Migration of Intergranular Boundaries in Cubic Zirconia-Yttria Induced by Magnesia Addition

When partially sintered cubic ZrO₂–10 mol% Y₂O₃ specimens are heat-treated at 1500°C with 5MgO·95(ZrO₂–10 mol% Y₂O₃) powder mixture in the pores, the intergranular boundaries, which may be either grain boundaries or thin glass films, migrate, leaving behind them a new solid solution slightly enriched with MgO but depleted of Y₂O₃. The boundary curvatures generally increase during the migration and the average migration distance initially increases linearly with the heat treatment time. The observed boundary migration behavior is similar to that observed previously in a number of metallic systems, and its driving force is believed to arise from the coherency strain in the thin solute diffusion zone ahead of the moving boundaries.     

Direction of Liquid Film Migration Induced by Chromic Oxide in Alumina-Anorthite

Three kinds of single-crystalline alumina plates with the crystallographic planes of C, m, and R were diffusion-bonded with liquid-phase sintered (98)alumina-(2)anorthite (in wt%) plates (P) and then heat-treated at 1600°C under a Cr₂0₃-containing atmosphere. During the heat treatment, for all of the specimens studied, the anorthite liquid films between alumina plates migrated to grains with surface orientation corresponding to higher coherency strain energy. This result is in better agreement with a prediction based on the coherency strain theory than the previous one obtained for grain-boundary migration. The discrepancy between the predicted and previously observed migration directions of some grain boundaries in alumina may therefore be attributed to an effect of grain-boundary structure and stress transmission across the boundary.     

Coarsening Process of Penetration-Twinned Grains in PMN-35 mol% PT Ceramics

In a Pb(Mg_1/3Nb_2/3)O₃-35 mol% PbTiO₃ (PMN-35PT) specimen with 5 mol% excess PbO, the pores trapped inside grains by heat treatment in a double cycle at 1150°3C have revealed the coarsening pattern of the grains. Using this technique, the coarsening process of abnormal large grains with typical characteristics of penetration twins could be determined. The results suggest that two-dimensional nucleation and lateral growth enhanced at the reentrant edges are the cause of zigzag-shaped σ3 incoherent twin boundaries.     

Precipitate Coarsening by Liquid Film Migration in (Mg,Ca)-PSZ's

Precipitate coarsening involving preferential growth of solute-poor tetragonal ZrO₂ precipitates along grain boundaries, combined with migration of grain boundaries containing a liquid silicate phase to form solute-rich precipitate-free cubic ZrO₂, has been observed in grain boundary regions of MgO–CaO–ZrO₂ and CaO–ZrO₂ alloys during isothermal aging at 1400°C. Studies of alloys with 9.5 mol% total solute, but with varying MgO/CaO ratios, demonstrated a marked composition dependence of the phase separation process, in that such phase separation was rare in MgO-rich materials. The migration process is analyzed using models developed for migration in less complex materials, and a mechanism incorporating orientation-dependent precipitate strain energies is proposed.     

Migration of Intergranular Liquid Films and Formation of Core-Shell Grains in Sintered TiC–Ni Bonded to WC–Ni

When TiC–20 wt% Ni powder mixtures are sintered at 1400°C, relatively large TiC grains possibly containing some Ni form with near-equilibrium shapes. When these specimens are heat-treated again at 1400°C in contact with sintered WC–20 wt% Ni pieces, the liquid films between the TiC grains in the contact region migrate against their increasing curvatures, forming (Ti,W)C solid solution behind them. These migrating liquid films reverse their directions on further heat-treatment. As in other alloys, this liquid film migration must be driven by the coherency strain energy produced by W diffusion at the surface of the dissolving TiC grains. Shells of (Ti,W)C solid solution also form around the cores of TiC grains near the contact region, and this process is probably driven by both coherency strain energy and free energy of mixing. At some contact regions, (Ti,W)C precipitates nucleate and grow, probably driven mainly by the free energy of mixing. In powder mixtures, the formation of core-shell grains is expected to be driven by the coherency strain energy, the free energy of mixing, and the capillary effect.     

