摘要:

Based on the mechanics analysis of crack‐interface interaction, a simple and direct micro‐indentation technique has been developed to evaluate the fracture energy of a bimaterial interface. The technique, when applied to a pristine planar SiC‐Si3N4interface at various angles of attack, is shown to provide a reasonable estimate of the interfacial fracture resistance. The experimentally obtained fracture energy has been compared favorably with a proposed atomic model of bond breaking between SiC and

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08195.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Composites of Al2O3(A) and cubic ZrO2(Z) (8 mol% Y2O3) (withc‐ZrO2volume fractions ranging from O to 1) were fabricated by pressureless sintering of mechanically mixed powders. The microstructures of the AZ composites were duplex, with the grains of both phases exhibiting similar size. Room‐temperature mechanical properties including Young's modulus (determined from elastic wave velocity measurements), strength and toughness (by indentation‐strength‐in‐bending), and Vickers microhardness were evaluated for the full range of compositions. All properties exhibited a linear decrease with increasingc‐ZrO2content, and noR‐curve behavior was seen in any of the composite compositions. Fracture morphology, observed from cracks emanating from microhardness indentations, changed from essentially intergranular for the tougher Al2O3to transgranular forc‐ZrO2, with AZ composites exhibiting m

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08196.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The main objective of this research was to develop an experimental and analytical methodology to predict the lifetime of internally pressurized SiC tubes from the lifetimes of simple specimens subjected to similar delayed failure modes. Finite element analysis was used to determine the strain rate distribution in a thick‐wall tube subjected to sustained internal pressure. This analysis was used along with the Monkman‐Grant creep rupture criterion (obtained from creep testing of tensile specimens) to predict the lifetime of SCRB210 tubes. Results showed a large difference between experimental and predicted lifetimes. This difference was due to the existence of severe defects in the SCRB210 tubes which failed at low stresses. These tubes failed in crack growth mode rather than creep mode. This result leads to the conclusion that creep rupture criteria obtained from testing simple specimens cannot be projected to predict the lifetimes of large and more complex compone

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08197.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Dense α‐sialon materials were produced by hot isostatic pressing (HIP) and post‐hot isostatic pressing (post‐HIP) using compositions with the formula Yx(Si12–4.5x, Al4.5x)‐(O1.5x,N16–1.5x) with 0.1 ≤x≤ 0.9 and with the same compositions with extra additions of yttria and aluminum nitride. X‐ray diffraction analyses show how the phase content changes from large amounts of β‐sialon (x= 0.1) to large amounts of α‐sialon (x= 0.4) and increasing amounts of mellilite and sialon polytypoids (x= 0.8). Samples HIPed at 1600°C for 2 h contained unreacted α‐silicon nitride, while those HIPed at 1750°C for 1 h did not. This could be due to the fact that the time is to short to achieve equilibrium or that the high pressure (200 MPa) prohibits α‐sialon formation. Sintering at atmospheric pressure leads to open porosity for all compositions except those with excess yttria. Therefore, only samples with excess yttria were post‐HIPed. Microstructrual analyses showed that the post‐HIPed samples had the highest α‐sialon content. A higher amount of α‐sialon and subsequently a lower amount of intergranular phase were detected atx= 0.3 andx= 0.4 in the post‐HIPed samples in comparison to the HIPed. The hardness (HV10) and fracture toughness (KIC) did not differ significantly between HIPed and post‐HIPed materials but vary with differentxvalues due to different phase contents. Measurements of cell parameters for all compositions show a continuous increase with increasingxvalue whi

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08198.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The paper explains the necessity and basic principles of fabricating slag glass‐ceramic based on special local antimony‐containing slag. A CaO‐Al2O3‐SiO2three‐component phase diagram was used to plan a base glass composition, to which four selected nucleating agents, Cr2O3, ZrO2, P2O5, and TiO2, were added separately. Studies focused on their roles in the crystallization of the present slag glass system with the aid of differential thermal analysis, X‐ray diffractometery, and scanning electron microscopy. Comparisons showed that the four nucleating agents behaved differently and therefore exerted different influences on crystallization. TiO2proved to be the best one, and its optimal concentration

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08199.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The growth of α‐Al2O3/metal composites by the directed oxidation of molten Al‐Mg‐Si alloys proceeds through four distinct stages. The first stage encompasses the early heating of the alloy ingot, melting, and continued heating to between 1123 and 1173 K. In this latter temperature range, the molten alloy surface rapidly oxidizes to form a MgO‐covered MgAl2O4layer. During further heating and initial soak at the composite growth temperature (1373 to 1573 K), the duplex layer slowly thickens (second stage). The start of the third stage, growth initiation, is marked by the spread of a metal‐rich zone over the duplex layer; this metal‐rich zone is believed to be connected to the molten alloy through microcracks in the thickened MgO/MgAl2O4layer. Small nodules of the oxide/metal composite nucleate from the metal‐rich layer. During the final rapid growth stage, the small composite nodules grow and coalesce to form a macroscopically planar growth front, which persists until growth is complete. Throughout the growth process, the external surface of the α‐Al2O3/metal composite is covered by a thin MgO layer. Immediately under this external layer and separating it from the α‐Al2O3is a thin l

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08200.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The growth kinetics of an Al2O3/metal composite by the directed oxidation of an aluminum alloy (10 wt% Si, 3 wt% Mg, balance Al) have been measured as a function of temperature (1398 to 1548 K) and oxygen partial pressure in O2/Ar gas mixtures. The growth rate exhibited an activation energy of ∼370 kJ/mol and a dependence on oxygen partial pressure consistent with aPo1/42relationship. A dissolution‐precipitation growth mechanism is proposed in which the growth rate is controlled by the electronic conductivity of an external Al2O3‐doped MgO surface layer in conjunction with grain boundary diffusion of magn

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08201.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Magnesia‐partially‐stabilized zirconia (Mg‐PSZ) is industrially important because of transformable metastable tetragonal precipitates which interact with and arrest cracks. This work addresses the precipitation of tetragonal phases at one composition, 9.5 mol% MgO, throughout a range of temperatures. High‐purity zirconia samples were sintered at 1700°C and rapidly quenched to heat‐treatment temperatures ranging from 1600° to 1100°C, then quenched to room temperature. Cooling rates through the tetragonal + MgO and the monoclinic + MgO two‐phase regions were found to affect the phase content. The kinetics of nonequilibrium phase transformation for high‐purity Mg‐PSZ are presented in terms of time‐temperature‐t

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08202.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

A vacuum‐pressure slip‐casting technique and hot isostatic pressing (HIPing) were used to prepare high‐strength and translucent alumina ceramics. A low‐viscosity and high‐solids‐content slurry (46 vol% solids) was prepared, and a dense green compact was formed. The samples were sintered and subjected to capsule‐free HIPing. Extremely high‐density (99.9%) and fine‐grained (0.7 to 15 μm in diameter) alumina ceramics were obtained. The HIPed samples showed high bend strength and translucency with in‐line transmittance of 30%

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08203.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Annealing at temperatures between 500° and 1000°C reverses the stress‐induced tetragonal‐to‐monoclinic phase transformation in precipitates within crack‐tip zones in MgO‐partially‐stabilized ZrO2(Mg‐PSZ). The stability of such reverse‐transformed tetragonal precipitates during subsequent room‐temperature aging has been examined using Raman spectroscopy, surface displacement measurements, and crack‐opening measurements. Partial spontaneous retransformation of material within the original zone from tetragonal‐to‐monoclinic symmetry occurred over

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08204.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY