Showing papers on "Grain size published in 1994"

Remanence and coercivity in isotropic nanocrystalline permanent magnets.

Abstract: Numerical micromagnetic calculations rigorously describe the correlation between the microstructure and the magnetic properties of nanocrystalline permanent magnets. In isotropic nanocrystalline permanent magnets exchange interactions override the anisotropy of the individual grains. Therefore the spontaneous magnetic polarization deviates from the easy axes in a region along the grain boundaries. For a fine grain structure with a mean grain size d20 nm the remanence is considerably enhanced, since the volume fraction of the boundary regions where the spontaneous magnetic polarization points towards the direction of the applied field becomes significantly high. The inhomogeneous ground state, however, favors the nucleation of reversed domains leading to a reduction of the coercive field with decreasing grain size. A uniform grain structure with a very small range in grain size avoids large demagnetizing fields and thus preserves a high coercivity. For a grain size of 10 nm isotropic two-phase permanent magnets based on ${\mathrm{Fe}}_{14}$${\mathrm{Nd}}_{2}$B and \ensuremath{\alpha}-Fe show remarkable high-energy products, because the volume fraction of the magnetically soft phase can be increased up to 50% without a significant loss of coercivity.

Grain destruction in shocks in the interstellar medium

Abstract: Destruction of interstellar dust occurs predominantly in supernova shock waves in the warm neutral/ionized medium (density approximately = 0.25/cu cm, temperature approximately = 10(exp 4) K). Recent theoretical developments and laboratory data for sputtering processes and grain-grain collisional vaporization allows us to better evaluate the grain destruction rate in interstellar shocks in the warm medium. We find that, independent of composition, grain denstruction in supernova blast waves is dominated by nonthermal sputtering for shock velocities greater than 50 km/s and less than or equal to 150 km/s and thermal sputtering at higher shock velocities. We use a detailed scheme for the vaporization of grains colliding at high velocities (v(sub s) greater than or equal to 20 km/s) and show that the grain-grain collision destruction process is only dominant for shock velocities of less than or equal to 50-80 km/s and is less important than previously assumed. Nevertheless, the grain-grain destruction rates are of order 30%-90% of the sputtering rates at v(sub s) greater than 100 km/s and less than 200 km/s and are important in vaporizing the cores of grains. Detailed results for grain destruction as a function of grain size and composition are presented. We also present results for silicon carbide, iron, ice, and porous test particles. For carbonaceous grains we find that the fractional destruction is less than or equal to 0.29, and for silicate it is less than or equal to 0.45, for v(sub s) less than or equal to 200 km/s. We have calculated grain lifetimes, using the three-phase model of the interstellar medium, and find lifetimes of 4 x 10(exp 8) yr for carbonaceous grains and 2.2 x 10(exp 8) yr for silicate grains. Given that the typical stardust injection timescale of 2.5 x 10(exp 9) yr, we conclude that efficient mechanisms for grain growth in the interstellar medium must exist in order that a significant fraction of the refractory elements be incorporated in dust, as observed. Therefore, although our improved model has less vaporization of dust due to grain-grain collisions, sputtering still destroys dust efficiently and grain mantle growth in the interstellar medium is required, a conclusion reached in previous models of grain destruction in the interstellar medium. Carbonaceous mantles on silicate grains can protect the silicate cores from sputtering destruction in interstellar shock waves, provided that the protective mantles can efficiently reform in the interstellar medium. Also, if the grains are porous the postshock grain velocities are lower than for solid particles, and grain destruction is reduced. Porosity and mantling may increase the grain lifetime by factors of approximately 3 and approximately 3-4, respectively. The fraction of interstellar silicon in silicate stardust is therefore less than or equal to 0.25, but more silicon might be depleted in the form of a grain mantle.

A unified approach to grain boundary sliding in creep and superplasticity

Abstract: Rachinger grain boundary sliding is a characteristic of high temperature deformation in both creep when the grain size is large ( d > λ ) and superplasticity when the grain size is small ( d λ ), where d and λ are the grain size and the subgrain size, respectively. An analytical procedure is used to determine the rate equation for Rachinger sliding when d > λ . Data for superplastic metals are examined to give the rate equation for Rachinger sliding when d λ . A unified model is developed for Rachinger sliding at all grain sizes, where the rate of sliding is controlled by the rate of accomodation through intragranular slip. It is demonstrated that the predictions of this model are in good agreement with experimental data under both creep and superplastic conditions.

