Showing papers on "Grain growth published in 2005"

Sintering: Densification, Grain Growth and Microstructure

Abstract: Basis of Sintering Science Solid State Sintering Models and Densification Grain Growth Microstructure Development Sintering of Ionic Compounds Liquid Phase Sintering

High Purity Anatase TiO2 Nanocrystals: Near Room-Temperature Synthesis, Grain Growth Kinetics, and Surface Hydration Chemistry

Abstract: High purity, spherical anatase nanocrystals were prepared by a modified sol−gel method. Mixing of anhydrous TiCl4 with ethanol at about 0 °C yielded a yellowish sol that was transformed into phase-pure anatase of 7.7 nm in size after baking at 87 °C for 3 days. This synthesis route eliminates the presence of fine seeds of the nanoscale brookite phase that frequently occurs in low-temperature formation reactions and also significantly retards the phase transformation to rutile at high temperatures. Heating the as-is 7.7 nm anatase for 2 h at temperatures up to 600 °C leads to an increase in grain size of the anatase nanoparticles to 32 nm. By varying the calcination time from 2 to 48 h at 300 °C, the particle size could be controlled between 12 and 15.3 nm. The grain growth kinetics of anatase nanoparticles was found to follow the equation, D2 − D02 = k0tme(-Ea/RT) with a time exponent m = 0.286(±9) and an activation energy of Ea = 32 ± 2 kJ·mol-1. Thermogravimetric analysis in combination with infrared an...

Microstructure evolution during FSW/FSP of high strength aluminum alloys

Abstract: Microstructural characteristics of friction stir processed (FSP) 7075 Al samples in different regions behind the pin tool were investigated. Furthermore, the microstructure evolutions during friction stir welding (FSW)/FSP has been revealed. Multi-mechanisms, including dDRX, grain growth, dislocation introduction, dynamic recovery (DRV) and cDRX, were found to be inherent in the process at different stages. Because inhomogeneous plastic deformation was introduced by the process, individual grains in the final microstructure have undergone different evolution mechanisms. These are either from growth of initially recrystallized grains or are the result of cDRX of subgrains formed during DRV. Grains within the processed region exhibit different densities of dislocations and are in various degrees of recovery.

Rapid stress-driven grain coarsening in nanocrystalline Cu at ambient and cryogenic temperatures

Abstract: There have been long-standing concerns about the stability of the internal structure of nanocrystalline metals. In this letter we examine grain growth in nanocrystalline Cu under the microhardness indenter, examining the influence of temperature of indentation and sample purity. Surprisingly, it is found that grain coarsening is even faster at cryogenic temperatures than at room temperature. Sample purity is seen to play an important role in determining the rate of grain growth. Fast grain coarsening can affect the outcome of mechanical tests, especially if they involve large stresses and high-purity samples.

