Loading...

Study of Nanocrystallization Kinetics, Microstructure and Magnetic Properties of Finemet-type Soft Magnetic Materials

Asghari Shivaee, Hossein | 2010

952 Viewed
  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 40756 (48)
  4. University: Sharif University of Technology
  5. Department: Institute for Nanoscience and Nanotechnology
  6. Advisor(s): Madaah Hosseini, Hamid Reza; nozad, Ahmad; Beitollahi, Ali
  7. Abstract:
  8. Nanocrystallization kinetics of amorphous Finemet-type alloys was studied using Isokinetic and Isoconversional methods (Kissinger model-fitting method, Vyazovkin advance method, Kissinger–Akahira–Sunose(KSA),Flynn–Wall–Ozawa(FWO),Tang and Starink) under isothermal and non-isothermal conditions. Effects of chemical composition on the nanocrystallization kinetics, microstructure and magnetic properties of Fe73.5Si13.5B9Nb3Cu1 and Fe77Si11B9Nb2.4Cu0.6 alloys were also investigated. Kinetic triplet (activation energy, frequency factor and reaction model) were determined by these methods and compared for the alloys. According to isoconversional methods, the activation energy is variable as a function of transformed fraction indicating reaction complexity. However, the mean values of 350 and 290 kJmol-1 were obtained for Fe73.5Si13.5B9Nb3Cu1 and Fe77Si11B9Nb2.4Cu0.6 alloys, respectively. Microstructures evaluations confirm that minor changes in chemical composition affect the kinetics and final microstructure of the alloy.The comparison of DSC results suggested that the smaller amount of Cu and Nb shifted the crystallization onset temperature of Fe77Si11B9Nb2.4Cu0.6 alloy towards lower temperatures. Thus, the values of crystalline volume fraction and grain size in each temperature for the annealed Fe77Si11B9Nb2.4Cu0.6 alloy are higher than the annealed Fe73.5Si13.5B9Nb3Cu1 alloy. The mean value of kinetic exponent n for Fe73.5Si13.5B9Nb3Cu1 alloy was 1.44±0.05 by considering instantaneous nucleation condition (n=m), which is consistent with one-dimensional growth mechanism. Kinetics and mechanistic predictions were also performed using numerical reconstruction of the experimental kinetic function. Comparison of the numerically reconstructed model with theoretical ones showed that no single model could perfectly fit the numerical values of kinetic function and the mechanism of transformation changes with conversion progress. Nevertheless it could be seen that nanocrystallization almost follows the one-dimensional diffusion mechanism The mean value of kinetic exponent n for Fe73.5Si13.5B9Nb3Cu1 alloy was 1.44±0.05 by considering instantaneous nucleation condition (n=m), which is consistent with one-dimensional growth mechanism. Theoretical isothermal curves calculated using impingement model can fit the experiments better than using the traditional JMA model, suggesting that the JMA equation is not suitable for investigating the crystallization kinetics of the Finemet metallic glass. Accurate kinetic parameters determined by considering the impingement and calculating variable activation energy showed that nanocrystallization in Finemet is a multi-step reaction. Almost similar results related to kinetic exponent and mechanisms of reaction have been obtained using both heating regime. 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. 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. The electrochemical corrosion behavior of Finemet alloy atvarious heat treatment temperatures was investigated. results showed that structural relaxation and nanocrystallization during the heat treatment improved corrosion behavior of the alloy. The heat-treated alloy at 650 ◦C showed a corrosion rate of 1.37×10-8 A cm-2 and appositive shift of +417 mV in the corrosion potential compared to the amorphous alloy and Also a higher charge transfer resistance up to 50kΩ due to corrosion resistance, compared with amorphous sample that showed a charge transfer resistance of 0.5 kΩ.
  9. Keywords:
  10. Nanocrystallization ; Finemet Alloys ; Isoconversional Kinetic ; Plasma Nitriding ; Electrochemical Behavior

 Digital Object List

 Bookmark

No TOC