Showing papers on "Grain growth published in 1995"

Ceramic Processing and Sintering

Abstract: Ceramic fabrication processes - an introductory overview synthesis of powders powder characterization science of colloidal processing sol-gel processing powder consolidation and forming of ceramics sintering of ceramics - fundamentals theory of viscous sintering grain growth and microstructural control liquid-phase sintering problems of sintering densification process variables and densification practice.

Experimental constraints on the dynamics of the partially molten upper mantle: Deformation in the diffusion creep regime

Abstract: Experiments have been conducted to investigate the effect of melt on the creep behavior of water-free olivine aggregates deformed in the dislocation creep regime. The influence of the melt phase is modest at melt fractions less than ∼0.04. However, at melt fractions > 0.04, the creep rate of melt-added samples is enhanced by more than an order of magnitude relative to melt-free aggregates. This unexpectedly large influence of melt on strain rate arises because deformation occurs by grain boundary sliding (GBS) accommodated by a dislocation creep process. Four observations support this hypothesis. (1) The strain rate enhancement observed in the dislocation creep regime can be related to the stress concentration caused by the reduction in the solid-solid grain boundary area. (2) Both melt-free and melt-added samples exhibit strain rates indicating that deformation is limited by slip on (010)[100], the easiest slip system in olivine. (3) The GBS mechanism occurs near the transition between diffusion and dislocation creep. (4) Grains in specimens deformed in the GBS regime are not significantly flattened, even after ∼50% shortening. In melt-free aggregates, a transition from the GBS mechanism to dislocation creep limited by slip on (010)[001], the hardest slip system, is observed with an increase in grain size. A transition to (010)[001] limited creep was not observed for partially molten aggregates because grain growth was inhibited by the presence of melt. The results of this study indicate that the viscosity of the upper mantle may decrease by at least an order of magnitude if the retained melt fraction exceeds 0.04 or if the onset of melting results in a reduction in grain size and a concomitant transition from (010)[001] to (010)[100] limited creep.

Thermal stability of nanocrystalline Ni

Abstract: Thermal stability of electroplated nanocrystalline Ni of 10 and 20 nm grain size was investigated by differential scanning calorimetry (DSC). The temperature dependence and heat release ΔH during grain growth have been determined by linear anisothermal measurements (linear heating at 10 K min −1 ). The corresponding change in microstructure has been monitored in the temperature range between 373 K and 693 K using transmission electron microscopy (TEM). The TEM and DSC studies identified three exothermic reactions: “nucleation” and abnormal grain growth (353–562 K), normal grain growth (562–593 K) and growth towards equilibrium (643–773 K). The grain growth behaviour, and the similar heat releases ΔH = 18 J g −1 , and 16 J g −1 measured for the 10 nm and 20 nm Ni nanocrystals respectively in the DSC experiments may be related to the observed sulphur segregation at grain boundaries and triple junctions.

General aspects and limits of conventional ultrafine WC powder manufacture and hard metal production

Abstract: Ultrafine WC/Co hard metals (average WC grain sizes ≤ 0.5 μm) can be successfully and reliably obtained by conventional hard metal manufacturing techniques. In this paper, some of the crucial aspects of conventional powder manufacture, powder milling and liquid phase sintering are discussed. Conventional ultrafine WC powder manufacture is based on the production of tungsten powder by hydrogen reduction of tungsten oxides and subsequent carburization. Alternatively, direct carburization can be carried out. However, inherent to the powder processing techniques used and the particle growth mechanisms involved (oxide precursors used, reduction and carburization history), there exists a lower limit beyond which-finer WC powders cannot be produced. This limit lies in the particle size range of 50–150 nm (0.05–0.15 μm). Powder milling is carried out to obtain an even dispersion of the Co binder in the ultrafine WC matrix. The more uniform the phase distribution (WC, Co, grain growth inhibitor) within the green powder compact, the more uniform will be the material transport during sintering, and hence the uniformity of the WC grain growth/growth inhibition during sintering. Enhanced WC grain growth occurs early in the sintering cycle, even below the temperature at which the liquid phase is formed. This growth can be largely restricted by the addition of VC. However, effective grain growth inhibition has to take place already during this early period of solid-state sintering. The ‘early’ availability of the grain growth inhibitor at the WC/Co interface can, therefore, determine the degree of growth inhibition. Ultrafine hard metals are in particular prone to discontinuous grain growth of the WC. Different reasons for this local growth mode are propounded relating to both the chemical as well as the geometrical departures from uniformity in the green powder compact. While it is still not possible to predict exactly an ultimate WC grain size limit, below which WC grain growth can no longer be restricted, even with proper inhibitor additions, experimental evidence indicates that this average WC grain size limit lies in the range of 200–300 nm. This limit is inherent to the existing conventional processing techniques (powder manufacture, milling, liquid phase sintering) and the WC growth mechanisms involved and can be overcome only by establishing a completely new route in hard metal manufacture.

