Massive transformation in bismuth oxide-based ceramics
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NTRODUCTION
THE massive transformation has been investigated for over the past three decades in a number of metallic alloys.[1–16] Several theoretical and experimental studies in metallic alloys have identified and categorized salient features of the massive transformation. These are listed as follows. (1) The massive transformation is composition-invariant. (2) The transformation is thermally activated. (3) The interface between the parent phase and the product phase, the interphase boundary, is incoherent, at least during growth. (4) The interphase-boundary motion is usually fast, typically on the order of 1 cm/s, and can be greater. The thermodynamics of the massive transformation defines a critical temperature (T0) at which the free energies of the parent and the product phases of the same composition are identical. For an alloy of a given composition, below this critical temperature, the free energy (per mole) of the
ANIL V. VIRKAR, Professor and Chair, is with the Department of Materials Science and Engineering, University of Utah, Salt Lake City, UT 84112. Contact e-mail: [email protected] POMIN SU, Scientist, is in Taiwan. KUAN-ZONG FUNG, Assistant Professor, is with the Department of Materials Science and Engineering, National Cheng Kung University, Taiwan, R.O.C. This article is based on a presentation made at the symposium entitled “The Mechanisms of the Massive Transformation,” a part of the Fall 2000 TMS Meeting held October 16-19, 2000, in St. Louis, Missouri, under the auspices of the ASM Phase Transformations Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
product phase (assuming that the parent phase is the hightemperature phase) is lower than that of the parent phase, thus making the transformation thermodynamically possible. This temperature obviously lies in the two-phase field comprising the original parent phase and the product phase. As a result, the occurrence of a massive transformation within the two-phase field is clearly a distinct possibility. Rapid transformation kinetics due to rapid diffusion in metallic alloys, by and large, has prevented an investigation of the massive transformation under isothermal conditions. As a result, whether or not massive transformation occurs within a two-phase field has been the subject of much controversy. As will be discussed in this manuscript, based on the investigation in Bi2O3-based materials and on a survey of published phase diagrams in many ceramic systems, there are likely many systems in which massive transformation may occur, and it probably initiates inside a two-phase field. Also, as will be discussed in this manuscript, the slowness of atomic (ionic) transport in ceramics facilitates a study of the kinetics of massive transformation under isothermal conditions. In the present work, for example, the maximum interphase-boundary speed measured was on the order of ⬃10⫺5 cm/s, which is five orders of magnitude slower than in many metallic alloys. The considerably slower mass transport in ceramics, as compared to that in metals, a
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