Abnormal Grain Growth in Alumina: Synergistic Effects of Yttria and Silica

Abnormal grain growth without strong anisotropy or faceting of the grains has been observed in high-purity yttria-doped alumina specimens, often starting at the surface and spreading right through the bulk at higher sintering temperatures. This appears to occur because of an interaction between Si contamination from sintering and the yttria doping; no such effect is seen for undoped samples. Similar microstructures were observed after deliberate Y/Si codoping. Analytical STEM showed that some grain boundaries bordering on large grains contained more Si than Y. HRTEM and diffuse dark-field imaging revealed thin (0.5–0.9 nm) disordered layers at some boundaries bordering large grains. It appears that Si impurities are accumulating at some boundaries and together with the Y inducing a grain boundary structural transformation that accounts for the dramatically increased mobility of these boundaries.     

Effect of Yttrium Aluminum Garnet Additions on Alumina-Fiber-Reinforced Porous-Alumina-Matrix Composites

The effects of incorporating yttrium aluminum garnet (YAG) into a porous alumina matrix reinforced with Nextel 610 alumina fibers were investigated. Composites with various amounts of YAG added to the matrix were prepared to determine its effect on retained tensile strengths after heating to 1100° and 1200°C. Strengths of YAG-containing composites were slightly lower than those of an all-alumina-matrix composite after heating for 5 h to 1100°C. However, after heating for 5 or 100 h at 1200°C, all the YAG-containing composites displayed greater strengths and greater strains to failure than the all-alumina composite. At the higher temperature, the presence of YAG is believed to inhibit the densification of the matrix, which helps to maintain higher levels of porosity and weaker interparticle bonding that allows for crack-energy dissipation within the matrix. A reduction in grain growth of the fibers by the presence of segregated Y was also observed, which may also contribute to higher fiber strength, thereby increasing the retained strengths of the YAG-containing composites.     

Influence of Moisture on Ultra-High-Temperature Tensile Creep Behavior of in Situ Single-Crystal Oxide Ceramic Alumina/Yttrium Aluminum Garnet Eutectic Composite

Tensile creep tests were conducted for an in situ single-crystal alumina/yttrium aluminum garnet (Al₂O₃/Y₃Al₅O₁₂ (YAG)) binary system eutectic composite at temperatures between 1773 and 1873 K in air and in a moist environment having a water-vapor pressure range of 0.06–0.6 MPa, under a constant tensile stress range of 100–160 MPa. The Al₂O₃/YAG eutectic composite exhibited a stress exponent of 8–13, indicative of tensile creep behavior characterized by a dislocation back-stress mechanism. Water-vapor pressures ≤0.4 MPa led to a significant acceleration of creep rates as a result of enhanced dislocation mobility in the Al₂O₃ and YAG phases.     

Grain Boundary Grooving Studies of Yttrium Aluminum Garnet (YAG) Bicrystals

Grain boundary grooving experiments were conducted with Σ5 (210) twist boundaries in Y₃Al₅O₁₂ (YAG) with the goal of extracting information on diffusion in YAG. Planar boundaries oriented 90° to the surface were annealed in air at various times and temperatures. Atomic force microscopy was used to characterize the subsequent grooves. The Mullins approach leads to the following expression for the diffusion coefficient: D (m²/s) = 3.9 × 10⁻¹⁰ exp[−330 ± 75 (kJ/mol)/ RT ]. The relatively low activation energy agrees well with earlier oxygen tracer diffusion measurements on YAG, suggesting that oxygen is the limiting diffusing species in boundary grooving of YAG.     

Processing, Microstructure, and Strength of Alumina–YAG Eutectic Polycrystals

Dense polycrystalline eutectics of alumina and yttrium aluminum garnet (YAG) were fabricated by hot-pressing powders of pulverized arc-melted buttons at homologous temperatures of 0.9 T _eu–0.93 T _eu (where T _eu is the eutectic temperature). The eutectic microstructure of the arc-melted buttons was retained after densification, although the grain boundaries were decorated with equiaxed grains of alumina and YAG ∼1–5 μm in size; possible causes for their formation have been discussed. A comparison of the measured strength of the polycrystalline eutectics (274 ± 61 MPa) with grain size and fracture toughness suggests that the strength-limiting flaws are significantly smaller than the mean grain size and larger than the mean eutectic spacing.     

热处理温度对钇铝石榴石(Y3Al5O12)析晶的影响

透明YAG多晶陶瓷具有优良的光学、机械与化学性能,逐渐成为新一代固体激光基质材料.分散均匀、团聚轻、超细颗粒的纳米前驱体粉末有利于制备出高度透明的激光陶瓷.以Y2O3,Al(NO3)3·9H2O和柠檬酸为原料,采用柠檬酸-凝胶法和低温自蔓延燃烧反应相结合制备出黑色粉末,经1100℃烧结出YAG纳米前驱体粉末.采用TG-DTA,XRD,FT-IR和TEM测试手段对YAG前驱体粉末进行表征,采用谢莱公式计算出不同烧结温度下晶粒尺寸.研究结果表明:随热处理温度升高,晶粒平均尺寸增加,标准偏差以小幅度增加,晶粒尺寸分布曲线保持一致,晶格参数减小.随热处理时间增加,晶粒主要以晶界扩散形式线性长大.纳米晶粒表面原子呈不规则分布,导致晶格参数增加,前驱体颗粒由单个晶粒所构成.     

Toughening Behavior of Silicon Carbide with Additions of Yttria and Alumina

The fracture-toughness-determining mechanism of silicon carbide with additions of yttria and alumina was studied. Observations of indentation crack profiles revealed that significant crack deflection had occurred. Median deflection angles increased with increased volume fractions of the second phases, which was accompanied by increased fracture toughness.     

Phase Transformation and Densification of an Attrition-Milled Amorphous Yttria-Partially-Stabilized Zirconia–20 mol% Alumina Powder

Phase transformations during consolidation treatments of an attrition-milled amorphous yttria-partially-stabilized zirconia (Y-PSZ: ZrO₂–3 mol% Y₂O₃)–20 mol% Al₂O₃ powder and the resulting microstructures have been investigated. A metastable cubic phase ( c -ZrO₂ solid solution) together with an α-Al₂O₃ phase is formed in the amorphous matrix by consolidation at temperatures below 1204 K. The metastable cubic phase transforms to a stable tetragonal phase ( t -ZrO₂ solid solution) with an increase in the consolidation temperature. Fully dense bulk samples consisting of extremely fine tetragonal grains together with a small amount of α-Al₂O₃ particles could be obtained by consolidation at temperatures above 1432 K. Important features concerned with the densification behavior are as follows: (1) Marked increase in the relative density occurs after cubic crystallization and subsequent cubic-to-tetragonal transformation. (2) All of the consolidated bulk samples show extremely fine grain structure with grain sizes of several tens of nanometers, irrespective of the consolidation temperature. (3) The regularity of the lattice fringe contrast in each tetragonal grain seems to be kept in the vicinity of grain boundaries. These results suggest that densification of the attrition-milled amorphous powder proceeds via superplastic flow and/or diffusional creep, rather than viscous flow of the initial amorphous phase before crystallization.     

钇铝石榴石纤维的制备和应用研究进展

钇铝石榴石纤维具有耐高温、抗氧化、低导热率、优异的抗高温蠕变性和良好的光学性能,是一种理想的结构增强材料、绝热耐火材料和光学材料.本文重点评述了近年来钇铝石榴石纤维制备和应用的研究进展,并展望了钇铝石榴石纤维制备和应用的发展趋势.     

Evolution of the Grain Size Distribution during the Sintering of Alumina at 1350°C

The objectives of this study are to provide some rare unfolded grain size distribution data for the sintering of alumina and to test for time invariance of the normalized grain size distribution as required by normal grain growth. The results show that ln ς doubled during the sintering times studied. The changes in the grain size distribution may be due to an increase in the number of relatively large grains combined with a reduction in the number of grain annihilation events compared with that required for time invariance of the normalized grain size distribution.     

Abnormal Grain Growth of Alumina: CaO Effect

The critical concentration of Ca required for the onset of abnormal grain growth in alumina was determined by controlled doping of Ca in ultrapure alumina (>99.999%), by sintering under clean contamination-free conditions, and by microstructural characterization. As in the case of Si, the excess concentration of Ca beyond its solubility limit was inversely related to the average grain size at the moment of first appearance of abnormal grains, which corresponds to the moment of sufficient enrichment of Ca in grain boundaries to form stable intergranular liquid films. However, the critical concentration of Ca was found to be in the range of only a few tens of ppm, which is lower than that of Si by almost 2 orders of magnitude. The equivalent silica concentration to form such a stable intergranular calcium aluminate glass film and its minimum thickness were estimated from the inverse relationship with the assumption that the glass composition is close to calcium hexaluminate.     

Bulk, Dense, Nanocrystalline Yttrium Aluminum Garnet by Consolidation of Amorphous Powders at Low Temperatures and High Pressures

Amorphous Al₂O₃–37.5% Y₂O₃ powders, prepared using spray pyrolysis followed by partial or complete thermal decomposition, were hot-pressed at 315°–640°C and 500 or 750 MPa uniaxial pressure. Hot pressing of fully decomposed amorphous powder at 450°–640°C at pressures up to 750 MPa led to densification (up to 96%) as well as nanocrystallization of yttrium aluminum garnet (YAG). When the pressure was applied during heating, instead of after reaching the final temperature, higher relative densities resulted. Fully crystalline starting powder did not densify. The low true density of the amorphous phase (3.1 g·cm⁻³) was believed to be responsible for the densification through enhanced ionic mobilities.     

Effect of Liquid Content on the Abnormal Grain Growth of Alumina

Alumina specimens with small amounts of CaO and TiO₂ were prepared and their microstructural evolution during sintering was investigated. Because of the appearance of a liquid phase during sintering, a duplex microstructure of a few abnormal grains and fine matrix grains was obtained when the CaO + TiO₂ content was small (≤0.04 wt%). When the CaO + TiO₂ content was relatively high (≥0.1 wt%), many grains grew and impinged upon each other. As a result, a rather uniform and homogeneous microstructure was observed.     

Effect of Yttrium and Erbium Ions on Epitaxial Phase Transformations in Alumina

The effect of low concentrations of Y, Er, and Cr solutes on the amorphous-to-γ transformation and on the γ-to-α transformation in aluminum oxide has been studied in situ by time-resolved reflectivity. The activation energies of the two transformations with these dopants are the same as in undoped alumina, being 4.1 ± 0.1 and 5.2 ± eV, respectively. Although not affecting the activation energies, Y, Er, and Cr do affect the transformation kinetics. Y and Cr ions decrease the γ-to-α transformation velocity and, over the limited range studied, do so in proportion to their concentration. Concentrations of Er as low as ∼6 ppm retard the γ-to-α transformation and concentrations of 32 ppm essentially stop the transformation occurring within the times and temperatures accessible within the present experiment, thereby preventing quantification of the effect of Er on the α-phase transformation. Erbium also retards the amorphous-to-γ transformation relative to undoped alumina whereas yttrium and chromium accelerate it.     

Deformation and Grain Growth of Low-Temperature-Sintered High-Purity Alumina

Through close control over green-state powder processing, pure alumina ceramics of 0.5-μm grain size were obtained by sintering at 1250°C. The static grain growth of this material was modest at temperatures below 1300°C. However, dynamic grain growth occurred rapidly during superplastic deformation. Therefore, although the ultrafine-grained alumina exhibited rather low initial flow stress at relatively low deformation temperatures, dynamic grain-growth-induced strain hardening gave rise to high flow stress causing cavitation and cracking. As a result, superplastic deformation could not be achieved for the ultrafine-grained pure alumina.     

