| New kind of nanocrystalline soft magnetic alloys FeCoHfBCu-HITPERM exhibit excellent high temperature soft magnetic properties,so it has become one of the research hotspot in recent years.The(Fe1-xCox)86Hf7B6Cu1(x=0.3?0.6)alloy ribbons were prepared by single-roller-quenching technique and then treated by medium-frequency magnetic pulse.The changes of microstructure and structural defects of specimens before and after medium-frequency magnetic pulse treatment were researched by X-ray diffraction(XRD),transmission electron microscope(TEM),Mossbauer spectra and positron annihilation lifetime spectra(PAS).The physical mechanism of magneto-nanocrystallization was explained by stochastic resonance theory.Then,the thermal stability and soft magnetic properties of the materials were measured by differential thermal analysis,homemade magnetostriction coefficient measurement system and vibrating sample magnetometer(VSM)respectively.Finally,in order to obtain better soft magnetic properties of(Fe1-xCox)86Hf7B6Cu1 alloys,the samples after medium-frequency magnetic pulse treatment were annealed at low temperature.The microstructure,structural defects and soft magnetic properties of the samples after treated by low temperature vacuum annealed were analyzed by Mossbauer spectra,PAS and VSM respectively.Through the analysis of XRD,TEM and Mossbauer spectra,it was found that the as-quenched(Fe1-xCox)86Hf7B6Cu1(x=0.3?0.6)alloys owned amorphous structure,but after the medium-frequency magnetic pulse treatment,the nanocrystalline phase?-Fe(Co)was born and dispersed in the residual amorphous matrix phase to form the so-called two-phase nanocrystalline alloys.By analysising the fitting data of Mossbauer spectra,we can know that the pulsed magnetic field parameters(intensity,frequency and time)were closely related to the hyperfine parameters and the amount of crystallization.With the increase of the intensity or time of the pulsed magnetic field,the crystallization amount increased,but with the increase of the pulsed magnetic field frequency,the crystallization amount first increased and then decreased.It illustrated that the pulsed magnetic field frequency had an an optimal response range for crystallization,and within this range the pulsed magnetic field frequency can most effectively promote the development of the crystallization process.In addition,the three conditions of random resonance theory were met in the magneto-nanocrystallization process of(Fe1-xCox)86Hf7B6Cu1 amorphous alloys,so this theory can be used to analyze the physical mechanism.The PAS analysis results showed that the positron can be annihilated in the free volume of the amorphous matrix,and the first lifetime value(short life)?1 was corresponding to the annihilation lifetime of positron in the smaller free volume,such as single vacancy;and the second lifetime value(medium life)?2 was corresponding to the annihilation lifetime of positron in the larger free volume,such as vacancy clusters or microvoids.With the change of the intensity,frequency and time of the pulsed magnetic field,the diffusion of atom and the migratory annihilation of single vacancy would be changed,which made the ?1-?2 and their relative intensity changed.In addition,?2 was also closely related to the volume fraction of crystallization.We can find out the structural defects changes in the interior of materials by analyzing the changes of lifetime values and their relative intensity.The thermal stability of(Fe1-xCox)86Hf7B6Cu1(x=0.3?0.6)amorphous alloys with different compositions and different roll speeds was studied by differential thermal analysis.According to the analysis,the(Fe1-xCox)86Hf7B6Cu1(x=0.3?0.6)amorphous alloys with x=0.4 and 49 m/s roll rate had the highest crystallization activation energy and the best thermal stability.Through the measurement and analysis for the magnetostriction coefficient of(Fe1-xCox)86Hf7B6Cu1 alloys,we can get that the total magnetostriction coefficient of the two-phase nanocrystalline alloys was determined by two parts,one part was the positive magnetostriction coefficient of amorphous matrix phase and the other part was the negative magnetostriction coefficient of ?-Fe(Co)nanocrystalline phase,so the total magnetostriction coefficient of the two-phase nanocrystalline alloy was less than that of the as-quenched specimens.And with the increase of pulsed magnetic field intensity,the saturation magnetostriction coefficient decreased.In addition,with the increase of applied DC magnetic field intensity,the magnetostriction coefficient of specimens treated by magnetic pulse first increased and then decreased.The Co content also had a certain influence on the magnetostriction coefficient.The(Fe1-xCox)86Hf7B6Cu1 amorphous alloys with x=0.4 had the smallest saturated magnetostriction coefficient and relatively better soft magnetic properties.According to the analysis of VSM,it was found that the soft magnetic properties of A2 sample with pulsed magnetic field parameters 250 Oe,1500 Hz and 10 min were relatively better and the Hc=0.42 Oe,Ms=150.43 emu/g.In order to optimize the soft magnetic properties of(Fe1-xCox)86Hf7B6Cu1 alloys,the A2 sample after magnetic pulse treatment were annealed at low temperature.The microstructure,structural defects and soft magnetic properties of the annealed samples were measured by Mossbauer spectra,PAS and VSM.It was found that the vacuum annealing technology could optimize the soft magnetic properties of the materials,but it was selective for the annealing temperature.In this paper,the comprehensive soft magnetic properties of the A22 sample obtained by 200?/30 min annealing treatment were the best. |
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