DIRECT OBSERVATION OF SOLIDIFICATION MICROSTRUCTURES AROUND ABSOLUTE STABILITY

This paper presents, for the first time, experimental evidence for abs olute stability in a transparent alloy. The succinonitrile-argon syste m was used to investigate the morphological transition from a cellular to a planar solid-liquid interface at high solidification velocities. Directional solidification of the organic contained in thin capillary tubes was observed in situ with an optical microscope. At the maximum interface velocity of 1.5 mm/s, a reduction of the solute content of the alloy resulted in a morphological transition from a cellular to a planar growth front. This transition developed in three stages: (i) tr ansition from deep cylindrical cells to mixed patterns of cylindrical and elongated cells; (ii) transition from cylindrical and elongated ce lls to low amplitude cells; (iii) transition from low amplitude cells to plane front. Within the limits of uncertainties with respect to the material properties and the thermal gradient, correspondence between the experimental results on plane front transition and linear stabilit y theory exists. It is shown that the transitions observed are located in the region near the limit point of the neutral stability curve on the branch for absolute morphological stability. Copyright (C) 1996 Ac ta Metallurgica Inc.