Solidification of Undercooled Monotectic Alloys
- PDF / 1,634,906 Bytes
- 11 Pages / 414.72 x 648 pts Page_size
- 56 Downloads / 226 Views
491
SOLIDIFICATION OF UNDERCOOLED MONOTECTIC ALLOYS C. GALAUP AND K. P. COOPER+ J. H. PEREPEZKO+, +University of Wisconsin-Madison, Department of Metallurgical and Mineral Engineering, 1509 University Avenue., Madison, WI 53706, U.S.A; *Centre de Recherches de Pont-a-Mousson, ?Iaidieres 54700, Pont-4-Mousson, France.
ABSTRACT During the processing of monotectic alloys large compositional segregation of the liquid can precede final solidification. Under normal treatment a coarse scale phase distribution is obtained as a result of convection and sedimentation effects which may be minimized by microgravity processing. In undercooled droplets (5-201) of Bi-Ga alloys with compositions near the monotectic point, a finescale segregation has been observed and is similar to that formed during a microgravity treatment of bulk samples. Near the critical point of the miscibility gap, Bi-rich alloys exhibit an undercooling prior to phase separation, while in Ga-rich alloys the onset of phase separation is at the miscibility gap boundary. Similarly, in Cu-Pb alloys an undercooling below the miscibility gap prior to phase separation is observed for Cu-rich alloys, but not for Pb-rich alloys near the critical point. These observations are consistent with the operation of a critical point wetting behavior which can modify the liquid segregation pattern during microgravity treatment.
INTRODUCTION In contrast to many solidification processes in alloys which involve the reaction of a homogeneous single phase liquid, the solidification of monotectic alloys can involve an initially inhomogeneous liquid. The factors that control the extent and size scale of liquid segregation can influence significantly the final solidification microstructure. For example, under controlled processing with directional solidification treatment fine, twophase composite structures can be grown with monotectic alloys (1-4). The morphology of the aligned composite array can be modified in a sensitive way by changes in growth conditions and by selected alloying additions (4-6). The liquid phase separation that is characteristic of the monotectic reaction is encountered often in undercooled systems. At subliquidus temperatures in eutectic systems a liquid alloy may enter the domain of a metastable miscibility gap so that the progress of the final solidification pattern will be altered from that developed under minimal liquid undercooling conditions. Metastable immiscibility regions are a fairly common feature in inorganic glass forming systems (7,8) and have been reported in a few metallic systems (9). One consequence of the microgravity environment of space is an expected enhancement in application of containerless processing techniques for solidification (10). In addition to a potential reduction of contamination and the associated opportunities for the study of reactive liquids, containerless processing is expected to promote melt undercooling. Recent experiments with a long drop tube have provided support for the effectiveness of contain-
492 erless pro
Data Loading...
