Bainitelike transformations in some oxide ceramics
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I.
INTRODUCTION
A C C O R D I N G to Nishiyama, m so-called lower bainite is made by a diffusionless transformation from austenite (3') to ferrite (a) in steels where the transformation requires not only a shear mechanism but also an individual atomic movement and accompanies small nonchemical energies. Lattice invariant deformation and interface energies are associated with the transformation. Therefore, if the temperature is lower than To (the equilibrium temperature between 3' and a phases), the lowerbainitic transformation can occur even at temperatures higher than the Ms temperature. Although the progress of this transformation is usually prevented by volume strains caused by an expansion of the martensitic transformation, the growth of the bainitic a phase will be able to continue when the expansion stress is released by the contraction due to the precipitation of carbide in the a region. This fact suggests that the activation energy for the growth of lower bainite is close to that for the carbon diffusion in the a region. A fairly large surface relief can usually be observed with the lower bainitic transformation. Simply said, the lower bainitic transformation requires a cooperative atomic shear movement together with the diffusion of solute atoms. 121 The "bainitelike" transformation, which the authors are proposing here in oxides, points out the following two cases: (1) A tetragonal-to-monoclinic transformation occurs isothermally in Z r O 2 - Y 2 0 3 ceramics, this being similar to the eutectoidal transformation, 3' ~ a + Fe3C, in steel. In other words, as shown in the phase diagram (Figure 1), the bainitelike transformation occurs during isothermal holding after a rapid cooling from high temperature into the two-phase region of monoclinic + cubic. The bainitelike transformation is associated with the following phenomena: (a) A surface relief appears in the N. NAKANISHI and T. SHIGEMATSU, Professors, T. FURUKAWA, Postdoctoral Fellow, and N. MACHIDA, Lecturer, are with the Faculty of Science, Konan University, Higashinada-Ku, Kobe 652, Japan. This article is based on a presentation made at the Pacific Rim Conference on the "Roles of Shear and Diffusion in the Formation of Plate-Shaped Transformation Products," held December 18-22, 1992, in Kona, Hawaii, under the auspices of ASM INTERNATIONAL's Phase Transformations Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
transformation from tetragonal to monoclinic; (b) this transformation, however, occurs not athermally but isothermally; and (c) the rate of isothermal transformation, as will be shown in Section I I I - A - 2 , is considered to be controlled by rotation or short-range diffusion of oxygen ions. This means that a fairly large strain can be released by these ionic movements. From the crystallographic point of view, the herringbone tetragonal phase of ZRO2-2.0 mol pct Y203 thin films has been observed 13j to undergo partial transformation to the monoclinic phase with an orientation relationship similar to that found in pure ZrO2: (100)m / / {1
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