Thermally Induced Microstructural Transformations of Fe 72 Si 15 B 8 V 4 Cu 1 Alloy
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CRYSTALLINE alloys obtained from amorphous precursors have been attracting great scientific interest in recent years due to their potential application in different fields, which are based on their favorable isotropic functional properties.[1–5] Due to kinetic and thermodynamic metastability, the amorphous materials are prone to structural stabilization under conditions of high temperature or pressure, or even during prolonged use at moderate temperatures, leading to formation of nanostructured materials with advanced mechanical and physical properties. Constructional Al-based alloys and magnetically soft and magnetically hard Fe-based alloys represent important groups of nanocrystalline materials obtained from amorphous precursors.[2] Their basic characteristic is crystallite diameter, while the optimal volume fraction of nanocrystals depends on the desired application. Thus, to obtain favorable, hard magnetic
MILICA M. VASIC´, and DRAGICA M. MINIC´ are with the Faculty of Physical Chemistry, University of Belgrade, Belgrade, Serbia. Contact e-mails: [email protected], dminic@ffh.bg.ac.rs RADOSLAV SURLA, NEBOJS˘A MITROVIC´, and ALEKSA MARIC˘IC´ are with the Faculty of Technical Sciences, University of Kragujevac, C˘ac˘ak, Serbia. DUS˘AN M. MINIC´ and LJUBICA RADOVIC´ are with the Military Technical Institute, Belgrade, Serbia. Manuscript submitted February 23, 2017. Article published online June 26, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A
properties, it is necessary to achieve full or almost full crystallization, but on the other hand, the optimal soft magnetic and mechanical properties are exhibited when the material is in partially crystallized form, consisting of nanocrystals embedded in amorphous matrix.[1–3] Although nanocrystalline materials of special technological interest can be produced using various methods and different starting phases,[2] the production from the amorphous precursor represents relatively simple way to obtain materials with desired properties. For this purpose, it is necessary to achieve fast nucleation and slow crystal growth rate.[2] These properties can be obtained by an appropriate choice of chemical composition of an initial alloy as well as thermal treatment procedure. The first nanocrystalline alloy which exhibits soft magnetic properties, obtained by crystallization of an amorphous alloy, was the one with chemical composition: Fe-Cu-Nb-Si-B, known under the trade name FINEMET.[6] The presence of Cu and Nb elements has a crucial role for creation of nanocrystalline structure. In this case, Cu atoms form clusters which serve as heterogeneous nucleation sites for crystallization of a-Fe(Si) phase, leading to higher nucleation rate.[7,8] On the other hand, Nb atoms, extruded out of the a-Fe(Si) grains, diffuse very slowly to the amorphous matrix due to the large atomic radius. In this way, concentration-gradient layer of Nb atoms grouping around grain hinders diffusion of Fe and Si atoms, slowing down the crystal growth rate.[7,9] Partial
VOLUME 48A, SEPTEMBER 2017—4393
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