INVESTIGATION OF THE HOMOGENIZATION BEHAVIOR OF ND-FE-NB-B ALLOYS

In order to establish the role of Nb-Fe-B precipitates, generated in the microstructure upon the addition of Nb to the stoichiometric Nd-Fe-B alloy, three alloys: Nd11.8Fe82.3B5.9 (A), Nd11.8Fe81.3Nb1B5.9 (B) and Nd11.8Fe80.3Nb2B5.9 (C) were characterised by studying the changes in the microstructure, the electrical resistivity and the microhardness during homogenisation. The microstructure of the as-cast alloys contains: Nd2Fe14B matrix phase (phi), Nd-rich phase, free iron and in alloys B and C Nb26Fe32B42 ternary phase. When alloys B and C are homogenised they contain predominantly phi and Nb26Fe32B42 with only very small amounts of the Nd-rich material. Nb26Fe32B42 was found to nucleate within the free iron, providing strong evidence for the role of Nb in the removal of free iron from the microstructure. The electrical resistivity and the microhardness of the alloys were measured at regular intervals during the homogenisation process at 1000 degrees C. These studies revealed that, after 90 h, alloy B was homogenised completely and alloy C was homogenised after 40 h; this was in good agreement with the microstructural studies. The resistance measurements also indicated a degree of anisotropy in the as-cast material. At an intermediate stage, both alloys exhibit maxima in the resistance and microhardness curves which could indicate an age hardening process.