Growth rates and misorientation relationships between growing nuclei/grains and the surrounding deformed matrix during recrystallization

Abstract: Average growth rates and misorientations between recrystallization nuclei (or grains) and neighbouring deformed matrix material have been studied for partially recrystallized samples by the electron back scattering pattern (EBSP) technique in heavily cold rolled aluminium and copper It was studied how the annealing time and the crystallographic orientation of nuclei/grains affects the growth rates and distribution of misorientations The two materials, aluminium and copper, develop a weak and a strong recrystallization cube texture respectively Information about effects of cube texture strength was therefore also obtained It was found that grains of cube orientation grow faster than grains of other orientations A wide distribution of misorientation relationships was observed to exist between the growing grains and the neighbouring deformed matrix, and this distribution was not significantly affected by the annealing time The faster growth of the cube oriented grains may be ascribed to a larger misorientation between cube grains and deformed matrix than that between other grains and the matrix

Epitaxial growth of PbTiO3 thin films on (001) SrTiO3 from solution precursors

Abstract: A mixed alkoxide liquid precursor was used to form epitaxial PbTiO3 thin films by spin-coating on cubic (001) SrTiO3 substrates. The films were heat-treated at temperatures between 380 °C/1 h and 800 °C/1 h. X-ray diffraction, atomic force microscopy, scanning and transmission electron microscopy were used to characterize the microstructure of the films and to evaluate the epitaxial phenomena. At ∼400 °C/1 h, a polycrystalline, metastable Pb-Ti fluorite crystallizes from the pyrolyzed amorphous precursor. At slightly higher temperatures (∼420 °C/1 h), the thermodynamically stable phase with the perovskite structure epitaxially nucleates at the film/substrate interface. A small number of epitaxial grains grow through the film toward the surface and consume the nanocrystalline fluorite grains. Coarsening of the perovskite grains leads to a reduction in mosaic spread during further heating. Pores, which concurrently coarsen with grain growth, produce a pitted surface as they disappear from within the film. At 800 °C/1 ha dense epitaxial PbTiO3 film with a smooth surface is observed. Parameters governing the formation of a- and c-domains are discussed as well as the small tilts of the domain axes away from the substrate normal.

Study of nanostructured WC-Co composites

Abstract: Nanostructured materials, also termed nanocrystalline materials, have emerged upon the horizon in the past few years. In the present research, a preliminary study has been performed regarding sintering, grain growth, and mechanical properties of nanostructured WC-Co. It was found that densification of nanostructured WC-Co powder could be completed in 5 min, or 15 min if grain growth inhibitors are added. Grains grow extremely rapidly, very likely via coalescence, during heating and the first few minutes (< 5) at temperature. After the initial rapid stage, grain growth followed the linear relationship of coarsening. It was also found that samples made from nanostructured powder had better surface crack resistance than samples made from standard submicron powder.