Grain Boundary Migration in Cubic Zirconia–Yttria Induced by Addition of Magnesia at Varying Concentrations

When partially sintered cubic ZrO₂–10 mol% Y₂O₃ specimens are heat-treated at 1500°C with powder mixtures of MgO and ZrO₂–10Y₂O₃ at varying ratios, the grain boundaries migrate, leaving behind new solid solutions enriched with MgO but depleted of Y₂O₃. With increasing MgO content in the solute source powder, the average migration velocity increases, and the MgO content increases and the Y₂O₃ content decreases slightly in these new solid solutions. With increasing MgO content in the solute source, the grain boundaries tend to be corrugated and faceted. Migration reversal is also observed at the corrugated boundaries. These variations of the grain boundary migration behavior with the MgO content in the solute source are consistent with the diffusional coherency strain energy as the driving force for the migration.     

Effect of Grain Coalescence on the Abnormal Grain Growth of Pb(Mg1/3Nb2/3)O3-35 mol% PbTiO3 Ceramics

Abnormal grain growth (AGG), which occurred during the heat treatment of Pb(Mg_1/3Nb_2/3)O₃-35 mol% PbTiO₃ (PMN-35PT) with excess PbO, was investigated. AGG has been suggested to be the consequence of grain coalescence that results in the formation of Σ3 coincidence site lattice and low angle grain boundaries. Because of reentrant edges appearing at the ends of these boundaries, the coarsening rate of grains was significantly enhanced and AGG occurred.     

Sinter-Forging Characteristics of fine-Grained Zirconia

Powder preforms of zirconia, containing 2.85 mol% yttria, were sinter-forged in simple uniaxial compression at 1400°C by applying constant displacement rates to the specimens. Shear and densification strains and the uniaxial stress were measured as a function of time. In contrast with alumina and silicon nitride, zirconia appears to densify by a dislocation mechanism. As a consequence, the densification rate is linked to the applied strain rather than to the applied hydrostatic pressure; the powder compact requires a critical amount of compressive strain to consolidate to full density, irrespective of the strain rate or the stress at which that strain is applied.     

Chemically Induced Migration of Liquid Films and Grain Boundaries in TiN—Ni—(TiC) Alloy

Liquid films and grain boundaries in the TiN–Ni system migrate when carbon atoms are added as TiC. Ti(NC) solid solution grows at the expense of TiN solid through liquid film migration (LFM), and its general characteristics are similar to those of the previously investigated Mo–Ni and W–Ni systems. But, in this system, precipitating solid–liquid interfaces of migrating liquid films show faceting, which is caused by orientation-dependent interface-controlled reactions. With an increase of added TiC, the migration rate increases parabolically with the observed lattice parameter difference between the TiN and Ti(NC) solid, which is in agreement with the prediction of the coherency strain model. Chemically induced grain-boundary migration (CIGM) also occurs, together with LFM. The results in this study show that LFM and CIGM occur in a system having relatively strong covalent bonding.     

Discontinuous Dissolution of a Liquid Phase in Ba(Ni1/3Nb2/3)O3 Ceramics

Discontinuous dissolution of a liquid phase accompanied by diffusion-induced grain-boundary migration (DIGM) in Ba(Ni_1/3Nb_2/3)O₃ (BNN) ceramics has been studied. When the liquid-phase-sintered BNN specimens are heat-treated at low temperatures, the liquid phase formed during sintering is dissolved discontinuously and during the reaction grain boundaries migrate. As the heat-treatment temperature is lowered, the degree of grain boundary migration and dissolution of the liquid phase increase. These results are qualitatively consistent with the coherency strain energy model proposed for the driving force for DIGM.     

Temperature dependence of the coarsening behavior of (Ba,Sr)TiO3 grains dispersed in a SiO2-rich liquid matrix

The shape of (Ba_0.35Sr_0.65)TiO₃ (BST) grains and their coarsening behavior were investigated as a function of temperature. The BST grains exhibit a morphological change at approximately 1410 °C; the grains were angular at 1390 °C and corner-rounded at 1420 °C. At 1390 °C, a few very large grains appear in the microstructure. Such abnormal grain growth is explained in terms of a coarsening advantage due to the Σ3 coincident site lattice (CSL) boundaries. On the other hand, at 1420 °C, normal grain growth was observed to occur. The change in coarsening behavior with temperature is related to the structural transition of the interface.     

Diffusion-Induced Grain-Boundary Migration in Ceria-Stabilized Tetragonal Zirconia Polycrystals

The microstructure and microchemistry of grain-boundary regions in (CeO₂+ La₂O₃)-stabilized tetragonal ZrO₂ polycrystals (Ce(La)-TZP) were studied by means of transmission electron microscopy (TEM). Evidence was found for the existence of crystalline and vitreous intergranular phases situated in small pockets at multiple grain junctions and in thin films along grain boundaries. In this ceramic system grain-boundary migration was observed in situ in the TEM in sample areas subjected to electron irradiation. Interfaces migrated away from their centers of curvature. Evidence was found for Ce de-alloying in the volume swept by the advancing boundaries. It is suggested that the coherency lattice strain brought about by a partial reduction of Ce, resulting in the diffusion of Ce³⁺ along grain boundaries to free surfaces, is the driving force for this phenomenon.     

Effect of SiO2 and TiO2 Addition on the Morphology of Abnormally Grown Large Pb(Mg1/3Nb2/3)O3–35 mol% PbTiO3 Grains

Abnormal grain growth (AGG) during the sintering of Pb(Mg_1/3Nb_2/3)O₃–35 mol% PbTiO₃ was investigated. When a small amount of SiO₂ was added, large abnormal grains with the penetration twin characteristic were observed to appear. AGG, in this case, is because of the twin plane re-entrant edges that provide coarsening advantage. When TiO₂ was added, on the other hand, the abnormal grains were simple cubic in morphology with well-developed {100} planes. In this case, the coarsening advantage is suggested to be because of grain boundary re-entrant edges.     

Deformation and Fracture of Polycrystalline Lithium Fluoride

Techniques for the fabrication of polycrystalline LiF test specimens were developed and evaluated using single-crystal LiF as a control. An etch was developed which revealed dislocations on all crystallographic faces of LiF. Large-grained polycrystalline specimens tested in four-point loading underwent 0.076 to 0.798% plastic strain before fracture. In most cases their yield stress was similar to that for single crystals favorably oriented for flow on {110}〈110〉 slip systems. Deformation was inhomogeneous among the grains because of differences in orientation with respect to the applied stress and within individual grains because of interactions at grain boundaries. Grain boundaries were barriers to slip, but stresses resulting from slip in one grain were transmitted to neighboring grains and often caused local deformation near the boundary. In one case, local boundary slip occurred on an (010) plane. Three-grain junctions were areas of high residual stresses, and fractures originated at boundaries at or near three-grain junctions. Fractures were mixed transgranular and intergranular.     

Intragranular Particle Residual Stress Strengthening of Al2O3–SiC Nanocomposites

Al₂O₃/SiC ceramic nanocomposites were fabricated from nanocrystalline Al₂O₃ (10 nm in diameter) and SiC (15 nm in diameter) powders, and a theoretical model of intragranular particle residual stress strengthening was investigated. The SiC nanoparticles in the Al₂O₃ grains create a normal compressive stress at the grain boundaries and a tangential tensile stress in the Al₂O₃ grains, resulting in the "strengthening" of the grain boundaries and "weakening" of the grains. The model gives a good explanation of the experimental results of the authors and others which are difficult to be explained by the existing strengthening models, i.e. the maximum strength is normally achieved at about 5 vol% of SiC particles in the Al₂O₃–SiC ceramic nanocomposites. According to the model, there exists an optimum amount of SiC for strengthening, below which the grain boundaries are not fully "strengthened" and the fracture is mainly intergranular, above which the grains are "weakened" too much and the fracture is mainly transgranular, and at which the fracture is a mixture of intergranular and transgranular.     

Microstructural stability at elevated temperatures

The factors affecting the diffusion-controlled coarsening of microstructures at elevated temperatures are reviewed in this paper. Coarsening can occur in a variety of different ways. Perhaps the most familiar of these is Ostwald ripening of a dispersed phase, e.g. precipitates in a 2-phase alloy. However, there are other related Ostwald-ripening-types of processes, including coarsening of dispersed phases in 2-dimensions (e.g. precipitates in thin films, ‘fibers’ in directionally solidified eutectics) and coarsening of particles that grow in three dimensions in a 2-dimensional diffusion field (e.g. grain-boundary precipitates, particles on a substrate). Coarsening by fault migration, which is an important issue in the microstructural stability of directionally solidified rod and lamellar eutectics, and discontinuous coarsening, which affects the stability of cellular microstructures produced by discontinuous precipitation, eutectoid decomposition and eutectic solidification, are also discussed. The principal predictions of theories of these various types of coarsening mechanisms are described, and exemplified where possible by reference to published data. Emphasis is placed on ability of current theory to explain existing data obtained from real as well as computer simulation experiments. Possible reasons for the shortcomings of theory are discussed.     

