Showing papers on "Grain growth published in 1993"

Probabilistic modelling of microstructure formation in solidification processes

Abstract: A new approach to the modelling of grain structure formation in solidification processes is proposed. Based upon a two-dimensional cellular automata technique, the model includes the mechanisms of heterogeneous nucleation and of grain growth. Nucleation occurring at the mould wall as well as in the liquid metal are treated by using two distributions of nucleation sites. The location and the crystallographic orientation of the grains are chosen randomly among a large number of cells and a certain number of orientation classes, respectively. The growth kinetics of the dendrite tip and the preferential 〈100〉 growth directions of cubic metals are taken into account. The model is then applied to small specimens of uniform temperature. The columnar-to-equiaxed transition, the selection and extension of columnar grains which occur in the columnar zone and the impingement of equiaxed grains are clearly shown by this technique. The calculated effect of the alloy concentration and cooling rate upon the resultant microstructure agree with experimental observations.

Thermal stability and grain growth behavior of mechanically alloyed nanocrystalline Fe-Cu alloys

Abstract: X-ray diffraction, transmission electron microscopy, and differential scanning calorimetry were used to study the thermal stability of highly supersaturated nanocrystalline FexCu100−x alloys (10 ~80. For 60<=x<=80 fcc and bcc phases coexist. Heating to elevated temperatures leads to structural relaxation, phase separation, and grain growth of the metastable nanocrystalline solid solutions. Single-phase fcc and bcc alloys undergo significant strain release but no appreciable grain growth prior to phase separation. After phase separation pronounced grain growth sets in. In contrast, samples in the two-phase region show some grain growth and significant chemical redistribution even at low temperatures. The phase separation of single-phase fcc and bcc alloys proceeds via different mechanisms: fcc solid solutions decompose by forming small Fe precipitates, while demixing in bcc alloys starts by segregation of Cu atoms to bcc grain boundaries before nucleation of Cu precipitates. These results show that the stability and grain growth behavior of nanocrystalline alloys is strongly affected by the microstructure of the material.

The Lattice-Parameters of Austenite and Ferrite in Fe-C Alloys as Functions of Carbon Concentration and Temperature

Abstract: Despite its relevance for various calculations involving phase transformations in Fe-C alloys, little information is available on the lattice parameter of austenite at elevated temperatures and its dependence on the carbon concentration. Furthermore, severe scatter exists in the literature for the lattice parameter at high temperature. Most literature data were acquired using X-ray diffraction, although neutron diffraction seems to be a more suitable technique penetrate and probe a large volume of material, the advantages of neutron diffraction over X-ray diffraction are its reduced sensitivity to surface decarbonization and improved crystal statistics (both decarbonization and grain growth can occur during diffraction experiments at high temperatures requiring long exposure times). In this work in-situ neutron diffraction experiments on Fe-C alloys were performed to determine the lattice parameter of the austenitic and ferritic phases in a temperature region from just below to just above the bi-phasic austenite/ferrite region.

Processing nanocrystalline ceramics for applications in superplasticity

Abstract: The production of nanocrystalline ceramics for subsequent use in superplastic forming operations requires that the ceramics be made in large quantities, with high densities, and under stringent grain growth control. To make large amounts of nanocrystalline starting powders, two popular wet chemical techniques (precipitation from salt solutions and alkoxide hydrolysis) can be used and are described in this paper. Unfortunately, pressureless sintering of these powders does not typically lead to the high densities and ultrafine grain sizes desired in the final product. Sintering data suggest that pore shrinkage occurs only when grains reach a critical size with respect to the pore size; thus, if the ceramic contains large pores, densification can require significant grain growth. Separation of large pores from grain boundaries may also occur and lead to incomplete densification, even at extremely large grain sizes. In all cases the pressureless sintering behavior of the nanocrystalline ceramics appears to adhere to well established theories used to explain the sintering of conventional, larger-grained ceramics. During both pressureless sintering and sinter-forging experiments, the grain size of a nanocrystalline ceramic is identical to the average spacing between open pores in the sample. Pressureless sintering results in the closure of these pinning pores by about 90% density and thus3leads to a substantial grain growth at densities greater than 90%. Sinter-forging, however, often allows one to maintain a stable population of small open pores (for pinning purposes) throughtout sintering, while preferentially eliminating the large pores which detract from the sample density. The deformation regime in which sinter-forging is performed has a decided effect on whether large pores or small pores are eliminated preferentially and, consequently, on whether a high density and fine grain size combination is achieved or not.

