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In conjunction with a previous investigation [Jung SH, Yoon DY, Kang SJL. Acta Mater 2013;61:5685], we have investigated the correlation between grain boundary structure and grain growth behavior in a model Ni system. Ultrafine Ni powder compacts of 180nm size were sintered in a wide range of temperatures from 500 to 1150°C and for various times in wet H2. Between 500 and 600°C, abnormal grain growth (AGG) readily occurred, with the formation of cube-shaped grains. Between 650 and 900°C, grain growth behavior was stagnant up to 6000min due to impingement of rapidly grown abnormal cube-shaped grains during heating. At 950°C, some of the impinged abnormal grains suddenly grew after 20min, showing secondary AGG behavior. As the sintering temperature was increased further, the grain growth behavior became quite normal. These changes in grain growth behavior with increasing temperature were accompanied by the structural transition of grain boundaries from fully faceted to partially faceted and defaceted. The observed grain growth behavior with respect to the grain boundary morphology is explained in terms of a coupling effect of the maximum driving force for grain growth and the critical driving force for appreciable migration of faceted boundaries. The present experimental results appear to support the validity of the previously suggested mixed control model of grain growth and the principle of microstructural evolution in polycrystals. [ABSTRACT FROM AUTHOR]

Grain growth in 8 mol% Y2O3-stabilized zirconia ceramics (8 YSZ) under an electric current has been investigated. Enhanced grain growth on the cathode side starts at 1150°C, well below the conventional sintering temperature, while grain growth is dormant on the anode side until 1400°C. In fully dense samples, the grain size undergoes an abrupt transition, differing by a factor of more than 10 on the two sides. Porous samples also experience faster densification on the cathode side, but grain growth is postponed until full density is first reached. Estimated grain boundary diffusivity on the cathode side has an apparent activation energy about 1 eV lower than that of normal grain boundary diffusion. These results are attributed to supersaturated oxygen vacancies accumulated on the cathode side, causing cation reduction that lowers their migration barrier. [ABSTRACT FROM AUTHOR]

This paper reports recent theoretical perspectives and experimental results on microstructural evolution during sintering in terms of the interface structure, which is either rough (atomically disordered) or faceted (atomically ordered). The paper presents theoretical predictions and calculations of grain growth during liquid-phase sintering based on crystal growth theories. It is shown that various types of grain growth behavior, which may be normal, abnormal, or stagnant, can appear as a result of the coupling effects of the maximum driving force for growth and the critical driving force for appreciable growth. The predictions are also shown to be valid in the case of solid-state sintering. A number of experimental observations showing the effect of some critical processing parameters have been found to be in excellent agreement with the predictions. Principles of microstructure development (grain growth control) during sintering are suggested. In addition, the effect of the interface structure on densification is briefly described and discussed. [ABSTRACT FROM AUTHOR]

Abstract: When 92WC–8Co (wt%) samples with WC powder of 0.4μm size were sintered by conventional liquid phase sintering at 1400°C, densification to more than 99% of the relative density was obtained after only 1h of sintering and abnormal grain growth occurred. However, when similar samples were pre-sintered in a solid state before liquid phase sintering, the samples were densified to more than 99% of the relative density without the formation of any abnormal grains within 30min of liquid phase sintering. As the grain growth behavior during liquid phase sintering was similar for the two kinds of samples, these findings show that pre-sintering in the solid state is an effective means of avoiding abnormal grain growth in submicron-sized WC–Co cemented carbides. As a result of the suppression of abnormal grain growth with full densification, the Vickers hardness of the samples pre-sintered and sintered for 30min was higher than that of conventionally sintered samples. [Copyright &y& Elsevier]

