Preferred orientation of beta-phase and its mechanisms in a fine-grained silicon-nitride-based ceramic
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Wonjoong Kim and Young-Wook Kim Department of Materials Science and Engineering, The University of Seoul, Seoul 130-743, Korea
Guo-Dong Zhan National Institute for Research in Inorganic Materials, Namiki 1-1, Tsukuba-shi, Ibaraki 305-0044, Japan (Received 26 June 2000; accepted 20 November 2000)
A quantitative texture analysis, including calculations of the orientation distribution function, is applied to investigate the preferred orientation of –Si3N4 in a fine-grained material containing almost equiaxed grains that has been hot-pressed, annealed, and plane-strain compressed. The results show that (i) plane strain compression can produce relatively strong textures that were dependent on the compressive strain; (ii) the basal plane of hexagonal –Si3N4 was normal to the hot-pressing direction for the hot-pressed and annealed samples, whereas it was parallel to the stress axis for deformed samples; and (iii) the mechanisms for texture development were preferred grain growth for the annealed sample and grain rotation for the hot-pressed and deformed samples, respectively.
I. INTRODUCTION
The preferred orientation of -phase and the related anisotropy in the physical and mechanical properties of silicon nitride ceramics have been well documented in literature.1–10 Thorough investigations of texture and its development mechanisms were first conducted by Lee and Bowman.4 They addressed the relationships between processing parameters, such as temperature, sintering additives, and stress states, and the preferred orientation in –Si3N4 that was hot pressed and hot worked. Preferred grain growth and grain rotation, as well as preferred nucleation or grain-growth mechanisms, for texture formation were demonstrated in their samples. Wu and Chen5 reported a very strong wire texture in a -sialon when it was deformed to large tensile strains, which they attributed to grain rotation, preferred grain-growth mechanisms, and a grain-welding process. These investigated materials were produced from ␣-phase starting powder and hence contained rodlike -phase with the c axis parallel to the long axis of the grain in the microstructure after hot pressing or hot working. Therefore, the textured –Si3N4 is usually observed as being aligned with the c
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J. Mater. Res., Vol. 16, No. 2, Feb 2001 Downloaded: 29 Mar 2015
axis normal to the hot-pressing direction or stress axis; i.e., the basal plane of the hexagonal –Si3N4 is parallel to the hot-pressing direction. Silicon nitride ceramics produced from ultrafine -phase powders exhibit a fine-grained microstructure consisting of equiaxed grains and have excellent microstructural stability.8–12 Recently, we observed superplasticlike behavior in a fine-grained –Si3N4 containing a transient liquid phase at a temperature as low as 1550 °C (⑀´ 艌 1 × 10−4 1/s).11,12 With cordierite as the sintering additive, this material underwent a significant microstructure evolution during super
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