Creep of Duplex Alumina–Titanium Nitride Polycrystalline Microstructures

Compression creep results in an inert atmosphere are presented for fine-grained Al₂O₃–TiN composites deformed under a stress range of 10–50 MPa stress and a temperature range of 1250°–1400°C. The effects of TiN content (10–70 vol%) and the preparation route (i.e., reactive versus nonreactive hot pressing) are considered. Al₂O₃ and TiN both are efficient grain-growth inhibitors, relative to each other; thus, deformation develops in stable equiaxed microstructures. Except for low TiN contents (10 and 20 vol%), the processing route does not markedly influence the observed strain rates, at least up to a strain of −0.16. In all cases, deformation occurs mainly via interfacial/grain-boundary sliding, and its rate is enhanced in approximate proportion to the amount of Al₂O₃/TiN interfacial area. The deformation rate seems to be limited by either some interface reaction or a complex boundary diffusional mechanism that occurs at the Al₂O₃ grain boundaries.     

High-Temperature Fracture Toughness of Duplex Microstructures

The temperature dependence of the fracture toughness of ceramics exhibiting duplex microstructures was studied relative to their single-phase constituents using two test methods: bend testing of chevron-notched beams, and the indentation-crack-length technique. The two materials systems studied were Al₂O₃: c -ZrO₂(Y) and A1₂O₃:Y₃A1_SO₁₂ (YAG), and the testing temperature ranged from room temperature to 1200°C. The study showed that in both systems the duplex materials showed higher toughness values than their single-phase constituents above 800°C. This result was attributed to the contribution of low-energy interphase boundaries to the overall composite toughness. Indentation crack length measurements gave toughness values and trends comparable to those determined by the chevronnotched beam method. By comparing the results of the two test methods it was possible to demonstrate that the indentation calibration constant (ξ) shows no significant temperature or material dependence. For the zirconia-containing materials, however, indentation at elevated temperatures is accompanied by significant localized plasticity, which suppressed the radial cracking. Under such conditions, some caution is warranted, since localized plasticity can lead to an overestimation of the fracture toughness.     

Base-Metal-Electroded BaTiO3 Capacitor Materials with Duplex Microstructures

The effect of dopants and processing conditions on the dielectric properties of base-metal-electroded materials was investigated. BaTiO₃ materials simultaneously doped with MgO and Y₂O₃ additives can achieve small capacitance variation (Δ C / C ), which meets the X7R specification, when the proportion of additives is abundant enough and the materials are not over-fired. Presumably, small Δ C / C values of thus obtained materials are the result of the formation of core–shell structure, which requires stringent control of material processing conditions. In contrast, X7R-type materials can be obtained in a much wider processing window, when prepared by mixing two BaTiO₃ materials of suitable dielectric constant–temperature ( K – T ) characteristics. Duplexed materials prepared from these two end-point BaTiO₃ materials with ratios ranging from 3:1 to 1:2 exhibit K – T behavior within the X7R specification, provided that one of the components possesses flat K – T behavior. Moreover, the dielectric properties of these materials were simulated using a simplified microstructural model. Simulation results indicate that the effective dielectric constant of core–shell materials, ( K _e)_CS, varies significantly not only with the dielectric properties of cores and shells, but also with the shell-to-core thickness ratio, whereas the effective dielectric constant of duplexed materials, ( K _e)_D, can be maintained at a very small Δ C / C value for a wide range of end-point constituent ratios, which agrees very well with the measured K – T properties for the materials.     

刚玉莫来石复相陶瓷热震及蠕变性能的影响因素分析

采用正交设计方法研究了硅微粉、氧化铝微粉及烧成温度对刚玉莫来石复相陶瓷热震稳定性及蠕变性能的影响机制。结果表明:烧结温度对复相陶瓷热震稳定性及蠕变性的影响最大,氧化铝微粉次之,硅微粉最小。高温抗折强度损失率在烧成温度为1650℃时最低,随氧化铝的含量增加而减小,并随硅微粉含量增加而增大。蠕变率随烧成温度的提高而减少,氧化铝质量分数为7%时最低,并随硅微粉含量增加而增大。通过调节硅微粉、氧化铝微粉及烧结温度,可控制颗粒与基质、莫来石的结合状态,气孔及残留α-Al2O3,从而改善材料的抗热震性及蠕变性。     

Computation of Effective Steady‐State Creep of Porous Ni–YSZ Composites with Reconstructed Microstructures

