Effect of the boron:rare-earth ratio on metastable phase formation in Nd2Fe14B-based nanocomposites

Magnetic nanocomposite alloys developed from amorphous precursors by annealing often exhibit metastable transition phases that ultimately control the kinetics of the nucleation and growth of the desired RE2Fe14B-type phase. It is thus important to understand the effect of starting composition on the formation of the metastable transition phases found in the nanocomposite system. The relationships between composition and the nature of metastable phase formation in the low-boron content melt-spun nanocomposite alloy Nd-2[Co-0.06(Fe1-xCrx)(0.94)](23.2)B-1.48 (0 less than or equal to x < 0.09) were studied and compared with results of other authors. The crystallization sequence undertaken by the quenched samples during annealing was followed by differential thermal analysis and x-ray diffraction. Differences in the nominal starting boron: RE compositional ratio of the alloy may produce significant changes in the nature and stability of the metastable phases present in the system. In general, a high compositional ratio (B:RE approximate to 1-4) promotes the initial formation of Fe3B and complex boron-rich intermetallic phases such as the cubic Nd2Fe23B3, the hexagonal NdFe12B6 and the cubic Y3Fe62B14-type phases in the quenched material, while a low compositional ratio (B:RE < 0.5) is favors the formation of alpha-Fe and a 2-17-type phase. It is suggested that the focus of further studies done to optimize the performance of these nanocomposite alloys should be on the effect that alloying additions have upon the metastable phases, and not on the final equilibrium phases. (C) 1999 American Institute of Physics. [S0021-8979(99)60008-1].