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

In order to establish the role of Nb-Fe-B precipitates, generated in t he 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) a nd Nd11.8Fe80.3Nb2B5.9 (C) were characterised by studying the changes in the microstructure, the electrical resistivity and the microhardnes s during homogenisation. The microstructure of the as-cast alloys cont ains: Nd2Fe14B matrix phase (phi), Nd-rich phase, free iron and in all oys B and C Nb26Fe32B42 ternary phase. When alloys B and C are homogen ised they contain predominantly phi and Nb26Fe32B42 with only very sma ll 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 resis tivity and the microhardness of the alloys were measured at regular in tervals during the homogenisation process at 1000 degrees C. These stu dies revealed that, after 90 h, alloy B was homogenised completely and alloy C was homogenised after 40 h; this was in good agreement with t he microstructural studies. The resistance measurements also indicated a degree of anisotropy in the as-cast material. At an intermediate st age, both alloys exhibit maxima in the resistance and microhardness cu rves which could indicate an age hardening process.