Grain growth behavior of cryomilled INCONEL 625 powder during isothermal heat treatment
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NTRODUCTION
NANOCRYSTALLINE materials may be broadly classified as those having spatial attributes (e.g., particle size, grain size, film thickness, etc.) that fall in the 1 to 100 nm regime, although strict adherence to the upper bound (100 nm) is not a requirement. The unique properties of this class of materials are often attributed to the high concentration of interfaces and the unusual behavior that emerges when the physical size of a structure falls in the “nanometric” range. For example, in a crystalline material nearly 50 pct of all atoms in a grain volume will be located at or near grain boundaries when the grain size is 10 nm.[1,2] Interest in this class of materials increased rapidly when it was realized that early results on the availability of commercial quantities of nanocrystalline powders were confounded. The original studies by Gleiter,[3] using the vapor condensation technique, opened the door to a wide range of approaches that to date have been successfully implemented to generate large amounts of nanocrystalline materials. These include mechanical alloying,[4] plasma processing,[5,6] electrochemical methods,[7] vapor methods,[8] and rapid solidification.[9] More recently, research efforts have been driven by the need to manufacture two (e.g., thin film or coating) or three (e.g., bulk material) dimensional structures that can retain KYUNG H. CHUNG, Postdoctoral Student, RODOLFO RODRIGUEZ, Ph.D Student, and ENRIQUE J. LAVERNIA, Professor and Chair, are with the Department of Chemical, Biochemical Engineering and Materials Science, University of California at Irvine, Irvine, CA 92697-2575. JONGSANG LEE, formerly Postdoctoral Student, Department of Chemical, Biochemical Engineering and Materials Science, University of California at Irvine, is Manager, Samsung SDI, Suwon, Korea. Manuscript submitted March 19, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
the attributes of the individual nanostructured particles.[10] In the case of nanocrystalline materials, it is important to understand the stability of the structure during thermal exposure, as consolidation typically involves some type of deformation/temperature cycle. It is, therefore, not surprising that a number of studies of the grain growth behavior of many types of materials[10–13] fabricated using varying approaches[14–17] are available. In recent studies, the grain growth behavior of nanocrystalline materials was reviewed by Malow and Koch[10] and Gleiter.[18] Moreover, on one hand, significant grain growth was observed at room temperature in Sn, Pb, Al, and Mg,[19] Cu and Pd.[20,21] On the other hand, grain structures with a high degree of stability were reported for nanocrystalline alloys[22,23,24] and some pure metals.[25] A number of phenomena were proposed in an effort to account for the observed grain stability, namely: grain-boundary segregation,[26] solute drag,[22] pore drag,[27] second phase drag,[23] and chemical ordering.[28] INCONEL 625,* a Ni-based superalloy, has excellent *INCONEL 625 is a trademark of INCO Alloys Interna
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