Effect of Grain Growth of β‐Silicon Nitride on Strength, Weibull Modulus, and Fracture Toughness

Abstract: [beta]-Si[sub 3]N[sub 4] powder containing 1 mol% of equimolar Y[sub 2]O[sub 3]-Nd[sub 2]O[sub 3] was gas-pressure sintered at 2,000C for 2 h (SN2), 4 h (SN4), and 8 h (SN8) in 30-MPa nitrogen gas. These materials had a microstructure of in-situ composites'' as a result of exaggerated grain growth of some [beta]-Si[sub 3]N[sub 4] grains during firing. Growth of elongated grains was controlled by the sintering time, so that the desired microstructures were obtained. SN2 had a Weibull modulus as high as 53 because of the uniform size and spatial distribution of its large grains. SN4 had a fracture toughness of 10.3 MPa[center dot]m[sup 1/2] because of toughening provided by the bridging of elongated grains, whereas SN8 showed a lower fracture toughness, possibly caused by extensive microcracking resulting from excessively large grains. Gas-pressure sintering of [beta]-Si[sub 3]N[sub 4] powder was shown to be effective in fostering selective grain growth for obtaining the desired composite microstructure.

Sintering kinetics and microstructure development of nanoscale Y-TZP ceramics

Abstract: Zirconia samples doped with 3 mol% yttria were prepared by gel precipitation from a metal chloride solution and their sintering behaviour compared with that of a commercial powder. Dense (relative density 97%) nanoscale ceramics with a mean grain size of 60 nm are obtained after sintering at 1050°C for 7h. Important densification mechanisms in the initial sintering stage are grain boundary sliding and grain boundary diffusion. Grain growth in the final sintering stage seems to be impurity drag controlled. Extremely low activation energies are obtained for both densification and grain growth in the initial sintering stages. Special attention has been paid to the effect of aggregate size of the precursor powder on the final grain size.

Grain Growth Studies of Silicon Nitride Dispersed in an Oxynitride Glass

Abstract: Isothermal growth of [beta]-Si[sub 3]N[sub 4] crystals dispersed in an oxynitride glass (Y-Si-Al-O-N) was studied by electron microscopy after heat treatment at temperatures between 1,550 and 1,640 C for 1 to 18 h. The [beta]-crystals exhibited growth striations introduced by intermediate coolings and these striations were used for developing a sophisticated technique for analysis of growth. It was determined that [alpha]/[beta]-transformation and Ostwald ripening can be treated as different kinetic stages of grain growth, while [beta]-nucleation was found to be insignificant. The mean diameter of the needle like [beta]-grains was almost constant during phase transformation, indicating negligible growth of the [beta]-prism plane; growth was mainly one-dimensional with the maximum mean length and aspect ratio just at the end of the phase transformation. The growth rate of the [beta]-basal plane was independent of diameter, indicating interface-controlled growth. During Ostwald ripening, the length distribution broadened and the aspect ratio of smaller grains decreased. Dissolution of small grains caused an increase in the mean diameter, while the mean aspect ratio decreased.

Sintering of Sol—Gel-Prepared Submicrometer Particles Studied by Transmission Electron Microscopy

Abstract: We studied the sintering process of sol-gel-prepared monodisperse submicrometer CeO2 spheres by examining the microstructural changes of single spherical particles after successive heat treatment in oxygen up to 850°C using transmission electron microscopy. Steps of organic phase removal, CeO2 crystallization, grain growth, and particle condensation were clearly illustrated. Grain-size and sphere-diameter changes were measured quantitatively using this technique. CeO2 particles were found to be highly porous until collapse, which occurred at~850°C.

