On the influence of in-situ reactions on grain size during reactive atomization and deposition
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I. INTRODUCTION
THE microstructure of spray-deposited materials has been extensively documented to consist of equiaxed grains, with an average grain size in the 10 to 100 m range, depending on the relevant thermodynamic and kinetic parameters.[1,2] However, in certain applications (e.g., superplastic forming), it is desirable to attain grain sizes that fall in the range of less than 10 m.[3,4,5] It then follows that it is essential to modify the spray-forming process in order to refine grain size in as-spray-deposited materials. In an effort to refine grain size, a modification of spray forming, namely, reactive atomization and deposition (RAD), was developed.[6–11] During RAD, the kinetic conditions that are present during spray processes (e.g., high surface area and elevated temperatures) are taken advantage of to promote the formation of in-situ dispersoids. In RAD, a reactive gas mixture is used instead of an inert gas, depending on the thermodynamics of the relevant system; target dispersoids include oxides, nitrides, and carbides. Oxide dispersoids are of particular interest given the affinity for oxygen of most metal systems, in combination with their inherent stability. The original development of RAD was based on the following two possibilities to refine the grain size in as-deposited materials: (1) enhanced heterogeneous nucleation in atomized droplets during flight and deposition caused by extraneous reaction products (e.g., oxides)[12] and (2) pinning of fine dispersoids (e.g., oxides) to grain boundaries.[6–11] In a related study,[11] Zeng et al. reported a similar average grain size in Ni3Al processed by RAD using 15 vol pct O2-N2 and by N2 spray deposition (SDN). However, Dai et al.[6] argued that, in RAD 5083 Al, the average grain size was significantly reduced with increasing oxygen concentration in the N2-O2 gas mixture. In addition, a number of other studies[9,13,14] have been completed in an effort to clarify the underlying kinetic mechanisms, which remain
YAOJUN LIN, Postgraduate Researcher, YIZHANG ZHOU, Associate Researcher, and ENRIQUE J. LAVERNIA, Professor, are with the Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616. Contact e-mail:[email protected] Manuscript submitted November 19, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A
heretofore poorly understood. As a result, it is essential to confirm the role of the RAD process on grain size. In this study, the influence of in-situ reactions during the RAD process on the evolution of grain size in the as-deposited 5083 Al is numerically and experimentally investigated in detail, by comparing RAD using 10 vol pct O2-N2 and the SDN process, which have the same processing parameters except for the atomization mediums used.
II. PROCESS DESCRIPTION AND PROBLEM STATEMENT During RAD or spray deposition using an inert gas, the evolution of grain size is governed by the events during two stages: flight and deposition. In the flight stage, atomized droplets are rapidly cooled by convection an
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