High-temperature plasticity of cubic bismuth oxide
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High-temperature plasticity of cubic bismuth oxide Anne Vilette and S. L. Kampe Department of Materials Science and Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0237 (Received 31 July 1995; accepted 4 January 1996)
Cubic (d) bismuth oxide (Bi2 O3) has been subjected to high temperature deformation over a wide range of temperatures and strain rates. Results indicate that bismuth oxide is essentially incapable of plastic deformation at temperatures below the monoclithic to cubic phase transformation which occurs at approximately 730 ±C. Above the transformation temperature, however, Bi2 O3 is extensively deformable. The variability of flow stress to temperature and strain rate has been quantified through the determination of phenomenological-based constitutive equations to describe its behavior at these high temperatures. Analysis of the so-determined deformation constants indicate an extremely strong sensitivity to strain rate and temperature, with values of the strain-rate sensitivity approaching values commonly cited as indicative of superplastic behavior.
I. INTRODUCTION
Bismuth oxide (Bi2 O3 ) undergoes a polymorphic transformation from its low temperature monoclinic form (a) to a face-centered cubic (fcc-d) form at approximately 730 ±C. The cubic phase is thereafter stable with increasing temperatures up to its melting point of 825 ±C. Associated with the monoclinic-to-cubic phase transformation is a large volume change— of a magnitude of approximately 17%.1 Johnson et al.1 utilized the dilatometric behavior of this transition to study transformational plasticity in Bi2 O3 and has advanced a model for transformational plasticity based upon these results. These researchers investigated the role of stress, grain size, and porosity on the resulting transformation strain attainable by thermal cycling through the phase transition temperature. The researchers report increased transformation strain with decreasing grain size and decreasing rates of cycling, both qualitatively consistent with accepted time-dependent models for creep and superplastic deformation processes. The magnitude of the transformational strains reported by these investigators (ø268%) has been subsequently reported in several reviews of superplasticity in ceramic materials2–6 as evidence of the specific existence of transformation superplasticity in Bi2 O3 . It is the intent of the present study to establish whether bismuth oxide is similarly capable of deformation via structural plasticity, i.e., from stresses imposed from an external source only and not relying on those developed microstructurally from the volume change associated with the a $ d phase transformation. Provided that the requisite microstructural and dilatometric accommodations are present, it is noted that both transformational and structural plasticity are fundamentally and mechanistically similar, differing only in the source J. Mater. Res., Vol. 11, No. 6, Jun 1996
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