Lateral Grain Growth of Poly-Si Films with a Specific Orientation by an Excimer Laser Annealing Method

Abstract: Dramatic lateral grain growth of nondoped poly-Si films (maximum grain size: ~4.5 µm, film thickness: 500 A) with strong crystallographic (111) orientation on glass substrates has been achieved using an excimer laser annealing method, namely at low temperature below 400°C and in a processing time shorter than a second, for the first time. The surface morphology of these poly-Si films was very smooth and the crystallinity was excellent with minimal internal defects. These poly-Si films have monomodally distributed grain sizes, with an average grain size of around 1.5 µm. As a result of experimental study, we speculate that the basic driving force behind this lateral grain growth was surface free energy anisotropy, as the same mechanism was observed in high-temperature furnace annealing of doped poly-Si thin films.

Sintering and grain growth of ultrapure alumina

Abstract: The kinetics of densification and grain growth of ultrapure alumina (> 99.999%) were measured for clean sintering conditions in a pure-sapphire tube, and compared with kinetics measured during normal sintering conditions in an alumina crucible of 99.8% purity. For the clean condition, the microstructure of sintered alumina remained homogeneous and only normal grain growth was observed up to 1900°C for 5 H. However, under the normal sintering condition, both normal and abnormal grain growth were observed depending on the sintering temperature and time. Thus, abnormal grain growth in alumina could be effectively suppressed without introducing sintering aids (such as MgO) by using an ultrapure powder and by preventing the introduction of any impurities throughout the sintering process. This result strongly suggests that abnormal grain in commercially pure alumina (⩽ 99.99%) is not an intrinsic property of alumina but an extrinsic property controlled by minor constituents that can be present in the original powder or introduced during powder processing and subsequent sintering.

Grain growth in three dimensions depends on grain topology.

Abstract: While the growth rate of the average volume in a three-dimensional polycrystal is well understood, the growth rates of individual grains (which determine material properties through the topological and volume distributions) are not known. Using a three-dimensional Q-state Potts model simulation, I find that the average canonical growth rate of a grain depends linearly on its number of faces and is independent of its volume, a surprisingly simple and unexplained result

Kinetics of β-Si3N4 Grain Growth in Si3N4 Ceramics Sintered under High Nitrogen Pressure

Abstract: The kinetics of anisotropic β-Si3N4 grain growth in silicon nitride ceramics were studied. Specimens were sintered at temperatures ranging from 1600° to 1900°C under 10 atm of nitrogen pressure for various lengths of time. The results demonstrate that the grain growth behavior of β-Si3N4 grains follows the empirical growth law Dn– D0n=kt, with the exponents equaling 3 and 5 for length [001] and width [210] directions, respectively. Activation energies for grain growth were 686 kJ/mol for length and 772 kJ/mol for width. These differences in growth rate constants and exponents for length and width directions are responsible for the anisotropy of β-Si3N4 growth during isothermal grain growth. The resultant aspect ratio of these elongated grains increases with sintering temperature and time.

Effect of Sintering Aids on Microstructures and PTCR Characteristics of (Sr0.2Ba0.8)TiO3 Ceramics

Abstract: The effects of liquid-phase sintering aids on the microstructures and PTCR characteristics of (Sr0.2Ba0.8)TiO3 materials have been studied. The grain size of sintered materials monotonically decreases with increasing content of Al2O3–SiO2–TiO2 (AST). The ultimate PTCR properties with ρht/ρrt as great as 105.61 are obtained for fine-grain (10-μm) samples, which contain 12.5 mol% AST and were sintered at 1350°C for 1.5 h. The quantity of liquid phase formed due to eutectic reaction between AST and (Sr,Ba)TiO3 is presumably the prime factor in determining the grain size of samples. The grains grow rapidly at the sintering temperature in the first stage until the liquid phase residing at the grain boundaries reaches certain critical thickness such that the liquid–solid interfacial energy dominates the mechanism of grain growth.

