On the role of magnesium and silicon in the formation of alumina from aluminum alloys by means of DIMOX processing
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I.
INTRODUCTION
THE DIMOX process has been actively studiedt~-91 for its relatively low cost, near-net-shape capability, and applicability of making several types of composites containing various fillers and controlled amount of metalsY ~ Furthermore, products of DIMOX have been proven to contain interconnected structure of ceramics and metals, 14,j21which provides the possibility of relatively high toughness as well as excellent strength and corrosion and wear resistance. In order to optimize the DIMOX process, it is of significance to understand not only what happened to the process, but also how and why the process could proceed under specific conditions. It has been widely accepted that both magnesium (Ms) and group IVA elements such as silicon (Si) are necessary for the growth of alumina from liquid aluminum alloys.15,6,131How and why these additional elements take effect seemed not as clear as is needed. From the application point of view, investigations on the effect LIU YANG, Professor, DEGUI ZHU and JUN ZHANG, Assistant Professors, and CHANGQING XU and JIAN ZHANG, Graduate Students, are with the Department of Materials Engineering, Southwest Jiaotong University, Chengdu 610031, People's Republic of China. This article is based on a presentation made in the "In Situ Reactions for Synthesis of Composites, Ceramics, and Intermetallics" symposium, held February 12-16, 1995, at the TMS Annual Meeting in Las Vegas, Nevada, under the auspices of SMD and ASM-MSD (the ASM/TMS Composites and TMS Powder Materials Committees).
2094--VOLUME 27A, AUGUST 1996
of additional metals on the microstructure show that the amount of these additions has an apparent effect on the microstructural morphology, especially the distribution and morphology of porosity,I3,4,~41and thus the properties of the products. Consequently, in order to optimize the utilization of additional metals, it is important to study the behaviors and arrangement of additions during and after the DIMOX processing in addition to studying the role of those additions on the nucleation kinetics of composites, which has been reported by many authors. This article thus reports the recent experimental investigation on the role of both Mg and Si in the process in air, particularly the distribution of them in the DIMOX process products from an aluminum alloy so as to explore the behaviors of these two crucial elements during processes. II.
EXPERIMENTAL PROCEDURE
For the purpose of elucidating the necessity and amount of additional elements, Mg and Si, an orthogonalized experimental procedure was designedY 5] The test layout, L~6145], is shown in Table I, in which the reaction temperature, reaction time, and amount of Mg and Si were chosen as the basic factors assumed to be the most important for the DIMOX process. After each reaction, both weight gain of the samples and the morphology (distribution of alumina, residual metals, and porosity) of the reaction products were determined as the judgement of the reaction. The original
METALLURGICAL AND MATERIALS TRANSA
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