Mechanical properties of nanophase metals

Abstract: Nanophase metals have grain-size dependent mechanical properties that are significantly different than those of their coarse-grained counterparts. Pure metals are much stronger and apparently less ductile than conventional ones; intermetallics are also strengthened, but they tend toward increased ductility at the smallest grain sizes. These property changes are primarily related to grain size limitations, but they are also affected by the large percentage of atoms in grain boundaries and other microstructural features. Strengthening appears to result from a limitation of dislocation activity, while increased ductility probably relates to grain boundary sliding. A brief overview of our present understanding of the mechanical properties of nanophase metals is presented.

Effect of Grain Size on Hertzian Contact Damage in Alumina

Abstract: The role of microstructural scale on deformation-microfracture damage induced by contact with spheres is investigated in monophase alumina ceramics over a range 3--48 [mu]m in grain size. Measurement of a universal indentation stress-strain curve indicates a critical contact pressure [approx] 5 GPa, above which irreversible deformation occurs in alumina. A novel sectioning technique identifies the deformation elements as intragrain shear faults, predominantly crystallographic twins, within a confining subsurface zone of intense compression-shear stress. The twins concentrate the shear stresses at the grain boundaries and, above a threshold grain size, initiate tensile intergranular microcracks. Below this threshold size, classical Hertzian cone fractures initiate outside the contact circle. Above the threshold, the density and scale of subsurface-zone microcracks increase dramatically with increasing grain size, ultimately dominating the cone fractures. The damage process is stochastic, highlighting the microstructural discreteness of the initial deformation field; those grains which lie in the upper tail of the grain-size distribution and which have favorable crystallographic orientation relative to local shear stresses in the contact field are preferentially activated. Initial flaw state is not an important factor, because the contact process creates its own flaw population. These and other generic features of the damage process will be discussedmore » in relation to microstructural design of polycrystalline ceramics.« less

Piezospectroscopic Determination of Residual Stresses in Polycrystalline Alumina

Abstract: The residual stresses created in polycrystalline aluminum oxide as a result of its constrained anisotropic thermal contraction are measured with the technique of piezospectroscopy using the fluorescence from trace Cr[sup 3+] impurities. The average residual stresses in the crystallographic a and c directions are determined as a function of grain size for a high-purity alumina, as is the width of the stress distribution (assuming it to be Gaussian). Over the range of grain sizes investigated, from 2 to 16 [mu]m, the residual stresses exhibit a dependence on grain size consistent with the prediction of the Evans-Clarke model of thermal stress relaxation by grain boundary diffusion.

Thermal instability at 10 Gbit/in/sup 2/ magnetic recording

Abstract: The limitation on recording density imposed by thermal stability is systematically studied by a method combining molecular dynamics and Monte Carlo computer simulations. The thermal decay for as long as 6 months has been simulated for written di-bits at the projected anisotropy, grain size, and bit length for 10 Gbit/in/sup 2/ magnetic recording. In the presence of demagnetizing field, a single layer film has little thermal effect at the upper limit of the projected grain sizes, while thermal decay is obvious when grain sizes are at the lower limit. The magnitude of the noise peak does not change significantly while the noisy region becomes wider after thermal decay. Compared with a single layer medium of the same total thickness, a double layer film has much more serious thermal decay and the negative interaction between layers tends to demagnetize the film, therefore making the thermal effect worse. The thermal decay in multilayer media tends to cancel the gain in noise reduction obtained by dividing the film layer at ultrahigh recording density. The effects of magnetostatic and exchange interaction, anisotropy, and grain volume on thermal stability are discussed. >