Abnormal grain growth in Pb(Mg½Nb2/3)O3-35 mol% PbTiO3 (PMN-35PT) ceramics doped with Li2O and PbO has been investigated. Replacing the PbO dopant with up to 2 mol% Li2O caused an increase in the number of abnormal grains. For the composition containing 2 mol% Li2O and 6 mol% PbO, the amount of abnormal grain growth decreased with increasing sintering temperature. Single crystals of ∼6 mm × 6 mm × 2 mm thickness were grown from the 2 mol% Li2O, 6 mol% PbO-containing composition via the templated grain growth method. Grain growth behavior with temperature is explained in terms of the effect of Li2O on interfacereaction-controlled grain growth and the critical driving force. [ABSTRACT FROM AUTHOR]

To investigate the effect of dislocations on grain growth in polycrystals, two sets of experiments were performed using SrTiO3 single crystals and SrTiO3 powder compacts. In the first set, with single crystals embedded in 2.0-mol%-TiO2-exess powder compacts, the growth of the single crystal was not affected by dislocations at 1300 °C, below the eutectic temperature, while it was enhanced by dislocations at 1470 °C, above the eutectic temperature, where the grain boundaries are wetted by an amorphous phase. In the second set, with 0.5-mol%-Nb2O5-doped powder compacts, the single crystal grew considerably into the SrTiO3 matrix grains in the presence of an amorphous film between the grains at 1470 °C both in 95N2-5H2 and in air, similar to the case of the TiO2-exess samples. However, when annealed at 1470 °C in 95N2-5H2 after pre-annealing at 1250 °C in 95N2-5H2, the amorphous phase remained at triple junctions and did not penetrate the grain boundaries, implying that this boundary configuration also is thermodynamically stable above the eutectic temperature. In this case, growth of the embedded single crystal was insignificant in spite of the presence of dislocations. These experimental observations indicate that the growth of SrTiO3 is promoted by dislocations only when an intergranular amorphous film is present at grain boundaries. The apparent ineffectiveness of dislocations on grain growth promotion without an intergranular amorphous film is discussed in terms of a low fraction of facetted grain boundaries and grain boundary drag by triple junctions. [Copyright &y& Elsevier]

Two series of experiments were performed to study the experimental conditions for the formation of {111} twins and related microstructures in barium strontium titanate ((Ba,Sr)TiO[sub 3]). In the first series, the phase equilibria in the BaTiO[sub 3]SrTiO[sub 3]-TiO[sub 2] system were determined. XRD and WDS analysis, done in the BaTiO[sub 3]-rich region, of 45(Ba,Sr)TiO[sub 3]-10TiO[sub 2] samples annealed at 1250°C for 200 h in air showed that (Ba,Sr)TiO[sub 3] was in equilibrium with Ba[sub 6]Ti[sub 17[O[sub 40] (B[sub 6]T[sub 17]) and Ba[sub 4]Ti[sub 13]O[sub 30] phases with strontium solubility (Sr/(Ba + Sr)) of ∼0.02 and 0.20, respectively. In the second series the microstructures of samples consisting of a mixture of (Ba,Sr)TiO[sub 3] and 2.0 mol% TiO[sub 2], were observed after sintering at 1250°C for 100 h in air. {111} twins formed only in the samples with faceted B[sub 6]T[sub 17] second phase particles, similar to the ease of BaTiO[sub 3]. In these samples, abnormal grain growth occurred in the presence of the {111} twins. In contrast, no {111} twins formed and no abnormal grain growth occurred in the samples containing second phase particles other than B[sub 6]T[sub 17]. With an increased substitution of strontium for barium, the aspect ratio of abnormal grains containing {111} twin lamellae was reduced. This result was attributed to a reduction in the relative stability of the {111} planes with the strontium substitution. [ABSTRACT FROM AUTHOR]

Examines the effect of sintering atmosphere on grain shape and growth in liquid-phase-sintered silicon carbide. Occurrence of abnormal grain growth; Application X-ray diffraction analysis; Formation of a 3C phase.

