The formation of lamellar-eutectic grains in thin samples
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THE fine microstructure of directionally solidified lamellar eutectics is the trace left behind in the solid by the periodic stationary pattern that the solid-liquid interface took on during growth. Such stationary growth patterns are the result of a dynamical balance between the competing effects of solutal diffusion in the liquid and capillary forces at the interface.[1] During the last decade, much progress has been made in the understanding of the dynamical characteristics of lamellar eutectic solidification patterns (stability range and modes of instability) thanks to numerous experimental and numerical studies,[2–11] but the mechanisms through which they appear during the early stages of directional solidification are still largely unclear, in spite of many valuable experimental investigations.[12,13] Usually, the solid in contact with the liquid is in a single phase at the onset of directional solidification. Before the final lamellar two-phase pattern becomes established, a complex transient, including a series of distinct processes (appearance of a seed of the initially absent phase, spreading of this phase onto the front, and rearrangement in order to form lamellae), has to take place. A good understanding of these processes is the key to a fine control of the eutectic microstructure. Lamellar eutectics are most generally composed of large regions (large compared to lamellar spacing), called eutectic grains, in which all the lamellae of either of the two phases have the same orientation (Fig. 1).[12] Recent studies have shown that there is a strong, although not obvious, connection between this crystallographic feature, and the regularity of the lamellar microstructure. Most of the often observed irregularities can be ascribed to the existence of eutectic grain boundaries.[4] Thus, the larger the eutectic-grain size is the more perfect the lamellar microstructure. Perfectly periodic lamellar patterns were obtained on a macroscopic scale in (thin) samples containing a single eutectic grain.[8] SILVE`RE AKAMATSU and GABRIEL FAIVRE, Senior Researchers (CNRS), are with the Groupe de Physique des Solides, CNRS UMR 7588, Universite´s Denis-Diderot et Pierre-et-Marie-Curie, 75251 Paris, Cedex 05, France. SE´BASTIEN MOULINET, PhD Student, is with the Laboratoire de Physique Statistique, Ecole Normale Supe´rieure, 75005 Paris, France. Manuscript submitted May 15, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
The formation of eutectic grains is thus an important aspect of the transient, which needs to be clarified. The experimental investigation of the processes occurring during the transient is made difficult by the broad ranges of characteristic lengths (from less than 1 m to several millimeters) and times (from less than 1 second to several minutes) involved. This difficulty can be surmounted by using the method of thin-sample directional solidification of a transparent analogue of metallic eutectics (CBr4-C2Cl6), which allows a continuous follow-up of dynamical processes with the adequate spatiotemporal
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