Improved mechanical properties in new, Pb-free solder alloys

Abstract: The mechanical properties of solders benefit from uniform dispersion of fine precipitates and small effective grain sizes. Metallurgical methods of attaining such a beneficial microstructure have been investigated in two new, near-eutectic, Pb-free solder alloys systems—Sn-Zn-In (m.p. ∼188°C) and Sn-Ag-Zn (m.p.∼217°C). It has been found that small alloying additions of Ag dramatically improve the mechanical properties of the ternary Sn-8Zn-5In alloy. The improvement is attributed to the elimination of the coarse and nonuniform distribution of plate-like dendrites and refining the effective grain size in the solidified microstructure. Also, small amounts of Cu dramatically improve the ductility in the ternary Sn-3.5Ag-lZn alloy. The quaternary Sn-3.5Ag-lZn-0.5Cu has better mechanical properties than the binary Sn-3.5Ag alloy because it has a uniform fine dispersion of precipitates and a small effective grain size. The combination of high mechanical strength and high ductility is likely to yield improved fatigue resistance properties in the interconnection of electronic components.

Surface energy characterization of unalloyed titanium implants.

Abstract: Osteointegration is dependent on a variety of biomechanical and biochemical factors. One factor is the wettability of an implant surface that is directly influenced by its surface energy. This investigation used the Zisman plot to determine critical surface tension as one representative measurement of surface energy. The effects of surface treatment, bulk grain size, and surface roughness on the critical surface tension of unalloyed titanium (Ti) were examined. Radio frequency glow discharge-treated Ti had the highest critical surface tension, followed by the passivated and heat-sterilized conditions. Titanium with no surface treatment had the lowest critical surface tension. The surface energy of Ti with an average grain size of 23 microns was not significantly different from that with a grain size of 70 microns. Surface roughness was shown to cause significant difference in measurements and definitely should be considered in studies of this kind.

The study of grain size dependence of yield stress of copper for a wide grain size range

Abstract: Room temperature yield strength of copper has been measured by means of miniaturized disk-bend test as a function of grain size ranging from about 30 nm to 180 μm. It has been established that grain size dependence of strength does not obey the Hall-Petch relation in the entire range of grain sizes studied. The results obtained suggest that a gradual change of deformation mechanism takes place with decreasing grain size. Nanostructured samples appear to be rather ductile.

Improvement of grain size and deposition rate of microcrystalline silicon by use of very high frequency glow discharge

Abstract: The influence of the plasma excitation frequency on the growth conditions and the material properties of microcrystalline silicon prepared by plasma enhanced chemical vapor deposition at low deposition temperature is investigated. It is found that an increase of the plasma excitation frequency leads to a simultaneous increase of the growth rate, the grain size, and the Hall mobility of microcrystalline silicon. This is attributed to an effective selective etching of disordered material creating more space to develop crystalline grains, while also more species for faster growth of the crystallites are available.

Analysis of grain size trends, for defining sediment transport pathways in marine environments

Abstract: An approach to grain size trend analysis is developed on the basis of a semi-quantitative filtering technique. Using this technique, grain size trends identified from a grid of surficial sediment samples ere transformed into a "residual pattern" representing net sediment transport paths. The method assumes that the grain size trends used for the analysis, have a higher frequency of occurrence in sediment transport directions than in the opposite directions but such dominance does not exist if there is no exchange of material between the sampling sites. The proposed method is applied to the analysis of grain size trends over the Christchurch Bay area, southern England. Mean grain size, sorting and skewness are used to form eight possible grain size trends; two of these are used to derive a residual pat tern. The pattern obtained shows general agreement with transport patterns derived from other sediment dynamics investigations undertaken for the region. Further, a residual pattern similar to that of transport paths on the basis of estimates of longshore transport rates, for the Rhone Delta, is derived. The present investigation indicates that the feasibility of grain size trend analysis depends upon the selection of appropriate grain size trends and the analytical approach.

