Microstructural analysis of strip cast Nd-Fe-B alloys for high (BH)max magnets

High energy density magnets >400 kJ/m(3) are increasingly used in many applications. Conventional casting techniques for sintered magnets reveal the formation of a high quantity of alpha-Fe and large Nd-rich regions, New techniques, like strip casting, produce homogeneous and fine scaled microstructures and are already used for producing high (BH)(max) magnets. The fast cooling rate during strip casting suppresses the formation of alpha-Fe dendrites and of large Nd-rich pockets. Directional solidification causes a formation of columnar grains containing a typical arrangement of hard magnetic Nd2Fe14B regions and Nd-rich regions, The Nd regions occur as intragranular platelets as well as intergranular phases. Intragranular lamellae show a periodicity which corresponds to a eutectoidal solidification according to the composition of the liquid and are directed parallel to the temperature gradient during solidification. The lamellae show an average width of 150 nm, a spacing of 3 mu m, and a length up to the size of the hard magnetic grains. The fine separation of the hard magnetic and Nd phases is advantageous for the milling of the alloy after hydrogen decripitation and improves sinterability of magnets. Although the microstructure of strip cast alloys is much finer than that of ordinary cast alloys, the alignment of the powder is not deteriorated and B-r is not reduced due to a sufficient large interlamellar spacing between the Nd-rich platelets that enables the formation of single crystal powder particles after milling. (C) 1998 American Institute of Physics. [S0021-8979(98)18911-9].