The structures expected in a simple ternary eutectic system: Part 1. Theory
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THE purpose of this paper is to discuss the structures expected during the steady state directional growth of alloys in simple ternary eutectic systems such as that shown in Fig. 1. It is shown that theideas developed to predict the ranges of structures in two component systems may be extended to predict the ranges found in three component systems. Considering the simple ternary system of Fig. 1 and assuming that each of the three primary phases a, fl and y grow in a nonfaceted fashion, and nucleate easily, a number of different structural regions are then to be expected. The composition ranges of each of these for a fixed growth velocity and temperature gradient are shown schematically in Fig. 2, and referring to this figure: Region 1--Near the pure component corners single phase growth occurs with a planar growth interface (e.g. Flemingsl); Region 2--Near the two component eutectic compositions two phase eutectic growth occurs, again with a macroscopically planar growth interface (e.g. Flemings2), and usually with a lamellar or rod-like structure2 The growth interface is shown schematically in Fig. 3(a); Region 3--For alloys near the eutectic valley two phase eutectic cells or dendrites are expected with the third component appearing near the ternary eutectic temperature either as a thin layer of this phase or as a three phase eutectic layer around each cell depending on the volume fraction of the ternary eutectic. The growth interface is shown schematically in Fig. 3(b). Region d Near the ternary eutectic composition a three phase eutectic structure is expected without any primary single phase or two phase eutectic (e.g. Kerr et a14). The three phase structure should grow with a (macroscopically) planar interface in a similar fashion to the pure single phase, and the pure two component eutectic. The growth interface is shown schematically in Fig. 3(c). Clearly the actual structure may be complex D. G. MCCARTNEY, formerly with Alcan Laboratories Ltd., Banbury, Oxon, United Kingdom is now Research Student, Department of Metallurgy, Oxford University, Oxford, United Kingdom, J. D. H U N T is Lecturer, Department of Metallurgy and Science of Materials, Oxford University, and R. M. JORDAN, formerly with Alcan Laboratories Ltd., is now with Alcan Canada Products, Arvida, Quebec, Canada. Manuscript submitted May 14, 1979. METALLURGICAL TRANSACTIONS A
and not necessarily a simple lamellar or rod-like one: Region 5--For alloys away from the eutectic valleys single phase cells or dendrites are to be expected, followed by the structures described in regions 2, 3 and 4, and can be as follows: a) Single phase dendrites can be followed by two
C A B
Fig. 1--Ternary eutectic phase diagram.
EUTECTIC VALLEYS
1
1 2
Fig. 2--Schematic representation of the composition boundaries of the various structural regions (dotted and solid lines) for a fixed velocity and temperature gradient. The numbers and letters refer to the regions described in the text.
ISSN 0360-2133/80/0811-1243500.75/0 9 1980 AMERICAN SOCIETY FOR METALS AND T
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