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

Magnetic nanocomposite alloys developed from amorphous precursors by anneal ing often exhibit metastable transition phases that ultimately control the kinetics of the nucleation and growth of the desired RE2Fe14B-type phase. I t is thus important to understand the effect of starting composition on the formation of the metastable transition phases found in the nanocomposite s ystem. The relationships between composition and the nature of metastable p hase 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) wer e studied and compared with results of other authors. The crystallization s equence undertaken by the quenched samples during annealing was followed by differential thermal analysis and x-ray diffraction. Differences in the no minal starting boron: RE compositional ratio of the alloy may produce signi ficant 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 interme tallic phases such as the cubic Nd2Fe23B3, the hexagonal NdFe12B6 and the c ubic Y3Fe62B14-type phases in the quenched material, while a low compositio nal 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 t he performance of these nanocomposite alloys should be on the effect that a lloying additions have upon the metastable phases, and not on the final equ ilibrium phases. (C) 1999 American Institute of Physics. [S0021-8979(99)600 08-1].