Grain growth of nanocrystalline Ni powders prepared by cryomilling
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NANOCRYSTALLINE (nc) materials, characterized by a microstructural length scale of 1 to 100 nm, have been in recent years found to exhibit unusual physical and mechanical properties.[1] These classes of materials inherently possess a significant fraction of high energy, disordered grain boundary regions that provide a strong driving force for grain growth.[2] Since the unique properties are closely related to the fine grain size and the large volume fraction of grain boundaries, it is of vital importance to maintain the microstructure at a nanometer scale during consolidation to produce bulk nc materials or service as structural components at elevated temperatures. With this regard, a number of studies on the grain growth behavior of nc materials were carried out in the past (e.g., see References 1 and 3 through 10). It has been thus far shown that various factors may affect the grain growth of nc materials, such as grain boundary segregation,[3,4] second-phase (Zener) drag,[5,6] solute (impurity) drag,[7,8] pore drag,[9] and chemical ordering.[10] Recently, cryogenic ball milling or cryomilling was found to produce nc structures with enhanced thermal stability.[4,11–13] Cryomilling, originally developed to process nanophase dispersion strengthened aluminum alloys, is a high/low energy mechanical milling technique, by which elemental and/or prealloyed powders are milled in cryogenic media, e.g., liquid nitrogen, liquid argon, etc., to form a slurry.[14] Reaction between powders and oxygen or nitrogen from the environment or milling slurry under energetic milling conditions leads to in-situ formation of ultrafine particles, such as oxides, nitrides, and oxy-nitrides. Luton et al.[14] J. LEE, formerly Postdoctor, Department of Chemical and Biochemical Engineering and Materials Science, University of California at Irvine, is Researcher, Samsung SDI, 575 Shin-Dong, Paldal-Gu, Suwon City, Kyungki-Do, Korea. F. ZHOU and K.H. CHUNG, Research Associates, and E.J. LAVERNIA, Professor, are with the Department of Chemical and Biochemical Engineering and Materials Science, University of California at Irvine, Irvine, CA 92697-2575. N.J. KIM, Professor, is with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Korea. Manuscript submitted May 21, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
reported the formation of nanometer-scale oxy-nitride particles of 2 to 10 nm in diameter during cryomilling. The presence of these stable second particles was found to effectively stabilize the aluminum grains against thermally activated growth. Furthermore, in cryomilled Fe-Al powders, Perez et al.[4] also showed enhanced thermal stability of nc structure, which was attributed to finely dispersed aluminum or iron oxides and aluminum nitride particles formed during cryomilling. Recently, the enhanced grain stability of nc structures in cryomilled powders was reported in diverse alloy systems, such as Ni-based superalloy,[12] pure Al,[13] and TiAl.[15] The present work was m
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