This paper investigates the effective steady‐state creep response of porous Ni–YSZ composites used in solid oxide fuel cell applications by numerical homogenization based on three‐dimensional microstructural reconstructions and steady‐state creep properties of the constituent phases. The Ni phase is found to carry insignificant stress in the composite and has a negligible role in the effective creep behavior. Thus, when determining effective creep, porous Ni–YSZ composites can be regarded as porous YSZ in which the Ni phase is counted as additional porosity. The stress exponents of porous YSZ are the same as that of dense YSZ, but the effective creep rate increases by a factor of 8–10 due to porosity. The relationship of creep rate and volume fraction of YSZ computed by numerical homogenization is underestimated by most existing analytical models. The Ramakrishnan–Arunchalam creep model provides the closest approximation among all analytical models.     

Creep and Creep Fracture in Hot-Pressed Alumina

The creep and creep fracture behavior of two hot-pressed aluminas are presented, for both flexural and tensile testing. Steady-state power-law creep is observed with a stress exponent of about 2 for each material. Three distinct fracture regimes are found. At high stress in flexure, fracture occurs by slow crack growth with a high stress dependence of the failure time. At intermediate stresses, in both flexure and tension, creep fracture occurs by multiple microcracking after modest strains. Failure times exhibit a modest stress dependence (stress exponent of 2.5 in tension and 3 in flexure), with a constant failure strain equal to 0.09. The failure times are considerably longer in flexure than in tension, because of the constraint imposed on crack growth by the bending geometry. We conclude that flexure cannot be used for creep lifetime assessment, even in simple, single-phase materials such as Al₂O₃. At low stresses, in tension, failure also exhibits a modest stress dependence but with a much higher failure strain. The material shows the onset of super-plastic behavior.     

Creep and Creep Rupture of an Advanced Silicon Nitride Ceramic

Creep and creep rupture behavior of an advanced silicon nitride ceramic were systematically characterized in the temperature range 1150° to 1300°C using uniaxial tensile creep tests. Absence of tertiary creep and the order-of-magnitude breaks in both creep rate and rupture lifetime at certain threshold combinations of stress and temperature were two characteristic features of the creep behavior observed. Thermal annealing was found to have enhanced both subsequent creep resistance and creep rupture life. The stress exponent (n) and the activation energy (Q) defined in the Norton relation were found to be 12.6 and 1645 kJ/mol for the material investigated. Both values appear to fall in the general range of those reported for other but similar types of Si₃N₄ ceramic materials. The stress exponent, m , equivalent to the slope of the Larson–Miller equation was found to be in the range 13 to 14.4, and that defined as p in the Monkman–Grant relation to be 0.91, based on the available experimental data. The values of m , n , and p obtained above approximately support the interrelationship of the three exponents given by p = m/n.     

ANFO炸药的微观结构

为了更好地认识铵油(ANFO)炸药的微观结构,采用扫描电镜(SEM)和密度测试方法,对4种不同状态硝酸铵(AN)和燃料油组成的ANFO炸药的微观结构进行表征,采用切片技术观察ANFO炸药颗粒的内部微观结构.结果表明,未膨化ANFO炸药颗粒内部及外部的孔洞和裂纹很少,颗粒表面比膨化ANFO炸药表面光滑;膨化ANFO炸药颗粒表面存在大量孔洞、裂纹和突起,表面凹凸不平,颗粒内部存在大量毛孔和孪晶;EANFO-34样品在颗粒孔洞和不规则方面具有一些鲜明特征,颗粒内部也存在大量孔洞.研究也表明,膨化AN颗粒的晶体生长并未完成.     

ANFO炸药的微观结构

为了更好地认识铵油（ANFO）炸药的微观结构，采用扫描电镜（SEM）和密度测试方法，对4种不同状态硝酸铵（AN）和燃料油组成的ANFO炸药的微观结构进行表征，采用切片技术观察ANFO炸药颗粒的内部微观结构。结果表明，未膨化ANFO炸药颗粒内部及外部的孔洞和裂纹很少，颗粒表面比膨化ANFO炸药表面光滑；膨化ANFO炸药颗粒表面存在大量孔洞、裂纹和突起，表面凹凸不平，颗粒内部存在大量毛孔和孪晶；EANFO-34样品在颗粒孔洞和不规则方面具有一些鲜明特征，颗粒内部也存在大量孔洞。研究也表明，膨化AN颗粒的晶体生长并未完成。     

Creep in polyamidobenzimidazole

Conclusions -- In rigid-chain polymers, just as in flexible-chain polymers, deformation develops with time because of conformational changes in the macromolecules.-- The difference from flexible-chain polymers consists in the fact that the conformational transitions take place not only in the intercrystallite amorphous regions (which dominates for flexible-chain polymers), but also in the crystallite lattice, wherein the latter makes a considerable contribution to the creep of the specimen.Translated from Khimicheskie Volokna, No. 5, pp. 37–38, September–October, 1991.     