Properties of ultra-fine grain binderless cemented carbide ‘RCCFN’

Abstract: Conventional binderless cemented carbide is known as WC-3% TiC-2% TaC cemented carbide with a mean WC grain diameter of about 2 μm, which does not include a binder phase. This alloy, however, has no binder phase and therefore low strength. The authors investigated the effects of mean diameter of raw WC powder on mechanical characteristics, and found that the smaller the mean diameter of raw WC powder, the lower the temperature at which sintering is possible and the higher the hardness and strength becomes. An investigation was also made on the effects of grain growth suppression additives on the alloy using 0.6 μm diameter WC powder, which offers the highest mechanical characteristics, with the objective of enhancing characteristics through finer grains. Hardness increased with additional amounts of Cr 3 C 2 and VC. Strength peaked at a certain additive amount, with superior values of H R A = 95.5 and transverse-rupture strength = 1.8 GPa. The microstructure was found to be composed of only very fine and uniform carbides; a mirror surface of 7 nm Rtm was obtained by mirror polishing. These characteristics make it possible for this alloy to be used in various optical applications.

The influence of high sintering temperatures on the mechanical properties of hydroxylapatite

Abstract: The kinetics of the thermal decomposition of stoichiometric hydroxylapatite (HA) has been studied up to 1500°C for the purpose of determining the maximum admissible combinations of temperature and time for sintering HA. The influence of the sintering temperature on shrinkage, density and grain growth is then investigated in the temperature range from 1000 to 1450°C. Nearly theoretical density was achieved above 1300°C. A maximum fracture toughness is obtained for the samples sintered at 1300°C whereas hardness increases up to a sintering temperature of 1400°C. These results are discussed in terms of the roles of porosity and grain size.

Grain Growth and Fracture Toughness of Fine‐Grained Silicon Carbide Ceramics

Abstract: Fine-grained silicon carbide ceramics with an average grain size of 0.11 {micro}m were liquid-phase sintered from fine {beta}-SiC powder by hot pressing. The hot-pressed materials were subsequently annealed to enhance grain growth. The diameters and aspect ratios of grains in the hot-pressed and annealed materials were measured on polished and etched surfaces. The bimodal grain size distribution in annealed materials was obtained at 1,850 C without appreciable phase transformation. The average diameter and average aspect ratio increased with annealing time. The fracture toughness of a fine-grained silicon carbide ceramic determined by the Vickers indentation method was 1.9 MPa {center_dot} m{sup 1/2}. The fracture toughness increased to 6.1 MPa {center_dot} m{sup 1/2} after grain growth by annealing at 1,850 C for 12 h. Higher fracture toughness of annealed materials is due to bridging by elongated grains as evidenced by R-curve-like behavior.

On the enhanced grain growth in ultrafine grained metals

Abstract: The static grain growth has been investigated in ultrafine grained copper with an initial grain size of 160 nm. It has been revealed that its kinetics follows to normal grain growth behaviour, but a grain growth starts at a relatively low temperature (0.32 T m ). Good fits with experimental data for several ultrafine grained metals have been obtained if the activation energy for grain boundary diffusion is assumed to be lower than for coarse grained materials, but increases during grain growth. It is suggested that this unusual behaviour of the activation energy is caused by the presence of non-equilibrium grain boundaries in ultrafine grained materials and their recovery during heating.

The Nature and behavior of grain boundaries

Abstract: Structure of Grain Boundaries.- Structure of Grain Boundaries - Theoretical Determination and Experimental Observations.- On Grain Boundary Dislocation Contrast in the Electron Microscope.- Some Properties of the Disclination Structure of Grain Boundaries.- Coincidence and Near-Coincidence Grain Boundaries in HCP Metals.- Computer Simulation of Asymmetric Grain Boundaries and their Interaction with Vacancies and Carbon Impurity Atoms.- Energetics of Grain Boundaries.- Grain Boundary Phase Transformations.- Behavior of Grain Boundaries Near the Melting Point.- The Interaction of Migrating Liquid Inclusions With Grain Boundaries in Solids.- An Electron Microscope Study of Configurational Equilibrium at Twin-Grain Boundary Intersections in Fcc Metals.- Grain Boundary Curvatures in Annealed Beta Brass.- Grain Boundary Motion and Related Phenomena.- On the Theory of Grain Boundary Motion.- The Behavior of Grain Boundaries During Recrystallization of Dilute Aluminum-Gold Alloys.- Mechanisms of Electromigration Damage in Metallic Thin Films.- Solute Effects on Grain Boundary Electro-Migration and Diffusion.- Growth Selection in High-Purity Cadmium.- A Crystallographic Alternative to the Coincidence Relationships in Copper.- Influence of Solutes on the Mobility of Tilt Boundaries.

Inhibited coarsening of solid-liquid microstructures in spray casting at high volume fractions of solid

Abstract: Experimental studies on coarsening in fine grained solid-liquid microstructures at high volume fractions of solid ( f s ) have been carried out to determine if inhibited coarsening under these circumstances could account for the anomalous fine cell sizes observed in spray castings. The materials investigated included a chill-cast dendritic binary alloy of Al-Cu, two spray cast alloys—AA2014 and Cu-Ti, whose grain size was the segregate spacing, and a d.c.-cast alloy Al-4.5 wt% Cu-1.5 wt% Mg in both coarse-grain and grain-refined conditions. The observed segregate spacings after coarsening were smaller than that predicted by empirical correlations of dendrite arm spacing and freezing time. In all cases, the coarsening was found to become slower as the temperature was reduced and f s increased. Conventional coarsening theories and experiments predict the opposite, i.e. faster coarsening at higher volume fractions of solid. Two additional coarsening models were developed for the grain growth at high volume fractions of solid by processes whose rates are limited by migration of liquid at grain boundaries as liquid films on 2-grain surfaces or liquid rods on 3-grain triple points. In both models, the conventional diffusion-limited t 13 coarsening law was reproduced, but the rate constant K contained the term 11−f s and so also predicted accelerated coarsening as f s → 1 . Three possible explanations for the observed lower K values at increasing f s are proposed. The first is the effect of the increasing difference between the solute contents of solid and liquid as the temperature is reduced. This produces a1/X 1 dependence of the coarsening rate constant K . The second inhibiting effect, specific to dendritic structures, is in-grain coalescence of dendrite arms at high f s which produces isolated liquid particles within the grains. The final possibility is particle-inhibition of grain boundary migration by minority (impurity) particles at the grain boundaries. Such particles were seen, however, for only two of the alloys, viz. the grain defined d.c. cast Al-4.5 wt% Cu-1.5 wt% Mg and the spray case AA2014, but they or gas-filled pores are proposed as strong possibilities to account for the fine grain sizes observed in all spray cast microstructures.

Study of grain growth in electrodeposited nanocrystalline nickel-1.2 wt.% phosphorus alloy

Abstract: The paper describes a transmission electron microscope study of the microstructural evolution of electrodeposited nanocrystalline Ni-1.2wt%P and pure nickel during heating experiments performed in situ. The grain structure of the Ni-1.2wt.%P alloy was found to be stable up to a temperature of 360 °C. The grain growth was concurrent with Ni3P precipitation, suggesting the important role of phosphorus in supersaturated solid solution in imparting the thermal stability to the microstructure. Normal grain growth was observed in Ni-1.2wt.%P at annealing temperatures up to 480 °C. Subsequent analysis revealed pinning of grain boundaries by Ni3P precipitates. The value of activation energy for grain growth in Ni-P alloy (2.25 eV), obtained from continuous scan rate DSC experiments, is consistent with the above observation. Electrodeposited nanocrystalline nickel, on the other hand, showed abnormal grain growth at temperatures as low as 260 °C. At 320 °C, the microstructure had largely transformed from the nanocrystalline to the microcrystalline state.

Dendrite coherency during equiaxed solidification in binary aluminum alloys

Abstract: Dendrite coherency, or dendrite impingement, is important to the formation of the solidification structure and castability of alloys. Dendrite coherency in the systems Al-xMn, Al-xCu, Al-xFe, and Al-xSi(x = 0 to 5 wt pct) has been studied by continuous torque measurement in solidifying samples. The fraction solid at the dendrite coherency point, fs*, varies with the alloy system and the solute concentration in the alloy, from 18 to 56 pct for the present alloys investigated. An increase in solute concentration decreases the coherency fraction solid,fs*. An alloy system with a large slope of the liquidus line has a high coherency fraction solid. A theoretical approach has been developed to account for the effects of the alloy system and solute concentration on the dendrite coherency in the alloy. The grain sizes of the alloys were evaluated using the parameters at coherency point.

Analytical modelling of grain growth in metals and alloys in the presence of growing and dissolving precipitates—II. Abnormal grain growth

Abstract: The present investigation is concerned with analytical modelling of grain growth in metals and alloys in the presence of growing and dissolving precipitates. In Part II the modelling methodology presented in Part I for normal grain growth has been further developed and applied to abnormal grain growth. The judicious construction of the constitutive equations makes full use of both dimensionless parameters and calibration techniques to eliminate poorly known kinetic constants. The results are presented in the form of novel “mechanism” maps (by utilizing the concept of group variables) which show the competition between the various processes that lead to abnormal grain growth during heat treatment. This approach has made it possible to fit many of the apparently conflicting results into a more consistent picture and establish a simple criterion for the initiation of abnormal grain growth under different pinning conditions. It is concluded that the key to a verified, quantitative understanding of the phenomenon of abnormal grain growth lies primarily in the recognition of the important effect of precipitate stability on the secondary recrystallization kinetics.

Processing and properties of al2o3/sic nanocomposites

Abstract: Summary Alumina/SiC nanocomposites were produced by mechanical mixture of commercial powders. The preparation steps involved the vigorous mixing of the powders and drying under conditions where the homogeneous mixture was kept stable. Pressureless sintering of die-pressed powders achieved reasonable densities (∼97% theoretical density) for 2·5wt% of SiC on sintering at 2073 K. Higher SiC contents strongly reduced the sintered density. The use of a more reactive alumina (finer matrix powder) gave similar results. Hot pressing at 1973 K/1 h/25 MPa produced high-density materials for SiC contents as high as 20 wt%. Transmission and scanning electron microscopy analysis showed that the SiC particles were well distributed and were situated both inside the grains and on the grain boundaries of the alumina matrix. The SiC strongly inhibited grain growth in the matrix in keeping with the Zener model. The bend strength increased as the SiC content increased, a result partly explained by the grain size refinement. The strength improvement of 20% over monolithic was explained in terms of the change to an intergranular fracture mode.

Simulation of curvature-driven grain growth by using a modified monte carlo algorithm

Abstract: The Monte Carlo (MC) algorithm that currently exists in the literature for simulating curvature-driven grain growth has been modified. The modified algorithm results in an acceleration of the simulated grain growth and an early estimate of the grain growth exponent that is close to the theoretical value of 0.5. The upper limit of grain size distributions obtained with the new algorithm is significantly lower than that obtained with the old, because the new algorithm eliminates grain coalescence during grain growth. The log-normal function provides an excellent fit to the grain size distribution data obtained with the new algorithm, after taking into account the anisotropy in grain boundary energy.

Retention of nanostructure in aluminum oxide by very rapid sintering at 1150 - C

Abstract: Aluminum oxide powders doped with MgO (300 to 500 nm) were sintered to almost theoretical density within just 10–15 min at 1150 °C using a plasma-activated sintering process based on charging the loosely filled powders with an electric discharge prior to densification by resistance heating. The microstructure of the consolidated disks was examined by high resolution transmission electron microscopy (HREM) and electron energy loss spectroscopy (EELS) and revealed excellent grain to grain contact with virtually no grain growth and structurally clean grain boundaries.

Anisotropic grain growth in TiO2-doped alumina

Abstract: Grain growth in TiO 2 -doped alumina was studied in a high density, ultrafine matrix (0.4 μm. Normal grain growth, anisotropic grain growth and abnormal grain growth were observed. With 0.15-0.4 wt.% TiO 2 , samples initially undergo normal grain growth until a crystal microstructure is attained and anisotropic grain growth in nucleated. Large anisotropic, platelet-shaped grains grow rapidly by a step growth process until impingement of the large grains essentially stops further growth. The volume fraction of anisotropic grains ranged from 20 to 100 vol.% suggesting that physical properties dependent on grain shape and volume fraction can be tailored. Critical requirements are proposed for the in situ growth of anisotropic grains.

Size-dependent solute segregation and total solubility in ultrafine polycrystals: Ca in TiO2

Abstract: Grain boundary segregation at ultrafine grain sizes has been studied. Using a STEM microanalysis technique to quantify the grain boundary coverage of calcium (0.34 mol%) in TiO2 ranging in grain size from 50 to 750 nm, it is found that below grain sizes of 150–350 nm, segregation deviates from conventional isotherms, exhibiting a clear size dependence. In this size regime the interfacial area to volume ratio is as important as temperature and composition in determining grain boundary coverage. In the present system, grain boundaries become saturated with calcium when the coverage reaches approximately one half of an equivalent monolayer. The experimental results can be modeled by a statistical thermodynamical treatment of segregation which takes into account the large density of grain boundary sites in this size range. We also find direct evidence of enhanced total solubility at very fine grain sizes due to grain boundary segregation.

A novel computer simulation technique for modeling grain growth

Abstract: In this paper, the author proposes a new computer simulation model for investigating grain growth kinetics, born from the recent work on the domain growth kinetics of a quenched system with many non-conserved order parameters. A key new feature of this model for studying grain growth is that the grain boundaries are diffuse, as opposed to previous meanfield and statistical theories and Monte-Carlo simulations which assumed that grain boundaries were sharp. Unlike the Monte-Carlo simulations in which grain boundaries are made up of kinks, grain boundaries in the continuum model are smooth. Below, he describes this model in detail, give prescriptions for computer simulation, and then present computer simulation results on a two-dimensional model system.

Atom Probe Studies of Nanocrystalline Microstructural Evolution in Some Amorphous Alloys

Abstract: This paper reviews our recent atom probe studies of nanocrystalline alloys fabricated by primary crystallization of amorphous alloys. Partitioning and segregation of alloying elements in the course of primary crystallization in Fe-Si-B-Nb-Cu, Fe-Ta-C, Fe-Zr-B and Al-Ni-Ce(-Cu) alloys were examined by atom probe field ion microscopy. Concentration fluctuations in alloying elements are commonly observed in the early stage of annealing, which is believed to induce a large number density of nucleation sites. During the growth stage of the crystalline phases, one or two following mechanisms work to control the crystal grain growth : (1) stabilization of the remaining amorphous phase by partitioning of amorphous forming elements, (2) segregation of slow diffusing solute element at the amorphous/crystal interface, (3) precipitation of thermally stable compound such as TaC.

Cooperative phenomena at grain boundaries during superplastic flow

Abstract: In situ observations in a scanning electron microscope (SEM) performed on different microstructural scales in Pb—62%Sn specimens, superplastically deformed in single shear and in simple tension, showed sliding of grain groups [cooperative grain boundary sliding (CGBS)], rotation of grain groups (cooperative grain rotation) and cooperative grain boundary migration (correlated migration of sliding grain boundaries). The observed macroscopic pattern of the CGBS surfaces is consistent with predictions of slip-lines field theory. The progress of the sliding of grain blocks at the scale of grain groups can be modeled in terms of cellular dislocations. The micromechanism for such sliding and the migration of sliding grain boundaries at the scale of the individual interface might be interpreted from the viewpoint of grain boundary dislocations.

Microstructure and high temperature mechanical properties of tin

Abstract: The mechanical properties of tin have been studied by tensile tests in the temperature range 293–463 K. Tensile tests were performed for cylindrical samples at a constant strain rate and varying strain rates during deformation. In-situ-tensile tests also were conducted in ribbon-form samples. At the strain rates studied, deformation takes place preferentially by slip, although some scattered twins also were observed at lower temperatures. Strong grain growth occurs at the higher temperatures. Microstructural observations of deformed samples show that dynamic recrystallization is not important in the temperature range investigated. The fracture surface of the cylindrical samples changes from a chisel type of fracture at the lower temperatures to a simple shear type of fracture at the higher temperatures. Both the tensile strength and ductility decrease with increasing temperature. An explanation is given for the loss of ductility at high temperatures. The activation energy for creep, obtained from strain-rate-change tests is 35 kJ mol−1 and the stress exponent is about 6. These values are related to a slip mechanism controlled by pipe diffusion.