Current-voltage characteristics of ultrafine-grained ferroelectric Pb(Zr, Ti)O 3 thin films

Abstract: Room-temperature current-voltage dependence of ultrafine-grained ferroelectric Pb(Zr, Ti)O3 thin films has been investigated. Both strong varistor type behavior and space charge limited conduction (SCLC) were observed. Differences in the current-voltage characteristics are attributed to differences in the nature of the grain boundaries resulting from variations in processing conditions. The strong varistor type behavior is believed to be due to the presence of highly resistive grain boundaries and thus may be termed grain boundary limited conduction (GBLC). A double-depletion-layer barrier model is used to describe the origin of high resistivity of the grain boundaries. It is suggested that the barrier height varies significantly with the applied field due to the nonlinear ferroelectric polarization, and that the barrier is overcome by tunneling at sufficiently high fields. In some other cases, the resistivity of the grain boundaries is comparable to that of the grains, and therefore the intrinsically heterogeneous films degenerate into quasi-homogeneous media, to which the SCLC theory is applicable. As such, a unified grain boundary modeling reconciles different types of conduction mechanisms in the ultrafine-grained ferroelectric thin films. This grain boundary modeling also well accounts for some other dc-related phenomena observed, including abnormal current-voltage dependencies, remanent polarization effect, electrode interface effect, and unusual charging and discharging transients. In addition, many other electrical properties of the ferroelectric films may be better understood by taking the effect of grain boundaries into account.

First- and second-order kinetics approaches for modeling the transport of colloidal particles in porous media

Abstract: We present results from experiments on the migration of inorganic colloids through laboratory columns containing clean quartz sand. Particle retention on the quartz collectors was found to be substantially less in experiments using negatively charged silica (SiO2) colloids than in experiments using positively charged anatase (TiO2) or boehmite (AlOOH) colloids. Analysis of these data with respect to two different advection-dispersion models indicates that deposition of colloidal silica follows a first-order, reversible kinetics process, while deposition of both anatase and boehmite is more closely depicted by second-order kinetics. Fitted values of the rate constant used to describe particle attachment vary consistently with the mean grain size of the sand and, for anatase and boehmite, are within a factor of 2 of the values predicted on the basis of colloid filtration theory.

The influence of particle size variations on the magnetic properties of sediments from the north-eastern Irish Sea

Abstract: This paper considers the magnetic properties of a range of recent and contemporary sediments from the north eastern part of the Irish Sea. Principal component ordinations of the results show a close link between magnetic property variations and particle size. The magnetic properties of a subset of samples, particles sized by a combination of sieving and pipette analysis, confirm that variations in ferrimagnetic (‘magnetite’) grain size parallel those in particle size, despite the fact that the magnetic grains in the fine grades have diameters 1–2 orders of magnitude smaller than those of the particle size fraction in which they occur. This is best explained by postulating that the fine magnetic grains occur in the clay fraction but are present in declining concentrations in the coarser grades up to 4o as an artefact of the pipette method. Most samples have a biomodal distribution of magnetic minerals, with a coarse mode associated with heavy minerals in the sands or coarse silts, and a fine mode in the clays. Magnetic susceptibility (x) and saturation isothermal remanent magnetization (SIRM) largely pick out the coarse mode where present; anhysteretic remanent magnetization (ARM) largely picks out the fine mode. The results open up the possibility of normalizing samples from these environments for particle size and, more specifically, clay content, by means of ARM or ARM/x values. The most likely source for the uniform and almost exclusive stable single domain magnetite, which dominates the magnetic properties of the clays, is thought to be bacterial magnetosomes. The measurements as a whole do not appear to hold much promise for discriminating sediment source types.

Effects of barrier layer and annealing on abnormal grain growth in copper thin films

Abstract: Abnormal (100) grain growth has been characterized in predominantly (111)‐textured Cu thin films as a function of deposition temperature, annealing temperature and the presence of a Ta or W underlayer. For films deposited at room temperature, bimodal grain size distributions are observed at annealing temperatures at or above 150 °C for Cu on Ta and 100 °C for Cu on W. Suppression of (100) abnormal grain growth was achieved by depositing Cu on either barrier layer at 150 °C. A bimodal grain size distribution was still observed for the film deposited on W at 150 °C but the large grains forming this distribution were found to be (111) oriented. These results are explained as the result of competition between strain energy minimization and surface and interface energy minimization. The (100) growth is shown to be driven by a reduction of the orientation‐dependent strain energy that builds up due to the elastic anisotropy of Cu. Films deposited at higher temperatures have a lower yield stress which limits the ...