Creep of aluminosilicate refractories

Conclusions Investigations were made of the creep of industrial aluminosilicate refractories of various types, differing as regards the original materials, phase composition, the properties of the glass phase, and the structure. The results of the investigations characterize the plastic properties of materials in a fairly broad range of temperatures and stresses.Translated from Ogneupory, No. 7, pp. 39–43, July, 1969.     

Diffusional Creep Mechanisms

Creep extension without dislocation movement across grains may be obtained in polycrystalline aggregates by diffusion through the grains (Nabarro-Herring) or through grain boundaries (Coble). These mechanisms are compared with a suggestion that creep can occur by grain-boundary sliding controlled by diffusion which can lead to movement along the boundary of grain-boundary protrusions and can be accommodated by diffusion around triple points.     

Microstructures of isotropic pyrolytic carbons

Microstructures of isotropie pyrolytic carbons have been examined by transmission electron microscopy, scanning electron microscopy and optical microscopy. Crystallite arrangements within the 0·5 μm dia growth features which make up these carbons differ greatly between high- and low-density carbons. Preferred orientations of crystallites are high in relatively large regions of the growth features of the high-density carbons while in the low-density carbons, the crystallites are in a tangled arrangement much as occurs in glassy carbons. The majority of the density defect in the low-density carbons is in the microporosity associated with this tangled structure and not in the large inter-growth-feature pores. Observations are made which suggest that the growth features are initially formed as free-floating particles before deposition on the substrate.     

Microstructures of Simulated Nuclear Waste

The microstructures of simulated tailored ceramic forms for the Idaho Chemical Processing Plant (ICPP) high-level waste were characterized by TEM. The ceramic forms were consolidated from simulated ICPP waste calcines with either silicayttria-based or silica–lithia-based additives by hot isostatic pressing. The hot isostatically pressed ceramic waste forms are composed of cubic CaF₂, monoclinic ZrO₂, stabilized cubic ZrO₂, tetragonal ZrSiO₄, and amorphous silicate phases which can be phase separated. The phaseseparated glass is a result of phase immiscibility in the soda aluminosilicate glass to the compositions of albite and mullite.     

Coarsening-Resistant Dual-Phase Interprenetrating Microstructures

Grain growth in a 50:50 (vol%) dual-phase mixture of Al₂O₃ and c -ZrO₂ (cubic zirconia) is severely limited compared with that for either of the single phases. At 1650°C, the growth rates in the duplex composition are 160 and 3500 times lower than that for single-phase Al₂O₃ and c -ZrO₂, respectively. The restriction of the grain growth is attributed to the limited mutual solubility and the physical constraint provided by the interpenetrating geometry of the two phases. Grain coordination number and dihedral angle are also considered as factor affecting grain stabilization in two-phase systems. A potentially important practical application of this work is the fabrication of grain-stabilized fibers for use as reinforcement in composite structures at elevated temperatures.     

Fired Porcelain Microstructures Revisited

The microstructure of standard commercial porcelain consists of alpha-quartz grains held in a complex matrix. Quartz grains are surrounded by amorphous silica-rich solution rims while the matrix contains clay relicts of small (~200 nm × 40 nm × 40 nm) primary (2Al₂O₃:1SiO₂) mullite crystals in aluminosilicate glass and feldspar relicts of acicular (>1 µm long) secondary (3Al₂O₃:2SiO₂) mullite in potassium aluminosilicate glass. A continuous increase in mullite crystal size from the clay-feldspar relict interface to the feldspar relict center and their compositions indicate a transformation from primary to secondary mullite. In aluminous porcelain, corundum grains are observed in addition to alpha-quartz and regions of clay and feldspar relicts. Small (~50 nm × 10 nm × 10 nm) tertiary mullite crystals (formed by precipitation from Al₂O₃-rich glass) were detected adjacent to these corundum grains.