Mechanism of Mechanically Induced Nanocrystallization of Amorphous FINEMET Ribbons During Milling
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IT has been known that FINEMET alloy with the composition of Fe73.5Si13.5B9Nb3Cu1 exhibits excellent soft magnetic properties. These properties originate from the heterogeneous structure consisting of nanocrystalline a-Fe(Si) grains embedded in an amorphous precursor produced by melt spinning.[1–4] Unfortunately, melt spinning ribbons are not suitable for applications where a large volume of soft magnetic materials with complex shapes is required. In contrast, Mechanical alloying can produce powders suitable for compaction and consolidation. This technique, which essentially consists of repeated cold welding and fracturing of powder particles to form new phases, has been used to synthesize FINEMET powder from pure elemental powders.[2,5] Localization of plastic deformation in the form of shear bands has been suggested as the main mechanism of grain size reduction in metallic powders.[5] Furthermore, crystallization on the regions with high plastic deformation in the amorphous materials is in contrast to the uniform distribution of crystallites in the amorphous matrix which is an essential condition for obtaining desired magnetic properties. Amorphization by mechanical alloying of elemental powders occurs by diffusion along primary component interfaces. During milling of the crystalline materials, the
T. GHEIRATMAND, Postdoctoral Researcher, and H.R. MADAAH HOSSEINI and P. DAVAMI, Professors, are with the Department of Materials Science and Engineering, Sharif University of Technology, Azadi Ave., P.O. Box 11155-9466, Tehran, Iran. Contact e-mail: [email protected]; [email protected] G. ABABEI, Senior Researcher, PhD, is with the National Institute of Research & Development for Technical Physics, 47 Mangeron Boulevard, 700050 Iasi, Romania. M. SONG, Professor, is with the State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, P.R. China. Manuscript submitted June 2, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
internal energy of system increases continuously due to the introduction of significant volume fraction of structural defects such as disordered grain boundaries, vacancies, and dislocations making amorphization more prone to occur.[5–7] Suryanarayana et al.[6] have stated two conditions needed to be satisfied for the formation of an amorphous phase: first, the large negative heat of mixing in the amorphous state which provides the thermodynamic driving force for the occurrence of the reaction and second, different diffusion coefficient of the primary constituents into each other and also in the amorphous phase.[6] Although solid state amorphization of FINEMETtype alloys has been extensively studied by many researchers,[7–11] very few investigations have been reported which explore the opposite; the crystallization transformation of amorphous FINEMET ribbons induced by mechanical milling. While the effect of the temperature increase during mechanical milling has been considered in some reports,[12,13] the effects of the high local pressure generated by the impac
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