Didymium-iron-boron system permanent magnets and their structure and properties

The structure-phase transformations in didymium-iron-boron magnets after isothermal annealing in vacuum were investigated in order to establish the structure-properties correlation. Didymium is a natural mixture of the rare-earth elements, such as Nd, Pr, Ce, La, Dy, Tb, Gd and other. The heavy rare-earth elements which are contained in the didymium structure are in the main magnetic tetragonal φ-phase (R2Fe14B) lattice and can increase its anisotropy and as a result its magnetic properties. The influence of isothermal annealing on magnetic properties was investigated. It has been shown that at annealing temperatures about 350-400 C for samples annealed without subsequent quenching and at 45-500 C for samples annealed with subsequent quenching in water the maximum of the coercitivity iHc=1248 kA/m is observed. Metallographical study has shown that at this temperatures of annealing the structural state of the researched alloy is optimum. The change of the coercitivity in a researched temperature interval of annealing correlates with the change of the average grain size of the main magnetic φ-phase, with its quantity in a unit of volume and with microhardness of a sample. Magnetic properties are lowered at the temperatures of annealing below or higher than given. It was established that during the annealing of permanent Dd-Fe-B magnets the complex processes of solution of main and doping elements, which at certain temperatures cause the formation of the structure optimum from the point of view of its magnetic properties: grains are with correct cut and a thin boundary of section, on the surface of which a non-ferromagnetic phase places. The formation of high values of coercitivity in annealed Dd-Fe-B magnets is determined by two mechanisms