Deformation-induced surface corrugation of superplastic ceramics
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Deformation-induced surface corrugation of superplastic ceramics Hiroyuki Muto, Takatoshi Futami, and Motosugu Sakaia) Department of Materials Science, Toyohashi University of Technology, Tempaku-cho, Toyohashi 441-8580, Japan (Received 6 February 2001; accepted 4 April 2001)
A new finding is reported for the surface corrugation induced in the superplastic deformation and flow of 3 mol% yttria-partially stabilized tetragonal zirconia polycrystal and -spodumene glass ceramic, the former including no secondary phase and the latter having a plenty of glassy phase at grain boundaries. A significant test specimen size effect on the resistance to superplastic deformation is also addressed. These findings are related to the microscopic processes and mechanisms of cooperative grain-boundary sliding.
The microscopic processes and mechanisms of superplastic deformation are still controversial, not only for metals but also for ceramics. In fact, the scientific definitions of “super” as well as “plasticity” have not yet been made, though numbers of papers referring to superplasticity have been published since C.E. Pearson reported his pioneering work in 1934.1 Some of superplastic materials actually deform and flow in a time-dependent “viscoelastic” manner. Models for grain-boundary sliding during superplastic deformation have also been proposed.2 –9 However, all the models failed to characterize an essential feature of superplasticity, i.e., no necking during large-scale elongation. One of the most plausible candidate of microscopic processes for the superplastic deformation of polycrystalline materials may be cooperative grain boundary sliding (CGBS), occurring through the cooperative movement of grain groups.4 –12 Since the work of Ball and Hutchinson in 1969,4 a series of studies for CGBS has been conducted by many scientists.4 –12 The experimental observations of CGBS have been mainly made using specimens with scratched marker lines.7,8 Offset of marker lines resulting from large elongation gave a definite evidence for the presence of CGBS. We have made detailed examinations for CGBS using model aggregates comprising rigid circular and/or hexagonal bars.11 Considering the principle of minimum
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 16, No. 7, Jul 2001
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energy dissipation, we concluded that the cooperative movements of grain groups along grain boundaries are essential to large-scale superplastic deformation. The intent of this paper is to present experimental evidence that conclusively supports the presence of CGBS in the large-scale deformations of polycrystalline ceramic systems: 3mol% yttria-partially stabilized tetragonal zirconia polycrystal (3Y-TZP) and -spodumene glass ceramic. 3Y-TZP and -spodumene glass ceramic are capable of superplastic elongations exceeding 100%.13 –15 The 3Y-TZP used in this work had a mean grain size of 0.3 m, supplied from Nikkato Co. Ltd., Ohtsu, Japan. As-received -spo
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