摘要:

Superplastic structural ceramics (Y‐TZP, Al2O3, Si3N4, and their composites) that can withstand biaxial stretching to large strains have been developed recently. Microstructural design of these ceramics first requires an ultrafine grain size that is stable against coarsening during sintering and deformation. A low sintering temperature is a necessary, but not a sufficient, condition for achieving the required microstructure. In many cases, the selection of an appropriate phase, such as tetragonal phase in zirconia orαphase in silicon nitride, which is resistant to grain growth, is crucial. The use of sintering aids and grain‐growth inhibitors, particularly those that segregate to the grain boundaries, can be beneficial. Second‐phase particles are especially effective in suppressing static and dynamic grain growth. Another major concern is to maintain an adequate grain‐boundary cohesive strength, relative to the flow stress, to mitigate cavitation or grain‐boundary cracking during large strain deformation. Existing evidence suggests that a lower grain‐boundary energy is instrumental in achieving this objective. The selection of an appropriate phase and the tailoring of the grain boundary or liquid‐phase composition can sometimes drastically alter the cavitation resistance. Related observations on forming methods, forming characteristics, and sheet formability are also reviewed. The basic deformation characteristics are similar to diffusional creep and are dominated by grain‐boundary diffusion. However, deformation characteristics are frequently altered by interface reactions, secondphase hardening/softening, and dynamic grain‐growth‐induced strain hardening. Ductility and formability, on the other hand, are controlled by the flow stress and flaw distribution, not by deformation instability as in superplastic metals. Analytical models and empirical correlations are presented to describe various constitutive relations pertaining to s

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb06734.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Two Al2O3/Al composites, grown by the directed oxidation of molten Al alloys at 1400 and 1600 K, were investigated by X‐ray diffraction, optical microscopy, scanning electron microscopy, transmission electron microscopy, and wet chemical analysis. The materials were found to contain a continuous network of Al2O3, which was predominantly free of grain‐boundary phases and was made up of nanometer‐ to micrometer‐sized crystallites, a continuous network of Al alloy, and isolated inclusions of Al alloy. No crystallographic orientation was observed in the metallic phase, whereas the Al2O3was oriented with itscaxis parallel to the growth direction. The higher process temperature yielded a lower metal content and less connectivity of the metallic con

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb06735.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

The directed oxidation of some molten aluminum alloys was studied. Alloys containing Si, Zn, and Mg are readily converted into Al2O3/Al composites with the characteristic directed metal oxidation process (DIMOX) metal‐channel structure. No reaction took place when alloys without Mg were used. However, when Mg‐free melts containing Si and Zn were in contact with bulk spinel (MgAl2O4), the typical reaction product grew from the metal pool, although without formation of the spinel starter layer which was thought to be a prerequisite for DIMOX growth. Furthermore, the growth front was more planar than when Mg‐containing alloys were

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb06736.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Ion exchange was used to introduce surface compression in soda‐lime‐silica glass (SLS) and aluminosilicate glass (ALS). In situ observations of the indentation cracking sequences on the modified surfaces were made and compared with those on the base glasses. The initiation point for each crack type formed during the loading and unloading cycle was monitored on the indenter load‐displacement traces. On the base glasses, median (in the SLS) and cone (in the ALS) cracking was observed during the loading cycle. “Modified” median, radial, and lateral cracking was observed in both glasses during unloading. The introduction of the residual compressive surface stress was found to completely suppress all cracking during the loading cycle (up to 100 N) in both the SLS and ALS. In addition, the initiation of the cracks formed during the unloading cycle was shifted to further down in the cycle, i.e., to lower values of the initiation load. A pronounced tendency for a shift in the cracking from median to radial was also observed on the residually stressed surfaces. A similar shift was observed on stress‐free surfaces during unloading as the value of the initiation load decreased for a give

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb06737.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

A strain gage technique, which relates the prior residual stress state in a material to the strain data obtained by fixing a strain gage on one surface and grinding off the other, has been proposed previously. In the current work, a generalized solution for obtaining an arbitray residual stress profile from strain gage data is presented. Numerical analysis using the solution indicates that the formulation is insensitive to random errors of 10% or less in the experimental data. Based on the results of the analysis, a procedure for determining stress profiles from strain gage data is outlined. Experimental data for tempered glass was analyzed using the technique proposed. The stress profiles predicted are in good agreement with independent observations using indentation and strength data.

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb06738.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

The crystal structure of Ce3B2N4(space groupImmm,α= 0.35653(5) nm,b= 0.63160(21) nm,c= 1.07131(50) nm) was derived from X‐ray powder photographs; precise atom parameters, atom occupancies, atom order, and interatomic distances were obtained from room‐temperature neutron powder diffraction data employing the Rietveld profile analysis method. The final reliability values of the refinement were RF= 0.059 and RI= 0.068. Boron atoms in triangular prismatic metal coordination [Ce6B] are at a distance of 0.1785(7) nm and form pairs. In a tetrakaidecahedral surrounding [Ce6B]BN2, each boron atom forms single bonds with two nitrogen atoms at a distance of 0.1464(4) nm. Nitrogen atoms thus are in rectangular pyramidal metal coordination [Ce5N] with additional boron atom completing a distorted octahedron [Ce5BN]. There are no direct N─N bonds. With respect to atom‐site occupancy, the crystal structure of Ce3B2N4is isopointal with the structure type of CeCr2B6(═(CeCr2)B2B4) and can furthermore be considered as a filled‐up version of the W2CoB2‐type ((W2Co)B2□4). Isotypic compounds, RE3B2N4, were synthesized with all the early and large rare‐earth metals (RE = La, Ce, Pr, Nd) and mischmetal (MM). Nd3B2N4was found to be unstable at temperatures higher than 1400°C. No RE3B2N4‐phase was observed with the smaller rare‐earth elements

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb06739.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Solid solutions in the AIN─Al2OC and SiC─AlN systems were fabricated by hot‐pressing powder mixtures in graphite dies. X‐ray diffraction showed the samples to be single phases of 2H structure. The samples were annealed between 1600° and 1900°C for up to 1000 h. In the SiC─AlN system, optical microscopy and X‐ray diffraction failed to reveal microstructural or phase changes. However, electron microscopy showed that samples had decomposed. Streaking of diffraction spots occurred along directions orthogonal to {012} planes (∼43° off thecaxis), which is approximately the direction along which the elastic energy function is a minimum. The orientation‐dependent Young's modulus was also a minimum along this direction. In AIN─Al2OC, optical microscopy and X‐ray diffraction indicated the occurrence of decomposition. The precipitates were disk‐shaped with [001] orthogonal to the disks. The occurrence of decomposition along the [001]direction suggests that it is the el

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb06740.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

The effects of nine metal cations (Li+, Na+, Mg2+, Ca2+, Sr2+, Cu2+, Al3+, La3+, and Y3+) on silica gel formation have been investigated by studying the hydrolysis and polycondensation of silicon tetraethoxide (TEOS) in the presence of metal nitrates. The influence of the water: TEOS mole ratio, metal ion concentration, and the reaction temperature has been investigated. The overall activation energy for gel formation has been determined from the temperature dependence of the time of gelation for each system. The activation energy fornetwork formation is 54.5 kJ/mol. The gel formation time as well as the activation energy sharply increases in the presence of Cu2+, Al3+, La3+, and Y3+. In contrast, the presence of Li+, Na+, Mg2+, Ca2+, or Sr2+lowers the gelation time but has no appreciable effect on the activation energy. This difference may be attributed to the participation or nonparticipation of the metal ions in the formation of the three‐dimensional polymeric network during polycondensation. The concentration of metal ion (Mg2+, Ca2+, Y3+) or the water: TEOS mole ratio had no appreciable effect on the gelation activation energy. A simple test has been proposed to determine whether a metal ion would act as a network intermediate or modifier in silica and other glassy network

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb06741.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

The shrinkage of ceramic injection molding suspensions caused by pyrolytic removal of the organic vehicle was measured for initial ceramic volume fractions from 0.48 to 0.64. Shrinkage was inversely related to initial ceramic volume fractionV, and maximum volume fraction after pyrolysisV*,maxwas 0.65. The factors which restrict shrinkage are discussed. The maximum volume fraction, obtained from semiempirical equations relating the viscosity of suspensions to volume fraction of powder (Vmax) was 0.73 to 0.76. Both the viscosity of the suspensions and the shrinkage on removal of the organic vehicle can be interpreted in terms of a free volume of fluid over and above that needed to fill interstices between contacting particles. Thermomechanical measurements also show that the free‐volume concept helps to interpret the transition from fluid to quasi‐solid properties as the organic vehicle is removed by pyrolysis from a molded b

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb06742.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Transformation zones surrounding cracks in several toughened magnesia‐partially‐stabilized zirconia ceramics are characterized by optical interference measurements of surface uplift and by Raman microprobe spectroscopy. The measurements demonstrate that the volume fraction of transformation is nonuniform within the zone and that the extent of the frontal zone is approximately the same as that of the wake. Results are used to evaluate the crack‐tip shielding stress intensity factor and to compare with the measured fracture toug

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb06743.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY