Interphase boundary structure with irrational orientation relationship formed in grain boundary precipitation
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RODUCTION
A GRAIN boundary precipitate holds a near-rational orientation relationship with respect to one of the two adjacent matrix grains but exhibits an irrational orientation relationship with respect to the other grain.[1] It was previously considered that the rationally related, precipitate/matrix interphase boundary is partially coherent, while the irrationally related, interphase boundary is incoherent.[1] However, it was found that a grain boundary precipitate forms partially coherent interphase boundaries on both sides of the grain boundary, as suggested by Aaronson and Russell,[2] through observation of ledges or misfit compensating defects on precipitate/matrix interphase boundaries in several systems.[3,4] Similar observations were made for the interphase boundaries formed in massive transformations[5,6] and pearlite transformations[7,8,9] in which the nucleation of the product phase also takes place at matrix grain boundaries. However, there has been no quantitative analysis of the interphase boundaries with such irrational orientation relationships so far. In the study of matrix/product interphase-boundary structures formed by diffusional transformations, geometrical models, such as the O-lattice model,[10] the invariant line model,[11] or the near-coincidence site (NCS) lattice model,[12–15] are generally used to analyze the observed structure. The application of such models is still limited to the interphase boundary at which a rational, low-energy orientation relationship is exactly or nearly satisfied. This is because the matrix notation of lattice correspondence in a
T. FURUHARA, Associate Professor, and T. MAKI, Professor, are with the Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan. Contact e-mail: [email protected] K. OISHI, Researcher, is with Matsushita Electric Industrial Co., Ltd., Moriguchi 570-8501 Japan. 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
structural change is hard to define for an arbitrary orientation relationship. Among those geometrical models, however, the NCS model does not need to define how lattice sites in the matrix transform to those in the product. The NCS model simply determines the coherent atom pairs with a small mismatch in the two crystal lattices interpenetrating each other with a given orientation relationship and then finds the boundary plane with the maximum density of coherent atom pairs. Thus, this model is suitable to analyze the interphase boundary structures with irrational orientation relationships. The present study aims to examine the interphase boundary with an arbitrary orientation relationship observed in grain boundary precipitation by analyzing the observed interphase boundary structures of bcc precipitates formed at fcc grain
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