Showing papers on "Grain growth published in 1989"
TL;DR: In this paper, a modified version of Hillert's grain growth model is presented, in which two distinct types of grains are present, and anisotropic grain boundary energies are modeled by assigning a higher energy to boundaries between like type than to boundaries of unlike type.
337 citations
TL;DR: In this paper, the microstructure of polycrystalline materials is digitized by dividing the polycrystaline material into small volume elements and storing the spatial location and crystallographic orientation of each element.
Abstract: Computer modelling has been carried out to study normal grain growth in three dimensions. The approach consists of digitizing the microstructure by dividing the polycrystalline material into small volume elements and storing the spatial location and crystallographic orientation of each element. An energy is assigned between each element and its neighbours, such that neighbours having unlike orientations provide weaker bonding than neighbours of like orientations. The annealing treatment during which grain growth occurs is simulated using a Monte Carlo technique in which elements are selected at random and thermally activated transitions to other orientations are attempted. With time, the system evolves so as to reduce the total grain interface area. The microstructures produced are in good correspondence to observations of pure metals and ceramics which have undergone grain growth. Power-law kinetics ([Rbar] = ct n) are observed, with a growth exponent in three dimensions of n = 0·48 ±0·04 in the...
332 citations
TL;DR: In this article, the authors studied grain growth kinetics in hot-pressed fine-grained olivine aggregates and found that the presence of water enhances grain growth when the amount of water is limited.
267 citations
TL;DR: In this paper, the main brass-type components of the rolling texture are retained, despite classical discontinuous recrystallization and grain growth, and possible solid solution effects (Mo) are invoked to explain this behaviour.
235 citations
TL;DR: In this paper, the authors developed two-dimensional vertex models which are expected to capture the essential ingredients of the long-time behaviour of 2D grain growth and permit efficient large-scale computer simulations.
Abstract: We develop two-dimensional vertex models which are expected to capture the essential ingredients of the long-time behaviour of two-dimensional grain growth and permit efficient large-scale computer simulations. Such simulations are carried out starting with the initial 24 000 randomly chosen Voronoi cells. We confirm the 1/2 power law for the growth of average grain size. We also find the normalized grain-size distribution and the edge number distribution which exhibit stationary behaviour at long times except for small-amplitude oscillatory behaviour for the latter.
198 citations
TL;DR: In this paper, the effects of chemical inhomogeneities and single-crystal seeds on normal and discontinuous grain growth were investigated in both undoped and MgO-doped Al2O3.
Abstract: Effects of chemical inhomogeneities and single-crystal seeds on normal and discontinuous grain growth were investigated in both undoped and MgO-doped Al2O3. The chemical impurities in the samples were exsolved at a lower temperature than the sintering temperature and measured by SEM/EDS to determine the correlation between the distribution of impurities and the microstructure in Al2O3. A feature of this study was the use of clean-room processing and firing procedures to maintain sample composition at the levels initially present in the starting powders. As the local concentrations of chemical impurities (i.e., Si, Ca) increased, the grain boundary–grain boundary dihedral angle distribution widened, with many angles at 180°, the grain-size distribution widened, and pore–boundary separation was enhanced. Discontinuous grain growth was observed in regions of undoped Al2O3 containing the largest Ca and Si concentrations. It is suggested that doping with MgO solute reduces the effects of impurities on grain growth by increasing the bulk solubility and decreasing interfacial segregation of impurities, especially Si, and by narrowing the distribution of grain boundary–grain boundary dihedral angles.
183 citations
TL;DR: In this paper, it was shown that grain growth kinetics within a sintering array can be controlled by coarsening kinetics, making it distinctly different from grain growth in the fully dense material which occurs by boundary motion.
Abstract: It is shown that grain-boundary motion in a sintering array of particles can be limited by the geometry of the sintered particle network. Despite its curvature, the grain boundary can encounter an energy barrier when it attempts to move through smaller grains toward its center of curvature. This energy barrier decreases to zero when the radius ratio (R) between adjacent grains approaches a critical value, R→Rc=− [cos (Ψe)]−1, where Ψe is the dihedral angle relating surface and grain-boundary energies. The change in radius ratio (R→Rc) can occur by interparticle mass transport, viz., coarsening. This result suggests grain growth kinetics within a sintering array can be controlled by coarsening kinetics, making it distinctly different from grain growth in the fully dense material which occurs by boundary motion. More significantly, it is shown that when a grain disappears from between larger grains by coarsening, the driving force for sintering between the newly joined grains is reinitiated. That is, grain growth via coarsening can continually drive sintering and array shrinkage. It is shown that coarsening by itself does not lead to significant array shrinkage, but can promote significant shrinkage when sintering is reinitiated. Densification is therefore expected to be predictably related to grain growth, and thus coarsening kinetics. It is shown that the relative density of a ring of spheres that undergoes cyclic coarsening and sintering is nearly linearly related to grain size. Conceptual relations between density and grain growth for real powder compacts are discussed and related to the function that describes the compact's distribution of pore coordination numbers. Finally, since coarsening can be the rate-limiting step for densification, it is suggested that theory and related experiments associated with densification kinetics should be reevaluated.
178 citations
TL;DR: In this article, the sintering process of highly pure powder was investigated with special interest in the evolution of α-β phase transformation, densification, and microstructure development, and it was observed that the transformation occurred without a liquid phase below 1730°C, which corresponds to the melting point of SiO2.
Abstract: Fully densified silicon nitride without additives was fabricated by means of hot isostatic pressing. The sintering process of highly pure powder was investigated with special interest in the evolution of α–β phase transformation, densification, and microstructure development. It was observed that the transformation occurred without a liquid phase below 1730°C, which corresponds to the melting point of SiO2. Above 1730°C, the densification and β-grain elongation accelerated concurrently because of the appearance of liquid SiO2. However, full densification was attained at 1950°C together with marked grain growth. Flexural strength, microhardness, fracture toughness, and Young's modulus of sintered bodies were measured as a function of temperature. In the sintered body started from highly pure powder, excellent MOR behavior was found up to 1400°C. Impurity content of a few hundred ppm was found to be sufficient to make densification easy and to degrade high-temperature strength.
170 citations
TL;DR: In this article, the influence of prior grain boundaries on nucleation of recrystallisation and associated texture development in iron has been investigated using deformed bicrystal specimens, and it was found that the texture evolution is interpreted in terms of a compromise between the frequency of potential nuclei and their respective growth velocities.
142 citations
TL;DR: In this paper, a methode de simulation permettant l'analyse de l'interaction entre les particules and les joints de grains, et de la relation entre la grosseur des grains and la fraction volumique des particules.
130 citations
TL;DR: In this article, both static and dynamic grain growth was studied during superplastic deformation of fine-grained yttria-stabilized tetragonal zirconia.
Abstract: Both static and dynamic grain growth were studied during superplastic deformation of fine-grained yttria-stabilized tetragonal zirconia. It was found that significant grain growth does not take place below 1300°C. Both static and dynamic growth were found to obey a similar equation of the form D3−D30=kt, where D and D0 are the instantaneous and initial grain sizes, respectively, t is the annealing time, and k is the kinetic constant for either static or dynamic grain growth. The activation energies were approximately 580 and 520 kJ/mol for static and dynamic grain growth, respectively.
TL;DR: The role of MgO in such a system is a grain growth inhibitor and a microstructural stabilizer as discussed by the authors, and it is proposed that the additive operates by reducing the mobility difference between clean (unwet) boundaries and grain boundaries wetted by thin amorphous films.
