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NANOCRYSTALLINE materials are defined as materials having grain sizes in the range of 1 to 100 nm, and these have a significant percentage of the total atoms located at the grain boundaries. As a result, nanocrystalline materials show quite different physical and mechanical properties as compared to coarsegrained materials.[1–6] The development of nanocrystalline soft magnetic materials paved a way for materials with high permeability, low coercive force, and high magnetic flux density.[6–10] The origin of magnetic softening via grain size reduction in the nanoscale region was explained on the basis of the random anisotropy model (RAM).[6,8,11,12] According to RAM, when the grain size is smaller than magnetic exchange length, the origin of the soft magnetic properties in the nanocrystalline materials is ascribed to the averaging of the magnetocrystalline anisotropy due to the random distribution of the nanoscale grains. Among the different methods of synthesis, the mechanical alloying of powders by high-energy ball milling has been considered as one of the most powerful methods for the synthesis of nanocrystalline materials S.K. VAJPAI, Research Scholar, and R.K. DUBE, Professor, are with the Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur, Uttar Pradesh, India, 208016. Contact e-mail: [email protected] A. TEWARI, formerly Assistant Professor, Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Senior Research Scientist, is with General Motors India Pvt. Ltd., Bangalore, Karnataka, India, 560066. Manuscript submitted February 16, 2008. Article published online July 30, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A

because of its simplicity, relatively inexpensive equipment, and the possibility of producing in large quantities. Grain refining by high-energy ball milling is capable of producing nanocrystalline metal powder for a range of metals, e.g., Cr, Fe, Nb, W, Hf, Co, etc.[13] Mechanicallyalloyed (MAed) micron-sized powders have been shown to possess nanoscale substructures for systems such as Co-Ni,[14] Co-Fe,[14] Fe-Ni,[15–19] Fe-based soft magnetic materials,[20–22] Fe-Si system,[23] and Ni-Fe-Mo.[24,25] In most systems, the nanograins or amorphous structure represents a ‘‘far from equilibrium’’ condition that is generally metastable in nature. For most commercial applications, consolidation of nanocrystalline powders is required for making a product. Due to highly metastable nature of MAed nanocrystalline powders, a processing route is required, which does not allow excessive grain coarsening. Conventional processing routes such as powder compaction-sintering, powder compaction-sintering-mechanical working, and hot isostatic pressing of powders involve heating of powders at elevated temperatures for a long period of time leading to the coarsening of grains. The explosivecompaction process has been found to be a suitable method to consolidate a variety of nanocrystalline powders.[26–35] During explosive compaction at ambient tem