فهرست مطالب امیرحسین تقوایی

Nowadays, Fe-based amorphous/nanocrystalline alloys have been greatly developed due to the excellent magnetic properties, as well as their relatively low cost. In this study, the effect of process control agent (PCA) addition on the microstructure and magnetic properties of Fe75Ta5Si10C10 alloy produced by mechanical alloying was investigated. The X-ray diffraction (XRD) results showed that the addition of 2 wt. % PCA notably increases the glass forming ability (GFA) in the present alloying system and the percentage of the amorphous phase eventually reaches a value of 94 wt.% after 120 h milling. The amorphization reaction upon addition of the PCA was further confirmed by the transmission electron microscopy (TEM) analyzes. The crystallite size and lattice strain after 50 h milling were calculated as 10 nm and 1.44% after PCA addition, and 15 nm and 1.37% before that, respectively. The scanning electron microscopy (SEM) results confirmed that the powder prepared with the PCA exhibits finer particles with a more spherical shape. Magnetic measurements demonstrated that that both saturation magnetization and coercivity decrease and the soft magnetic behavior is improved with the PCA addition. Therefore, overall results in this study contain improvement of glass forming ability and soft magnetic properties with the PCA addition.

Co-based metallic glasses have attracted the attention of many researchers due to their excellent magnetic softness, as well as their high corrosion resistance. In the present research, Co-Fe-Ta-Hf-B glassy ribbon with a high thermal stability was prepared by melt-spinning and its isochronal crystallization kinetics at different heating rates was systematically investigated. The apparent and local activation energy of crystallization, the fraction of crystals at different temperatures and other kinetics parameters were calculated according to Ozawa method. In addition, the devitrification kinetics of the produced glass was studied based on the well-known Jahnson-Mehl-Avrami (JMA) and Sestak-Berggren (SB) models. The results indicated that the local activation energy increases with progress of crystallization. Furthermore, the average activation energy calculated based on the Kissinger model was 664 kJ/mol, which is larger than many other metallic glasses, and confirms a high thermal stability of this alloy. It was shown that the JMA model cannot be used for analysis of the crystallization behavior of the present alloy and due to a complex and autocatalytic nature of crystallization, the SB model gives more liable results for investigation of crystallization kinetics.

Nowadays, soft magnetic materials such as nanocrystalline and amorphous alloys with unique physical, mechanical and magnetic properties have attracted much attention. Recently, Fe-based amorphous alloys have been greatly developed due to their excellent magnetic properties and relatively low cost. In this study, effect of milling time on microstructure of Fe-C-Ta alloy prepared by mechanical alloying was studied. Besides, the possibility of glass formation was investigated according to thermodynamic calculations, performed based on advanced Miedema model. The X-ray diffraction (XRD) results proposed that the fraction of amorphous phase enhances by increasing milling time up to 70 h, and then it becomes unchanged up to 90 h milling. Moreover, the differential scanning calorimetry (DSC) results confirmed the formation of amorphous phase with a crystallization temperature of 678 K, after 70 h of milling. The thermodynamic calculations with respect to the advanced Miedema model revealed that the Gibbs free energy changes for glass formation (-42.35 kJ/mol) are larger than those of solid-solution formation (-28.5 kJ/mol) and consequently, the amorphous phase has a larger tendency to form after milling.