Structureand magnetic properties of Mechanically alloyed nanocrystalline Fe-Si (-M) [M=AI, B, Cr] Powders

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Fe-Si based alloys are traditionally well known soft magnetic materials which find applications in split-core current transformers, magnetic cores of electrical appliances, magneto-fluids, magnetic shielding, electromagnetic noise suppression, etc. Nanocrystalline materials prepared from melt-spun amorphous precursors with the average crystallite size less than the ferromagnetic exchange length exhibit ultra soft magnetic properties with very low coercivity ~10-3 Oe. Mechanical alloying (MA) is an alternate and commercially viable route for preparing nanocrystalline materials. Preparing mechanically alloyed soft magnetic nanocrystalline Fe-Si powders and understanding the correlation between the structure and magnetic properties of these materials are important from basic physics as well as application view points.This thesis work aims to understand (i) the correlation between the microstructure and magnetic properties of mechanically alloyed Fe-Si based nanocrystalline materials and (ii) to explore the possibility of improving their soft magnetic properties. The following atomic compositions were studied in the present work: Fe75Si25-xMx (x = 0, 5, 10; M = Al, B, Cr). The thesis work is presented in six chapters. The first chapter serves as a general introduction to the materials of interest to this thesis work, the theoretical formalisms relevant to them and the content of the thesis. The experimental techniques used in the preparation, processing and characterization of Fe-Si-M powders are discussed in the second chapter.In the third chapter, the results and discussion on Fe-Si and Fe-Si-M (M = Al, B, Cr) powders milled for various time periods have been presented. Milling was carried out for 80hours in a planetary ball mill. MA was found to produce non-equilibrium solid solution aDFe(Si) in case of Fe75Si25 powders with an average crystallite size of about 10 nm and dislocation density of 1018 m-2 after 80 hours of milling. The 80 hours milled powders showed coercivity of 128 Oe. An increase of coercivity with increasing milling time periods was observed, which has been mainly attributed to the introduction of dislocations and the reduction of average particle sizes in the course of the milling process. Domain wall pinning results increased coercivity in the milled powders. The Curie temperature (TC) of the powders milled for 80 hours was found to be higher than that of melt-spun ribbons of similar compositions. Such higher TC has been attributed to the lattice distortions caused by the presence of strain in the powders, induced during MA process. Similar studies were carried out on Fe75Si25-xMx (x = 5, 10) powder compositions, which also showed good correlations between the structure and magnetic properties. However, the structural and magnetic parameters were different for different alloy compositions. In the fourth chapter, the results and discussion on heat-treated nanocrystalline Fe-Si and Fe-Si-M (M = Al, B, Cr) powders have been detailed. Annealing has been found to have a significant effect on the microstructure and improvement of the soft magnetic properties of the powders. Annealing resulted in a gradual transformation of the disordered a-Fe(Si) phase of as-milled powders to ordered DO3 superlattice phase in the case of Fe75Si25 powders, as revealed by X-ray diffraction (XRD) studies. In a particula..
Supervisor: A. Perumal