Superplastic Flow in Ceramic Microfiber Specimens
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SUPERPLASTIC FLOW IN CERAMIC MICROFIBER SPECIMENS R. LAPPALAINEN AND R. RAJ Cornell University, Dept. of Materials Science Bard Hall, Ithaca, NY 14853-1501, U.S.A.
and Engineering,
ABSTRACT Tensile superplastic deformation was studied using ceramic specimens that had a cross section of approximately 1 gm x 10-50 Rm, and a length of 4 mm. These fibers were prepared by physical vapor deposition of oxides on a predefined pattern made by lithography. Experiments with Y2 03 stabilized ZrO2 fibers and spinel fibers showed two characteristic features of flow behavior: serrations and a threshold stress. The serrations were explained by a flow model based on discrete sliding events distributed in time and flow space throughout the polycrystal. These fluctuations in stress were detected in our microfiber specimens due to relatively few grains in the gage section. Experimental results of the effect of sample size, grain size and strain rate on the serration behavior are discussed in the frame of the new model.
INTRODUCTION Superplastic behavior described by the equation
of
ceramic
materials
is
generally
* e -Q/RT dapwhere i is the strain rate, a is the stress, d is the grain size, n and p are stress and grain size exponent, Q is the activation energy, T is the absolute temperature, R is the gas constant and C is a constant. This is based on the classical equations for deformation by lattice diffusion (Nabarro-Herring) [1] or grain boundary diffusion (Coble) [2] which correspond to n=l and p=2 or 3, respectively. Similar terms are obtained in a model which involve the grain boundary sliding accommodated by diffusion [3]. not a perfect sink or source for If the grain boundary is vacancies, grain boundary sliding is assumed to be controlled by the interface-reaction which leads to the stress exponents larger than 1. All these models give an impression that the superplasticity is a simple smooth process driven by diffusion. However, as superplastic deformation pointed out by Ashby and Verrall [4], events that occurs locally through discrete grain-switching produce spurts of local strain. In our recent paper [4] we introduced a new approach in modeling serrated flow. The model is based on the idea of discrete sliding events which can be due to classical diffusional flow, phase transformation or grain growth which are considered to be additive mechanisms of superplastic deformation. The fluctuations curves are in the flow stress of experimental stress-strain attributed to these discrete sliding events that are distributed in time and space throughout the sample. The size of fluctuations (1)
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=
Mat. Res. Soc. Symp. Proc. Vol. 239. D1992 Materials Research Society
134
is expected to depend on such factors as sample size, grain size and strain rate. The aim of this paper is to study these effects in two superplastic systems: yttria stabilized zirconia and platinum doped spinel. The serrated flow model [4] will be first described briefly. Then the experimental results of serration behavior will be discussed in the frame
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