| The purpose of this work is to provide the engineer with an understanding of the principles and mechanisms involved in the superplastic forming of some technologically important ceramic materials, i.e. Si(,3)N(,4) based ceramics, SiC based alloys, and the commercially available glass-ceramics. Compressive and tensile tests were performed on model materials, Corning code 9608 and 0329 systems. These were made by controlled crystallization of bulk glasses, and contained a residual glass (usually 2% to 5% by volume) because of incomplete crystallization. The glass resides in the grain boundaries and triple junctions. Hot-pressed Si(,3)N(,4) and SiC based alloys have nearly identical microstructures in their finished forms. It is proposed that if superplastic forming can be successfully applied to the chosen model materials, then superplastic forming should also be applicable to the nitrogen-based ceramic materials and the SiC based poly-phase alloys.; In the present study we showed that large elongations can be attained in the glass-ceramic materials. Furthermore, we have found that the flow is superplastic in the classical sense, that is, the strain-rate sensitivity of the material is close to unity. The mechanism of flow has been identified as being similar to diffusional creep. It is different only because the rate of transport of matter is enhanced by the liquid phase present in the grain boundaries. The stress, grain size, and temperature dependence of the flow behaviour was studied to establish the validity of the liquid phase enhanced diffusional mechanism. Reasonable agreement between experimental results and theoretical prediction was obtained.; The remarkable result from the experiments was that strain rates of about 10('-4)S('-1) to 10('-3)S('-1) could be obtained at moderate stresses, about 1 to 10 MPA, at temperatures of about 1400K. In compression, the material was infinitely ductile while in tensile testing, elongation as large as 400% was obtained.; In addition to flow, the failure mechanism was also examined in detail. Two types of failure mechanisms were investigated: (1) grain boundary cavitation through the polycrystal, (2) the initiation and propagation of crack(s) from the surface. Three types of experiments were done to differentiate between these two mechanisms: (1) compression tests, (2) tension tests and (3) tension tests under biaxial loading. |
