Atomic structure of the crystalline/amorphous interface in a directionally crystallized Pd 80 Si 20 alloy
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INTRODUCTION
ALTHOUGH the theory of directional solidification of single and multiphase alloys is well-established and a number of experiments have verified many aspects of the t h e o r y , tl-7] o n e important aspect that remains to be compared with theory is the atomic structure of the solid/liquid interface. Since the atomic structure of the solid/liquid interface determines both the growth kinetics and resulting morphology of the crystalline phases, it is important to be able to examine this structure experimentally. This investigation was undertaken to determine the atomic structure of a solid/liquid interface in a lamellar eutectic system. Work is currently in progress to connect these atomistics with the kinetics and thermodynamics of the transformation. However, in keeping with the theme of this symposium, this paper emphasizes results concerning the atomic structure of the solid/liquid interface. Several different theoretical approaches and computational techniques have been developed to investigate the atomic structure and mechanisms of attachment at solid/liquid interfaces and the role of these features in growth kinetics. These include early analytical treatments of solid/liquid interfacial structure based on factors such as the kinetic undercooling and its relationship with the crystallography and ledge structure of the interface, 12'4's'9] the entropy of fusion of the solid, tt~ and the driving force for the solid to liquid transformation, t12,13] One important prediction from these W.H. BREARLEY, formerly Graduate Research Assistant, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, is Manufacturing Process Engineer with International Business Machines Corporation, East Fishkill Facility, Hopewell Junction, NY 12533. P.-C. SHIEH, Graduate Research Assistant, and J.M. HOWE, Alcoa Associate Professor, are with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213. This paper is based on a presentation made in the symposium "The Role of Ledges in Phase Transformations" presented as part of the 1989 Fall Meeting of TMS-MSD, October 1-5, 1989, in Indianapolis, IN, under the auspices of the Phase Transformations Committee of the Materials Science Division, ASM INTERNATIONAL. METALLURGICAL TRANSACTIONS A
analyses is that the solid/liquid interface will be atomically flat or faceted for materials with high entropies of fusion, i.e., ASjR > 2 (where ASs is the entropy of fusion and R is the universal gas constant), as illustrated in Figure l(a), and nonfaceted or diffuse for materials with relatively low entropies of fusion, tl~ as illustrated in Figure l(b). Another important prediction is that a faceted interface will become increasingly rough as the driving force for growth increases and the critical size for two-dimensional nucleation decreases, [13] as illustrated in Figure l(c). At very high undercoolings, the structure of the rough interface in Figure 1(c) may eventually approach that of the nonfa
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