Preparation of Freestanding Zn Nanocrystallites by Combined Milling at Cryogenic and Room Temperatures
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INTRODUCTION
MECHANICAL milling or ball milling was established as a flexible technique for synthesis of a wide range of nanostructured metals, alloys, and ceramics.[1–4] The process of mechanical milling is carried out in a high energy ball mill. Mechanical milling of a ductile constituent at the ambient temperature involves repeated deformation, cold welding, fracturing, and dynamic recrystallization, leading to the formation of nanocrystalline aggregates.[1,2] The impact forces imposed by the balls cause plastic deformation, work hardening, and ultimately the fracture of the particles. The main disadvantage of this process is the generation of structural defects in the microstructure. On the other hand, the cryomilling is a variation of mechanical milling in which powders undergo ball milling in a cryogenic slurry (mixture of the power with liquid nitrogen) or at cryogenic temperatures.[5] It takes advantage of both extremely low temperature and conventional milling. The cooling of the powders to vary low temperature often promotes fracture. In addition, the recovery and dynamic recrystallization processes can be suppressed at cryogenic temperature, causing further grain refinement.[5] Moreover, mechanical milling at cryogenic temperature for shorter duration effectively reduces the contamination from the milling environment and the milling media.[5] The experimental and theoretical studies suggest that for any material, there is a minimum grain size attainable CHANDRA SEKHAR TIWARY, SANJAY KASHYP, Research Students, and KAMANIO CHATTOPADHYAY, Professor, are with the Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, Karnataka, India. KRISHANU BISWAS, Associate Professor, and AKASH VERMA, Undergraduate Student, are with the Department of Materials Science and Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, Uttar Pradesh, India. Contact e-mail: [email protected] Manuscript submitted May 25, 2012. Article published online November 27, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
during mechanical milling. According to Mohamed,[6] the minimum grain size during mechanical milling is decided by the balance of two factors: (1) defect generation and (2) defect recombination or annihilation. These two factors are strong functions of temperature. The first factor depends on the ability of a material to deform plastically and thereby to form dislocations, twins, and other defects in the microstructure. On the other hand, the second factor is controlled by recovery. The dynamic recovery can effectively be suppressed by low-temperature milling for many materials.[7] In addition, as already mentioned, low temperature aids the process of fracturing and thereby leads to reduction of the crystallite size. However, the plastic deformation is also limited at extremely low temperatures; therefore, defect generation will not suffice to get fine grain size. In contrast, room temperature milling (RT milling) will cause defect generation and recombination. Therefore, a combined milling a
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