Showing papers on "Grain growth published in 1985"
[...]
TL;DR: In this article, both the relatively simple problem of the interaction between a single particle and a grain boundary, and the much more complicated problem of deriving a restraining force from the many particles on the grain boundary have been considered.
680 citations
TL;DR: In this paper, the equations and procedures for constructing hot-isostatic pressing diagrams are greatly simplified and clarified, and two further mechanisms are added: diffusional deformation of the particles themselves when the grain size is much smaller than the particle size, and the separation of pores from boundaries when grain growth occurs.
658 citations
TL;DR: In this article, a model for growth of secondary grains into a uniform matrix of columnar normal grains is presented, which indicates that secondary grain growth rates should increase with grain boundary energy, surface energy anisotropy, grain boundary mobility, and temperature.
Abstract: Secondary grain growth in thin films can lead to grain sizes much greater than the film thickness. Surface energy anisotropy often provides an important fraction of the driving force for secondary grain growth, especially in the early stages of growth. Surface‐energy‐driven secondary grain growth leads to the development of large grains with restricted crystallographic textures. A model is presented for growth of secondary grains into a uniform matrix of columnar normal grains. The model indicates that secondary grain growth rates should increase with grain boundary energy, surface energy anisotropy, grain boundary mobility, and temperature. While final secondary grain sizes will decrease with film thickness, their growth rates will increase. The final secondary grain sizes and orientations will be strongly affected by grain sizes and orientations in the initial film. The models presented here provide analytical tools for experimental study of secondary grain growth in thin films. They will be used in for...
263 citations
TL;DR: In this paper, the growth of abnormally large grains is modelled under two conditions: 1) where the driving force is provided solely by curvature and 2.1 where the difference in the gas-metal surface energy between grains of different crystallographic orientation.
262 citations
TL;DR: In this paper, a Monte Carlo computer simulation technique has been developed which models grain growth for the case in which the grain boundary energy is anisotropic, and the grain growth kinetics, as represented by the growth exponent n ( R = Ct n ), is found to decrease continuously from 0.42 ± 0.02 to 0.25 ± 1.02 as the anisotropy is increased.
229 citations
TL;DR: In this paper, the porosity and microchannels developed as a result of grain growth and of plastic deformation (grain-boundary sliding, diffusion creep, etc.) under compressive stresses in the scales.
Abstract: It is well known that oxide scales develop porosity and microchannels that permit inward transport of molecular species from the ambient gas even under conditions when there is no evidence of cracking of the scales. It is proposed that such porosity and microchannels develop as a result of grain growth and of plastic deformation (grain-boundary sliding, diffusion creep, etc.) under compressive stresses in the scales. The presence of small amounts of impurities enriched at grain boundaries in the scales may greatly affect deformation and mechanical and transport properties in scales.
132 citations
TL;DR: In this article, the authors investigated the dynamic recrystallisation mechanisms in a magnesium alloy during uniaxial large strain compression at T = 0.7−0.95 Tm (Tm = melting temperature).
127 citations
TL;DR: The microstructural aspects of the superplastic phenomenon are reviewed in this article, where experimental results of a very large number of investigations are critically analysed in the context of: grain shape and size; grain growth; grain boundary sliding and migration, grain rotation and rearrangement; diffusion and dislocation activity.
Abstract: The microstructural aspects of the superplastic phenomenon are reviewed. The experimental results of a very large number of investigations are critically analysed in the context of: grain shape and size; grain growth; grain boundary sliding and migration, grain rotation and rearrangement; diffusion and dislocation activity. It is shown, that in spite of often conflicting evidence in the literature, a common pattern of microstructural behaviour emerges for all the materials and conditions investigated to date.
124 citations
TL;DR: In this article, the kinetics of grain growth in fully dense Al2O3 with and without MgO solute additions were measured for high-purity samples containing no liquid phases.
Abstract: The kinetics of grain growth in fully dense Al2O3 with and without MgO solute additions were measured for high-purity samples containing no liquid phases. The MgO was found to suppress the grain-boundary migration rate by a factor of 50. Compensating lattice defects are suggested to play a role in grain-growth inhibition. Implications of these results to the sintering of Al2O3 are discussed.
