Interaction of iron particles with a solid-liquid interface in lead and lead alloys
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I.
INTRODUCTION
IN many studies of the interaction of foreign particles with advancing solid-liquid interfaces in water and organic materials, it has been shown that particles are rejected by the interfaces at low growth velocities.~-7 Theories have been proposed which predict the critical growth velocities below which the particles are rejected. ~.~6However, different particle and matrix materials which are solidified below critical velocities behave in different ways 145 which is not accounted for in the theories proposed. For example, carbon particles are pushed by an advancing interface in diphenylamine, and are not pushed in s a m under the same conditions.~ Accordingly, factors other than the growth velocity influence the particle-interface interaction. In metals there is no clear evidence that pamcles are rejected by an advancing solid-liquid interface at any velocity. In deoxidized steel ingots s'9'~~and E.S.R. ingots II l,, particle inclusions are observed to be preferentially situated in interdendritic regions. However, the mechanism by which the particles are segregated to these regions has not been established. The present investigation was undertaken to determine if particles in a metal are rejected by an advancing plane solid-liquid interface. In addition, cellular and dendritic interfaces were examined to establish the extent to which particles are concentrated in the intercellular and interdendritic regions and to try to account for this segregation. II.
PROCEDURE
A. Metal Particles in a Metal Matrix
Attempts were made to produce a uniform distribution of metal particles in lead using a number of different metal powders. In most cases these attempts were unsuccessful, in that the particles would not enter the melt even with extensive stirring and fluxing. It was found that iron particles
C. E SCHVEZOV is Graduate Student, Department of Metallurgical Engineering, University of British Columbia, on leave from Umversidad Nacional de Mislones, Argentina. E WEINBERG is Professor and Head, Department of Metallurgical Engineering, University of Brmsh Columbia. Vancouver, BC V6T 1W5, Canada. Manuscript submitted March 15, 1984. METALLURGICAL TRANSACTIONS B
could be introduced in lead with very little dissolution of the iron during a normal test. This system was therefore adopted for the present experiments. The iron particles were obtained by screening Armco iron powder. The particles were approximately spherical in shape and after screening (+400 mesh) were of 3 to 30 g m in diameter. The experiments with pure lead used lead of 99.999 pct purity. The lead alloys were prepared from material of 99.99 pct purity. Master alloys containing a high density of iron particles were produced by vigorously mixing the particles in the melt, using a ZnC12 flux, and rapidly casting the melt in a chill mold. Test samples were then made by adding pieces of the master alloy to the required melt. stirring vigorously, and chill casting into vycor tubes. The tubes were 200 mm in length, had an internal diameter of 11 mm
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