline materials exhibit a variety of properties that are different and often considerably improved in comparison with those of conventional coarse-grained polycrystalline materials [2]. This change in the microstructure can affect mechanical and magnetic properties, particularly, coercivity, Hc, and squareness ratio, Mr/Ms, which are very important parameters from an industrial point of view [2,3]. Microstructural [4-6] and magnetic properties [7,8] of ball milled Co have been previously reported. In the case of cobalt based alloys, produced by mechanical alloying magnetic data on these powders shows behaviour typical of a soft ferromagnetic alloys [9]. This paper present the first results in our research group on the effect of three principle BM parameters : plateau rotation, vial rotation and ball-milling time on the microstructure and consequently on the magnetic properties. Experimental procedure Nanocrystalline Co and Fe-Co were produced by ball milling. Elemental powders, with an average particle size of 50 mm and a mean grain size estimated as 20 nm were introduced into a cylindrical tempered steel container (vial) of capacity 50 ml. Each container was loaded with three quenched steel (type 100C6) balls (diameter1.5 cm, mass 14g). The containers were sealed with a Teflon O-ring and the milling conducted in stationary air without exchange with laboratory atmosphere. The milling was carried out using the so-called G5 specially designed planetary high energy machine which allows the independent choice of the shock frequency and of the shock energy [10,11].

Experimental evidences of lattice distortion in Nanocrystalline Materials

Lattice parameters for nanocrystalline (nc) materials were found to be alerted with respect to the corresponding coarse-grained polycrystalline counterparts in several systems. In this work, experimental evidences of the lattice structure distortion in various nc materials processed by means of different approaches will be summarized and discussed. Strong evidences showed the lattice structure distortion in nc samples which is manifested by an evident change in the lattice parameters, Debye-Waller parameter, and the thermal properties of the nanocrystalline lattice. These results imply that the property changes of nc materials should be attributed not only to the numerous non-equilibrium grain boundaries, but to the distorted nanocrystallites as well.

Mechanical properties, ductility, and grain size of nanocrystalline iron produced by mechanical attrition

The main goal of this investigation is to determine the influence of grain size on the mechanical properties and, specifically, the intrinsic ductility of nanocrystalline (nc) Fe. Ball-milled nc Fe was consolidated into compacts of near theoretical density by uniaxial warm pressing. Compaction parameters and annealing treatments resulted in a range of grain sizes for subsequent mechanical testing. The miniaturized disk bend test, hardness, and the automated ball indentation (ABI) method were used to test nanocrystal (nc) iron in compression and tension. The deformation and fracture morphologies of the tested samples were characterized by light and scanning electron microscopy. The hardness, as a function of the grain size, was described with a Hall-Petch slope, which was smaller than that in coarse-grained Fe. In tension, the material failed in a macroscopically brittle manner, while local ductility in very concentrated shear bands was observed. The compressive characteristics of the nc Fe were similar to those of a perfectly plastic material. The results are discussed in the context of the mechanical behavior of coarse-grained polycrystalline metals and alloys.

Electron-irradiation-induced phase segregation in crystalline and amorphous apatite; a TEM study

Single crystals of natural F-rich apatite and 800 keV Kr (super 2+) ion-beam-amorphized apatite were irradiated by an electron beam in a transmission electron microscope over a range of beam energies and beam currents. Irradiation of crystalline apatite using a high current density (16 A/cm 2 ), caused the precipitation of cubic CaO from the crystalline apatite matrix. Using a lower beam current (1.6 A/cm 2 ), the formation of nanometer-sized voids was observed, but CaO did not crystallize even after prolonged irradiation. Amorphous apatite crystallized to a coarse-grained polycrystalline assemblage of apatite crystallites at 85-200 keV. Increasing the beam current through the sample caused the formation of finegrained cubic CaO, and the crystallization of apatite was not observed, even at high doses. In each case, many beam-induced bubbles formed and were typically larger at the edge of the beam. Thermal annealing at 450 degrees C resulted in epitaxial crystallization from the thick portions of the TEM foil and resulted in a single crystal with a high defect density. Electron-beam irradiations at 300 degrees C confirmed that the difference in microstructural evolution as a function of current density is driven by dose-rate effects. In fact, temperature and dose rate are competing effects in the precipitation of CaO.

X-ray Stress Measurement of Coarse-Grained Polycrystalline Materials by Imaging Plate Method.

X-ray Stress Measurement of Coarse-Grained Polycrystalline Materials by Imaging Plate Method.

Heat-capacity comparison among the nanocrystalline, amorphous, and coarse-grained polycrystalline states in element selenium.