Cyclic martensitic transformations and damage evolution in shape memory zirconia: Single crystals vs polycrystals

In zirconia-based shape memory ceramics (SMCs), cracking during the martensitic transformation can be avoided in structures that reduce the relative presence of grain boundaries where high levels of transformation mismatch stress develop. This approach has been well established in single crystals, but only for sample sizes below about 5 microns. In this paper, we extend the strategy of eliminating grain boundaries to bulk specimen scales by fabricating mm-scale single crystal SMCs via cold crucible induction melting. For 1.5 and 2.0 mol% Y₂O₃-ZrO₂, we study the cyclic martensitic transformation of both single crystal and polycrystal structures. Whereas single crystals have very repeatable transformation behavior in terms of hysteresis and strain amplitude, polycrystals degrade dramatically as they accumulate cracking damage with repeated cycling. As the polycrystal evolves from a pellet to granular packing of loose single crystals/grains, the energy dissipation converges with that of the single-crystal structure, and the energy spent on cracking throughout that process is captured by calorimetry analysis. These results verify that grain boundaries play a key role in damage evolution during martensitic transformation and that microstructural control can extend the size-scale of viable single crystal or oligocrystal SMCs from the micro- to the millimeter scale.     

Intergranular Fracture of Lithium Fluoride–22% Calcium Fluoride Hypereutectic Salt at 800 K

Constant-velocity compression tests were conducted at 800 K on as-cast LiF-22 mol% CaF₂ hypereutectic salt with engineering strain rates varying between 1.8 × 10⁻⁶ and 2.3 × 10⁻¹ s⁻¹. Considerable stain hardening was observed during the initial stages of deformation, and the true stress-strain curves exhibited maxima. Plots of the true strain rate against the flow stress at the proportional limit and the peak stress exhibited a power-law relation with stress exponents of 7.7. Microstructural examination of the deformed specimens showed extensive grain-boundary cracking and cavitation. These results suggest that grain-boundary cracking and interfacial sliding is important for cavity nucleation at the grain boundaries and at the LiF-CaF₂ interfaces, and cavity growth and interlinkage, which appear to depend on the morphological differences between different grain boundaries, occur through the preferential failure of the weaker LiF phase.     

Discontinuous Dissolution of Iron Aluminate Spinel in the Al2O3–Fe2O3 System

When sintered 85Al₂O₃–15Fe₂O₃ (in wt%) specimens consisting of corundum grains and spinel particles were annealed at temperature where only a corundum phase was stable, phase transformation of spinel into metastable FeAIO₃ and subsequently complete dissolution of the metastable phase occurred together with the migration of grain boundaries at the surface of the specimens. Since the grain boundary migration was induced by grain boundary diffusion of Fe₂O₃ from the transforming and dissolving particles, the boundary migration by temperature decrease corresponds to a discontinuous dissolution of the spinel particles and a chemically induced grain boundary migration by temperature change. Inside the specimens, however, the transformation—dissolution and the grain boundary migration were suppressed because of unavailable accommodation of the volume expansion due to the transformation.     

Preparation of Highly Dense PZN–PZT Thick Films by the Aerosol Deposition Method Using Excess-PbO Powder

Lead zinc niobate–lead zirconate titanate thick films with a thickness of 50–100 μm were deposited on silicon and alumina substrates using the aerosol deposition method. The effects of excess lead oxide (PbO) on stress relaxation during postannealing were studied. Excess PbO content was varied from 0 to 5 mol%. The as-deposited film had a fairly dense microstructure with nanosized grains. The films deposited on silicon were annealed at temperatures of 700°C, and the films deposited on sapphire were annealed at 900°C in an electrical furnace. The annealed film was detached and cracks were generated due to the high residual compressive stress and thermal stress induced by thermal expansion coefficient mismatch. However, the film deposited using powder containing 2% of excess PbO showed no cracking or detachment from the substrate after the postannealing process. The PbO evaporation at elevated temperature during the postannealing process seemed to have reduced the residual compressive stress. The remanent polarization and relative dielectric constant of the 50 μm thick films annealed at 900°C were 43.1 μC/cm² and 1400, respectively, which were comparable with the values of a bulk specimen prepared by a powder sintering process.