Grain growth kinetics of Si3N4 during α/ß-transformation

Abstract: The growth of s-Si3N4 particles, highly diluted in an oxynitride glass, was analysed by SEM and the growth features of individual crystals were discussed. The observed linear time law for the length growth during α/s-transformation was correlated with results of a theoreticl analysis taking into account the diffusion in the liquid phase as well as the interface reaction at α- and s-crystals. The number of initial nuclei was found to be important for a linear time law of growth. Finally, the considerations of the dilute particle system were applied to a Si3N4 ceramic. The opposing influence of growth anisotropy of sSi3N4 in the presence of a melt and the packing requirements demanding a reduction of the specific surface area is discussed.

Three-dimensional probabilistic simulation of solidification grain structures: Application to superalloy precision castings

Abstract: A two-dimensional (2-D) probabilistic model, previously developed for the prediction of microstructure formation in solidification processes, is applied to thin section superalloy precision castings. Based upon an assumption of uniform temperature across the section of the plate, the model takes into account the heterogeneous nucleation which might occur at the mold wall and in the bulk of the liquid. The location and crystallographic orientation of newly nucleated grains are chosen randomly among a large number of sites and equiprobable orientation classes, respectively. The growth of the dendritic grains is modeled by using a cellular automaton technique and by considering the growth kinetics of the dendrite tips. The computed 2-D grain structures are compared with micrographie cross sections of specimens of various thicknesses. It is shown that the 2-D approach is able to predict the transition from columnar to equiaxed grains. However, in a transverse section, the grain morphology within the columnar zone differs from that of the experimental micrographs. For this reason, a three-dimensional (3-D) extension of this model is proposed, in which the modeling of the grain growth is simplified. It assumes that each dendritic grain is an octaedron whose half-diagonals, corresponding to the crystallographic orientations of the grain, are simply given by the integral, from the time of nucleation to that of observation, of the velocity of the dendrite tips. All the liquid cells falling within a given octaedron solidify with the same crystallographic orientation of the parent nucleus. It is shown that the grain structures computed with this 3-D model are much closer to the experimental micrographie cross sections.

Oxidation resistance of PVD Cr, Cr-N and Cr-N-O hard coatings

Abstract: The plasma-beam-sputtering process was used to deposit coatings 3 μm thick of various compositions from pure chromium to stoichiometric CrN and a few CrN x O y coatings on steel and sapphire substrates at a temperature of 200°C. The formation of the coatings, their microstructure, morphology, composition and microhardness were determined. Their oxidation behaviour in an oxygen flow in an oven was studied at temperatures between 500 and 800°C by the weight gain method, X-ray diffraction and Auger electron spectroscopy depth profile analysis. The surface morphology and crystallite growth were observed by scanning electron microscopy and roughness measurements. The results show a pronounced minimum in weight gain of Cr-N x coatings as a function of partial pressure of nitrogen after heating in oxygen at 700 and 800 °C. The protective layer on all the Cr-based coatings studied was a stable Cr 2 O 3 oxide. Of all the hard coatings investigated, the stoichiometric CrN coating had the lowest oxidation rate in pure oxygen, also after a long-term test of 200 h. Oxidation in oxygen also induces the growth of crystallites, 0.05 μm in size in as-deposited coatings. After heating for 4 h at 700 °C in oxygen, the grain size increased to 0.4 μm, while at 800 °C it increased to 0.75 μm. There was no essential difference in grain growth between tool steel and sapphire substrates.

Thermal stability of ultrafine-grained metals and alloys

Abstract: Grin growth in ultrafine-grained elemental metals (Cu, Ag, Pd) and alloys has been studied by differential scanning calorimetry and transmission electron microscopy. The samples were prepared via the inert gas condensation technique, followed by uniaxial high pressure compaction. Abnormal grain growth is observed in all the pure elemental samples. The onset of this secondary recrystallization is at or slightly below the recrystallization temperature known from the respective conventionally cold-worked metals. The activation energies for grain growth were determined by the Kissinger method and correspond to the typical values for grain boundary diffusion. For samples measured directly after preparation, a two-step process was found. The low temperature reaction can be attributed to relaxation processes, because no microstructural changes can be observed. The reaction at higher temperature is due to abnormal grain growth. Principally the same behaviour was found in samples with increased residual porosity (up to 15 vol.%) and concentration of substitutional solutes (up to 5 at.% Au). Gaseous impurities (oxygen) increase the onset temperature for abnormal grain growth. Normal grain growth is observed in duplex microstructures, as demonstrated using AgCu as a model system.

