In situ observation of interactions between gaseous inclusions and an advancing solid/liquid interface
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between Gaseous Inclusions and an Advancing. Solid/Liquid Interface DONGKAI SHANGGUAN and DORU M. STEFANESCU It is well known that the distribution of foreign particles in the matrix is one of the major microstructural features determining the properties and performance of particle-reinforced metal-matrix composites. For metalmatrix composites produced by solidification processing, the particle distribution in the matrix is determined by the various interactions involved in the different stages of processing, i.e., transfer of particles from the atmosphere to the melt, particle-particle interaction in the melt, and interactions between foreign inclusions and an advancing solid/liquid interface. Of these three issues, the kinetics of the inclusion/interface interaction is the most crucial but also the most complex. Understanding the inclusion/interface interaction is of great fundamental as well as technical importance and has therefore been a subject of experimental and theoretical study over the past two decades. [1-5'7] In metallic systems, however, the study on this subject is obstructed by the opaqueness of the process. In this work, in situ observations have been made of the interaction between gaseous inclusions and an advancing faceted solid/liquid interface. It was thought from a fundamental point of view that these observations could facilitate understanding of the kinetics of the interaction between foreign inclusions and an advancing solid/ liquid interface in particle-reinforced metal-matrix composites. In this Communication, results of these observations are presented, and the relevance to the processing of metal-matrix composites is discussed. The observations were made with a transparent, o r g a n i c compound, salol (phenyl salicylate, 2-(HO)C6H4CO2C6Hs), which is a facetting material with a melting point of 43 ~ The as-received material was purified by zone refining prior to the experiment. The purified material was then sandwiched between a pair of glass slides, and a thin cell thus made containing the material under investigation was driven, at a constant velocity, V, across a temperature gradient, G, which was maintained by a hot plate and a cold plate separated by a certain distance, on a temperature gradient stage which is similar to that described elsewhere. [61 The experimental setup was horizontal. With this arrangement, salol crystals were grown unidirectionally under various growth conditions (growth velocity V and temperature gradient G), and the growth process was observed directly under a transmission optical microscope. Further details of the experimental observations can be found elsewhere. I31 DONGKAI SHANGGUAN, Assistant Research Engineer, and DORU M. STEFANESCU, Professor, are with the Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487-0202. Manuscript submitted June 21, 1990. METALLURGICAL TRANSACTIONS B
Salol solidifies with a faceted solid/liquid interface. When an advancing interface encounters a gas bubble, which is probab
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