A structural transition of Ba6Ti17O40/BaTiO3 interfaces from faceted to rough was induced by reducing oxygen partial pressure in the atmosphere. As the oxygen partial pressure decreased, the number densities of {111} twins and abnormal grain decreased. TEM observation showed that the twin formation was governed only by the faceting of the interface. Experimental evidence of {111} twin-assisted abnormal growth of faceted BaTiO3 grains was also obtained. [ABSTRACT FROM AUTHOR]

To study the effect of oxygen partial pressure on grain growth in BaTiO[sub 3], TiO[sub 2]-excess samples have been sintered in air with and without a prior Hz heat treatment. Without prior H[sub 2] treatment, abnormal grain growth occurs below and above the eutectic temperature (T[sub e]). An introduction of Hz treatment before air sintering, however, increases the average grain size and suppresses the formation of abnormal grains during subsequent air sintering below and above T[sub e]. This H[sub 2] treatment effect has been explained in terms of a decrease of the driving force for the growth of faceted grains below a critical value for formation of abnormal grains. The observed graingrowth behavior under various atmospheres demonstrates the possibility of having various microstructures via control of oxygen partial pressure and initial grain size. [ABSTRACT FROM AUTHOR]

Abstract: During annealing of ultrafine nickel powder compacts, abnormal grain growth (AGG) occurred with the formation of cube-shaped grains. The observed AGG is similar to the late-stage abnormal grain growth (LS-AGG) that is commonly observed in electrodeposited (ED) Ni sheets. The cube shape of the abnormal grains with {100} facets was identical to that of late-stage abnormal grains in ED Ni. As the temperature increased, the time to the appearance of abnormal grains decreased. After impingement of abnormal grains, little further grain growth occurred, indicating stagnant grain growth behavior. The formation of abnormal grains in the present study, however, cannot be explained by the previously suggested AGG mechanisms, i.e. the S phase wetting mechanism and initial texture mechanism, because no amorphous phase containing S was present and no texture developed in our samples. In contrast, the observed grain growth behavior can be well explained in terms of the coupling effect of the maximum driving force for grain growth and the critical driving force for appreciable migration of faceted boundaries. It is concluded that the AGG in ultrafine-grained Ni occurs via mixed migration mechanisms, i.e. diffusion and interface reaction-controlled, of faceted boundaries with respect to the driving force. [Copyright &y& Elsevier]

Abstract: The effect of boundary faceting on densification has been studied in a 5mol.% TiO2–excess BaTiO3 model system. It was possible to inhibit grain growth during sintering in moderately reducing atmospheres after a hydrogen presintering treatment and, thus, to exclude the effect of grain growth on densification. For a given oxygen partial pressure () there was a saturated relative density which decreased with increasing and boundary faceting. The fraction of small pores was also higher in the samples sintered under high than in those sintered under low . These experimental observations demonstrate the presence of a critical driving force for densification in systems with faceted boundaries and its dependence on boundary faceting, i.e. the critical driving force is larger for systems with greater faceting. The result further confirms that a faceted boundary, either fully or partially faceted, is an imperfect atom source for densification; this idea is in contrast to the conventional understanding. [Copyright &y& Elsevier]

When 1 wt.%-Fe2O3-doped Al2O3 samples were sintered at 1600 °C in a reducing atmosphere, the additive remained as Fe precipitates at the triple grain-junctions. On annealing the sintered samples in an oxidizing atmosphere, however, the Fe precipitates dissolved into the Al2O3 grains at the surface region and induced grain-boundary migration. This diffusion induced grain-boundary migration (DIGM) resulted in the corrugation of grain boundaries and the formation of misfit dislocations in the migration region. The mechanical properties of the samples were evaluated by the Hertizian indentation technique under static and cyclic loading. The sample with DIGM showed better mechanical properties than that without DIGM: improvement in the critical load for cone crack initiation under static loading and in the number of cycles for crack initiation under cyclic loading. In terms of microstructure, the cone crack propagation was suppressed and a quasi-plastically deformed region appeared under static loading, and the surface chipping at the contact area was much reduced under cyclic loading. An insignificantly low value of residual stresses at the surface of the sample with DIGM suggested that the mechanical property improvement was due to the microstructural changes, the grain boundary corrugation and the misfit dislocations, rather than to any compressive stresses introduced in the migration layer. DIGM appears to be a possible means of improving the mechanical properties of Al2O3. [Copyright &y& Elsevier]

A structural transition of Ba6Ti17O40/BaTiO3 interfaces from faceted to rough was induced by reducing oxygen partial pressure in the atmosphere. As the oxygen partial pressure decreased, the number densities of {111} twins and abnormal grain decreased. TEM observation showed that the twin formation was governed only by the faceting of the interface. Experimental evidence of {111} twin-assisted abnormal growth of faceted BaTiO3 grains was also obtained. [ABSTRACT FROM AUTHOR]

Abstract: Grain coarsening behavior in the 95Na1/2Bi1/2TiO3–5BaTiO3 system has been studied as a function of the addition of TiO2. As the amount of added TiO2 was increased, the grain shape changed to a more faceted cube, indicating an increase in the step free energy of the facets, and hence a rise in the critical driving force for appreciable growth of grains. Grain coarsening behavior also changed from pseudo-normal to abnormal with an increasing TiO2 concentration and thus increased faceting. The pseudo-normal behavior observed in the system without TiO2 addition also changed to quite abnormal behavior during extended sintering. These observations support our theoretical prediction based on the coupling effects between the maximum driving force for growth and the critical driving force for appreciable growth. [Copyright &y& Elsevier]

Abstract: The effect of an intergranular amorphous film on grain growth behavior has been studied in a faceted model system, BaTiO3. We prepared two kinds of samples with and without intergranular amorphous films but with the same grain size and density. During annealing the samples at 1350°C in air, abnormal grain growth occurred in samples with intergranular amorphous films while grain growth was inhibited in samples with dry boundaries, indicating the presence of a pinning force in samples with dry boundaries. To compare the mobilities of dry and wet boundaries, single crystal and polycrystal bilayer samples with or without amorphous films were prepared and annealed at 1340°C. In contrast to the observed grain growth behavior in polycrystals, the growth of the single crystal into the polycrystal with dry boundaries was faster than that into the polycrystal with wet boundaries, demonstrating the higher mobility of a dry boundary, unlike the conventional understanding. [Copyright &y& Elsevier]

Abstract: The effect of additions of up to 1mol% Al2O3 on abnormal grain growth in BaTiO3 samples sintered at 1200 and 1250°C has been studied. Samples with and without additions of 0.4mol% TiO2 were prepared. For the samples without added TiO2, addition of 0.1mol% Al2O3 increases the number density of abnormal grains, with further additions reducing the number density. The initial increase in number density is caused by Al2O3 forming a solid solution with BaTiO3 and releasing TiO2 to the grain boundaries. This excess TiO2 then reacts with BaTiO3 to form Ba6Ti17O40, which promotes {111} twin formation and abnormal grain growth. Further additions of Al2O3 react with BaTiO3, Ba6Ti17O40 and excess TiO2 to form Ba4Al2Ti10O27 and BaAl2O4 second phases, neither of which are growth sites for abnormal grains. For the samples with added TiO2, addition of Al2O3 decreases the number of abnormal grains due to the Al2O3 reacting with the excess TiO2 and BaTiO3 to form Ba4Al2Ti10O27 and BaAl2O4 instead of Ba6Ti17O40. [Copyright &y& Elsevier]

Abstract: The effect of additions of up to 1mol% Al2O3 on abnormal grain growth in BaTiO3 has been studied. For samples sintered at temperatures ≤1250°C addition of 0.1mol% Al2O3 increases the number density of abnormal grains, with further additions reducing the number density. For samples sintered at 1350°C, addition of Al2O3 caused the primary abnormal grain size to decrease and then increase. The results are explained by considering the effect of Al2O3 addition on the TiO2 content of grain boundaries. [Copyright &y& Elsevier]