112 citations
TL;DR: In this paper, very small aggregates appear to be the smallest microstructural units which determine the ultimate packing situation after compaction, and the sintering process is seen to proceed via several stages of micro-structural development.
Abstract: Ultra-fine stabilized zirconia powders, which only contain extremely small aggregates were prepared. The control of agglomerates and aggregates in these powders is of utmost importance in order to obtain highly sinter-reactive ceramics. The very small aggregates appear to be the smallest microstructural units which determine the ultimate packing situation after compaction. Resulting green microstructures and sintering behaviour were studied extensively. The sintering process is seen to proceed via several stages of micro-structural development. During the most important stage, where the ceramic material approaches full density, the observed occurrence of abnormal grain growth strongly influences the ultimate grain size. The extent of abnormal growth is highly dependent on aggregate sizes present in the starting powder.
110 citations
TL;DR: A review of studies of the synthesis and densification of mullite ceramics suggests that reaction sintering could be the best way to obtain high purity, dense mullite from common materials (amorphous silicon dioxide and α-aluminum oxide) as discussed by the authors.
TL;DR: In this paper, a lattice model for 2-dimensional grain growth was developed in which topology and local environment are included, and the results of this model were used to simulate grain growth in thin encapsulated films.
TL;DR: Proposition d'un modele theorique de simulation de la croissance des grains applicable aux dernieres etapes de la recristallisation lors de la migration de joints de grains de grand angle sous l'effet de forces de tension superficielle as mentioned in this paper.
TL;DR: In this paper, the effect of adding B, Al, and B + Al on the pressureless sintering of β-Sic was examined, and the influence of the Sintering atmosphere and heating schedule on densification behavior, polytype transformation, and microstructure development was also studied.
Abstract: The effect of additions of B, Al, and B + Al on the pressureless sintering of β-Sic was examined. The influence of the sintering atmosphere and heating schedule on densification behavior, polytype transformation, and microstructure development was also studied. High densities were obtained at 1940°C by the simultaneous addition of B and Al. The decrease in the sintering temperature is attributed to the presence of a liquid phase which results in the formation of platelets (up to 200 # in size) of an α-polytype, predominantly 4H and 6H. Polytype transformation and exaggerated grain growth could be prevented by annealing the compact at 1650° to 18500°C for 0.5 to 1 h. This procedure results in a better redistribution of the sintering aids, giving a fine-grained microstructure, constituted primarily of the cubic 3C polytype.
TL;DR: In this paper, the Wagner-Zlyosov-Hillert dynamic grain growth model is applied to a polycrystalline material and the relationship between the contact areas between the grains and the grain size distribution is investigated.
TL;DR: In this paper, Boron carbide compacts sintered between 2220° and 2250° C showed abnormal grain growth that was associated with twinning and micro-cracking.
Abstract: Boron carbide compacts sintered between 2220° and 2250° C showed abnormal grain growth that was associated with twinning and microcracking. The twin direction was 〈011〉. The microcracking was linked to thermal expansion anisotropy determined by high-temperature lattice constant measurement.
TL;DR: In this article, the deformation properties of two oxide dispersion strengthened (ODS) alloys, MA 754 and MA 6000, with initial grain sizes of 0.67 µm and 0.26 µm, respectively, have been studied.
Abstract: The high temperature deformation properties of two oxide dispersion strengthened (ODS) alloys, MA 754 and MA 6000, with initial grain sizes of 0.67 µm and 0.26 µm, respectively, have been studied. Tensile tests have been conducted at 1173, 1273, and 1373 K at strain rates ranging from 2 X 10−4 to 5 s−1. Tension creep tests were conducted on MA 6000 at 1273 K to extend the strain rate regime to 3 X 10−8 s−1. Microstructures of both undeformed and deformed samples have been characterized by transmission electron microscopy. MA 754 exhibits a maximum elongation of 200 pct and a maximum strain rate sensitivity of 0.30 at 1273 K. MA 6000 is superplastic, exhibiting a maximum elongation of over 300 pct and a maximum strain rate sensitivity of 0.47 at 1273 K. The microstructure of MA 754 is unstable during deformation, showing recrystallized grains and grains which have grown to 1 µm in diameter. No evidence for ordinary recrystallization is found for MA 6000, and grain growth is slight. For both alloys, strain rates less than about 1 s−1 alter the initial microstructure and prevent grain coarsening on subsequent annealing at higher temperature. Deformation of the fine-grained MA 6000 can be described as a combination of power law creep and diffusional (Coble) creep, with a threshold stress caused by the presence of γt’ particles existing only for the diffusional creep process. Structural instabilities do not permit a simple description of deformation of MA 754.