Theory of grain boundary motion in the presence of mobile particles

Abstract: A theory of grain boundary motion in the presence of mobile particles is put forward. It is shown that the boundary-particle-interaction leads to a hysteresis in the velocity-driving relationship. The extent of the hysteresis depends on particle mobility, which is very sensitive to particle size. The effect of particles is discussed for planar and curved boundaries as well as volume particle distributions. The theory accounts for a smaller limiting grain size during grain growth than predicted by Zener drag. The concept can be generalized to include all kinds of mobile obstacles for boundary migration. In such cases not the distribution of obstacle spacing rather the distribution of obstacle mobilities will control microstructure evolution.

Pulsed laser‐induced melting followed by quenching of silicon films

Abstract: Amorphization and crystallization were studied through laser‐induced melting of silicon films formed on quartz substrates induced by irradiation with a 30 ns XeCl excimer laser. Homogeneous and rapid solidification occurs and amorphous solid state can be formed when the melt duration is long enough to make the temperature gradient in liquid silicon lower than 1×105 K/cm at the Si/quartz interface. The solid state after homogeneous solidification is governed by recalescence caused by latent heat released at solidification. A completely amorphous state is formed when film thickness is thinner than 24 nm because latent heat reduces as film thickness decreases. Both crystalline and amorphous states were observed for film thickness above 24 nm because recalescence can cause crystalline grain growth. Complete crystallization occurs through interface controlled growth when the temperature gradient is higher than 1×105 K/cm. The velocity of liquid/solid interface is 0.6 m/s, which is too low to cause amorphization.

Microstructures and hardness of ultrafine-grained Ni3Al

Abstract: The microstructural evolution of the ultrafine-grained intermetallic compound Ni 3 Al is studied as a function of annealing at different temperatures. The ultrafine microstructure is produced by a high plastic torsional straining. Transmission electron microscopy, X-ray diffraction and differential scanning calorimetry are used to characterize the microstructural evolution and microhardness is used to determine mechanical behaviour. The as-deformed microstructure exhibits an almost fully disordered crystalline structure with coherent domain size of about 18 nm, a strong torsional texture and high internal elastic strains. On annealing the as-deformed samples at different temperatures, the recrystallization of the material into a granular type structure containing non-equilibrium grain boundaries is first observed. This is followed by the transformation from non-equilibrium grain boundaries with simultaneous grain growth. This transformation is correlated with an increase of hardness. A new concept of non-equilibrium grain boundaries transparency is presented to interpret this singular behaviour. The results are compared to those obtained on an ultrafine-grained Al-1.5% Mg alloy produced by the same technique and which exhibits the same mechanical behaviour.

Microstructural origin of the perpendicular anisotropy in M-type barium hexaferrite thin films deposited by RF magnetron sputtering

Abstract: Barium hexaferrite thin films deposited by RF magnetron sputtering have exhibited saturation magnetization 90% of that of bulk single crystals, whereas the perpendicular uniaxial anisotropy is only 60% of that of the bulk. X-ray diffraction spectra suggest good c-axis orientation perpendicular to the film plane. However, M-H hysteresis loops show a fairly large in-plane hysteresis. Scanning electron microscopy and transmission electron microscopy show that the films are made of a mixture of platelet-shaped grains and acicular grains. Microdiffraction studies in a transmission electron microscope indicate that the platelets have their c-axis oriented perpendicular to the plane and that the acicular grains have c-axis orientation in the plane. Preferential grain growth in the basal plane of the crystal is believed to be responsible for these grain geometries. >

Computer modelling of anisotropic grain microstructure in two dimensions

Abstract: Assuming an anisotropic factor for geometrical grain microstructure, known as the Voronoi or Johnson-Mehl model, several grain microstructures have been simulated in two dimensions employing nucleation seeds that are assumed to be elliptical and polygon like shapes. By varying the shape of these seeds, six different microstructures have been generated based on growth and impingement model in both simultaneous and continuous nucleation cases. In addition, morphology such as the aspect ratio distribution, grain size distribution and number of grain edges in each case have been analyzed and are correlated qualitatively.