A lead-free high-Curie-point ferroelectric ceramic, CaBi2Nb2O9

Abstract: Nano-powders of BaTiO3, SrTiO3, Ba0.6Sr0.4TiO3, a mixture of the composition (BaTiO3)0.6(SrTiO3)0.4 with particle sizes in the range of 60 to 80 nm, and Bi4Ti3O12 with an average particle size of 100 nm were consolidated by spark plasma sintering (SPS). The kinetics of reaction, densification and grain growth were studied. An experimental procedure is outlined that allows the determination of a “kinetic window” within which dense nano-sized compacts can be prepared. It is shown that the sintering behaviour of the five powders varies somewhat, but is generally speaking fairly similar. However, the types of grain growth behaviour of these powders are quite different, exemplified by the observation that the kinetic window for the (BaTiO3)0.6(SrTiO3)0.4 mixture is 125 oC, ~75 oC for Bi4Ti3O12, ~25oC for BaTiO3 and SrTiO3, while it is hard to observe an apparent kinetic window for obtaining nano-sized compacts of Ba0.6Sr0.4TiO3. During the densification of the (BaTiO3)0.6(SrTiO3)0.4 mixture the reaction 0.6BaTiO3+0.4SrTiO3 → Ba0.6Sr0.4TiO3 takes place, and this reaction is suggested to have a self-pinning effect on the grain growth, which in turn explains why this powder has a large kinetic window. Notably, SPS offers a unique opportunity to more preciously investigate and monitor the sintering kinetics of nano-powders, and it allows preparation of ceramics with tailored microstructures.The dielectric properties of selected samples of (Ba, Sr)TiO3 ceramics have been studied. The results are correlated with the microstructural features of these samples, e.g. to the grain sizes present in the compacts. The ceramic with nano-sized microstructure exhibits a diffuse transition in permittivity and reduced dielectric losses in the vicinity of the Curie temperature, whereas the more coarse-grained compacts exhibit normal dielectric properties in the ferroelectric region.The morphology evolution, with increasing sintering temperature, of bismuth layer-structured ferroelectric ceramics such as Bi4Ti3O12 (BIT) and CaBi2Nb2O9 (CBNO) was investigated. The subsequent isothermal sintering experiments revealed that the nano-sized particles of the BIT precursor powder grew into elongated plate-like grains within a few minutes, via a dynamic ripening mechanism.A new processing strategy for obtaining highly textured ceramics is described. It is based on a directional dynamic ripening mechanism induced by superplastic deformation. The new strategy makes it possible to produce a textured microstructure within minutes, and it allows production of textured ferroelectric ceramics with tailored morphology and improved physical properties.The ferroelectric, dielectric, and piezoelectric properties of the textured bismuth layer-structured ferroelectric ceramics have been studied, and it was revealed that all textured samples exhibited anisotropic properties and improved performance. The highly textured Bi4Ti3O12 ceramic exhibited ferroelectric properties equal to or better than those of corresponding single crystals, and much better than those previously reported for grain-orientated Bi4Ti3O12 ceramics. Textured CaBi2Nb2O9 ceramics exhibited a very high Curie temperature, d33-values nearly three times larger than those of conventionally sintered materials, and a high thermal depoling temperature indicating that it is a very promising material for high-temperature piezoelectric applications.

Texture Development by Templated Grain Growth in Liquid-Phase-Sintered α-Alumina

Abstract: The distribution and orientation of platelet-shaped particles of α-alumina in a fine-grained alumina matrix is shown to template texture development via anisotropic grain growth. The textured microstructure ranges from 4 wt% oriented platelet particles in calcined samples to nearly 100% oriented α-Al2O3 grains after sintering at 1400°C. A CaO + SiO2 liquid phase creates favorable thermodynamic and kinetic conditions for anisotropic grain growth and grain reorientation during sintering. Important criteria for templated grain growth include (1) anisotropic crystal structure and growth, (2) high thermodynamic driving force for template grain growth, and (3) modification of diffusion in the system to continuously provide material to the anisotropically growing template grains.

Mechanism for Developing the Boundary Barrier Layers of CaCu3Ti4O12

Abstract: CaCu 3 Ti 4 O 12 (CCTO) was prepared by solid-state reaction and identified by X-ray diffractometry. The evolution of the microstructure was observed by scanning electron microscopy (SEM). It was found that discontinuous grain growth developed during sintering, and large abnormal grains played a very significant role in the dielectric behavior. Cu ions segregated to the boundaries and CuO located at the triple-point sites of the abnormal large grains were observed by electron energy loss and energy-dispersive X-ray spectroscopy, respectively. Moreover, two kinds of morphologies, i.e., terraces with ledges and bump domains, were discovered inside the grains under the thermal etching conditions at 960°C for 72 min, which solves the contradiction related to the interpretation of the giant dielectric response between ceramic and single-crystal CCTO. Complex impedance spectroscopy was used to analyze the conductivity of ceramic CCTO, which suggests that it consists of conducting domains with two kinds of insulating barrier layers of domain and grain boundaries. The insulating domain and grain boundaries were attributed to orderly arranged dislocations and segregation of Cu ions, respectively. The conduction of CCTO was found to be related to the porosity, the grain size, and the thickness of the insulating boundary layers. For a sample sintered at 1065°C for 3 h, the anomalously low resistivity and temperature-dependent dielectric constant were due to the fact that domain boundaries were not substantially formed. The possible reasons for the development of barrier layers and the variation of the dielectric constant with the sintering time are discussed. A barrier-layer model with dielectric response based on the Maxwell-Wagner type of relaxation for ceramic CCTO is proposed. Two kinds of dielectric responses occur, depending on the microstructure: they are dominated by the domain and domain boundary and by the grain and grain boundary for large grains and fine grains, respectively.

