Modeling of oxide breakup during equal channel angular pressing of reactively gas-atomized Al powders
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THOUGH ultrafine-grained (UFG) materials (0.1 to 1 mm in grain size) produced by severe plastic deformation (SPD) techniques possess high strength at room temperature,[1,2] they have such a low thermal stability that recrystallization and grain growth readily occur when this class of materials is exposed to elevated temperatures, leading to an average grain size of approximately 10 mm.[3,4] In an effort to improve thermal stability of UFG materials, precipitates (e.g., carbides[5,6] and Sc-containing intermetallics[7,8,9]) at grain boundaries are used to retard grain boundary migration. However, coarsening and growth of precipitates at high temperature restrict the extensive application of this approach for improving thermal stability. A better method to improve the thermal stability is to introduce fine ceramic particles (e.g., oxides, nitrides, and carbides) by either chemical reactions or artificial addition. These ceramic particles are so stable that they neither decompose nor coarsen at high temperature. As a consequence, the thermal stability of UFG materials can be retained until a high temperature. This method has been extensively validated in nanostructured materials synthesized by ball milling.[10,11,12] However, no research efforts have been reported to use this method to improve the thermal stability of UFG materials synthesized by SPD. In the current study, an approach to introduce fine oxide particles into SPD Al alloys and other materials is proposed, which can be described as follows. Metal powders are first produced by oxygen-containing gas atomization to generate oxides on the powder’s surfaces, and then these powders are consolidated to a bulk material by a SPD method, equal channel angular pressing (ECAP), which not only produces ultrafine grains but also fragments the introduced oxides into fine oxide particles. In fact, ECAP has been used for effective consolidation of powders to produce bulk materials.[13,14,15] However, fragmentation behavior of the oxides on the powder’s surface during ECAP has received no attention. According to the Zener pinning theory,[16] higher oxide content or smaller oxide particle size benefits a smaller limiting grain size. However, YAOJUN LIN, Postdoctoral Research Associate, formerly with the Department of Chemical Engineering and Materials Science, University of California, Davis, is with the Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843-3123. Contact e-mail: [email protected] ENRIQUE J. LAVERNIA, Professor, is with the Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616-5294. Manuscript submitted November 14, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
a high oxide volume fraction reduces the material’s ductility. A smaller oxide particle size not only brings about a smaller limiting grain size, but also benefits the material’s ductility by minimizing the stress concentration level.[17] The objective of this numerical study is to investigate the influences of ECAP conditions, includi
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