Growth of silicon particles in an aluminum matrix
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I.
INTRODUCTION
THE process
by which silicon particles, embedded in aluminum, become larger and fewer, in the course of isothermal heat treatment, has been variously called conglobation, spheroidization, and Ostwald ripening. None of these terms is fully descriptive in the sense that it identifies any one geometric property which can be monitored to describe the course of the process definitively. The reason for this is that simultaneous changes in the number of particles, in particle size and particle shape are all in progress together. Some idea of the nature of this evolution can be had from the scanning electron microscope pictures of Figures l(a), 1(b), and 1(c), which depict the forms of particles that have been separated by dissolving the aluminum matrix with hydrochloric acid from a modified 7 pct silicon casting. Successive pictures in this series represent the same material in the cast state and after different lengths of heat treatment at 540 ~ In the cast state the particles are fragments of dendrites. As heating proceeds, the particles become coarser and tend toward more rounded forms. At all times there is a wide diversity in both particle size and shape. In an effort to sort out these factors and, thus, to describe explicitly the changes that are in progress, a study has been made by the use of the shape-insensitive global parameters of quantitative microscopy. Aluminum-silicon alloys containing nominally 4, 7, and 12 pct of silicon, both unmodified and Na modified, were heated for times up to 2500 hours at temperatures of 500, 540, and 566 ~ The resulting microstructures were analyzed optically to determine for each condition the volume fraction (Vv) of silicon particles, the total surface area in unit volume (Sv) of the silicon particles, and their total curvature in unit volume E N. RHINES (deceased) was Distinguished Service Professor Emeritus, Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611. M. ABALLE, formerly Adjunct Associate Professor, Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, is Investigador Cientifico, Centro Nacional de Investigaciones Metalfirgicas, Av. Gregorio del Amo, 8, 28040 Madrid, Spain. Manuscript submitted November 5, 1984. METALLURGICAL TRANSACTIONS A
(Mr). In addition, the particles contained in a specified volume of alloy were both counted (Nv) and classified with respect to the volume of the individual particles, using a Coulter Counter for these latter purposes. By way of these observations it has been learned that the dispersion of silicon particles has several geometric characteristics that display simple, regular behavior in the course of heat treatment. The distribution of particle volume can be represented as being log-normal with a geometric standard deviation that remains constant throughout each heat treatment and is the same at all temperatures studied. Particle growth, as measured by the increase in volume of the particle of average volume, is a lineal function o
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