Effect of Yttrium and Lanthanum on the Tensile Creep Behavior of Aluminum Oxide

Abstract: The tensile creep behavior of two rare-earth dopant systems, lanthanum- and yttrium-doped alumina, are compared and contrasted in order to better understand the role of oversized, isovalent cation dopants in determining creep behavior. It was found that, despite some microstructural differences, these systems displayed qualitatively a similar improvement in creep resistance, supporting the hypothesis that creep is strongly influenced by segregation. Differences in primary creep behavior and activation energy for steady-state creep were, however, observed for these systems. Given these results, it is expected that creep behavior can be further optimized by adjusting the dopant level and by controlling the microstructure.

Abnormal Grain Growth of Alumina

Abstract: Abnormal grain growth (AGG) is not one of the intrinsic properties of alumina but rather is an extrinsic property that is controlled by certain impurities that are introduced during powder synthesis, processing, or sintering. When small amounts of glass-forming impurities are introduced, some portion beyond their solubility limits will accumulate at grain boundaries at the final stage of densification, form thin intergranular glass films of thermodynamically stable thickness, and induce the sudden appearance of abnormal grains by increasing the rate of grain-boundary migration abruptly. The proposition has been tested experimentally with small, but varying, amounts of silica in ultrapure alumina (99.999%) that has been sintered in a contamination-free condition. Average grain sizes for the appearance of AGG are inversely related to the doping concentration of silica. The thickness of intergranular silicate glass films at the onset of AGG in alumina is constant and estimated to be }3.7 nm.

Deformation mechanisms responsible for the high ductility in a Mg AZ31 alloy analyzed by electron backscattered diffraction

Abstract: The microstructural evolution during tensile deformation of an AZ31 alloy with grain size ranging from 17 to 40 µm, at intermediate temperatures, has been studied using electron backscattered diffraction (EBSD) and optical microscopy (OM) as the main characterization tools. Two deformation regimes could be distinguished. In the high-strain-rate regime, the stress exponent was found to be about 6, and the activation energy is close to that for Mg self-diffusion. These values are indicative of climb-controlled creep. In the lower strain rate range, elongations higher than 300 pct were measured. In this range, significant dynamic grain growth takes place during the test, and thus, the predominant deformation mechanisms have been investigated by means of strain-rate-change tests. It was found that the stress exponent varied during the test between 1.7 and 2.5, while the activation energy remains close to that for grain-boundary diffusion. The EBSD analysis revealed, additionally, the appearance of low to moderately misoriented boundaries that tend to lay perpendicular to the tensile axis. The enhanced ductility of this AZ31 alloy in this regime is attributed to the operation of a sequence of deformation mechanisms. Initially, grain-boundary sliding governs deformation; once dynamic grain growth occurs, dislocation slip becomes gradually more important. Dislocation interaction gives rise to the appearance of new interfaces by continuous dynamic recrystallization (CDRX).

The temperature ranges for maximum effectiveness of grain growth inhibitors in WC–Co alloys

Abstract: In order to more effectively control the WC grain growth during liquid phase sintering of submicron WC–Co alloys, the temperature ranges where common grain growth inhibitors are most active have been determined. In the study, small additions (0.5% by weight) of TiC, TaC, NbC, VC, and Cr3C2 were added to WC–6Co and WC–10Co alloys. In some cases, a small amount of coarse WC powder was added to simulate the effect of discontinuous grain growth. The powder compacts were vacuum sintered at temperatures ranging from 1300 to 1575 °C. The magnetic coercivity and microstructure results indicated that VC is equally effective in limiting continuous grain growth for all temperatures within the temperature range while the other inhibitors become more effective with temperature. VC was also found to be the most effective inhibitor of discontinuous grain growth. The results of the investigation are expressed in terms of the effectiveness of grain growth inhibition as determined by microstructural analysis, magnetic coercivity and hardness.

Thermodynamic stabilization of nanocrystallinity

Abstract: Nanocrystalline materials are polycrystals made up of nanometer-sized grains separated by a network of interfaces - grain or phase boundaries - that generally make a positive contribution to the total energy of the system. Consequently, there exists a thermodynamic driving force for reducing the overall interface area, which renders such systems intrinsically unstable against coarsening. The latter process entails interface migration and the concomitant deterioration of any property enhancements effected by the ultrafine grain size. We describe a strategy for significantly reducing or even eliminating the driving force for grain growth in nanocrystalline materials via the deliberate segregation of solute atoms into the core region of boundaries. Applied to Pd-Zr solid solutions containing up to 20 at.% Zr, the strategy yields nanocrystalline specimens manifesting an unusually high thermal stability with respect to grain growth, extending to the vicinity of the melting point. Parallels are drawn between the migration of grain boundaries in segregation-stabilized systems and antiphase domain boundaries in ordered alloys.

Effect of Yttrium and Lanthanum on the Final-Stage Sintering Behavior of Ultrahigh-Purity Alumina

Abstract: Final-stage sintering has been investigated in ultrahigh-purity Al2O3 and Al2O3that has been doped individually with 1000 ppm of yttrium and 1000 ppm of lanthanum. In the undoped and doped materials, the dominant densification mechanism is consistent with grain-boundary diffusion. Doping with yttrium and lanthanum decreases the densification rate by a factor of ˜11 and 21, respectively. It is postulated that these large rare-earth cations, which segregate strongly to the grain boundaries in Al2O3, block the diffusion of ions along grain boundaries, leading to reduced grain-boundary diffusivity and decreased densification rate. In addition, doping with yttrium and lanthanum decreases grain growth during sintering. In the undoped Al2O3, surface-diffusion-controlled pore drag governs grain growth; in the doped materials, no grain-growth mechanism could be unambiguously identified. Overall, yttrium and lanthanum decreases the coarsening rate, relative to the densification rate, and, hence, shifted the grain-size-density trajectory to higher density for a given grain size. It is believed that the effect of the additives is linked strongly to their segregation to the Al2O3grain boundaries.

A novel approach to sintering nanocrystalline barium titanate ceramics

Abstract: A novel approach to pressureless sintering based on the combination of rapid-rate sintering, rate-controlled sintering, and two-step sintering under a controlled atmosphere is proposed. This combined sintering method facilitates control of grain and pore morphology. The application of this sintering approach for pure nanocrystalline barium titanate powder enables the suppression of grain growth during the intermediate and final stages of sintering and the production of fully dense ceramics with 108 nm grain size. The grain growth factor is 3.5, which is three and 17 times smaller than rate-controlled and conventional sintering, respectively.

Overview Inclusion assisted microstructure control in C–Mn and low alloy steel welds

Abstract: Inclusion assisted microstructure control has been a key technology to improve the toughness of C–Mn and low alloy steel welds over the last two to three decades. The microstructure of weld metals and heat affected zones (HAZs) is known to be refined by different inclusions, which may act as nucleation sites for intragranular acicular ferrite and/or to pin austenite grains thereby preventing grain growth. In the present paper, the nature of acicular ferrite and the kinetics of intragranular ferrite transformations in both weld metals and the HAZ of steels are rationalised along with nucleation mechanisms. Acicular ferrite development is considered in terms of competitive nucleation and growth reactions at austenite grain boundary and intragranular inclusion nucleation sites. It is shown that compared to weld metals, it is difficult to shift the balance of ferrite nucleation from the austenite grain boundaries to the intragranular regions in the HAZ of particle dispersed steels because inclusion de...

Grain-size dependence of the relationship between intergranular and intragranular deformation of nanocrystalline Al by molecular dynamics simulations

Abstract: The strength of nanocrystalline aluminum has been studied using molecular dynamics simulation. Nanocrystalline models consisting of hexagonal grains with grain size $d$ between 5 nm and 80 nm are deformed by the application of tension. A transition from grain-size hardening to grain-size softening can be observed in the region where $d\ensuremath{\approx}30\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, which is the optimum grain size for strength. In the grain-size hardening region, nanocrystalline models primarily deform by intragranular deformation. Consequently, a pile-up of dislocations can be observed. When the grain size becomes less than 30 nm, where the thickness of the grain boundaries cannot be neglected in comparison to the grain sizes, the dominant deformation mechanism of nanocrystalline metals is intergranular deformation by grain boundary sliding. Further, geometrical misfits by grain boundary sliding are accommodated by the grain rotation mechanism. Moreover, cooperative grain boundary sliding occurs in the 5 nm model. The optimum grain size is controlled by the relationship between resistance to intergranular deformation by grain boundary processes and intragranular deformation resisted by the grain boundary. Therefore, the primary role of the grain boundary changes in the region where the optimum grain size is observed.