Interface control for resistance to intergranular cracking

Abstract: Theoretical and experimental results are presented, with the primary objective of improving the resistance of convectional polycrystalline alloys to intergranular degradation phenomena, through the application of grain boundary design and control. Geometric considerations are discussed, which show that, as a consequence of both energetic and crystallographic constraints associated with twinning, a grain boundary character distribution (GBCD), consisting entire of low σ grain boundaries, is attainable. A geometric model of crack propagation through active intergranular paths is used to evaluate the potential effects of σ grain boundary fraction and grain size on intergranular cracking. The effect of the GBCD on intergranular stress corrosion cracking and intergranular corrosion in a nickel-based alloy 600 (Ni16Cr9Fe) is determined. Important factors in achieving microstructural optimization of alloy 600 are presented. These results provide direct experimental support for the model of intergranular crack propagation, and demonstrate the importance of grain boundary structure control for enhancing the resistance of a material to intergranular degradation.

Clean grain boundaries in aluminium nitride ceramics densified without additives by a plasma-activated sintering process

Abstract: High-purity aluminium nitride (AlN) powders of submicron size were consolidated to almost theoretical density in minutes at 1800°C using a plasma-activated sintering (PAS) process. The structure and microchemistry of grain boundaries in this polycrystalline material (average grain size, about 0.43 μm), sintered without additives commonly used in the densification of ceramics, was examined by high-resolution electron microscopy and high-spatial-resolution electron-energy-loss spectroscopy (EELS). The grain boundaries were ‘clean’ down to atomic-scale resolution and revealed AIN grain-to-grain contacts throughout the material; EELS data show no evidence for significant oxygen at grain boundaries or at grain triple junctions. A low density of oxygen-containing inversion domain boundaries were found in the grains, and they are thought to be formed to accommodate small amounts of oxygen (about 0.82 at.%) during the PAS consolidation process. The observed clean grain boundaries suggest that the plasma ...

Grain Growth Kinetics in Alumina–Zirconia (CeZTA) Composites

Abstract: Grain growth kinetics in alumina-rich, zirconia-alumina composites have been examined as a function of volume fraction of zirconia. Both the microstructural scale and character alter during grain growth. Grain growth exponents between n = 3 and 4 were observed and have been rationalized in terms of current models for grain growth and particle coarsening. Grain growth constants were measured and compared with available data. Coupled grain growth was observed in that the ratio of the alumina-to-zirconia grain size was constant, for a given zirconia content. This behavior is discussed in terms of the topological constraint present during growth and the observed microstructural evolution. The measured activation energies were consistent with boundary-diffusion-controlled grain growth.

Texture in multilayer metallization structures

Abstract: The effects of thin Ti, TiN, or Ti/TiN underlayers on the development of the crystallographic texture and the grain structure are explored. Metal layers ∼0.5 μm in thickness of Al‐0.5Cu or of Cu are deposited on these underlayers and on amorphous SiO2 as a reference. A strongly textured underlayer such as Ti〈0002〉 or Ti〈0002〉/TiN〈111〉 induces a similarly strong 〈111〉 texture in the AlCu. In copper with 〈111〉, 〈200〉, and random texture components, an underlayer induces a stronger 〈111〉 component compared to an analogous film deposited on SiO2. A nearly random texture in TiN significantly weakens the texture in subsequent metal films. Grain size distributions in all AlCu films are monomodal reflecting a process of normal grain growth. The grain size distribution for Cu sometimes deviates from lognormal. The bimodal distribution implies that grain growth is abnormal even though the median grain size does not exceed a low multiple of the film thickness.

Comprehensive Study of Lateral Grain Growth in Poly-Si Films by Excimer Laser Annealing and Its Application to Thin Film Transistors

Abstract: Lateral grain growth in nondoped poly-Si films was studied by using Si thin films (500 A) with different structures as a starting material for excimer laser crystallization. It was clarified that the lateral grain growth phenomenon (micron-size grains with strong (111) orientation) upon excimer laser annealing was strongly affected by both the microstructure and the orientation of the initial Si thin films. This result supports our previous speculation that the principal driving force of the lateral grain growth phenomenon is surface energy anisotropy. Poly-Si thin-film transistors using these films show a high field effect mobility of 440 cm2/Vs, achieved through a low-temperature process below 600° C. This excellent electrical characteristic is thought to be due to the large grain size of poly-Si thin film with controlled orientation, good crystallinity, and a smooth surface.

