Microstructural characterization of Ni 3 Al processed by reactive atomization and deposition
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INTRODUCTION
THE use of particle dispersions to improve the mechanical properties of metals at elevated temperatures was first reported in 1910 by Coolidgem for thoriated tungsten. The first oxide-dispersion-hardened alloys, sintered aluminum powders (SAP), were developed in 1952 by a Swiss research group, t2J The principal accomplishment of this work was the incorporation of the dispersoid, A1203, into the matrix aluminum powder during a grinding operation. One of the important characteristics of these dispersion-strengthened materials was the observed increase in strength with increasing oxide content up to approximately 12 to 15 wt pct AlzO3. In 1970, the application of the mechanical alloying technique to produce dispersion strengthened superalloys was reported by Benjamin. t3j Since then, a large number of oxide-dispersion-strengthened (ODS) systems have been successfully developed using a variety of techniques. The elevated temperature strength and thermal stability typically associated with ODS alloys are generally derived from the presence of refractory oxide (e.g., A1203, Y203) particles less than 0.1 /xm in size, which are evenly distributed in such a manner as to lead to an interparticle spacing of less than 0.5/zm in the consolidated product. In a previous investigation conducted by the authors,[41 a novel processing methodology, reactive atomization XIAOLU ZENG, Graduate Student, and ENRIQUE J. LAVERNIA, Associate Professor, are with the Department of Chemical Engineering and Materials Science, University of California, Irvine, CA 92717-2575. STEVEN R. NUTT, Professor, is with the Department of Materials Science, University of Southern California, Los Angeles, CA 90089-0241. Manuscript submitted February 9, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
and deposition (RAD), was developed to combine atomization, reaction, and consolidation into a single-step process and thereby to synthesize ODS and other dispersion-strengthened materials. In RAD processing, a stream of molten material is energetically atomized into a dispersion of micron-sized droplets using reactive-gas jets. The atomized droplets undergo chemical reactions with the surrounding gas media while simultaneously being rapidly solidified in the spray cone. The solid, liquid, and mushy droplets are subsequently deposited on a cooled substrate and collected as a thick preform. The microstructure of RAD processed materials is largely governed by the temperature and extent of solidification of the droplets. A schematic diagram of RAD processing is shown in Figure 1. This synthesis approach offers the opportunity for in situ continuous control over alloy composition and chemical reactions between atomized droplets and atomization gas. By carefully selecting the combinations of alloying additions and reactive gas on the basis of thermodynamic considerations, it is possible to use RAD processing to synthesize materials containing in situ dispersoids, such as oxides, carbides, and nitrides. The mechanical behavior of RAD processed materia
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