Abnormal Grain Growth in Alumina with Anorthite Liquid and the Effect of MgO Addition

Abnormal grain growth (AGG) in alumina with anorthite liquid has been observed with varying anorthite and MgO contents, at 1620°C. When only anorthite is added to form a liquid matrix, the grain–liquid interfaces have either flat or hill-and-valley shapes indicating atomically flat (singular) structures. The large grains grow at accelerated rates to produce AGG structures with large grains elongated along their basal planes. This is consistent with the slow growth at low driving forces and accelerated growth above a critical driving force predicted by the two-dimensional nucleation theory of surface steps. With increasing temperature, the AGG rate increases. The number density of the abnormally large grains increases with increasing anorthite content. The addition of MgO causes some grain–liquid interfaces to become curved and hence atomically rough. The grains also become nearly equiaxed. With increasing MgO content the number density of the abnormally large grains increases until the grain growth resembles normal growth. This result is qualitatively consistent with the decreasing surface step free energy associated with partial interface roughening transition.     

Effect of Alumina Addition on the Tetragonal-to-Monoclinic Phase Transformation in Zirconia–3 mol% Yttria

The tetragonal-to-monoclinic martensitic phase transformation in ZrO–3 mol% Y₂O₃ (PSZ) containing 0 to 12 wt% Al₂O₃ was investigated by dilatometry, XRD, and SEM-EDS methods. The propagation of the transformation into the specimen interiors was suppressed by the addition of Al₂O₃. The grain size was independent of the addition of Al₂O₃. Both Y₂O₃ and Al₂O₃ segregated at grain boundaries. From this segregation behavior, it was suggested that a certain compound or phase of Y₂O₃–Al₂O₃ could be formed at grain boundaries, which would presumably prevent the propagation of the transformation into interiors of PSZ-containing Al₂O₃.     

Effect of Coarse-Powder Portion on Abnormal Grain Growth during Hot Pressing of Commercial-Purity Alumina Powder

By classification, two powder portions, one consisting of coarse particles and the other consisting of fine particles, were separated from a MgO-doped (1000 ppm) commercial-purity Al₂O₃ powder. Examinations of microstructure evolution during hot pressing showed that extensive abnormal grain growth occurred for the coarse portion. For the fine portion, although there was an indication that grain-size distribution deviated from normal distribution on prolonged hot pressing, such extensive abnormal grain growth did not occur. Extensive abnormal grain growth also occurred when the coarse portion was mixed into a high-purity powder that exhibited no abnormal grain growth alone. Chemical analyses revealed that the coarse portion contained the higher concentration of impurities but lower concentration of magnesium than the fine portion. It was discussed that particle aggregates in the coarse portion might have been responsible for the higher concentration of impurities but lower concentration of magnesium and, thus, for the extensive abnormal grain growth.     

Thermal Expansion Characteristics of Alumina–Magnesia and Alumina–Spinel Castables in the Temperature Range 800°–1650°C

The thermal expansion of Al₂O₃–MgO castables containing 5.5 wt% MgO and 1.36 wt% CaO and Al₂O₃–spinel castables containing 20 wt% spinel having 95 wt% Al₂O₃ and 1.7 wt% CaO was measured in the temperature range of 800–1650°C by dilatometry. A sharp increase in expansion from around 1425° to 1525°C, followed by a sharp decrease with further increasing temperature, is characteristic of Al₂O₃–MgO castables. The sharp increase in expansion is believed to be caused by the bond linkage between the CA₆ and spinel grains in the bonding matrix, while the sharp decrease is apparently related to liquid-phase sintering. The sharp increase and decrease in expansion were not observed in Al₂O₃–spinel castables because of the much lower MgO (around 1 wt% MgO) and impurity contents. The magnitude of thermal expansion of calcium aluminate bonded castables containing self-forming or preforming spinels or both is dictated by the MgO content of the castables.     

Effect of Sintering Atmosphere on the Densification and Electrical Properties of Alumina

The effect of sintering atmosphere on the final density and electrical properties of alumina compacts has been investigated using two different oxygen pressures: air and CO/CO₂. Measuring of electrical behavior has been considered a tool for determining the mechanism responsible for densification. Finally, the importance of a reducing atmoshphere on the electrical behavior of polycrystalline alumina is pointed out.