摘要:

Duplex (two‐phase) microstructures and laminar composites offer some unique opportunities for improving the room‐temperature mechanical reliability (e.g., flaw tolerance) and the high‐temperature microstructural stability (e.g., resistance to grain growth and creep damage) of structural ceramic materials. Examples illustrating the approach to designing novel multiphase microstructures and laminar composites with enhanced structural reliability are given. These are based on current work on various alumina‐based ceramics as well as zirconia‐, silicon nitride‐, and silicon carbide‐containing ceramics. Critical issues and areas for future work

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07188.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The stress–strain behavior of a variety of duplex ceramics was measured with the aid of strain gauges attached to the tensile surface of beams in four‐point flexure. The duplex materials consisted of a matrix of alumina and, alternatively, of yttria–tetragonal‐zirconia composites without (2Y‐TZP) and with alumina (3Y‐TZP20A). Up to 20 vol% pressure zones of different sizes containing unstabilized zirconia were dispersed within the matrices. Different types of stress–strain behavior were distinguished. The results were compared with microcrack and shear band observations of indented andR‐

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07189.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The creep behavior of single‐crystalline MgO tested in the 〈100〉 direction is reviewed in the temperature range 1300° to 1800°C. At low stresses, the stress exponent is equal to about unity, and the deformation process is attributed to Harper–Dorn creep. At high stresses, the stress exponent is equal to approximately 5 and the deformation process is attributed to dislocation glide controlled by climb. The creep behavior in both regions is successfully predicted by an internal stress model for Harper

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07190.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Vickers hardness in spinel increases by 23% after neutron irradiation to a fluence of 8.3 × 1022n/m2at 100°C. Annealing at high temperatures above 100°C gradually decreases the hardness. Above 500°C, the hardness is reduced to almost the same value as that of unirradiated material. The hardness of spinel, irradiated at 470°C to a fluence of 2.4 × 1024n/m2, is unchanged both after irradiation and after annealing up to 1000°C. The length change during annealing was also measured and is similar to the hardness change. Frank dislocation loops with a density of 3 × 1014/cm3are induced by neutron irradiation at 470°C but they apparently do not affect the hardening in spinel. Thus, point defects are concluded to act as obstacles against dislocation movement. The yield stress measured at 1400°C is also unchanged after irradiation. It is believed that not only the point defects but also the loops are annihilated by annealing

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07191.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

In this investigation quantitative texture analysis, including calculation of the orientation distribution function, is used to demonstrate the degree of preferred orientation inβ‐Si3N4which has been hot‐pressed or hot‐worked. The results indicate that plane strain compression can produce strong textures. The texture is decided by the processing parameters including temperature, sintering additives, and stress state. Grain rotation and preferred grain growth apparently both contribute to texture development inβ‐Si3N4. Basal (00l) pole figures obtained from the orientation distribution function are consistent with microstructural observations and are reflected in indentation fracture toughness anisotropy. In plane strain the ratio of maximum to minimum fracture toughness is great

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07192.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The high‐temperature deformation behavior of a fine‐grained polycrystalline yttrium aluminum garnet (YAG) was studied in the temperature range of 1400° to 1610°C using constant strain rate compression tests under strain rates ranging from 10−5/s to 10−3/s. The stress exponent of the creep rate, the activation energy in comparison with that for single‐crystal YAG, and the grain size dependence suggest that Nabarro–Herring creep rate limited by the bulk diffusion of one of the cations (Y or Al) is the operat

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07193.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Crack growth behavior under creep conditions was studied in SiC‐whisker‐reinforced mullite and silicon nitride. Tests of four‐point bend specimens with indentation cracks were periodically interrupted to observe the creep behavior. At each interruption the bulk creep strain of the specimen, the growth of the indentation cracks, and the nucleation and growth of creep‐induced cracks were measured. A strong linear correlation was observed in both materials between the crack growth rate and the creep strain rate. For a given strain rate, cracks in the silicon nitride composite propagated at velocities about an order of magnitude greater than those in the mullite composite. On the other hand, for similar nominal stresses, creep rates in the silicon nitride composites were about an order of magnitude less than with the mullite co

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07194.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The microwave dielectric properties and crystal structure of Ba(Zn1/3Ta2/3)O3– (Sr,Ba)(Ga1/2Ta1/2)O3ceramics were investigated in the present study. TheQvalue of Ba(Zn1/3Ta2/3)O3was improved by adding 5 mol% Sr(Ga1/2Ta1/2)O3. The maximumQvalue ofQ×f= 162000 GHz was obtained at 0.95Ba(Zn1/3Ta2/3)O3. 0.05Sr(Ga1/2Ta1/2)O3. For this composition, a lattice super structure caused by hexagonal ordering was observed. A further improvement in theQvalue was attained when some Sr was replaced with Ba, and 0.95Ba(Zn1/3Ta2/3)O3· 0.05(Sr0.25Ba0.75)(Ga1/2Ta1/2)O3exhibited a maximumQvalue such thatQ×f= 210000 GHz. Despite the increasedQvalue with the replacement of Sr by Ba, thec/avalue, which indicates the degree of lattice distortion, remained constant near3/2. TheQvalue thus improved without lattice distortion in the system Ba(Zn1/3Ta2/3)O3‐(Sr,Ba)(Ga1/2Ta1/2)O3, whereas the improvement ofQvalue increased with lattice distortion in the solid solution system with Ba(Zn1/3Ta2/3)O3as an end

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07195.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

An anomalous positive temperature coefficient (PTC) of resistance was found in a conductor–insulator ceramic composite. The composite materials was composed of randomly dispersed conducting graphite and insulating cristobalite‐form SiO2particles. The PTC effect occurred at exactly 230°C, that is, at a reversible phase inversion point with a large thermal expansion of cristobalite. The magnitude of the PTC anomaly was high in composites located in the middle of the percolation curve, and loaded with large graphite and fine SiO2parti

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07196.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

R‐curves for two in‐situ reinforced silicon nitrides A and B of nominally the same composition are characterized using the Griffith equation and indentation fracture mechanics. TheseR‐curves are calibrated against fine‐grained silicon nitrides which have a known chevron‐notch (long‐crack) toughness and with a nearly flatR‐curve behavior. Silicon nitride A, with its coarser microstructure and higher chevron‐notch toughness, shows lower resitance to crack growth than silicon nitride B if the crack size is less than ∼200 μm. These results are consistent with the indentation–Strength measurements which show a crossover of strength between the two materials at an indentation load between 49 and 98 N, and below the crossover A has a lower strength. The toughening behavior is explained using an elastic‐bridging model for the short crack, and a pullout model for the long crack. The effects ofR‐curve properties

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07197.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY