As-received, yttria-doped (4.2 wt % Y203) single crystals of zirconia were heated to ?21OO0C in air. Cube-shaped samples with faces perpendicular to (100) axes on the basis of the pseudocubic symmetry were cut from the cryst:als. X-ray and electron diffraction indicated that the crystals are polydomain with [OOl] axes, on the basis of the tetragonal symmetry, in three mutually orthogonal directions (pcerpendicular to the cube faces). The cube-shaped crystals were tested in compression at temperatures as high as 1400°C. X-ray diffraction indicated that ferroelastic domains underwent reorientation (switching) in compression. Subsequently, notched samples with the long direction of the beams along (100) on the hasis of the pseudocubic symmetry were fractured in three-point bending at temperatures as high as 1000°C. X-ray diffraction from fracture surfaces showed that domain reorientation had occurred and that no monoclinic phase was observed on fracture or ground surfaces. The fracture toughness at room temperature and at 1000°C was -12 and -8 MPa . m"*, respectively. Preliminary experiments on po1:ycrystalline tetragonal zirconia samples containing 5.4 wt% Y 2 0 3 and sintered at 22100°C also showed no evidence of the nionoclinic phase on fracture or ground surfaces. The toughness of the polycrystalline samples was typically 7.7 MPa . m"'. These results indicate that ferroelastic domain switching can occur during fracture and may contribute to toughness. [
On heating at ≤ 1900 °C for up to 320 h SiC‐AlN solid solutions of the 2H crystal type containing 50 and 75 mol% AlN, prepared by hot‐pressing SiC/AlN powder mixtures, decompose into modulated structures.
Samples in the pseudobinary AIN-A120C were fabricated by hot-pressing mixtures of AIN, A14C3, and Al2O3 powders in graphite dies in an atmosphere of nitrogen. The resulting dense samples were subjected to thermal treatments over a range of temperatures from 1550" to 1950°C. The hot-pressed as well as annealed samples were examined using optical microscopy, scanning transmission electron microscopy, and X-ray diffraction. An electron microprobe was used to determine composition. X-ray diffraction showed that AlzOC dissolved in AIN up to 44 mol% at 1800°C. Thermal treatment at lower temperatures led to the decomposition of the solid solution into two isostructural phases. In samples containing -14 to 60 mol% AlN, the morphology of the precipitates was lenticular. Diffraction contrast analysis showed that the precipitates were rich in AIN. Lattice images showed that the (001) planes of the 2H structure were continuous between the matrix and the precipitates. These precipitates appear to be similar to Guinier-Preston zones observed in metallic alloys. When annealed for long periods of time, interface dislocations formed, signifying partial loss of coherency. In some compositions (-61 to 64 mol% AIN), a lamellar microstructure developed similar to that observed in cellular phase separation. Also, in some of the compositions, an additional phase was observed whose composition and structure were not determined. [
Single-phase solid solutions of 2H crystal type in the Sic-A1N system containing 50 and 75 mol% AlN were fabricated by hot-pressing mixtures of S i c and AlN powders. Dense samples were subjected to thermal treatments over a range of temperatures between 1600" and 1900°C for a period of time up to 320 h. Transmission electron microscopy revealed the existence of modulated structures indicative of spinodal decomposition.ORK by Cutler et a/.' showed that the w2H polymorphs of S i c and AIN form an extensive solid solution. Ruh and Zangvil'.' and Rafaniello ef ~1 .~ fabricated dense specimens in the Sic-AlN pseudobinary by hot-pressing mixtures of S i c and AlN from commercial sources and SiCAlN derived from carbothermal reduction of SiO,-AI,O, mixture in nitrogen, respectively. These authors also examined various physical and mechanical properties of the resulting ceramics. In the work of Ruh and Zangvil,* a complete 2H solid solution over the entire range of compositions was not observed although 2H was the predominant polytype (along with other polytypes) present. By contrast, Rafaniello et ~1 1 .~ observed a complete solid solution over the range of compositions from = 10 to 100 mol% AIN with 2H as the only polytype present. This difference was rationalized on the premise that carbothermal reduction of Si02-A120, in nitrogen yields an intimate mixture of S i c and AIN which facilitates the formation of the solid solution.In later work on annealed samples of Sic-AlN, Rafaniello et noted that twophase microstructures evolved with both phases being of the 2H polymorph. Precipitation occurred by a cellular mechanism as well as by nucleation and growth. Further, preliminary evidence indicated the formation of possible modulated structures.6 These results indicated that perhaps a miscibility gap exists below about 2000°C. Analysis of some of the grain-boundary precipitates indicated the existence of spinel-type structure. Kuo et al.' subsequently showed that excess oxygen may be accommodated by the formation of A1303N spinel. Consequently, precipitation reactions observed were in samples containing oxygen as an impurity.
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