Showing papers on "Grain growth published in 2007"

Sintering of Ceramics

Abstract: Sintering of Ceramics: Fundamentals The Sintering Process Driving Force for Sintering Defects in Crystalline Solids Diffusion in Crystalline Solids The Chemical Potential Diffusional Flux Equations Diffusion in Ionic Crystals: Ambipolar Diffusion Solid-State and Viscous Sintering Mechanisms of Sintering Effects of Grain Boundaries Theoretical Analysis of Sintering Herring's Scaling Law Analytical Models Numerical Simulation of Sintering Phenomenological Sintering Equations Sintering Diagrams Sintering with an Externally Applied Pressure Stress Intensification Factor and Sintering Stress Alternative Derivation of the Sintering Equations Grain Growth and Microstructure Control General Features of Grain Growth Ostwald Ripening Topological and Interfacial Tension Requirements Normal Grain Growth in Dense Solids Abnormal Grain Growth in Dense Solids Grain Growth in Thin Films Mechanisms Controlling the Boundary Mobility Grain Growth and Pore Evolution in Porous Solids Simultaneous Densification and Grain Growth Fabrication Principles for Ceramics with Controlled Microstructure Liquid-Phase Sintering Elementary Features of Liquid-Phase Sintering Stages of Liquid-Phase Sintering Grain Boundary Films The Basic Mechanisms of Liquid-Phase Sintering Numerical Modeling of Liquid-Phase Sintering Hot Pressing with a Liquid Phase Use of Phase Diagrams in Liquid-Phase Sintering Activated Sintering Vitrification Special Topics in Sintering Inhomogeneities and their Effects on Sintering Constrained Sintering I: Rigid Inclusions Constrained Sintering II: Adherent Thin Films Constrained Sintering III: Multilayers Constitutive Models for Porous Sintering Materials Morphological Stability of Continuous Phases Solid Solution Additives and the Sintering of Ceramics Sintering with Chemical Reaction: Reaction Sintering Viscous Sintering with Crystallization Sintering Process Variables and Sintering Practice Sintering Measurement Techniques Conventional Sintering Microwave Sintering Pressure-Assisted Sintering Appendix A Physical Constants Appendix B SI Units - Names and Symbols Appendix C Conversion of Units Appendix D Ionic Crystal Radii (in units of 10-10m) Appendix E Density and Melting Point of Some Elements and Ceramics

Densification mechanisms in spark plasma sintering of nanocrystalline ceramics

Abstract: Effect of the particle size on the possible electric discharge during the SPS was examined. Nanoparticle compacts enable accumulation of high electric charge, and discharge under conventional voltages used for the SPS. The critical particle size for the electric discharge is both morphological and material dependent. The early stages of densification of the nanocrystalline powder compact proceed either by the plastic deformation or grain-rotation coalescence and sliding, aided by softening of the particle surfaces. The active densification mechanism depends on the changes both in the mechanical and electrical properties with temperature. Densification of 11 nm nc-MgO particles with low yield stress proceeds by plastic deformation already at 700 °C. However, densification of 34 nm nc-YAG particles with high yield stress proceeds by nano-grain rotation aided by particle surface softening. Densification at the final stages of SPS is associated with diffusional processes, where curvature driven grain growth predominates.

The von Neumann relation generalized to coarsening of three-dimensional microstructures

Abstract: Cellular structures or tessellations are ubiquitous in nature. Metals and ceramics commonly consist of space-filling arrays of single-crystal grains separated by a network of grain boundaries, and foams (froths) are networks of gas-filled bubbles separated by liquid walls. Cellular structures also occur in biological tissue, and in magnetic, ferroelectric and complex fluid contexts. In many situations, the cell/grain/bubble walls move under the influence of their surface tension (capillarity), with a velocity proportional to their mean curvature. As a result, the cells evolve and the structure coarsens. Over 50 years ago, von Neumann derived an exact formula for the growth rate of a cell in a two-dimensional cellular structure (using the relation between wall velocity and mean curvature, the fact that three domain walls meet at 120 degrees and basic topology). This forms the basis of modern grain growth theory. Here we present an exact and much-sought extension of this result into three (and higher) dimensions. The present results may lead to the development of predictive models for capillarity-driven microstructure evolution in a wide range of industrial and commercial processing scenarios--such as the heat treatment of metals, or even controlling the 'head' on a pint of beer.

Consolidation enhancement in spark-plasma sintering: Impact of high heating rates

Abstract: Spark-plasma sintering (SPS) provides accelerated densification and, in many cases, limited grain growth compared to regular hot pressing and sintering. Possible mechanisms of this enhancement of the consolidation in SPS versus conventional techniques of powder processing are identified. The consolidation enhancing factors are categorized with respect to their thermal and nonthermal nature. This paper analyses the influence of a major factor of thermal nature: high heating rates. The interplay of three mechanisms of material transport during SPS is considered: surface diffusion, grain-boundary diffusion, and power-law creep. It is shown that high heating rates reduce the duration of densification-noncontributing surface diffusion, this favors powder systems’ sinterability and the densification is intensified by grain-boundary diffusion. Modeling indicates that, besides the acceleration of densification, high heating rates diminish grain growth. The impacts of high heating rates are dependent on particle sizes. Besides SPS, the obtained results are applicable to the broad spectrum of powder consolidation techniques which involve high heating rates. The conducted experiments on SPS of an aluminum alloy powder confirm the model predictions of the impact of heating rates and initial grain sizes on the shrinkage rates during the electric current-assisted consolidation. It is noted, that this study considers only one of many possible mechanisms of the consolidation enhancement during SPS, which should stimulate further efforts on the modeling of field-assisted powder processing.

Microstructural evolution during the heat treatment of nanocrystalline alloys

Abstract: Nanocrystalline alloys often show exceptional thermal stability as a consequence of kinetic and thermodynamic impediments to grain growth. However, evaluating the various contributions to stability requires detailed investigation of the solute distribution, which is challenging within the fine structural-length-scales of nanocrystalline materials. In the present work, we use a variety of techniques to assess changes in the grain size, chemical ordering, grain-boundary segregation, and grain-boundary structure during the heat treatment of Ni–W specimens synthesized over a wide range of grain sizes from 3 to 70 nm. A schematic microstructural evolution map is also developed based on analytical models of the various processes activated during annealing, highlighting the effects of alloying in nanocrystalline materials.

Two‐Step Sintering of Nanocrystalline ZnO Compacts: Effect of Temperature on Densification and Grain Growth

Abstract: Two-step sintering (TSS) was applied on nanocrystalline zinc oxide (ZnO) to control the accelerated grain growth occurring during the final stage of sintering. The grain size of a high-density ( > 98%) ZnO compact produced by the TSS was smaller than 1 μm, while the grain size of those formed by the conventional sintering method was ∼ 4 μm. The results showed that the temperature of both sintering steps plays a significant role in densification and grain growth of the nanocrystalline ZnO compacts. Several TSS regimes were analyzed. Based on the results obtained, the optimum regime consisted of heating at 800°C (step 1) and 750°C (step 2), resulting in the formation of a structure containing submicrometer grains (0.68 μm). Heating at 850°C (step 1) and then at 750°C (step 2) resulted in densification and grain growth similar to the conventional sintering process. Lower temperatures, e.g., 800°C (step 1) and 700°C (step 2), resulted in exhaustion of the densification at a relative density of 86%, above which the grains continued to grow. Thermogravimetric analysis results were used to propose a mechanism for sintering of the samples with transmission electron micrographs showing the junctions that pin the boundaries of growing grains and the triple-point drags that result in the grain-boundary curvature.

Sintering and properties of lead-free (K,Na,Li)(Nb,Ta,Sb)O3 ceramics

Abstract: Two different stoichiometric (K0.44Na0.52Li0.04)(Nb0.86Ta 0.10Sb0.04)O3 and non-stoichiometric (K0.38Na0.52Li0.04)(Nb0.86Ta 0.10Sb0.04 )O 2.97 compositions were prepared by the conventional mixed oxide and carbonate route. Low temperature synthesis process at 700 ◦ C for 2 h, and further attrition milling process resulted in agglomerated powder of 500 nm of primary particles in the range of 50–70 nm. The ceramics sintered at 1125 ◦ C showed the highest densification that decreased for higher sintering temperatures. Non-stoichiometric composition densification process was assisted through the presence of a liquid phase that promotes grain growth and the appearance of a secondary ferroelectric phase with tungsten–bronze type structure. The samples showed a relaxor type behaviour that diminished because of the compositional homogeneity improvement of the liquid phase in the non-stoichiometric samples. The higher piezoelectric properties were obtained for the samples with high ferroelectric type dielectric constant versus temperature behaviour. In the non-stoichiometric samples piezoelectric constant d33 values reach ∼200 pC/N. © 2007 Elsevier Ltd. All rights reserved.

Preparation of [110] Grain Oriented Barium Titanate Ceramics by Templated Grain Growth Method and Their Piezoelectric Properties

Abstract: [110]-oriented barium titanate (BaTiO3) ceramics were prepared by templated grain growth (TGG) method using [110]-oriented BaTiO3 platelike particles as a template and hydrothermal BaTiO3 sphere particles with different particle sizes as a matrix. The degree of orientation along the [110] direction, F110, was measured using an X-ray diffraction (XRD) pattern by the Lotgering method. To obtain both a high density and a high F110, the preparation conditions were optimized as functions of matrix particle size, volume fraction of the template to the matrix, and sintering temperature. As for the results, BaTiO3-grain-oriented ceramics with a high density of more than 96% were successfully prepared despite various F110 values from 0 to 98%. Scanning electron microscopy (SEM) revealed that their average grain sizes were always approximately 75 µm despite various F110 values and there were no anisotropic microstructures. These grain-oriented BaTiO3 ceramics were poled at 100 °C, and their piezoelectric properties were measured using a resonance–antiresonance method and a piezo d33 meter for d31 and d33 piezoelectric constants. As for the results, the d31 values were almost constant at -50 pC/N despite various F110 values, while the d33 values increased with increasing F110 values, and at around an F110 of 85%, d33 reached a maximum of 788 pC/N.

Microstructure and mechanical properties of nanocrystalline WC–12Co consolidated by spark plasma sintering

Abstract: Cemented carbide powders based on WC–12Co with a grain size of 40–250 nm were generated by high-energy ball milling. The powders with different extents of VC and (VC + Cr3C2) addition were consolidated to full density by spark plasma sintering (SPS). The density, microstructure, grain size and fracture toughness KIc of the SPS consolidated samples were measured and compared with samples made by liquid phase sintering. Dense samples with a pore rating

Two‐Stage Sintering of Alumina with Submicrometer Grain Size

Abstract: This work verifies the applicability of two-stage sintering as a means of suppressing the final stage grain growth of submicrometer alumina. The first heating step should be short at a relatively high-temperature (1400°–1450°C) in order to close porosity without significant grain growth. The second step at temperatures around 1150°C facilitates further densification with limited grain growth. Fine-grained alumina with a relative density of 98.8% and a grain size of 0.9 μm was prepared by two-stage sintering. A standard sintering process resulted in ceramics with identical relative density and a grain size of 1.6 μm.

The deformation, recrystallisation and texture of three magnesium alloy extrusions

Abstract: The deformation, recrystallisation and texture of three magnesium alloy extrusions, AZ31, WE43 and ZC71, have been investigated. The extruded microstructures were partially recrystallised; the nucleation of new grains was associated with boundaries, deformation features and second phase particles. The unrecrystallised grains were orientated with directions parallel to the extrusion axis. In AZ31 and ZC71 the recrystallised grains also exhibited a component, which evolved into the major component during annealing. In WE43 recrystallised grains were orientated with their basal planes at ∼45° to the extrusion axis; this texture component dominated the texture following annealing. The recrystallisation grain growth textures appear to be governed by growth selection where alloying elements change the grain boundary behaviour and orientation relationships for high mobility. The tensile and compressive strengths of the extrusions were affected by the texture.

Growth behavior of Fe2Al5 during reactive diffusion between Fe and Al at solid-state temperatures

Abstract: The solid-state reactive diffusion between Fe and Al was experimentally observed using Al/Fe/Al diffusion couples prepared by a diffusion bonding technique. The diffusion couples were isothermally annealed in the temperature range between T = 823 and 913 K for various times up to 120 h. Due to annealing, Fe 2 Al 5 is produced as a layer at the Fe/Al interface in the diffusion couple. Although FeAl, FeAl 2 and FeAl 3 are also stable intermetallic compounds in this temperature range, these compounds were not recognized in the diffusion couple within the experimental annealing times. The mean thickness of the Fe 2 Al 5 layer is proportional to the square root of the annealing time, and grain growth occurs in the Fe 2 Al 5 layer. This means that the growth of the Fe 2 Al 5 layer is controlled by the volume diffusion of the constituent elements in each phase. On the basis of the observation for the growth rate, we may estimate that the interdiffusion coefficient is about one order of magnitude smaller for Fe 2 Al 5 than for Al but more than three orders of magnitude greater for Fe 2 Al 5 than for Fe. If nucleation occurs sufficiently fast for all the compounds, the interdiffusion coefficient should be more than two orders of magnitude smaller for FeAl, FeAl 2 and FeAl 3 than for Fe 2 Al 5 .

Hot pressing of tantalum carbide with and without sintering additives

Abstract: Densification of tantalum carbide (TaC) was studied by hot pressing at temperatures ranging from 1900° to 2400°C with and without sintering additives. Without sintering additives, the relative density increased from 75% at 1900°C to 96% at 2400°C. A microstructural examination showed no observable grain growth up to 2300°C. Densification was enhanced with carbon (C) and/or B4C additions. TaC with a 0.78 wt% C addition achieved a relative density of 97% at 2300°C. Additions of 0.36 wt% B4C or 0.43 wt% B4C and 0.13 wt% C increased the relative density to 98% at 2200°C, accompanied by rapid grain growth at 2100°C and higher temperatures.

Analysis of high-temperature deformation and microstructure of an AZ31 magnesium alloy

Abstract: High-temperature plastic deformation and dynamic recrystallization of AZ31 extruded (EX) and heat treated (FA) alloy was investigated in the temperature range between 200 and 400 °C. High-temperature straining resulted in partial dynamic recrystallization above 250 °C; in the EX alloy recrystallization was complete at 300 °C, while a moderate grain growth was observed at 400 °C. The peak flow stress dependence on temperature and strain rate are described by means of the conventional sinh equation; the calculation of the activation energy for high temperature in the whole range of temperature deformation gives Q = 155 kJ/mol, i.e. a value that was reasonably close but higher than the activation energy for self diffusion in Mg. The microstructure resulting from high-temperature straining was found to be substantially different in EX and FA alloys; in particular, the EX alloy was characterized by a lower flow stress, a higher ductility and by a finer size of the dynamically recrystallized grains. These results are then discussed on the basis of the “necklace” mechanism of dynamic recrystallization.

Evolution of microstructure and microtexture in fcc metals during high-pressure torsion

Abstract: Pure nickel and commercially pure (CP) aluminium were selected as model fcc materials for a detailed investigation of the experimental parameters influencing grain refinement and evolution of microstructure and microtexture during processing by high-pressure torsion (HPT). Samples were examined after HPT using microhardness measurements, transmission electron microscopy and orientation imaging microscopy. Processing by HPT produces a grain size of ∼170 nm in pure Ni and ∼1 μm in CP aluminium. It is shown that homogeneous and equiaxed microstructures can be attained throughout the samples of nickel when using applied pressures of at least ∼6 GPa after 5 whole revolution. In CP aluminium, a homogeneous and equiaxed microstructure was achieved after 2 whole revolutions under an applied pressure of 1 GPa. For these conditions, the distributions of grain boundary misorientations are similar in the centre and at the periphery of the samples. It is shown that simple shear texture develops in fcc metals subjected to high-pressure torsion. Some grain growth was detected at the periphery of the Al disk after 8 revolutions. The factors influencing the development of homogeneous microstructures in processing by HPT are discussed.

Densification of SiC by SPS-effects of time, temperature and pressure

Abstract: Temperature, holding time and conditions of pressure application, three of the most important spark plasma sintering (SPS) parameters, have been reviewed to assess their effect on the densification and grain growth kinetics of a pure commercially available submicrometer-sized silicon carbide powder. Experiments were performed in the 1750–1850 °C temperature range with holding time from 1 to 10 min. Two pressure setups were used: one with pressure (75 MPa) applied at 1000 °C and the other with ultimate pressure applied at sintering temperature. Experimental data highlighted the fact that temperature and holding time have a different impact on grain growth and densification. Diffusion and migration mechanisms that promote grain growth were found to be strongly dependent on temperature, the latter being linked to pulsed current intensity. Conditions of pressure application suggest that the ultimate pressure applied at higher temperature increases densification by keeping small surface contact between particles.

Abnormal Grain Growth and New Core–Shell Structure in (K,Na)NbO3-Based Lead-Free Piezoelectric Ceramics

Abstract: A unique core–shell structure was observed in coarse grains in (K,Na)NbO3 (KNN)-based lead-free piezoelectric ceramics. It is morphologically different from the chemical inhomogeneity-induced core–shell grain structure reported previously in BaTiO3-based ceramics. The core region is composed of highly parallel nanosized subgrains, whereas the shell region consists of larger-sized but similar self-assembled subgrains. The electron-backscattered diffraction analysis and selected area electron diffraction pattern confirmed that coarse grains with a core–shell structure were single-crystalline-like grains. The formation process of such coarse grains was then discussed based on mesocrystal growth along with the classical theory of grain growth. The two studied KNN-based systems showed a similar grain growth transformation: from self-assembled aggregation clusters with nanosized subgrains to a typical core–shell grain structure when the sintering temperature was increased only by a range of 10°–20°C. The volatilized alkali metal oxides and liquid phase were supposed to accelerate such grain growth transformation. When abnormal grown grains with a core–shell structure occurred, both systems showed the highest densities and dielectric constants along with the lowest dielectric losses, while their piezoelectric properties tended to decline.

Thermal stability of nanostructured superhard coatings: A review

Abstract: It is important to evaluate the thermal stability of hard coating because at high working temperatures the mechanical and tribological properties are deteriorated. The temperature operating on the cutting tool tip during work may reach temperatures as high as 1000 °C. Environmental considerations limiting the use of lubricants and coolant liquids, increase the necessity of finding coatings that can function at such high temperature. Coatings can be differentiated by their hardness, H , into three main categories: hard with H H > 40 GPa; and ultra-hard coatings with H > 80 GPa. There are two main reasons in the high hardness coatings: either high compressive stresses or nano-scale structure. The application of high biaxial compressive stress acts as a driving force for recovery, i.e. the higher the compressive stress, the lower is the thermal activation energy needed to initiate recovery. High biaxial compressive stress increases superhardness, but reduces the coating thermal stability. Dislocations increase the micro-scale compressive stress inside the coating and consequently, enhance recovery. In nano-scale coatings, the small nanometric scale grain size restricted grain growth and boundaries sliding, and therefore the thermal stability is enhanced. This study treats the thermal stability of several types of superhard materials, i.e. nanocomposite coatings and those consisting of a hard transition-metal nitride and a soft metal. It focuses on formation mechanisms, materials and phase composition.

Preparation and Characterization of Samaria-Doped Ceria Electrolyte Materials for Solid Oxide Fuel Cells

Abstract: The microstructure, thermal expansion, mechanical property, and ionic conductivity of samaria-doped ceria (SDC) prepared by coprecipitation were investigated in this paper. The results revealed that the average particle size ranged from 10.9±0.4 to 13.5±0.5 nm, crystallite dimension varied from 8.6±0.3 to 10.7±0.4 nm, and the specific surface area distribution ranged from 62.6±1.8 to 76.7±2.2 m2/g for SDC powders prepared by coprecipitation. The dependence of lattice parameter, a, versus dopant concentration, x, of Sm3+ ion shows that these solid solutions obey Vegard's rule as a (x)=5.4089+0.10743x for Ce1−xSmxO2−1/2x. For SDC ceramics sintered at 1500°C for 5 h, the bulk density was over 95% of the theoretical density; the maximum ionic conductivity, σ800°C=(22.3±1.14) × 10−3 S/cm with minimum activation energy, Ea=0.89±0.02 eV, was found in the Ce0.80Sm0.20O1.90 ceramic. A dense Ce0.8Sm0.2O1.9 ceramic with a grain size distribution of 0.5–4 μm can be obtained by controlling the soaking time at 1500°C. When the soaking time was increased, the microhardness of Ce0.8Sm0.2O1.9 ceramic increased, the toughness slightly decreased, which was related to grain growth with the soaking time.

Transparent yttrium aluminum garnet (YAG) ceramics by spark plasma sintering

Abstract: Commercial nanocrystalline yttrium aluminum garnet (nc-YAG) powders were used for fabrication of dense and transparent YAG by spark plasma sintering (SPS). Spherical 34 nm size particles were densified by SPS between 1200 and 1500 °C using 50 and 100 MPa pressures for 3, 6, and 9 min durations. Fully dense and transparent polycrystalline cubic YAG with micrometer grain size were fabricated at very moderate SPS conditions, i.e. 1375 °C, 100 MPa for 3 min. Increase in the SPS duration and pressure significantly increased the density especially at the lower temperature range. The observed microstructure is in agreement with densification by nano-grain rotation and sliding at lower densities, followed by curvature driven grain boundary migration and normal grain growth at higher densities. Residual nanosize pores at the grain boundary junctions are an inherent microstructure feature due to the SPS process.

Role of grain boundary and grain defects on ferromagnetism in Co:ZnO films

Abstract: The annealing effects on magnetism, structure, and ac transport for Co:ZnO films have been systematically investigated. The room temperature saturation magnetization (Ms) varies drastically with Ar or Ar∕H2 annealing processes. By using the impedance spectra, the change in grain boundary and grain defects of these films can be analyzed. The results demonstrate that Ar annealing produces mainly the grain boundary defects which cause the enhancement of Ms. Ar∕H2-annealing creates not only grain boundary defects but also the grain defects, resulting in the stronger enhancement of Ms. Ferromagnetism for Co:ZnO films is influenced by both grain boundaries and grain defects.

Evidence for segregation of Te in Ge2Sb2Te5 films: Effect on the “phase-change” stress

Abstract: The authors present direct evidence for Te segregation to the grain boundaries in chalcogenide Ge2Sb2Te5 films by using transmission electron microscopy scans with a 0.5nm diameter focused probe. This finding is consistent with the observed impeded grain growth and with the post-transition relief of a “spikelike” stress, fully to the pretransition level. Te motion shows up in void formation below 200°C, a pileup of Te at the surface and its loss at higher (above 400°C) temperatures. Tuning the driving force for this segregation may be key for the optimal phase-change material design.