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Study of Active Screen Plasma Nitriding Treatment On the Nanocrystallization Process in Finemet Alloys

Memarzadeh Lotfabad, Elmira | 2009

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 39887 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Madaah Hosseini, Hamid Reza
  7. Abstract:
  8. In this work, the effect of Active Screen Plasma Nitriding (ASPN) treatment on structural, mechanical, electrical and magnetic properties of nanocrystalline Finemet-type alloys with the composition of Fe73.5Si13.5B9Nb3Cu1 and Fe77Si11B9Nb2.4Cu0.6 was investigated. The samples were prepared by vacuum arc melting of high purity constitution in order to obtain a nanocrystalline structure. The rapidly solidified samples on a chilling wheel, were subjected to the ASPN treatment in different temperatures ranging from 410 °C to 560 °C for 3 h and in two gas mixtures of 75% N2-25% H2 and 25% N2-75% H2 at 5 mbar atmosphere. The amorphous ribbons were then annealed under vacuum with the same conditions of temperatures and time to compare both techniques. The structural and thermal properties of the samples were analyzed using various techniques such as X-ray Diffraction (XRD), Atomic Force Microscopy (AFM) and Differential Scanning Calorimetry (DSC). Microhardness measurements, electrical resistivity and Vibrating Sample Magnetometer (VSM) were used to study mechanical, electrical and magnetic properties of the samples, respectively. Scanning Electron Microscopy (SEM) with energy dispersive spectroscopy (EDS) was employed to the qualitative composition analyses and elements distribution from surface to the center of the ribbons. It was observed that the ASPN treatment leads to finer grain size and higher crystalline volume fraction and modifies the structural features of Fe(Si) phase. The lattice parameter of a part of the Fe(Si) phase where nitrogen has diffused in the nitrided samples, was larger than that in the annealed ones in which, it decreased up to 520 °C and then was raised at 560 °C. In ASPN treatment of Fe73.5Si13.5B9Nb3Cu1, the lowest magnitude of coercivity and maximum saturated magnetization were obtained at 440 °C in 75% N2-25% H2 and at 560°C in 25% N2-75% H2 gas mixtures, respectively. Our proposed method (ASPN treatment) increased the microhardness and electrical resistivity of the samples. The comparison of the DSC data for the alloys suggested that the smaller amount of Nb as a growth inhibitor shifted the crystallization temperatures towards lower temperatures. The electrical resistivity for the annealed and nitrided Fe77Si11B9Nb2.4Cu0.6 alloy was lower compared to the annealed and nitrided Fe73.5Si13.5B9Nb3Cu1 alloy at 440 °C and temperatures above, due to the larger grain size and lower Si content in Fe(Si) phase of Fe77Si11B9Nb2.4Cu0.6 alloy. The VSM results showed that the maximum saturation magnetization and coercivity in Fe77Si11B9Nb2.4Cu0.6 alloy were larger compared to Fe73.5Si13.5B9Nb3Cu1 alloy.
  9. Keywords:
  10. Nanocrystal ; Active Screen Plasma Nitriding ; Nanocrystallization ; Finemet Alloys

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