TL;DR: In this paper, the microhardness and electrical resistivity have been measured on thin Hf•N films, covering the entire composition range from pure Hf to overstoichiometric HfN.
Abstract: The microhardness and electrical resistivity have been measured on thin Hf‐N films, covering the entire composition range from pure Hf to overstoichiometric HfN. Influence of substrate bias on the properties of stoichiometric HfN films has also been studied. All films have been prepared by high‐rate reactive magnetron sputtering at a substrate temperature of 400 °C. Both hardness and resistivity increase as nitrogen is added to the α‐Hf phase. For the cubic HfN phase the hardness has a maximum, ≊3500 HV, and the resistivity a minimum, 225 μΩ cm, at a composition close to stoichiometry. However, both values are considerably higher than those reported for bulk samples. This is explained in terms of nonequilibrium growth conditions, giving rise to high densities of dislocations and interstitially incorporated nitrogen atoms. For films with a nitrogen content above 50 at. % a very high‐resistivity value is found, 2.0 Ω cm at maximum. By applying a low substrate bias voltage the resistivity of stoichiometric HfN was decreased with a factor of about 3, and simultaneously a grain growth and/or reduced defect concentration was observed. At higher bias voltages the resistivity increases and the grain sizes decrease.
TL;DR: In this paper, the synthesis of thermal-shock-resistant material from zirconyl phosphate, 2ZrO2P2O5, was investigated; only the a phase appears to be thermodynamically stable.
Abstract: The synthesis of thermal-shock-resistant material from zirconyl phosphate, 2ZrO2P2O5, was investigated. Modifications of the compound were examined; only the a phase appears to be thermodynamically stable. Sintering of a-phase powder to a dense ceramic was promoted by adding a metal oxide. Crystalline zirconyl phosphate has a low average thermal expansion coefficient. The relation of microcracking to grain size and strength was studied using specimens sintered under varied conditions. A strong low-thermal-expansion polycrystalline ceramic was obtained by controlling grain growth.
TL;DR: In this article, the effects of composition on recrystallization and grain growth were interpreted in terms of excess solute atoms, suggesting the same diffusion-controlling mechanism for the two processes.
TL;DR: In this paper, a comparison of β-Al2O3 composites containing either tetragonal (t-) ZrO2 or cubic (c-) ZRO2 with the single phase material showed that the major strengthening mechanism is the reduction in critical flaw size.
Abstract: In order to fabricate transformation-toughenedβ″-Al2O3 and optimize its mechanical and electrical properties it was found to be necessary to carefully control the particle size distribution of the starting powders and their mixing. The ionic resistivity of the composites depended primarily on the volume fraction of ZrO2. Additions between 10 and 20 vol% produced materials with ionic resistivities (300° C) between 6 and 10 Ωcm and eliminated exaggerated grain growth of theβ″-Al2O3. Comparison ofβ″-Al2O3 composites containing either tetragonal (t-) ZrO2 or cubic (c-) ZrO2 with the single phase material showed that the major strengthening mechanism is the reduction in critical flaw size. This occurred by the elimination of the flaw population associated with abnormally large grains. For maximum increases in fracture toughness and strength, however, the use of t-ZrO2 (transformation toughening) as a second phase is preferred.
01 Mar 1985-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: In this article, in-situ backscattering measurements and channeling (in the single crystals) have been carried out using TEM, STEM and EDS, and it was found that 80% of the Ag is substitutional at the Ag saturation concentration (7-8 at.% max. concentration). Annealing at 623 K. for 1 h, decreases the substitutional fraction to 60%.
