摘要:

Detailed microscopy of two crept aluminas, one with (AD99) and one without (Lucalox) a grain boundary glassy phase, has been performed to determine the pertinent damage mechanisms during creep. Evidence is presented for a nucleation‐controlled cavitation process where creep cavities nucleate primarily on two‐grain facets, followed by cavity growth and coalescence to form grain‐facet‐sized cavities and microcracks. A variety of creep cavity morphologies were observed in Lucalox, including spheroidal and irregularly shaped cavities. The latter finding implies a strong influence of crystallographic orientation and the corresponding surface energy of the cavitated planes on the cavity shape. In contrast, classical spheroidal cavities were observed in AD99 due to the presence of a viscous phase along grain boundaries. Direct evidence for grain boundary sliding as the process driving force for cavitation in Lucalox is presented together with evidence for the nucleation of creep cavities at grain boundary ledges. These findings are compared to the grain boundary sliding (GBS) and small‐angle neutron scattering (SANS) measurements performed previously on the same systems. Based on this study, the cavity nucleation process in the glassy‐phase‐ and non‐glassy‐phase‐containing aluminas is apparently similar as both involve the nucleation of rows of equally sized and equa

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb06631.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Thermal residual stresses due to the thermal expansion mismatch in a ceramic–interlayer–metal joint are calculated analytically. A qualitative estimate of the stress distribution is obtained by assuming the joint consists of one or two elastic slabs sandwiched between two semi‐infinite isotropic elastic solids. These elastic solids and the interlayer(s) are perfectly bonded to each other at the planar interfaces. The region where temperature change takes place is assumed to be a cylinder with its axis normal to the interface. A simple equation is obtained for the stress value at the center of the ceramic‐interlayer interface as a function of the thermal expansion coefficients, elastic constants of the constituent materials, and the thickness of the interlayer(s). The results obtained by this simple model agree well, qualitatively, with those obtained experimentally and numerically (finite element calculations). An appropriate interlayer material to reduce the residual stress in ceramic‐metal joints is

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb06632.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

The superplastic deformation behavior of fine‐grained, yttria‐stabilized tetragonal zirconia under conditions of biaxial gas‐pressure deformation is described. Sheet specimens were deformed into hemispherical caps at temperatures ranging from 1450° to 1600°C and at imposed gas pressures ranging from 345 to 2760 kPa. For the forming conditions examined, hemispherical caps were formed at times ranging from 2 to 476 min. A gas‐pressure‐forming apparatus used to conduct the experiments is described and a mechanical analysis of the deformation process is presented. Also discussed is the role of grain growth and the relationship between data obtained in uniaxial testing and the behavior observed during the biaxial deformation experiments of this study. A comparison is made between the observed thickness distribution in the hemispherical caps with predictions from an analy

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb06633.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Results of dynamic fatigue experiments in water at room temperature on an indented Al2O3/25 wt% SiC whisker composite material have shown that this composite has a high resistance to slow crack growth. Aging tests in water revealed that the residual stress due to the indentation does not play an important role, and interrupted fatigue tests showed that the crack starts to grow at very low stress intensities, but the velocity is very low. Detailed fractography indicated that the slow crack growth path is intergranular in the whisker composite. The slow crack growth behavior in the whisker composite is discussed in association with the indentation residual stress, the change of the crack shape during the bending test. These results are compared with a bio‐grade Al2O3with lower resistance to slow crack growth, and important differences are pointed ou

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb06634.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Amorphous silicate grain boundary phases of varying chemistry and amounts were added to 3Y‐TZP in order to determine their influence on the superplastic behavior between 1200° and 1300°C and on the room‐temperature mechanical properties. Strain rate enhancement at high temperatures was observed in 3Y‐TZP containing a glassy grain boundary phase, even with as little as 0.1 wt% glass. Strain rate enhancement was greatest in 3Y‐TZP with 5 wt% glass, but the room‐temperature hardness, elastic modulus, and fracture toughness were degraded. The addition of glassy grain boundary phases did not significantly affect the stress exponent of 3Y‐TZP, but did lower the activation energy for superplastic flow. Strain rate enhancement was highest in samples containing the grain boundary phase with the highest solubility for Y2O3and ZrO2, but the strain rate did not scale inversely with the viscosity of the silicate phases. Grain boundary sliding accommodated by diffusional creep controlled by an interface reaction is proposed as the mechanism for superplastic deformation in 3Y‐TZP with and without glassy grain

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb06635.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Anhydrous glasses in the seriesxLi2O + (1 ‐x)P2O5have been prepared and characterized in the range 0 ≤x≤ 0.5. FT‐IR spectroscopy and glass transition temperature measurements have been used to explore the structure and a key physical property of the low‐alkali phosphate glasses. The structure ofv‐P2O5is proposed to consist of a 3‐D network of trigonally connected tetrahedra decorated with a P=O unit. Contrary to what has long been proposed for these glasses, the addition of alkali degrades the 3‐D network through the generation of nonbridging oxygens rather than strengthen the network through the proposed alkali ion bridging. TheTgofv‐P2O5is ∼653 K and decreases some 130 K with the addition of 10 mol% Li2O.Tgthen reaches a minimum value at 20 mol% Li2O and increases with further Li2O additions. The increase inTg, even though the fraction of nonbridging oxygens is still increasing, is interpreted in terms of an increasing entanglement of long‐chain PO2groups in the glass. Such a structural transition from a 3‐D network of interconnected PO4groups for P2O5to a 1‐D chain structure for LiPO3is one of the first examples of the importance of intermediate‐range order in govern

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb06636.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

A theoretical approach is forwarded to predict the electromechanical properties of porous piezoelectric ceramics. The analysis is able to account for the effects of porosity shape and concentration and is applicable to piezoelectric ceramics of arbitrary material symmetry. By coupling the exact solution for a single ellipsoidal pore embedded in an infinite piezoelectric matrix with an effective medium approximation, the theory considers, in an approximate manner, interaction effects at finite porosity concentrations. The theoretical estimates are developed using a matrix formulation which enables all elastic, dielectric, and piezoelectric moduli of the porous solid to be readily computed. Numerical results are presented to illustrate the effects of the shape and concentration of the porosity on the effective electroelastic moduli and transducer parameters of practical importance. Particular attention is devoted to assessing the sensitivity of the effective electromechanical properties to the accuracy of the input data. Finally, theoretical estimates are shown to be in good agreement with existing experimental results for porous piezoelectric ceramics with various microstructural geometries.

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb06637.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

α‐C2SH can be synthesized by hydrothermal treatment of lime and silicic acid for 2 h at 200°C. When heated to 390–490°C, α‐C2SH dissociates through a two‐step process to form an intermediate phase plus some γ‐C2S. This appears to be a new dicalcium silicate different from known dicalcium silicates—α, α′L, α′H, β, and γ phase—and is stable until around 900°C. At 920–960°C, all the phases are transformed to the α′Lphase. The intermediate phase has high crystallinity and is stable at room temperature.29Si MAS NMR measurements indicate the possibility that it contains both protonated and unprotonated monosilicate anions. The intermediate phase that has passed through the α’phase at higher temperature yields β‐C2S on cooling. The intermediate phase is highly active, and completed its hydration in 1 day (w/s= 1.0, 25°C). Among the crystalline calcium silicate hydrates with Ca/Si = 2.0, it is hillebrandite th

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb06638.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

A phenomenological thermodynamic treatment of multicomponent, elastically stressed ionic crystals is presented and the conditions for thermodynamic equilibrium derived using a Gibbsian variational approach. By assuming that the electrostatic and electronic contributions to the internal energy density are determined by the local electric displacement and the distribution of unassociated charge between the valence and conduction bands, respectively, (metallic) conductors, semiconductors, and insulators can be treated. This approach is compatible with thermodynamic treatments of stressed metallic systems and the structure‐element description of ionic crystal

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb06639.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Unidirectional and cross‐ply Nicalon fiber‐reinforced calcium aluminosilicate (CAS) glass‐ceramic composite specimens were subjected to tension–tension cyclic fatigue and static fatigue loadings. Microcrack densities, longitudinal Young's modulus, and major Poisson's ratio were measured at regular intervals of load cycles and load time. The matrix crack (0° plies) density and transverse crack (90° plies) density increased gradually with fatigue cycles and load time. The crack growth is environmentally driven and depends on the maximum load and time. Young's modulus and Poisson's ratio decreased gradually with fatigue cycles and load time. The saturation crack densities under fatigue loadings were found to be comparable to those under monotonic loading. A matrix crack growth limit strain exists, below which matrix cracks do not grow significantly under fatigue loading. This limit coincides with the matrix crack initiation strain. Linear correlations between crack density and moduli reductions obtained from quasi‐static data can predict the moduli reductions under cyclic loading, using experimentally measured crack densities. A logarithmic correlation can predict the Young's modulus reduction in a limited stress range. A fatigue crack growth model is proposed to explain the presence of two distinct regimes of crack growth and Young's modulu

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb06640.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY