Study On The Low-Frequency Magnetic Pulsing Effect Of Amorphous Alloy Fe₇₈Si₉B₁₃ As Well As Its EET Simulation

Fe-based soft magnetic alloys containing nanocrystalline precipitates embedded in an amorphous matrix are generally prepaared by partial devitrification of rapidly solidified amorphous phase. In order to improve the soft magnetic properties of the double-phase nanocrystalline alloy, the grain sizes and volume fractions of the nanocrystalline will be adjusted by controlling the crystallization process. Therefore, the research on the formation and properties of the double-phase nanocrystalline alloy has attracted widespread attention in recent years and become a focus in soft magnetic materials.In this thesis the low frequency pulse magnetic field (LFPMF) treatment was adopted to nanocrystallize Fe78Si9B13 amorphous alloy to form the double-phase alloy. The microstructures and properties of untreated as well as treated alloys were examined by Mossbauer spectra, X-ray diffraction (XRD), transmission electron microscopy (TEM) and differential scanning calorimetric (DSC). The activation energy in phase transition process was determined through DSC profiles by Kissinger’s equation, and phase transformation thermodynamics and kinetics in nanocrystallization of amorphous alloy treated by LFPMF were studied. Then the models of amorphous and microcrystalline phase were established with the empirical electron theory (EET) and the microcrystalline model of amorphous alloy. The phase structure factors, interface structure factors and some other physical parameters of the Fe78Si9B13 amorphous alloys before and after the LFPMF treatment were calculated with BLD method and compared with the experimental values. The effect of magnetic pulses parameters, such as field intensity Hp, frequency fp, time t, etc., on the amount of nanocrystalline, the structure and properties of the alloy was determined. And the micromechanism in nanocrystallization of amorphous alloy by LFPMF treatment was discussed in the electronic level.Experimental results showed that the maximum temperature risingΔt in the LFPMF treatment process is no more than 10℃. and the internal and external temperature rising did not show significant difference. There did not exist high temperature or transient high temperature and temperature uniformity was good. Therefore, the LFPMF treatment on amorphous alloy for nanocrystallization at low temperatures is an innovative process because it is not only different from ordinary isothermal annealing, and also superior to the pulse electric field treatment in which the Joule heating effect always cause a high temperature rising.Treated by pulse magnetic field, the amount of crystalline and the magnetic moment of the Fe78Si9B13 alloys increased linearly with the magnetic field intensity and treating time, but did not show determined tendency with the magnetic field frequency. The optimal frequency range in this study was determined as 30-35Hz. At the same time, the hyperfine magnetic field of the alloy was affected after the LFPMF treatment due to the occurrence of nanocrystallization. The distribution of hyperfine magnetic field of the original amorphous samples is a single-peak located at about 260kOe. After the LFPMF treatment, this single-peak shifted to lower magnetic field and there was a sign of appearance of another single-peak at higher magnetic field. The peaks position varied with the pulse magnetic field parameters.There appeared two exothermic peaks in the DSC profiles in the nanocrystallization process of Fe78Si9B13 alloy treated by LFPMF, indicating that two phase transitions took place and the transformation were both primary crystallization. The crystallized phase was single-phase b.c.c a-Fe(Si). The activation energy of the first exothermic peak of the original Fe78Si9B13 amorphous alloy was determined by Kissinger’s equation as 433.6 kJ/mol. While the activation energy was only less than 219.3 kJ/mol when treated by LFPMF. The sharp decrease of the activation energy implied that the phase transition barrier was reduced and phase transition took place more easily when treated by LFPMF. This may be ascribed to that the LFPMF treatment decreases the critical nucleating radius rc, the critical nucleating free energyΔGc, increase of the nucleation rateⅠ, and then faster nucleation and growth, thus promoted the crystallizations of the amorphous alloys.The covalence electronic structures of amorphous and crystalline phases before and after LFPMF treatment were calculated using the bond length difference (BLD) method in the empirical electron theory (EET). The empirical calculation formula of the total magnetic moment were established according to the atoms proportions, phase structure factor nA and the contribution of each cell iron atoms to the magnetic moment of the Fe78Si9B13 alloy. The total magnetic moment of amorphous phase was calculated as 1.895μB and that of crystallized phase was 2.5579μB-Then the magnetic moment of the double-phase alloys after the nanocrystallization by LFPMF treatment was calculated based on the alloy composition and the total magnetic moment obtained above, and compared with experimental values. The error of these two is less than 10%, indicating that it is possible and reliable to calculate the magnetic moment of amorphous alloy from covalence electronic level in the first approximation. This can be applied to guide the optimization of soft magnetic properties of Fe78Si9B13 amorphous alloy. But the relationship between the magnetic moment and pulse magnetic filed parameters require further investigation.The out-of-phase interface valence electron parameters (P(hkl),P(uvw) andΔp, etc), those are related to the properties of amorphous and crystalline phases, were calculated and the micromechanism of nanocrystallization of Fe7gSi9B13 alloy by LFPMF treatment was explained using phase structure factor and interface conjunction factors. For Fe78Si9B13 amorphous alloy, the nA ofα-Fe in amorphous matrix is smaller than the others, then the driving force of phase transition is smaller while the nucleation rate is larger. Soα-Fe crystal is first crystallized. And then isα-Fe-Si crystal. They form a-Fe(Si) solid solution. Moreover, the electronic densities of the amorphous a-Fe (110) planes//crystallized a-Fe (110) planes, amorphous a-Fe (110) planes//crystallized a-Fe-Si (110) planes, amorphous a-Fe-Si (110) planes//crystallizedα-Fe-Si (110) planes, crystallized a-Fe (110) planes//crystallizedα-Fe-Si (110) planes are continuous under the first approximation, and the electronic density of both sides are high. When Fe78Si9B13 amorphous alloy was treated by LFPMF, the planes with certain phase difference and high electronic density, small density difference in the amorphous matrix can be ordered by diffusion and form the crystal nucleus of a-Fe andα-Fe(Si). Then with the movement of particle phase interface and the increase of phase interface area, the grains grow up. If the electronic density difference of phase interface is larger, the change of atomic size on interface will be more significant, the atoms states on interface or in phase will change much more, the resistance of interface movement will be higher, the grain’s growth will be more difficult. In another word, the electron density difference makes their accumulation and growth inhibited. Therefore, the crystallization under pulse magnetic field can only form tiny crystallized phase a-Fe-(Si) well dispersed in the amorphous matrix.