摘要:

Constant compressive stress creep experiments in the temperature and stress ranges of 1770 to 2020 K and 35 to 373 MPa have been performed on polycrystalline alpha SiC. Specimens having average grain sizes of 3.5, 4.9, and 7.5 μm have been studied in order to determine the grain size dependence of steady‐state creep in this material and to distinguish the relative contributions of diffusion‐accommodated grain boundary sliding and dislocation glide. Analysis of steady‐state creep rate data revealed different kinetics of creep below and above 1920 K as follows: activation energies of 387–541 and 838–877 kJ/mol, stress exponents of 1.0–1.4 and 1.4–2.5, and inverse grain size exponents of 2.5 and 4. Themvalue of 2.5 indicated Coble‐controlled creep with a contribution of Nabarro–Herring creep in the low‐temperature range. Themvalue was less reliable for the high‐temperature range because of the large differences in the microstructures of the three materials. The large number of high aspect ratio grains with increasing average grain size resulted in different contributions of diffusion‐accommodated grain boundary sliding and dislocation glide

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07198.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

For the first time direct evidence for Mg segregation to the surface of pores within translucent polycrystalline alumina grains has been found using energy dispersive X‐ray spectroscopy (EDXS) and convergent beam electron diffraction (CBED) in an analytical electron microscope (AEM) on a submicrometer scale. This supports the model that MgO dopant increases the surface diffusivity which, in turn, increases the pore mobility. The MgO dopant's role in retarding grain growth in combination with the enhanced pore mobility allows the achievement of nearly full density and translucency in alumin

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07199.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Porcelains for tableware in the quartz–feldspar (sericite)–kaolin system were investigated to study the effect of firing temperature on the bending strength. The maximum bending strength of a body was attained at about 75°C below the temperature of complete sintering; therefore, its apparent porosity and the total porosity were almost 0% and 8.0%, respectively. Control of the finely dispersed, round, and isolated pores in the body was found to be necessary to attain strengthening of the body despite its relatively large porosity, and at the same time it was important that a large amount of fine quartz particles remain undissolved. Furthermore, a dense layer, formed close to the surface, was observed to strengthen the body more than that expected from its relative density when the body was fired above 12

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07200.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Using a Fourier transform approach, the micromechanical stress distributions in and around inclusions in ceramic composites were calculated and were found to be in excellent agreement with the Eshelby approach when applied to an ellipsoidal geometry. Both inhomogeneous modulus and crystalline anisotropy were incorporated into the technique. The calculations were then extended to a more realistic inclusion shape, namely, the octahedron, and the effects of faceted geometries and stress concentration sites were delineated. Calculations show that the approximation of ellipsoidal inclusion shape for a faceted inclusion in ceramic composites yields the general features but can be misleading in predicting the micromechanical properties. Our model is applied to predict nucleation of cracks at faces, edges, and corners of octahedrally shaped SiC inclusions in Al2O3and nucleation sites of the room‐temperature phase transformation of octahedrally shaped particles of zirconia (from tetragonal to monoclinic) in an alumina matrix, where the martensitic nucleation is governed by strain field

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07201.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

After showing that india (In2O3) resisted high‐temperature reaction with SO3/Na2SO4and vanadate melts, we prepared india‐stabilized zirconia (ISZ) by a proprietary sol–gel process, and tested the material for corrosion resistance to 700–900°C molten vanadates. ISZ was superior to yttria‐stabilized zirconia (YSZ) in vanadate resistance at 700°C, and essentially equivalent at 900°C. Certain differences were observed between the vanadate‐induced corrosion/destabilization of ISZ a

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07202.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Mechanisms of creep deformation have been investigated for a commercial grade of siliconized carbide containing ≅33%/silicon. Microstructural studies of both tensile and compressive test specimens indicate dislocation damage generation in both the silicon carbide and the silicon phases as a consequence of creep. In the silicon carbide, dislocation damage was normally restricted to contact sites between the silicon carbide grains resulting from high intergranular contact stresses during deformation. Dislocation damage was also observed in the silicon. Although dislocation damage was heavy in some regions of the specimens, most regions of the specimens, most regions were free of dislocations. This result is consistent with the hypothesis that deformations occurs by the motion of clusters of grains during deformation. In tension, creep at high strain rates,1 × 10−8S−1, was accompanied by the formation of cavities at Si/SiC interfaces within the intergranular silicon phase. As cavities were not associated with dislocations, their growth was probably controlled by diffusional processes. Based on observations of the microstructure, a model of deformation is proposed to explain the fact that siliconized silicon carbide creeps faster in tension than in compression, at the same applied stress. The model is based on soil mechanics concepts. It is suggested that creep is controlled by intergranular friction between aggregate particles of the com

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07203.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

A novel polymer precursor method for the preparation of silicates including enstatite‐free forsterite is being investigated. Precursors to crystalline materials were prepared via the copolymerization of the K and/or Mg salts of methacrylic acid with various combinations of a siloxane‐methacrylate and a methoxysilyl‐methacrylate Thermogravimetric/differential thermal, pyrolysis mass spectrometry, and high‐temperature X‐ray diffraction analyses were used to monitor the conversion of the precursors to inorganic oxide during air pyrolysis. Some precursors to crystalline oxides showed yields as high as 77% of th

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07204.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Structural ceramics are susceptible to microcrack damage by thermal shock. There is a critical temperature for thermal shock damage initiation with damage severity increasing at greater shock temperatures. In this work the applicability of an ultrasonic method to determine the critical temperature and the accumulated damage is demonstrated in alumina. Information is obtained via velocity and attenuation measurements using surface and obliquely incident bulk ultrasonic waves. The elastic anisotropy effect due to preferred crack orientation has been estimated. The critical temperature for the alumina is about 200°C. The damage increases steeply from 200° to 400°C and grows significantly above 400°C. Changes of up to 17% from the original values in the effective shear moduli and up to 45% in the longitudinal effective modulus in the direction transverse to crack orientation are measured at high thermal shock temperatu

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07205.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Two dense ceramics, Bi2Sr2CaCu2O8+Xand 2Y‐TZP: 10 wt% Al2O3, were hot‐extruded through conical dies under a wide range of conditions. Extrusion of Bi2Sr2CaCu2O8+Xwas performed at 775° to 825°C and speeds of 0.0005 to 0.2 mm/s, with the die semi‐angles from 30° to 60° and extrusion ratios from 4 to 9. Extrusion of 2Y‐TZP:10 wt% Al2O3was performed at 1650°C and 0.017 mm/s. During hot extrusion of Bi2Sr2CaCu2O8+X, a strong texture was developed with the basal plane aligned along the extrusion direction, whereas the zirconia/alumina composite underwent grain growth which resulted in strain hardening. A simple analytical extrusion model for strain‐rate‐sensitive materials has been developed using a combined slab method and upper‐bound method and a power‐law material flow equation. The model predictions are in good agreement with the experimental results and they further serve to delineate the limitation of the extrusion technique for

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07206.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

We measured the Raman spectrum associated with the E2‐TOquasiand LOquasimodes of 6H‐SiC particles as a function of hydrostatic pressure using a diamond anvil cell. The results of this calibration experiment were used to analyze the residual stress in 6H‐SiC particles within Al2O3/SiC composites with 12%, 20%, and 30% SiC by volume. The Raman spectra show that residual stress in the SiC near the surface of the composites is −2040 ± 120, −1841 ± 110, and −1615 ± 100 MPa for the 12%, 20%, and 30% SiC composites, respectively. The measured decrease in stress with increasing packing fraction is consistent with theoretical predictions based on

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07207.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY