An analytical model for the oxide size in Al alloys synthesized by reactive atomization and deposition
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INTRODUCTION
IN spray forming processes, a molten metal stream is disintegrated into a dispersion of micrometer-sized droplets by energetic gas jets (typically inert), and then the droplets are directed toward a substrate where they accumulate into a bulk material of predetermined size and geometry.[1,2] Convective cooling of the droplets during flight, which yields a limited amount of liquid phase during impingement, generally promotes the formation of a microstructure that is characterized by limited segregation, as well as relatively fine equiaxed grains, in the 10- to 100-m range.[1,2] Noteworthy is the observation that these microstructural characteristics have been widely reported for numerous alloy systems, regardless of the spray forming parameters or apparatus employed.[1,2] Reactive atomization and deposition (RAD) was originally developed in an effort to take advantage of the kinetic conditions that are present during spray processes (e.g., high surface area, elevated temperatures, and a controlled atmosphere) to promote the formation of in-situ dispersoids.[1,3–6] 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 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 December 3, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A
for oxygen of most metal systems, in combination with their inherent stability. One of the primary motivations to study this synthesis approach is to improve thermal stability and strength of spray-deposited materials. The strength, as well as other characteristics (e.g., thermal stability), of deposited materials can be improved via dispersion strengthening and grain size refinement arising from fine oxide dispersoids. The presence of these particles can also enhance thermal stability via retardation of recrystallization and grain growth at elevated temperatures.[7,8,9] The extent to which thermal stability and strength can be improved strongly depends on the amount, size, and distribution of oxide dispersoids in the final spray-deposited material.[10] As a consequence, thermomechanical processing (e.g., rolling, extrusion, forging, and hot isostatic pressing) is required for an as-spray-deposited material in order to obtain fine oxide dispersoids,[11,12] as well as to achieve full density.[1,2] However, as the starting point of oxide fragmentation during thermomechanical processes, the problem posed by the oxide size in as-deposited materials has received only limited attention. In related work, Zeng et al.[4] indicated that the size scale of Y2O3 and Y3Al5O12 is 0.1 to 0.2 m in width and 0.2 to 0.6 m in length in as-sprayed deposited RAD Ni3Al. However, Dai et al.[6
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