Effect Of Low-Frequency Magnetic Pulsing And It’s Positron Annihilation Technique Reseaching For Fe-Co-Hf-B-Cu Amorphous Alloy

HITPERM-type soft magnetic alloys Fe(Co)-M-B-Cu(M=Nb, Zr, Hf, etc.) have special structures, so they exhibit excellent soft magnetic properties. They are best promising for the more-electric aircraft (MEA) and other high-temperature application areas. In this paper, we use lots of research methods, such as X-ray diffraction (XRD), Mossbauer spectroscopy (MS), Positron annihilation spectroscopy (PAS), Transmission electron microscopy (TEM) and Differential thermal analysis (DTA) to research microstructure, structural defects and magnetic properties of (Fe1-xCox)86Hf7B6Cu1(x=0.4) amorphous alloy mainly before and after low-frequency magnetic pulsing treatment.The results of MS and TEM experiments show that low-frequency magnetic pulsing treatment can achieve single-phase nanocrystallization of (Fe1-xCox)86Hf7B6Cu1(x=0.4) amorphous alloy. During crystallization processes, the mean size of10-15nm nanocrystalline phase α-Fe(Co) is born and dispersed in the residual amorphous matrix phase to form the so-called two-phase nanocrystalline alloys. Under the different treatment conditions, the volume fraction of the crystallization is changed at the range of1.169%～13.272%. The hyperfine magnetic field of nanocrystalline two-phase alloys generated primarily from the contribution of the three areas, i.e., the crystallization phase, the interface and the remaining amorphous phase. Under the different treatment conditions, the mean hyperfine magnetic field of nanocrystalline two-phase alloys slightly lower than the value286.17kOe of the quenched alloys’. But the crystalline phase corresponding to hyperfine magnetic field is approximate330kOe. The hyperfine magnetic field of the remaining amorphous phase is noticeable lower than the quenched amorphous alloys’.After low-frequency magnetic pulsing treatment, the XRD pattern of nanocrystalline two-phase alloys is overall consistent with the quenched alloys, i.e., both of them show a diffuse diffraction peak. This phenomenon shows that the resolution capability (or sensibility) of XRD is relatively lower. And small amount of nanocrystalline is not easy to be detected by this means. During studying microstructure of materials intensively, this method has some limitations. After being treated under the different low-frequency magnetic pulse treatment conditions, the activation energy of nanocrystalline alloys slightly lower than the value280.9kJ/mol of the quenched alloys. This indicates that the thermal stability of nanocrystalline two-phase alloys decrease slightly compared with the quenched alloys.We use fast-fast coincidence ORTEC system to measure PAS. Time resolution of the lifetime spectrometer is about240ps. In order to acquire positron annihilation lifetime τi and the corresponding annihilation intensity Ii(i=1,2,3), we use three-exponent damped components to free fitting PAS. The results indicate that after being treated under the different treatment conditions, each kind of the positron annihilation lifetime value τi of alloys exhibits decline trends. And the each kind of lifetime component corresponding to the strength Ii; also appears smaller amplitude fluctuations. This means that during the process of low-frequency magnetic pulsing treatment, atomic magnetic moments occur to reciprocate magnetization/demagnetization under the effect of pulsing magnetic field, which leading to periodic magnetostriction vibration and short-range migration of the atoms so that Fe, Co magnetic atoms together to form nanocrystalline. This leads to structure of alloys relaxes continuously and various defects occur to annihilation, formation and recombination, etc.. In addition, some non-magnetic atoms (such as B, Hf, etc.) will also migrate with the migration of the magnetic atoms during the process of low-frequency magnetic pulsing treatment. This also leads to microstructure defects of alloys changed, for example, non-magnetic atoms fill in vacancies or micro-holes leading to the size of defects reduction, which resulting to reduce the value of positron annihilation lifetime. Overall, the values of all lifetimes take on a declining trend, which can also explain the migration of atomic vibrations leading to structure of the alloys more ordering with the treatment conditions being improved.