Abstract: Microstructural morphology in A1203 containing trace amounts (∼0.25 vol%) of a silicate-based amorphous phase was studied, as a finction of MgO additions, using transmission electron microscopy. Incipient abnormal grains develop during hot-pressing of undoped Al2O3 powder and are characterized by a large aspect ratio (>3) and long basal plane {0001} facets. The facets are completely wet by a thin (∼3 nm) amorphous phase in contrast with grain ends, which appear to be devoid of an amorphous phase. The shape of the grains is believed to result from differences in mobility between clean (i.e., unwet) grain boundaries and intergranular films which are liquid at the firing temperature. Doping with trace additions of MgO (Mg/A1=250 ppm) results in smaller, more uniform grain structures. Many grains in the MgO doped material exhibit all the features of the incipient abnormal grains in the undoped material, with the exception of the large aspect ratio. It is concluded that the role of MgO in such a system is a grain-growth inhibitor and a microstructural stabilizer. It is proposed that the additive operates by reducing the mobility difference between clean (unwet) boundaries and grain boundaries wetted by thin amorphous films. It is believed the additive accomplishes this primarily by reducing the mobility of the clean grain boundaries via a solid-solution pinning mechanism.
TL;DR: The grain-boundary mobility in magnesium aluminate spinels (MgO · nAl2O3) of magnesia-rich (n 1) compositions has been measured from normal grain growth in dense, hot-pressed samples as mentioned in this paper.
Abstract: The grain-boundary mobility in magnesium aluminate spinel (MgO · nAl2O3) of magnesia-rich (n 1) compositions has been measured from normal grain growth in dense, hot-pressed samples. Over the temperature range 1200° to 1800°C, the mobility in magnesia-rich compositions is found to be greater than that in alumina-rich compositions by a factor of 102 to 103. Within the alumina-rich field, the mobility varies by less than a factor of 10 over the composition range 1 > n > 1.56. Nearly stoichiometric spinels (1 < n < 1.07) form a variety of sources and starting materials exhibit boundary mobilities within a factor of 5 at fixed temperature, showing an impurity tolerance which has not been found in other ionic solids.
TL;DR: In this article, the effects of silicon ion implantation on the crystallization kinetics and grain size of low-pressure chemical vapor deposited amorphous silicon on oxidized silicon substrate have been studied by x-ray diffraction and transmission electron microscopy.
Abstract: The effects of silicon ion implantation on the crystallization kinetics and grain size of low‐pressure chemical vapor deposited amorphous silicon on oxidized silicon substrate have been studied by x‐ray diffraction and transmission electron microscopy. The most effective grain size enhancement was achieved by deep silicon ion implantation with the projected range located beyond the bottom interface to allow the maximum kinetic energy transfer at or near that interface. The grain size enhancement was due to a decrease of nucleation rate and an increase of the nucleation activation barrier from 3.9 to 4.9 eV for the Si+‐implanted sample. The amorphous‐to‐crystalline grain growth activation barrier of 3.2 eV was not altered by Si+ implantation, but the growth rate was decreased. Retardation of nucleation and enhancement of grain size are attributable to the implant‐recoiled oxygen. The average grain size increases from ∼0.2 to ∼2.0 μm by using 2×1015 cm−2 of Si+ implantation at 92 keV for 82‐nm‐thick films.
TL;DR: In this article, it is shown that cubic-shaped particles are generally more effective than spherical particles in holding up grain growth and are also more effective if particles are fully coherent, which is the case in this paper.
TL;DR: In this paper, an estimate of the geometrical rate constant G3 in the parabolic grain growth law was made, where G3 = 0.5 ± 0.l.
TL;DR: In this paper, it was shown that the proportion and clustering of CSLs at surface grains is instrumental in the initiation of anomalous grain growth and the nature of the grain misorientation texture after grain growth.
