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Investigation the Correlation Between Nanocrystallization and Consolidation Mechanisms and Their Effect on Magnetic Properties of Bulk Finemet Type Alloys

Gheiratmand, Tayebeh | 2014

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 46492 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Madaah Hosseini, Hamid Reza; Davami, Parviz
  7. Abstract:
  8. Finemet soft magnetic alloys in the form of toroidally winded ribbons are not suitable for industrial applications where a large volume of magnetic materials is required. Production of Finemet bulk alloy by powder metallurgy techniques is an applicable method to produce complex component with isotropic magnetic properties which are the same as ribbons. In this research, Finemet bulk magnetic alloy with composition of has been produced by consolidation of amorphous powders obtained by milling of melt-spun ribbons. At the all stages, the structure and magnetic properties were studied using X-ray diffraction, differential scanning calorimetry, transmission electron microscopy, scanning electron microscopy, Mössbuaer spectroscopy and a SQUID magnetometer. At the first step, the effect of quenching wheel speed on the structure and Curie temperature of Finemet alloy has been investigated. Ribbons were melt-spun at different wheel speeds and then were annealed to nucleate nano crystals embedded in the amorphous matrix. The results showed that at higher wheel speeds the greater potential energy triggers the formation of Fe(Si) crystallites. TEM observations confirmed the presence of an α-Fe(Si) phase with 11 nm crystallite size in the amorphous matrix of annealed ribbons. It was found that increasing the wheel speed results in the reduction of the Curie temperature in as-spun ribbons. Moreover, the Curie temperature of the intergranular amorphous region in the annealed ribbons was at least 80 ºC higher than that of corresponding amorphous phase in as-spun ribbons due to exchange interaction penetration of the adjacent Fe(Si) crystallites and relaxation processes. At the second step, Finemet amorphous ribbons were milled by high energy vibrational mill for different periods up to 65 min. The results showed that partial crystallization of the amorphous powder occurs during milling. TEM observations confirmed that an α-Fe(Si) phase with a mean crystallite size of ~9 nm nucleates inhomogenously on the plastically deformed regions in the ribbon milled for 45 min. DSC analysis indicated that under high energy vibrational milling the Fe23B6 phase becomes unstable and Fe2B, Fe3B and α-Fe phases could form instead in the amorphous matrix. Gibbs free energy calculations explained the increase of crystalline phases’ nucleation rates under the high pressures resulting from the mechanical milling impacts. Thermo-magnetic measurements showed the enhancement of the Curie temperature of amorphous phase during milling which is due to the annihilation of free volumes and microstructural ordering. The Hopkinson effect leads to the monotonic increase of magnetization with respect to the temperature before reaching the Curie temperature of amorphous phase. Moreover; the magnetization increases with the formation of the Fe(Si) phase while the coercivity decreases. Mössbauer spectroscopy revealed the formation of 13at% crystalline phase with composition of Fe-16.5Si. Finally, Finemet bulk alloy was produced by spark plasma sintering of the milled ribbons. Spark plasma sintering was carried out at super-cooled liquid region for short time of 7 minutes using powders milled for 36 min. The results indicated that during sintering the Fe(Si) phase with grain size of 9 nm forms in the amorphous matrix. The amount of Fe(Si) phase was calculated as 75% in the sample consolidated from fully amorphous powder. The relative density of this sample was measured as 96%. It was shown that at initial times of consolidation, the increase of density was due to the particles rearrangement. Afterwards, viscous flow with activation energy of 23.4 kJmol-1 was distinguished as the main mechanism of densification.
    In another research, the effects of relaxation and nanocrystallization on magnetic properties of (Fe0.5Co0.5)73.5Si13.5B9Nb3Cu1 ribbons have been investigated. Melt-spun Ribbons were annealed at different temperatures. The results indicated that the relaxation processes shift the Curie temperature of amorphous phase to the higher temperatures. It was also found that through crystallization phenomena the saturation magnetization increases due to the super-exchange between Fe and Co atoms in the crystalline phase. A slight variation in magnetization was observed at 700 ºC during heating due to the ordering transition in FeCo system
  9. Keywords:
  10. Milling ; Compression ; Magnetic Properties ; Finemet Alloys ; Melt Spinning ; Spark Plasma Sintering ; Nanocrystalline Finemet Bulk Alloys ; Amorphous Ribbons

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