Showing papers on "Grain growth published in 1982"
TL;DR: The mechanism of dynamic recrystallisation at temperatures between 425 and 600 K is reported to not involve significant high angle boundary migration, but to be similar to a mechanism reported for several minerals.
752 citations
TL;DR: Grain boundary conductivities were determined by complex impedance measurements (1-106 Hz) on high-purity ceramics prepared by the alkoxide synthesis and on less pure Ceramics obtained from a commercial powder as discussed by the authors.
403 citations
TL;DR: In this paper, the authors proposed that the grain-size dependence of the yield stress of metals is due to the elastic incompatibility stresses at the grain boundaries, and the generation and motion of these "geometrically necessary" dislocations attenuated the stress concentrations; a work-hardened grain-boundary layer is formed.
Abstract: It is proposed that the grain-size dependence of the yield stress of metals is due to the elastic incompatibility stresses at the grain boundaries. For clarity, the process of yielding is divided into three stages. In the first stage (prior to microyielding), the differences in elastic properties arising from the elastic anisotropy of adjacent grains establish localized stress concentrations at the grain boundaries. In the second stage, the stress concentrations at the grain boundaries result in localized plastic flow; this is the microyield region. The generation and motion of these 'geometrically necessary' dislocations attenuates the stress concentrations; a work-hardened grain-boundary layer is formed. As the stress is increased, the polycrystalline metal acts as a composite material consisting of a continuous network of a work-hardened grain-boundary material and discontinuous bulk material. The bulk is prevented from flowing plastically because the continuous network of work-hardened grain-...
269 citations
TL;DR: In this article, a set of poorly-known kinetic constants which appear in such a treatment are determined by fitting the equations to data from real or simulated welds, at certain fixed points.
226 citations
TL;DR: In this paper, the internal strains associated with the martensitic phase transformation of zirconia were used to introduce microcracks into Al2O3/ZrO2 composites.
Abstract: The internal strains asSociated with the martensitic phase transformation of zirconia were used to introduce microcracks into Al2O3/ZrO2 composites. The degree of transformation was found to be dependent on the volume fraction of ZrO2 and its size, the latter of which could be controlled by suitable heat treatments. The microstructural changes that occurred during the heat treatments were studied using quantitative microscopy and X-ray diffraction. For materials containing more than 7.5 vol% Zr02, the ZrO2 particles were found to pin the Al2O3 grain boundaries, thus limiting the Al2O3 grain growth. The limiting grain size was found to be dependent on size and volume fraction of ZrO2. Heat treatments for the higher volume fraction materials (>7.5 vol% ZrO2) caused micro-structural changes which resulted in increased amounts of monoclinic ZrO2 at room temperature; elastic modulus measurements indicated that this was occurring concurrently with microcracking. By combining the ZrO2 grain-size distributions with the X-ray analysis it was possible to calculate the critical ZrO2 size required for the transformation. The critical size was found to decrease with increasing amounts of ZrO2. Hardness and indentation fracture toughness were measured on the composites. Grain fragmentation was observed at the edge of the indentations and microcracks were observed directly, using an AgNO3 decoration technique, near the indentations.
207 citations
TL;DR: The grain volume distribution of recrystallized aluminum, determined by separating and weighing the individual grains, has been found to be log normal, its spread in size being expressed by the standard deviation of the distribution (In σv), which remains constant during steady state grain growth as discussed by the authors.
Abstract: The grain volume distribution of recrystallized aluminum, determined by separating and weighing the individual grains, has been found to be log normal, its spread in size being expressed by the standard deviation of the distribution (In σv), which remains constant during steady state grain growth. The value of In σv is established by the degree of cold working that precedes annealing, being smaller the greater the degree of cold work. Grain growth proceeds the more rapidly the larger the value of In σv that is, the smaller the degree of prior cold work. The distributions of the numbers of faces per grain and of edges per face are also log normal and are proportional to the grain volume distribution. Thus, the relative number of three-edged faces increases with In σv and accounts for the observed increase in the rate of grain growth.
155 citations
TL;DR: Grain growth rates for novaculite, flint and jasper have been determined between 1000° and 800°C and 15 to 2 kb water pressure, and there is no significant difference between the three samples although the jasper contained 5-10 vol % iron oxide as mentioned in this paper.
