Particle engulfment and pushing by solidifying interfaces: Part 1. Ground experiments
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I.
INTRODUCTION
THE phenomenon of interaction of particles with melt interfaces has been studied since the mid 1960s. While the original interest in the subject was mostly theoretical, researchers soon realized that understanding particle behavior at solidifying interfaces may yield practical benefits. The experimental evidence on transparent organic materials[1] demonstrates that there exists a critical velocity of the planar solid/liquid (SL) interface, below which particles are pushed ahead of the advancing interface and above which particle engulfment occurs. The solutal and thermal field, as well as fluid motion at the SL interface, influence both interface morphology and the particle/interface interaction itself. It is, thus, imperative to fully understand the solidification science and transport phenomena aspects associated with the process in order to control it in commercial applications. The scientific objectives of the work supported by NASA at The University of Alabama are (1) to enhance the fundamental understanding of the physics of interaction between inert particles and the SL interface, (2) to generate experimental data for model validation, (3) to further develop existing theoretical models, and (4) to investigate aspects of melt processing of particulate metal matrix composites (MMCs) in the unique microgravity environment that will yield some vital information for terrestrial applications. The research includes ground and microgravity experimentation. This article reports on the ground experiments FRANK R. JURETZKO, Graduate Research Assistant, Metallurgical and Materials Engineering, and DORU M. STEFANESCU, University Research Professor and Director of Solidification Laboratory, are with The University of Alabama, Tuscaloosa, AL 35487. BRIJ K. DHINDAW, formerly Visiting Scholar, Solidification Laboratory, The University of Alabama, is Professor, IIT, Kharagpur, India. SUBHAYU SEN, Staff Scientist, USRA, and PETER A. CURRERI, Metals and Alloys Group Leader and USMP Mission Scientist, are with the NASA Marshall Space Flight Center, Huntsville, AL 35806. Manuscript submitted October 24, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
(Part I) and some initial microgravity experiments (Part II) performed during the Life and Microgravity Science (LMS) Mission on the space shuttle Columbia between June 22 and July 6, 1996. II.
BACKGROUND
Even though the experimental and theoretical research has lately been focused on possible applications for MMCs produced by casting[2,3] or spray-forming[4,5] techniques, other applications of this concept have emerged. One recent application of particle pushing/engulfment is in the growing of Y1Ba2Cu3O7-d (123) superconductor crystals from an undercooled liquid.[6] The oxide melt contains Y2Ba1Cu1O5 (211) precipitates which act as flux pinning sites. These sites enhance the critical current density of the superconductor. Upon solidification, the 211 precipitates segregate at the crystal growth front. Optimal properties can only be obtained if uniform precipita
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