The Mechanism And Effects Of Low-frequency Pulsed Magnetic Field On The Fe-based Amorphous Alloys

Magnetic field, as an important energy field, is often used alone or in combination with other energy fields to change the microstructure and to improve the performances of materials. Crystallization of amorphous alloys is one of the methods to prepare nanocrystalline materials, In this thesis, two Fe-based amorphous alloys (Fe78Si9B13and Fe73.5Cu1Nb3Si13.5B9) have been treated by the low-frequency pulsating magnetic field. By applying the pulsed magnetic field(PMD), the amorphous samples can be nanocrystallized in a short period of time at a temperature below the crystallization temperature. This is a novel method to prepare nanocrystalline alloys by a low-temperature rapid crystallization method, which is different from the ordinary isothermal annealing, magnetic annealing and other methods. This method is called amorphous’s magneto-nanocrystallization.The effects of the low-frequency PMD on the crystallization, microstructure and magnetic performance of Fe78Si9B13amorphous alloy have been investigated in the present work. By using non-contact infrared thermometer and other temperature measurement devices, the temperature rise of the samples were measured; by means of Mossbauer spectrometer, transmission electron microscopy (TEM), positron annihilation lifetime spectroscopy (PAT), the microstructure and crystallization behaviors of the samples were investigated; the magnetic properties of the specimens were analyzed by an alternating gradient magnetometer (AGM). The present work presents shed a new light on the microstructure and characteristics of the nanocrystalline alloys which were produced by amorphous magneto-nanocrystallization method. The detailed results are mentioned as follows.The temperature rise of the pulsating-magnetic-field treated samples were measured by a thermocouple and a non-contact infrared thermometer separately. The results revealed that the amorphous sample can be nano-crystallized by PMD treatment when the temperature rise ΔT is below10℃. The calculated AT is△T=5.54℃as estimated by the adiabatic method.Mossbauer spectrometer, TEM and PAT have revealed that:(1) The amorphous alloy Fe78sSi9B13can crystallize in a nanocrystalline single phase by low-frequency PMF. The crystalline phase consists of α-Fe(Si) with a grain size of2-10nm, and the crystalline volumeic fraction is3%-7%;(2) After the low-frequency PMF treatment, the Si concentration in the a-Fe(Si) phase reaches about10%in the slightly crystallized samples Si atoms occupy the substitutional sites in the crystal lattice of the α-Fe(Si) phase. B atoms in the sample locate in the residual amorphous phase or the interfaces between the crystalline phase and the residual amorphous phase;(3) Treated by PMF, atomic magnetic moment shows a certain preferred orientation, with an increase of the normal opposite;(4) With treatment of the low-frequency pulsating magnetic field, the changes of the microstructure in the amorphous alloy depend on the synergistic effect of pulse frequency, magnetic field intensity, and the acting time of the magnetic field,①with the increase of the magnetic field intensity, the isomer of the crystallization phase decreases, the absolute value of the quarter-pole splitting diminishes, the crystallization hyperfine magnetic field increases, the content of the crystallization phase increases, the short-lived annihilation value τ1of the types in the sample decreases, the intensity I1reduces with the increase of the pulsating magnetic field intensity;②The volume fraction of nano-crystals, which precipitated from the amorphous matrix, increases with the time increase of low-frequency PMF;③There is an optimal magnetic field frequency for producing the amorphous magneto-crystalline transition. In the region, a balance of free volume attenuation increase; the crystallization phases form in the region, with the maximum nucleation rate and maximum volume fraction of grains.The studies of magnetic properties show that:the soft magnetic properties of the amorphous alloys have been significantly improved when no crystallization phase separated from amorphous matrix, by the PMF treatment, at100Oe,10Hz and a time period (t) of1200s; The soft magnetic properties can be improved in the presence of a small amount of precipitated nanocrystallites.The optimal parameters for PMF treatment are H=379Oe,f=40Hz and t=180s. The coercivity variation rate (△HC/Hc0) can reach79.6%.In the case of amorphous alloy Fe73.5Cu1Nb3Si13.5B9, low-frequency PMF can induce a rapid crystallization into the nanocrystalline phase at a low-temperature, and the soft magnetic properties were improved.Based on the experimental results mentioned above, the physical mechanism of magnetoism induced crystallization into nanocrystalline phase for Fe-based amorphous alloys was preliminarily discussed by the stochastic resonance in the present work.