Synthesis, processing, and deformation of bulk nanophase Fe-28Al-2Cr intermetallic

Abstract: Nanophase Fe-28Al-2Cr powder was obtained by ball milling and consolidated by shock wave compaction Fully-dense well-bonded compacts were produced with a diameter of 32 mm and a thickness of [approx]6 mm The grain size in the compacts was [approx]80 nm In tension, the nanophase intermetallic is brittle with a failure strength ([sigma][sub f] = 065 GPa) comparable to a coarse-grained intermetallic with similar composition In compression, the nanophase material exhibits superplastic-like flow during room temperature quasistatic deformation to true strains greater than 14 The compressive flow strength is 21 GPa and no macroscopic strain hardening is observed This behavior is compared to the coarse-grained material that has a yield strength of 025 GPa, displays significant work hardening and little tension-compression anisotropy TEM examination of the nanophase material before and after deformation shows a refinement of the microstructure during deformation The microstructure refines to [approx]10 nm grains surrounded by amorphous material

Tin dioxide thin-film gas sensor prepared by chemical vapour deposition : Influence of grain size and thickness on the electrical properties

Abstract: Tin dioxide films are elaborated by a chemical vapour deposition (CVD) method. An accurate control of deposition parameters (temperature, total pressure, duration) so that appropriate annealing conditions (duration, temperature) can be used to modify the structural properties of the films: grain size, thickness, and stoichiometry. Important modifications of electrical performances in tin dioxide films for gas-sensing applications are observed. A correlation between structural properties of CVD films and their electrical behaviour is proposed. The main results are: (i) a sharp increase in the electrical conductance under pure air G0 from a critical value of the grain size D=2L, due to the apparition of a conduction channel between adjacent grains; the depletion layer L is evaluated to 35 A; (ii) a dependence of the electrical conductance G0 with stoichiometry observed for various deposition temperatures and various annealing conditions; the predominant effect of stoichiometry variations for films deposited at high temperature (100–300 A grain size range) is responsible for the decrease of G0, and (iii) a strong influence of film thickness e, with a maximum of sensitivity for the thinnest films, in which tin dioxide is more discontinuous and disordered, and an increase in G0 with e due to the increase of the number of percolation paths up to 3000 A corresponding to a percolation threshold.

Finite element simulations of shear localization in plate impact

Abstract: S hear band development in a tungsten heavy alloy (WHA) during pressure-shear plate impact is analysed numerically. The alloy has a microstructure of hard tungsten grains embedded in a soft alloy matrix. A two-dimensional, plane strain model of the alloy microstructure is used in the computations. For this model microstructure a fully coupled thermo-mechanical initial boundary value problem is formulated and solved, accounting for finite deformations, inertia, heat conduction, thermal softening, strain hardening and strain-rate hardening. Calculations are carried out for distributions of uniform grains and for micro-structures obtained from digitized micrographs of the actual alloy. The effects of variations in grain volume fraction and grain size are considered. Experiments and the numerical calculations show that the two phase alloy is more susceptible to shear banding than either of the constituent phases. While the onset of shear localization depends on the grain distribution and volume fraction, the shear band width is found to be set by heat conduction and is insensitive to the grain volume fraction and the grain morphology, The shear band width obtained from the calculations is in good agreement with what is observed in the experiments. Furthermore, the computed shapes of the deformed tungsten grains inside the band resemble closely the observed shapes of the deformed grains in the experiments.

Application of grain boundary engineering concepts to alleviate intergranular cracking in Alloys 600 and 690

Abstract: Nickel-based alloys used in nuclear steam generator tubing have been found to be susceptible to intergranular stress corrosion cracking while in service. Following a recently developed model, grain boundary engineering concepts may be used to alleviate these concerns. This paper presents a report on the grain refinement approach in the proposed model which results in a higher probability of arresting stress corrosion cracks before reaching a critical length at which failure occurs. For this purpose, an electroplating system was developed to produce ternary Ni-Cr-Fe alloy coatings. Electroplating conditions are given for the production of Alloys 600 and 690 having an average grain size in the range 100-250 nm.