α-Fe/NdFeB Nanocomposite Magnets Prepared By Annealing Amorphous Alloy And High Pressure Torsion Deformation

The control of the microstructure of nanocomposite magnets is of importance for improving their magnetic properties. In present study, the crystallization kinetics of amorphous alloys with different initial states, the formation of nanocrystals and the magnetic properties of annealed alloys were investigated by employing x-ray diffraction(XRD), differential scanning calorimeter(DSC), transmission electron microscopy(TEM), and vibrating sample magnetometer(VSM). Theα-Fe phase in amorphous R7.5Fe81.5Co3Nb1B7 possesses a large crystallization activation energy of 349.4±4.1 kJ/mol. After annealing at 750℃, the grain size ofα-Fe and Nd2Fe14B phases in the R7.5Fe81.5Co3Nb1B7 is 27.0 nm and 40.0 nm, respectively. The large grain size leads to a low maximum energy product of 14.4 MGOe. The grain size ofα-Fe and Nd2Fe14B phases in partially crystallized R7.5Fe81.5Co3Nb1B7 after annealing at 650℃is 16.1 nm and 27.3 nm, respectively, which results in a maximum energy product of 17.8 MGOe. At the initial stage of crystallization, amorphous R7.5Fe81.5Co3Nb1B7 and partially crystallized R7.5Fe81.5Co3Nb1B7 show a similar activation energy for theα-Fe crystalline phase. However, the partially crystallized alloy shows a large increase in the crystallization activation energy of theα-Fe phase in subsequent crystallization process, which is helpful in impeding the growth ofα-Fe crystals and getting a uniform microstructure with a small grain size.The microstructure, magnetic properties and crystallization kinetics of amorphous Nd9Fe85B6 alloy after high-pressure-torsion-deformation(HPTD) have been investigated. The HPTD prior to annealing produces numerousα-Fe nanograins. At the torsion coefficient n=3, the volume fraction and grain size ofα-Fe phase are about of 40% and 10 nm, respectively. In HPTD samples annealed at 600℃, the grain size ofα-Fe and Nd2Fe14B phases is uniform and small, d = 16 nm and 26 nm, respectively. This results in an increase incoercivity by 26% and in remanence by 13%. The growth activation energy of theα-Fe and Nd2Fe14B phases is determined to be 91.9±14.4 kJ/mol and 46.4±2.4 kJ/mol, respectively.