Structure and magnetic properties of Dd-Fe-B alloys

Permanent magnets Nd-Fe-B have the highest magnetic energy (> 400 kJ/m(3)). Use of permanent magnet in various industries extends recently to improve product quality and miniaturize electric devices. That's why there is need in materials with high magnetic properties and sufficiently low cost. The alloys on didymium (Nd+Pr) basis meet the best these requests. The present paper deals with microstructure, phase composition, micro hardness and magnetic properties of the alloys Dd-Fe-B with stoichiometric composition Dd(15)Fe(77)B(8) - Dd(16)Fe(76)B(8). Analysis of experimental samples revealed essential differences in their structure and phase composition connected with both melt temperature and conditions of alloy crystallization. Metallographic study showed that all ingots of cast alloys has multi-phase structure. Its basis (more than 80 %) is formed by faceted columnar crystallites Dd(2)Fe(14)B oriented perpendicular to solidification front. The grains of main phase are separated by intergranular intervals with non-uniform structure. The reason of such structure formation during alloy solidification is caused, probably, by the fact that the studied intergranular intervals are the most non-equilibrium due to the big differences in solidification point and different composition of main and intergranular phases. Obviously, the particles Nd similar to 13-21 % appear at first in this fluid; their separation enriches the rest fluid by neodymium. At the temperature about 655 degreesC fluid in equilibrium conditions has to crystallize as triple eutectic mixture. However, such process does not complete in non-equilibrium conditions; that's why only Nd individual sparse particles are formed, and the areas with composition close to triple cutectic are solidified. Metallographic study of the alloys Dd-Fe-B produced at different crystallization conditions showed their applicability for manufacturing permanent magnets with average magnetic parameters: residual induction B-r 0,8 - 1,25 T, coercivity H-C more than 800 kA/m and energy product (BH)(max) up to 350 kJ/m(3).