Abstract: Microtexture (i.e. grain-specific) measurements have been made from grains both before and after grain growth in an austenitic stainless steel. Such measurements allow computation of grain misorientation texture in terms of the proportions of coincident site lattice (CSL) boundaries. After 2% postrecrystallization strain the grain growth is surface initiated in this alloy, and so, during the grain growth incubation period to, data were acquired just from those grains contiguous with the free surface. A clear trend emerged that the proportion and clustering of CSLs (particularly ‘special’ types which tend to have higher than average mobilities) increased both during to and after to when grains were actually growing. The CSL fractions were consistently greater for surface than for interior grains. It is therefore proposed that firstly the clustering of CSLs at surface grains is instrumental in the initiation of anomalous grain growth and secondly the nature of the grain misorientation texture after...
TL;DR: In this article, the addition of silver oxide has been found to accelerate the decomposition and melting of the Y-Ba-Cu-O superconductor, which exhibits, after sintering near 980°C, morphology of very large stacked-plate grains for the YBa 2 Cu 3 O 7−δ phase and the presence of decomposition products.
TL;DR: In this article, a small addition (2 wt%) of ZrO2 to BaTiO3 ceramics was shown to improve microstructural uniformity and retarded grain growth depending on sintering temperature.
Abstract: Dielectric properties and structural characteristics of BaTiO3 ceramics are significantly influenced by small addition (2 wt%) of ZrO2. SEM and TEM observations revealed enhanced microstructural uniformity and retarded grain growth depending on sintering temperature. Above 1320°C, Zr diffusion into the BaTiO3 lattice resulted in a chemical modification of the tetragonal structure and the development of core–shell grains. Below 1320°C, TEM analysis showed ZrO2 at the grain boundaries as discrete particles (∼0.03μm). X-ray diffraction analysis revealed a decrease in the axial (c/a) ratio with decreasing grain size. A corresponding decrease in the spontaneous polarization, and twinned domain structures, were also observed in the fine-grained ceramics. These samples also showed a flattened permittivity response with temperature and significantly lower losses.
TL;DR: In this paper, the effect of the Bi2O3-doped ZnO was studied with a special emphasis on the effect on the Bi 2O3 content, which increased the probability of the formation of skeleton structure and reduced the grain growth rate and sintered density.
Abstract: Densification and microstructure de velopment in Bi2O3-doped ZnO have been studied with a special emphasis on the effect of the Bi2O3 content. A small amount of Bi2O3 in ZnO (0.1 mol%) retarded densification, but the addition of Bi2O3 to more than 0.5 mol% promoted densification by the formation of a liquid phase above the eutectic temperature (∼740°C). The liquid phase increased grain-boundary mobility, which was responsible for the formation of intragrain pores and the decrease in the sintered density. The increase in the Bi2O3 content increased the probability of the formation of skeleton structure, which reduced the grain growth rate and the sintered density.
01 Dec 1989-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, microstructural investigations of ZrO28wt.%Y2O3 coatings, air plasma-sprayed on Hastelloy substrate with low-pressure plasma-Sprayed NiCrAlY bond coat, have been performed using transmission electron microscopy on transversal thin foils, electron and X-ray diffraction and scanning electron microscope.
Abstract: Plasma-sprayed yttria partially stabilized zirconia (YPSZ) is currently the most advanced system envisaged for thermal barrier coatings to be applied on hot turboengine components. Although this paticular system is now well-known for its excellent thermomechanical resistance at elevated temperatures, the relationship between these performances and the microstructure of such complex coatings is not fully understood. As a first step towards this understanding, microstructural investigations of ZrO28wt.%Y2O3 coatings, air plasma-sprayed on Hastelloy substrate with low-pressure plasma-sprayed NiCrAlY bond coat, have been performed using transmission electron microscopy on transversal thin foils, electron and X-ray diffraction and scanning electron microscopy. The evolution of the coating microstructure after air annealing at 1100°C, 1200°C and 1400°C has been studied. Particular attention has been paid to the microstructure evolution of the ceramic-bond coat interface as well as to the Al2O3 scale which develops under oxidizing conditions. The as-sprayed PSZ is essentially composed of columnar, microcracked, very fine grain, metastable, high yttrium content, tetragonal phase (t′), formed by a fast diffusionless transformation during plasma spraying. On annealing, grain growth and polygonization occur and new finer microstructures appear: chevron-like elongated precipitates and a very fine tweed-like microstructure (t″). At 1400°C, evidence of low yttria content, transformable (t) phase has been found; the martensitic transformation of this phase into monoclinic is, however, very much limited by the presence of non-transformable matrix around the transformable precipitates. At the ceramic-bond coat interface, a dense Al2O3 scale develops whose morphology changes from the bond coat to the ceramic. These observations give valuable insights into the temperature evolution of these coatings. This should, in future, allow the more accurate formulation of a relationship between temperature, (t′) phase content and coating lifetimes.
