Crystallography of the Simple HCP/FCC System
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BODY-CENTERED-CUBIC (BCC), face-centeredcubic (FCC), and hexagonal-close-packed (HCP) are three common simple crystal structures in metallic materials, either as the matrix or as the precipitate. Because the crystallographic features of precipitates, orientation relationship (OR) and the corresponding habit plane (HP) in particular, significantly affect the mechanical properties of the material, understanding the crystallography of phase transformations is a fundamental part of materials science. This is particularly important for precipitation hardening alloys. In the past decades, the FCC/BCC system has been the most actively investigated using almost all available crystallographic models, such as the structure ledge model, the invariant line model, and O-lattice theory.[1–10] Most of the crystallographic features in this system have been adequately understood or explained using one or more these models. However, none of these models can actually predict the ORs and the habit planes without relying on experimental results or assuming an OR. The M.-X. ZHANG, Senior Lecturer, and P.M. KELLY, Honorary Professor, are with the Division of Materials, School of Engineering, University of Queensland, St Lucia, QLD 4072, Australia. Contact e-mail: [email protected] S.-Q. CHEN, formerly Postdoctoral Research Fellow, Division of Materials, School of Engineering, University of Queensland, and formerly Research Scientist, CSIRO, Colling-Wood, Brisbane, QLD 4072, Australia, is deceased. H.-P. REN, formerly Visiting Scholar, Division of Materials, School of Engineering, University of Queensland, is Professor, School of Materials Science and Engineering, Inner Mongolian University of Science and Technology, Baotou 014010, P.R. China. Manuscript submitted May 24, 2007. Article published online March 28, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
only model with truly predictive capabilities is the recently developed edge-to-edge matching model,[11–14] which has been successful in predicting all experimentally reported ORs and habit planes in both FCC/ BCC[11] and HCP/BCC systems.[12,14] Compared with the FCC/BCC and HCP/BCC systems, the HCP/FCC system has attracted much less attention. Ramanujan and co-workers[15,16] proposed a discrete lattice plane model to analyze the composition profile and surface energy between HCP and FCC structures in an Al-Ag alloy. Using the atomic site correspondence concept, Howe[17] considered that both the martensitic transformation and diffusion-controlled phase transformation could lead to the formation of surface relief in the FCC to HCP transformation. Aaronson and co-workers[18] used the structural ledge model to explain the crystallographic features of transformation in the FCC/HCP system based on experimental observations. Looking through most of the published work, it is clear that previous studies tried to explain or to understand the experimental observations in a single system or in only a few particular systems. Following its successful application to a range of simple
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