High Strain, 〈001〉 Textured 0.675Pb(Mg1/3Nb2/3)O3–0.325PbTiO3 Ceramics: Templated Grain Growth and Piezoelectric Properties

Abstract: Lead magnesium niobate–lead titanate, 0.675Pb(Mg1/3Nb2/3)O3–0.325PbTiO3 (PMN–32.5PT) ceramics were textured (grain-oriented) in the 〈001〉-crystallographic direction by the templated grain growth process. The textured PMN–32.5PT ceramics were produced by orienting {001}-SrTiO3 (ST) platelets (∼10 μm in diameter and ∼2-μm thickness) in a submicron PMN–32.5PT matrix. The templated growth of 〈001〉-oriented PMN–32.5PT grains on the ST platelets resulted in textured ceramics with ∼70% Lotgering factor and >98% theoretical density. Unlike most lead-based ceramics, excess PbO was not needed for sintering or grain growth. Based on unipolar stain-field measurements at 0.2 Hz, the textured samples displayed >0.3% strain at 50 kV/cm. Low-field d33-coefficients of >1600 pC/N (<5 kV/cm) were measured directly from unipolar measurements. The low drive field d33-piezoelectric coefficient of the highly textured samples is two times greater than polycrystalline PMN–32.5PT.

Microwave dielectric properties of low-temperature sintered Mg3(VO4)2 ceramic

Abstract: In order to develop a new low-temperature co-fired ceramics (LTCC), the microwave dielectric properties and microstructure of (Mg 3− x Co x )(VO 4 ) 2 ceramics were investigated. The grain growth of Mg 3 (VO 4 ) 2 ceramic was observed as the sintering temperature was increased from 750 to 1050 °C; the maximum Q · f value of 65,440 GHz was obtained at the sintering temperature of 1050 °C. At a sintering temperature of 1075 °C, the Mg 3 (VO 4 ) 2 ceramic decomposed to form the MgO and liquid phases; the Q · f value of the sample decreased. As for the Co substitution for Mg, the XRPD patterns of (Mg 3− x Co x )(VO 4 ) 2 ceramics showed a single phase over the whole composition range; the individual curves indicated the maximum values for density which gradually converge as the sintering temperature is increased but the values shift toward the lower sintering temperatures with Co substitution for Mg. As a result, a Q · f value of 78,906 GHz with a dielectric constant of 9.5 was obtained at x = 2 when the sample was sintered at 900 °C for 5 h in air, whereas the temperature coefficient was −94.5 ppm/°C.

Pressureless Sintering of Titanium Diboride with Nickel, Nickel Boride, and Iron Additives

Abstract: The effect of relatively small additions (1-5 wt%) of nickel, nickel boride (NiB), and iron to promote the liquid-phase sintering of titanium diboride (TiB2) has been studied. Carbon also was added to some samples, to reduce the amount of oxygen impurities in the TiB2 ceramics. Green bodies that were formed by uniaxial pressing were sintered in a graphite furnace at 1300°–1700°C, both under vacuum and in a 500 mbar argon atmosphere, and high densities (>94% of theoretical density) were obtained at temperatures greater than or equal to 1500°C. The weight loss of the samples during sintering was shown to be dependent on the densification rate and the final density and was not governed only by the thermodynamics of the system. Significant exaggerated grain growth was observed in samples with nickel, NiB, and iron during sintering at 1700°C. The exaggerated grain growth was observed to be closely related to the oxygen content of the samples and to temperature. The addition of carbon strongly reduced the density and the oxygen content and, thereby, inhibited grain growth. We have proposed that the exaggerated grain growth is enhanced by surface diffusion in a titanium-oxide-rich layer on the TiB2 grains.

Anisotropic Grain Growth in Diphasic‐Gel‐Derived Titania‐Doped Mullite

Abstract: Densification and anisotropic grain growth in diphasic-gel-derived, titania-doped mullite were studied. Titania enhanced initial and intermediate stage densification in diphasic mullite gels by reducing the glass viscosity. Rodlike anisotropic mullite grains started to grow in titania-doped diphasic mullite gels once a dense, equiaxed microstructure was achieved. The onset temperature for anisotropic grain growth decreased with increasing titania concentration because the sintering temperature for final-stage densification decreased. The lowest onset temperature for anisotropic grain growth was ∼1500°C in 5 wt% titania-doped mullite. The aspect ratio and area fraction of anisotropic mullite grains increased with higher titania concentration and were strongly dependent on the initial titania particle size. Kinetic studies demonstrated that anisotropic grain growth in titania-doped diphasic mullite gels followed the empirical equation Gn - G0n=Kt, with growth exponents of 3 and 6 for the length [001] and thickness [110] directions, respectively. The activation energies for grain growth were 690 kJ/mol for the length and 790 kJ/mol for the thickness directions.