TL;DR: In this article, the effect of Fe and B4C on the sintering behavior and mechanical properties of TiB2 ceramics have been investigated in an Ar atmosphere at 2000° using attrition-milled powder (mean particle size = 0.8 μm).
Abstract: The effect of Fe and B4C on the sintering behavior and mechanical properties of TiB2 ceramics have been studied. Sintering was performed in an Ar atmosphere at 2000° using attrition-milled TiB2 powder (mean particle size = 0.8 μm). When a small amount of Fe (0.5 wt%) was added, abnormal grain growth occurred and the sintered density was low. In the case of B4C added along with 0.5 wt% Fe, however, abnormal grain growth was remarkably suppressed, and the sintered density was increased up to 95% of theoretical. But with excess Fe addition (5 wt%), B4C grains did not act as a grain growth inhibitor, and B4C grains were frequently trapped in large TiB2 grains. The best mechanical properties were obtained for the TiB2–10 wt% B4C–0.5 wt% Fe ceramics, which exhibited a three-point bending strength of 400 MPa and a fracture toughness of 5.5 MPa · m1/2.
TL;DR: In this article, a Monte Carlo computer simulation technique was used to simulate pictorially grain growth and grain interactions during solidification, and it was shown that columnar to equiaxed transition occurs when the volume fraction of equiaaxed grains ahead of the columnar interface is 0·50.
Abstract: A Monte Carlo computer simulation technique, previously applied to the simulation of a number of solid state processes involving microstructural evolution, has been employed to simulate and represent pictorially grain growth and grain interactions during solidification. By careful control of the number, location, and time of origin of grain nuclei it has been possible to simulate a wide range of features of grain structure in castings. Also, in support of an existing statistical argument, it has been demonstrated that the columnar to equiaxed transition occurs when the volume fraction of equiaxed grains ahead of the columnar interface is 0·50.MST/912
Patent•
23 Oct 1989TL;DR: In this article, a process for refining the grain size of α and α-β titanium alloys through forging and recrystallization above the alloy's β-transus temperature is described.
Abstract: Fine grain titanium forgings and to a process for refining the grain size of α and α-β titanium alloys through forging and recrystallization above the alloy's β-transus temperature. Specifically, the method employs an isothermal press in which a billet heated above the alloy's β-transus temperature, forged to produce an elongated, flattened grain structure, is held above the alloy's β-transus temperature for a predetermined time to allow fine grains to nucleate and grow through recrystallization, and then is quenched to arrest grain growth and to establish a fine grained titanium alloy. A second forging step may be employed to attain an aspect ratio of the grains. The fine grained titanium forgings made by this process have a maximum prior β-grain size of 0.5 mm throughout the workpiece.
TL;DR: In this article, the authors compared the curvature-driven vacancy diffusion flux with that arising from thermal diffusion, driven by the temperature gradient (grad T/T) during the final stage of sintering.
Abstract: The vacancy diffusion flux, driven by the neck growth curvature (1 /ρ) which contributes to conventional sintering of a powder compact, is theoretically compared with that arising from thermal diffusion, driven by the temperature gradient (grad T/T) during the final stage of sintering. Under conditions experienced in ultrarapid sintering, the thermally driven vacancy flux can exceed the curvature-driven flux by a factor of 1000. This effect may explain the phenomenon of enhanced densification rates observed in plasma sintering.