Microstructural evolution in two-dimensional two-phase polycrystals

Abstract: In two-dimensional polycrystals composed of α-phase and β-phase grains the stability of ααα, βββ, ααβ and αββ three-grain junctions and αβαβ four-grain junctions depends on the α−α, β−β and α−β interfacial energies. A computer simulation which generates thermodynamically consistent microstructures for arbitrary interfacial energies has been utilized to investigate microstructural evolution in such polycrystals when phase volume is not conserved. Since grain shapes, phase volume, and phase arrangements are dictated by interfacial energies, clustered-, alternating-, isolated-, and single-phase microstructures occur in different interfacial energy regimes. Despite great differences in microstructure, polycrystals which contain only three-grain junctions evolve with normal grain growth kinetics. In contrast, structures containing flexible four-grain junctions eventually stop evolving. We conclude that two-dimensional polycrystals continually evolve when grain junction angles are thermodynamically fixed, while grain growth ultimately ceases when grain junction angles may vary. Predictions concerning three-dimensional and phase-volume conserved systems are made.

Effect of grain size on yttrium grain boundary segregation in fine-grained alumina

Abstract: In this work, the effects of the mean grain size and of the total yttrium content on the distribution of this element in fine-grained alumina are studied. The yttrium distribution is determined in the TEM using energy dispersive X-ray analysis by measuring the mean grain boundary composition as well as the mean size and number of precipitates per unit volume. Two different types of microstructure are observed, depending on both the grain size and the total yttrium content: in the first, only grain boundary segregation is observed, whereas in the second, the grain boundaries are saturated with yttrium, resulting in the intergranular precipitation of an yttrium-rich second phase. For a given composition, the microstructure changes from the first kind of microstructure to the second kind during grain growth. Predictions using a simple geometrical model based on the mass conservation of yttrium are in good agreement with the experimental results for both types of microstructure. It is shown that the kinetics of grain growth are not significantly affected by the yttrium when it is used along with magnesia in alumina.

Phase transformations and microstructure evolution in sol-gel derived yttrium-aluminum garnet films

Abstract: Diphasic yttrium-aluminum garnet (Y3Al5O12, YAG) sols were made by hydrolysis of aluminum and yttrium isopropoxides. The sols were gelled across TEM grids to make films. The films were heat-treated up to 1550 °C for as long as 300 h. Heat-treatments of bulk gel were also done. Microstructure and phase evolution were observed by TEM. Some observations were done in situ in a TEM hot-stage. YAG fraction and grain size, matrix grain size, nuclei/area, and film thickness were measured. Bulk samples were characterized by x-ray, DTA, and TGA. Yttrium-aluminum monoclinic (YAM) and transition alumina appeared at 800 °C. YAG nucleated between 800 °C and 950 °C. Nucleation was weakly correlated with the transient presence of YAlO3 garnet, and was eventually site-saturated at 0.3/μm3. The change in grain growth rate of the YAM and transition alumina matrix correlated with the change in the growth rate of YAG. Between 850 °C and 1000 °C YAG growth had t1/2 dependence and 280 kJ/mole activation energy. Below 850 °C nucleation was continuous, and growth had t0.85 dependence. Above 1000 °C YAG growth had t1/4 dependence, and the matrix grains coarsened with t1/4 dependence. Thicker films reacted faster because the nuclei/area and the growth rate after nucleation scaled with thickness. YAG growth was accompanied by formation of 20–100 nm subgrains. In the late stages of matrix grain coarsening there was also some reaction to YAG by a different process. Nucleation and growth kinetics are compared with other systems. Possible mechanisms are discussed.