Abstract: 40–130 keV Ag + ions have been implanted at 46 K and 300 K into Ni films (25–38 nm thick) deposited on NaCl and Ni single crystals (100) and (110) in the dose range 10 11 to 4 × 10 16 cm −2. In-situ backscattering measurements and channeling (in the single crystals) have been carried out. The thin films were also analysed using TEM, STEM and EDS. For the single crystal samples it is found that 80% of the Ag is substitutional at the Ag saturation concentration (7–8 at.% max. concentration). Annealing at 623 K. for 1 h, decreases the substitutional fraction to 60%. Bombardment of the thin films at both 46 K and 300 K results in significant recrystallization and grain growth when the ion dose ≳ 10 15cm −2. It is found that D ¯ ∞ ϕ and Δ D ¯ / Δ ϕ increases with E·ν(E), where D ¯ is the average grain diameter, φ is the total ion dose and E·ν(E) is the ion energy deposited in elastic collisions. When the average Ag concentration is ≲ 4 at.%, the Ag atoms are in solution in the Ni matrix; at higher concentrations Ag precipitates (∼ 10nm diameter) are formed. Also, for the high dose implants, the Ni film exhibits a preferred orientation with a 〈110〉 fibre texture. The high density of defects and high strain energy produced by the Ag ion bombardment along with the tendency to reduce the surface energy are considered to cause recrystalization and grain growth of the Ni, and segregation of Ag in the thin film samples.
TL;DR: In this paper, the influence of temperature and grain size on the tensile ductility of AISI 316 stainless steel has been examined in the temperature range 300-1223 K for specimens with grain sizes varying from 0.025 to 0.650 mm at a nominal strain rate of 3 × 10 −4 s −1.
TL;DR: In this paper, a finite element model is proposed to analyze in-pile fission gas release from UO2 fuel by simultaneous mechanisms of diffusive flow and boundary sweeping, considering the effect of irradiation-induced resolution in grain boundaries.
Abstract: A finite element model is proposed to analyze in-pile fission gas release from UO2 fuel. The model describes fission gas transfer from grain interior to grain boundaries by simultaneous mechanisms of diffusive flow and boundary sweeping, considering the effect of irradiation-induced resolution in grain boundaries. The onset of gas release is predicted when gas amount in grain boundaries is saturated. The grain is assumed to be spherical and is radially divided into a number of elements, in each of which the diffusion problem is approximated by describing the gas concentration in terms of a quadratic function. The effect of grain growth is treated by updating the nodal radii incrementally. This model is applicable to variable irradiation histories and its required computer time is acceptable for use in fuel rod performance codes.
TL;DR: The effect of cold work in the range of 21.9 pct to 94.2 pct reduction in area by swaging on the recrystallization behavior and grain size distribution in Ti was investigated in this paper.
Abstract: The effect of cold work in the range of 21.9 pct to 94.2 pct reduction in area by swaging on the recrystallization behavior and grain size distribution in Ti (0.2 pct Oeq) was investigated. In keeping with commonly observed behavior, increasing amounts of cold work lead to an increase in hardness prior to annealing and a decrease in the subsequent recrystallization temperature and grain size. Also, there occurred for the recrystallized grains a decrease in the standard deviation of the grain volume distribution In Σv deduced from the one-dimensional linear intercept measurements. The decrease in In Σv with cold work in swaged Ti is in good accord with that reported by Rhines and Patterson following uniaxial deformation of Al. Considering the results for both Al and Ti, it appears that the largest decrease in In Σv occurs for true strains up to R ~0.5, the change with larger strains being more gradual. The reason for the decrease in In Σv with increased cold work is not completely clear. The increased rate of grain growth with reduction in amount of cold work can be understood in terms of the larger number of three-edged grain faces which occur with the greater spread in the recrystallized grain volume distribution.
TL;DR: In this article, the role of calcination and sintering schedules on the final sintered density and microstructure of Y2O3 compacts was studied and the optimum calcination procedure appeared to be at 1000°C and for one hour in air atmosphere.