TL;DR: In this article, vanadium oxide films were deposited with different vanadium oxygen ratios for substrate temperatures between 250 and 550 C by dc reactive magnetron sputtering and the resistivity ratios between a semiconducting phase and a metallic phase were limited to 1000 order by voided boundaries and oxygen vacancies.
Abstract: Voided growth structures of sputter-deposited films affect strongly their optical and electrical properties Vanadium dioxide is an interesting material to study effects of film microstructure and nonstoichiometry on electrical properties because its phase transition makes it possible to investigate electrical behavior both in a semiconducting phase and in a metallic phase Vanadium oxide films were deposited with different vanadium oxygen ratios for substrate temperatures between 250 and 550 C by dc reactive magnetron sputtering The resistivity ratios between a semiconducting phase and a metallic phase are limited to 1000 order by voided boundaries and oxygen vacancies The voided boundaries are defined by columnar structure and agglomerated grain growth The results emphasize the necessity of a combination of deposition to obtain the film with a favorable structure and postdeposition annealing to control the film stoichiometry
TL;DR: In this article, the authors used an elastic-viscoplastic constitutive model of a ductile porous solid to characterize the evolution of room temperature damage along the grain boundary.
TL;DR: In this paper, the microstructure development in Sb2O3-doped ZnO was studied at doping levels up to 2.0 mol% and the volume fraction of trapped inclusion particles decreased with increasing doping level.
Abstract: The microstructure development in Sb2O3-doped ZnO was studied at doping levels up to 2.0 mol%. Dopant Sb2O3 reacted with ZnO to form inclusion particles,α-Zn7Sb2O12, and inhibited the grain growth of ZnO. With increasing doping level of Sb2O3, the growth rate of ZnO decreased whereas that of inclusion particles increased. Some inclusion particles were trapped in ZnO grains at low doping levels of Sb2O3, but the volume fraction of trapped inclusion particles decreased with increasing doping level. Stereological analysis of the size and number ratios of ZnO grains and inclusion particles indicated that a compatible assumption is needed to evaluate Zener effect in two-phase sintering.
TL;DR: In this paper, a UV pulsed excimer laser was irradiated onto Si+-implantated thin silicon films, and the resultant crystallized state was investigated by UV reflectance measurement and TEM observation.
Abstract: A UV pulsed excimer laser was irradiated onto Si+-implantated thin silicon films. The resultant crystallized state was investigated by UV reflectance measurement and TEM observation. As a result, we found that the grain size depended on the irradiated pulse energy, and that the grain growth was more effective for a smaller Si+ dose, in the low-pulse-energy region of less than 200 mJ/cm2. Moreover, for efficient annealing, the back-side irradiation through the quartz substrate was preferable to the front-side irradiation upon the silicon surface. Using this method, TFT's were fabricated in a low-temperature process, and excellent device characteristics were obtained. The leakage current was below 1×10-13 (A/µm).
TL;DR: The effect of sintering aids SiO2, Al2O3, and TiO2 on the microstructure and electrical properties of Sb3+−doped BaTiO3 materials is examined in this paper.
Abstract: The effect of sintering aids SiO2, Al2O3, and TiO2 on the microstructure and electrical properties of Sb3+‐doped BaTiO3 materials is examined. The SiO2 acts as a liquid‐phase former, which not only enhances the atomic diffusion, but also purifies the grain interior and increases the grain conductivity. The room‐temperature resistivity of the materials is lowered. An abundance of the liquid phase due to excess TiO2 will, however, result in a clustering of particles and the formation of a nonuniform microstructure. The Al3+ ions inhibit the grain growth and lead to a small and uniform grain size distribution. These ions also act as electron traps which increase the Schottky barrier height and in turn, raise the high‐temperature resistivity of the samples.