Abstract: Grain growth rates for novaculite, flint and jasper have been determined between 1000° and 800°C and 15 to 2 kb water pressure. A pre-treatment established an equilibrium microstructure and a nearly uniform mean grain diameter ($\bar{D}_{0}$). At 1000°C and 15 kb the grain growth data are linear on a log ($\bar{D}-\bar{D}_{0}$) vs. log time plot, and there is no significant difference between the three samples although the jasper contained 5-10 vol % iron oxide. At lower pressures and temperatures the growth curves are approximately parallel to the 1000°C and 15 kb data, but growth is slower and that for jasper is slower than for the other two. The growth rate is strongly dependent on the solubility of quartz in the intergranular fluid, and the activation energy is about 11 kcal/mole. The data are used to estimate the growth rate of quartz at metamorphic conditions and to help evaluate the conditions where growth of dynamically recrystallized grains may be significant. The growth rate of calcite grains (S...
153 citations
TL;DR: A commercial Al-Zn-Mg-Cu alloy was processed for superplastic forming and tested in uniaxial equibiaxial and plane-strain conditions as mentioned in this paper.
83 citations
TL;DR: In this paper, the cyclic oxidation behavior of a fine-grained, rapidly solidified 303 stainless steel was determined at 900 °C in pure oxygen, and it was shown that the fine grain size and the MnS dispersion increased the resistance of the alloy to oxidation.
Abstract: The cyclic oxidation behavior of a fine-grained, rapidly solidified 303 stainless steel was determined at 900 °C in pure oxygen. The rapidly solidified alloy exhibited superior resistance to oxidation compared with that of a wrought 304 stainless steel; its oxidation resistance was as good as that of a wrought 310 stainless steel, even though the latter alloy contained more Cr and Ni. The matrix of the rapidly solidified steel contained a uniform dispersion of fine MnS precipitates (0.2 to 0.5 μm), which were effective in inhibiting grain growth at elevated temperatures. The enhanced resistance to oxidation of the rapidly solidified alloy is attributed to two factors: (1) the formation and growth of protective Cr2O3 and SiO2 scales were promoted by the fine alloy grain size (5 to 8 =gmm) and by the presence of the MnS dispersion, and (2) the adherence of the scale was increased by the formation of intrusions of SiO2 from the external scale into the alloy, which formed around MnS precipitates and along closely-spaced alloy grain boundaries, and which acted to key the scale mechanically to the alloy.
79 citations
TL;DR: In this article, a grain growth model has been developed and implemented in the SUPREM process simulator, and the simulated results are compared to those obtained by transmission electron microscopy for As, P and B-doped polysilicon.
Abstract: The electrical characteristics of polycrystalline silicon layers are closely related to their grain structure. This paper describes a comprehensive study of the grain growth of polysilicon under a wide range of doping and processing conditions. A grain‐growth model has been developed and implemented in the SUPREM process simulator, and the simulated results are compared to those obtained by transmission electron microscopy for As‐, P‐ and B‐doped polysilicon. These results indicate that n‐type dopants increase the growth rate whereas the p‐type dopant has a negligible effect.
69 citations
TL;DR: In this paper, the resistance to normal grain growth offered by a dispersion of spherical particles is calculated and an allowance is made for grain boundary flexibility and for the tendency for some particles to assist the motion.
TL;DR: The effects of exogenous cytokinins were consistent with those due to spikelet removal suggesting that this hormone participates in a joint regulation of source and sink activity during grain filling and influenced grain weight by creating different intrinsic growth potentials in the grains.
Abstract: The effects of changes in source-sink relation and of cytokinin applications were examined with regard to the development of flag leaves and grain growth in two contrasting spring wheats, Triticum aestivum L., cv. Kolibri and Solo, grown in hydroculture at mean day/night temperatures of 21/19°C.
Lowering temperatures during the phase of ear differentiation decreased the ratio of source and sink size (flag leaf area/grains per ear) but hardly affected maximum chlorophyll content of the flag leaf (source activity), maximum rates of grain growth (sink activity), or final grain weight. Nevertheless, considerable differences were produced with regard to the duration of grain filling, the development and coordination of source and sink activities after maxima had been attained.