Nanocrystalline element selenium ~nc-Se! samples were synthesized by completely crystallizing a meltquenched amorphous selenium ~a-Se!. Heat capacities ( Cp) of the nc-Se ~with a mean crystallite size of 10 nm!, a fully relaxed a-Se, and a coarse-grained polycrystalline Se were experimentally determined in a temperature range of 225‐500 K. Measurement results show that there is no pronounced difference between the Cp values of the nc-Se and the a-Se below 250 K. Heat capacities of the nc-Se are found to be only about 2% higher than those of the coarse-grained polycrystalline selenium. The Cp enhancement for the nc-Se, which is much lower than those reported in other nanocrystalline elements, is discussed in terms of the grain-boundary configurations in the nanostructures. @S0163-1829~96!02030-9# Heat capacity of a material is directly related to its atomic structure, or its vibrational and configurational entropy which is significantly affected by the nearest-neighbor configurations. Nanocrystalline ~nc! materials are structurally characterized by the ultrafine crystalline grains, and a large fraction of atoms located in the metastable grain boundaries in which the nearest-neighbor configurations are much different from those in the crystallites. Or in other words, the grain-boundary possesses an excess volume with respect to the perfect crystal lattice. Therefore, heat capacities of the nc materials are expected to be higher than those of the corresponding coarse-grained polycrystalline counterparts. Experimental measurements of heat capacities at constant pressure, Cp in several nc materials synthesized by means of different methods indicated that C p values are frequently higher than those for the coarse-grained polycrystalline counterparts, and are even higher than those for the amorphous states as well. 1‐3 For example, the Cp values of a nc-Pd synthesized by consolidation of ultrafine metal powders were reported to be as much as 60% higher than those for the coarse-grained polycrystalline Pd. 1 The heat-capacity values of a ball-milled nc-Ru sample were found to be about 20% higher than those of its coarse-grained polycrystalline counterparts. 2 Nevertheless, these results should be accepted cautiously because the measured Cp values might be influenced by many factors. Porosity and possible gaseous contamination @e.g., the sample has a density of only 80% and a total impurity content of about 1 at. % in the case of the nc-Pd ~Ref. 1!# in the ultrafine-powder-consolidated nc materials may contribute to the enhanced Cp . While in the ballmilled nc specimens, contamination from the milling media and the subsequent consolidation conditions might alter the heat-capacity results. Up to now, solid experimental results of heat-capacity comparison among the amorphous, the coarse-grained polycrystalline, and the nc states are still lacking.

Nanocrystalline metals crystallized from amorphous solids: nanocrystallization, structure, and properties

Polycrystalline materials with nanometer-sized grains, termed nanocrystalline materials, can be formed by crystallizing completely amorphous solids under proper heat treatment conditions. The crystallized nanocrystalline materials exhibit some unique structural characteristics and novel properties which are fundamentally different from those of the conventional coarse-grained polycrystalline materials. This article reviews the present state of the art in this field. The current status of research and developments on the nanocrystallization, microstructure and properties of the materials will be summarized. Comparisons of structural characteristics and properties are made between the crystallized nanocrystalline materials and those prepared by other methods. Further considerations of the development and applications of this new class of materials will also be presented.

Nanostructured Ni 3Al produced by magnetron sputtering

Intermetallic Ni 3Al has been deposited by means of magnetron sputtering on substrates with an amorphous surface structure. The source was a homogeneous mixture of NiAl 75:25 % in the solid phase. The lateral and perpendicular grain sizes are below 20 nm. The morphology of the layer was studied by transmission electron microscopy. The hardness of a single Ni 3Al layer and of a multilayered Ni 3AllNi are measured by nanoindentation and compared with the hardness of a bulk coarse-grained polycrystalline Ni 3Al.

The structure and property characteristics of amorphous/nanocrystalline silicon produced by ball milling

The structural transformation of polycrystalline Si induced by high energy ball milling has been studied. The structure and property characteristics of the milled powder have been investigated by x-ray diffraction, scanning electron microscopy, high-resolution electron microscopy, differential scanning calorimetry, Raman scattering, and infrared absorption spectroscopy. Two phase amorphous and nanocrystalline Si has been produced by ball milling of polycrystalline elemental Si. The nanocrystalline components contain some defects such as dislocations, twins, and stacking faults which are typical of defects existing in conventional coarse-grained polycrystalline materials. The volume fraction of amorphous Si is about 15% while the average size of nanocrystalline grains is about 8 nm. Amorphous elemental Si without combined oxygen can be obtained by ball milling. The distribution of amorphous Si and the size of nanocrystalline Si crystallites is not homogeneous in the milled powder. The amorphous Si formed is concentrated near the surface of milled particles while the grain size of nanocrystalline Si ranges from 3 to 20 nm. Structurally, the amorphous silicon component prepared by ball milling is similar to that obtained by ion implantation or chemical vapor deposition. The amorphous Si formed exhibits a crystallization temperature of about 660 °C at a heating rate of 40 K/min and crystallization activation energy of about 268 kJ/mol. Two possible amorphization mechanisms, i.e., pressure-induced amorphization and crystallite-refinement-induced amorphization, are proposed for the amorphization of Si induced by ball milling.

Thermal expansion behavior of nanocrystalline NiP alloys of different grain sizes

Nanocrystalline (NC) NiP alloy samples with average grain sizes ranging from a few to 127 nm have been produced by crystallizing the amorphous NiP alloy at various annealing temperatures. The linear thermal expansion coefficients ( α L) of these NC samples were accurately measured in a temperature range of 300–400K. Experimental results showed that α L is heavily dependent on the grain size and decreases from about 20.7 × 10 −6 to 15.5 × 10 −6 K −1 with an increase in the average grain size from 7.5 to 127 nm. From the α L dependence of grain size in these NC samples and the α L value of the as-cast coarse-grained polycrystalline NiP sample we deduced that the difference in thermal expansion coefficients of the interfaces and of the nm-crystallites decreases with a reduction of grain size.

Comparison of structure and properties of alloys in amorphous, nanocrystalline and coarse-grained polycrystalline states

Some properties, including the microhardness, H(V), and electrical resistivity of a nanocrystalline, two coarse-grained polycrystalline and one amorphous alloys of Fe-Mo-Si-B of the same composition, were measured experimentally and compared. The structures of amorphous, nanocrystalline and coarse-grained Fe-Mo-Si-B alloys are also compared and discussed.