Changing the ratio of source and sink size during grain filling by spikelet removal affected final grain weight the less, the later the treatments began. Comparing, plants with identical source activities demonstrated that differences in grain weight were mainly due to different growth rates between day 16 and 40. Source activities were raised only temporarily (day 23) by spikelet removal done at day 2 or 14.
From this it may be concluded that changing the ratio of source and sink size influenced grain weight by creating different intrinsic growth potentials in the grains. These potentials might be estimated 3 weeks after ear emergence by DNA and RNA content per grain.
The effects of exogenous cytokinins were consistent with those due to spikelet removal suggesting that this hormone participates in a joint regulation of source and sink activity during grain filling.
TL;DR: In this paper, annealing experiments in order to study grain boundary migration (GBM) were carried out at temperatures of 1513-1773 K from 10 min to 100 hours at atmospheric pressure.
TL;DR: In this article, BaTiO3-SrTiOO3 solid solution during sintering of powder mixtures is characterized by preferential diffusion of Ba2+ ions.
Abstract: Formation of BaTiO3-SrTiO3 solid solution during sintering of powder mixtures is characterized by preferential diffusion of Ba2+ ions. As a consequence, several nonequilibrium phases are temporarily formed; they were identified by X-ray and microprobe analysis. Eutectic liquid appears below 1300°C, which may explain exaggerated grain growth during sintering of BaTiO3-SrTiO3 mixtures. Disturbance in neck growth and Kirkendall-type porosity hamper densification in the heterogeneous system as compared with the pure titanates.
TL;DR: Grain-oriented Bi2WO6 ceramics were fabricated by normal sintering techniques as discussed by the authors by using a fused salt process using an NaCl-KCI melt.
Abstract: Grain-oriented Bi2WO6 ceramics were fabricated by normal sintering techniques. Platelike crystallites were initially synthesized by a fused salt process using an NaCl-KCI melt. When calcined at 850°C, =100 μm. After dissolving away the salt matrix, the Bi2WO6 particles were mixed with an organic binder and tapecast to align the platelike crystallites. Large particles were easily oriented by tapecasting but the sinterability of the tape was poor. Preferred orientation of small particles was increased by tapecasting and grain growth during sintering further improves the degree of orientation. Sintering above the 950°C phase transition, however, results in discontinuous grain growth and low densities. Optimum conditions for obtaining highly oriented ceramics with high density occur at sintering temperatures of 900°C using fine-grained powders which yield orientation factors of =0.88 and densities of 94% theoretical.
01 Oct 1982
TL;DR: In this article, the growth of faceted crystals is modeled using the rounded grains as sources and the faceted grains as sinks, and the temperature distribution is shown to be quasi-linear at steady state in homogeneous snow.
Abstract: : Ice grains in a snow cover with a low temperature gradient assume a well-rounded equilibrium form. However, at temperature gradients of 0.1 to 0.2 C/cm (depending somewhat on temperature and snow density), the rounded grains recrystallize into a faceted kinetic growth form. The large temperature gradient must play a decisive role in moving the vapor fast enough to sustain the rapid growth rate associated with the kinetic growth form. Once the large temperature gradient is removed, the grains recrystallize back to the equilibrium form. the recrystallization occurs in either direction without a change in bulk density. The growth of faceted crystals begins at the warmer base of the snow cover where the excess vapor pressure is largest. A transition between the overlying rounded grains moves upward in time. Faceted crystals also grow just below crusts of reduced permeability, where the increased vapor accumulation can sustain the excess vapor pressure neded for kinetic growth. The heat and vapor flows are described using a model based on thermodynamic equilibrium. The temperature distribution is shown to be quasi-linear at steady state in homogeneous snow. The recrystallization of the snow is modeled using the rounded grains as sources and the faceted grains as sinks. In the future this model should be extended to account for different temperatures among the sources and sinks.
TL;DR: In this article, changes in microstructure and texture are described in polycrystalline ice compressed at temperatures of −10 and −1℃ and annealed for various times at −1 ℃ without a load.
Abstract: Changes in microstructure and texture are described in polycrystalline ice compressed at temperatures of –10 and –1℃ and annealed for various times at –1℃ without a load. The
microstructures, obtained under these conditions, were studied by means of polarized light
microscopy, both mean and maximum grain sizes were determined. In the colder finer grained
samples (–10℃) the grain size increase during annealing was not as rapid as the coarser hot
worked (–1°) samples. Little textural change was observed in the annealed hot worked samples,
however the strong preferred orientation of the colder samples was destroyed with annealing.
The change in texture corresponded to a decrease in the grain growth rate and the formation
of a stable grain structure. Increasing strain and decreasing the deformation temperature both
reduce annealing time and the annealed grain size.
TL;DR: The authors experimentally deformed polycrystalline ice with plane strain at a temperature of −1°C, under a constant load and with shortening strains varying from 2 to 27%.
Abstract: Specimens of polycrystalline ice were experimentally deformed in plane strain at a temperature of –1°C, under a constant load and with shortening strains varying from 2 to 27%. The random c-axis orientation of the initial ice aggregates changes continuously with increasing strain towards a steady-state, small-circle containing two maxima lying about the shortening direction. The ice microstructure is sequentially transformed from undeformed grains to smaller, irregular, deformed grains containing recrystallized grains. Grain growth then occurs forming a coarse interlocking-grain aggregate. These microstructural changes are attributed to solid-state processes occurring during deformation. The previously deformed specimens were then annealed without a load at –1°C. Annealing resulted in a substantial grain-size coarsening. However, no c-axis preferred-orientation difference developed between annealed and unannealed samples.
TL;DR: In this article, it is suggested that this is caused by an acceleration of the rate controlling processes, namely, particle dragging or particle coarsening, resulting in material which cannot be subsequently recrystallized.
Abstract: An experimental charge of the oxide dispersion and γ′ precipitation strengthened superalloy MA 6000 was hot worked in the as-extruded, fine grained condition. The recrystallization during post-deformation annealing was followed by optical and electron microscopy. Coarse grain formation occurs by secondary rather than primary recrystallization (abnormal or discontinuous grain growth) as indicated by the driving forces involved, the inhibition of normal grain growth, the well-defined coarsening temperature, and the dependence of secondary grain size on temperature. Straining at high temperatures and low strain rates produces material which cannot be subsequently recrystallized. The reason is the loss of driving force during straining due to normal grain growth. The influence of deformation rate and deformation temperature can be properly accounted for by using the diffusion compensated strain rate for description of deformation conditions. Normal grain growth is accelerated by straining. It is suggested that this is caused by an acceleration of the rate controlling processes, namely, particle dragging or particle coarsening.
TL;DR: In this article, it was shown that upon annealing, warm-worked NiAl recrystallizes and undergoes subsequent grain growth in a manner typical of metals and alloys.
Abstract: Experiments have established that upon annealing, warm-worked NiAl recrystallizes and undergoes subsequent grain growth in a manner typical of metals and alloys. The recrystallization and grain growth kinetics, respectively, can be described by the expressionsX v = 1 - exp(-Bt k ) and −d =Ct n where Xv is the fraction recrystallized,t is time, −d is the average grain size, andB, k, C, andn are parameters whose magnitudes depend upon temperature.
TL;DR: In this article, the effect of initial grain size on cavitation during superplastic deformation in two commercially available Zn-Al eutectoid alloys has been studied using metallography and precision density measurements.
Abstract: The effect of initial grain size on cavitation during superplastic deformation in two commercially available Zn-Al eutectoid alloys has been studied using metallography and precision density measurements. Cavitation was found to be minimal for initial grain sizes below about 5 μm. Superplastic deformation caused grain growth in both alloys under all testing conditions, and when the grain size exceeded about 8 μm a significant level of cavitation was produced. The grain size and extent of cavitation increased with increasing strain along the specimen gauge length, with cavities concentrated in regions adjacent to the fracture tip. Although never very large, the cross-sectional area at fracture increased with increasing levels of cavitation. It was concluded that cavitation in Zn-Al eutectoid results from incomplete accommodation of grain-boundary sliding when excessive grain growth leads to restricted grain-boundary diffusion and/or to restricted grain-boundary migration.
TL;DR: In this paper, the microstructure, transverse rupture strength, and electrical resistivity of tantalum carbide specimens produced by sintering at various temperatures were studied, and it was shown that to obtain sintered tantalum carbonide of fine-grained structure, low porosity, and adequate strength, it is necessary to employ very fine starting powders.
Abstract: A study was made of the microstructure, transverse rupture strength, and electrical resistivity of tantalum carbide specimens produced by sintering at various temperatures. It is shown that, to obtain sintered tantalum carbide of fine-grained structure, low porosity, and adequate strength, it is necessary to employ very fine starting powders. Raising the sintering temperature and increasing the duration of isothermal holding lower the porosity and electrical resistivity of sintered tantalum carbide, but at the same time induce substantial grain growth.
TL;DR: In this article, the effects of second phase particles, e.g. M23C6, MC and M(C, N) carbides on the grain growth phenomenon of FeNiCr alloys have been determined.
TL;DR: In this paper, the authors summarized the results of some recent research on a thermomechanical method of refining the grain size in precipitation hardenable aluminum alloys and illustrates the infuence of grain refinement on several material properties.
Abstract: This paper summarizes the results of some recent research on a thermomechanical method of refining the grain size in precipitation hardenable aluminum alloys and illustrates the infuence of grain refinement on several material properties. Grain refinement is achieved by deliberately introducing a large number of nucleation sites for recrystallization and by controlling grain growth after recrystallization. Recrystallization to a relatively small and equiaxed grain size has been achieved in a number of commercial aluminum alloys using these concepts. The influence of the fine recrystallized grain size on such properties as superplastic deformation, room temperature tensile properties, fatigue life, and exfoliation corrosion resistance is discussed. The results show that refinement to a grain size of 8–14 µm is sufficient to develop extensive superplasticity and to yield a small increase in tensile properties in alloys such as 7075 and 7475.
TL;DR: In this article, the effect of nitrogen on grain growth of Ti(C,N)−Ni alloys was investigated by comparing TiC−Ni and TiN−Ni, in which carbon in the titanium carbide is replaced by nitrogen, from the viewpoint of grain size control.
Abstract: In order to improve the mechanical properties of cemented carbides, Ti(C,N)–Ni–Mo alloys, in which carbon in the titanium carbide is replaced by nitrogen, are of potential interest from the viewpoint of grain size control. Since grain size control by nitrogen was also observed in Ti(C,N)–Ni alloys containing no molybdenum, the effect of nitrogen on grain growth of Ti(C,N)–Ni alloys was investigated by comparing TiC–Ni and TiN–Ni alloys. The grain growth rate of Ti(C,N)–Ni alloys showed a minimum value at the carbon content C/C+N = 0·5. From the results of chemical and grain size analyses, it is considered that the growth rate depends on the solubility of Ti(C,N) in the nickel solid solution and the degree of coalescence because the solubility decreases with decreasing carbon content. With nitride, the amount of liquid increases by denitrification of TiN, owing to the formation of the nickel solid solution or TiNi3 phases, and growth of TiN grains by Ostwald ripening is observed. The apparent activ...
TL;DR: In this article, the authors investigated the optimum casting rate for large-grained CdxHg1-xTe ingots with 0.15
TL;DR: In this article, the effect of chromium dopant on surface diffusion of MgO was investigated using the grain-boundary grooving method, and the surface diffusion was determined using a grain-grain-based method.
Abstract: Surfaces diffusion affects processes such as sintering, grain growth, catalysis, and pore migration. An investigation was conducted to determine the effect of chromium dopant on the surface diffusion of MgO. For comparison, surface diffusion was also studied on undoped MgO from two sources. In all cases, surface diffusion was determined using the grain-boundary grooving method. 9 refs.
TL;DR: In this article, the recrystallization process in two concentrated aluminium-manganese alloys has been examined in the unhomogenized condition and it has been shown that recurstallization nuclei are initially associated with constituent particles originally situated in the as-cast cell walls.
Abstract: The recrystallization process in two concentrated aluminium–manganese alloys has been examined in the unhomogenized condition It is shown that recrystallization nuclei are initially associated with constituent particles originally situated in the as-cast cell walls After 80% reduction, the constituent particles were still aligned and associated with a particle-free zone Initial growth of nuclei occurred along these particle-free zones rather than out into the matrix, which contained a large volume fraction of fine precipitate particles Only when normal grain growth had occurred in the as-cast cell walls to produce large elongated grains could the particle pinning of the matrix be overcome and the high-angle boundaries migrate outwards to consume the remaining matrix