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 appl ications. Conventional casting techniques for sintered magnets reveal the f ormation of a high quantity of alpha-Fe and large Nd-rich regions, New tech niques, like strip casting, produce homogeneous and fine scaled microstruct ures and are already used for producing high (BH)(max) magnets. The fast co oling rate during strip casting suppresses the formation of alpha-Fe dendri tes and of large Nd-rich pockets. Directional solidification causes a forma tion of columnar grains containing a typical arrangement of hard magnetic N d2Fe14B regions and Nd-rich regions, The Nd regions occur as intragranular platelets as well as intergranular phases. Intragranular lamellae show a pe riodicity which corresponds to a eutectoidal solidification according to th e composition of the liquid and are directed parallel to the temperature gr adient 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 grai ns. The fine separation of the hard magnetic and Nd phases is advantageous for the milling of the alloy after hydrogen decripitation and improves sint erability of magnets. Although the microstructure of strip cast alloys is m uch 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 interlam ellar spacing between the Nd-rich platelets that enables the formation of s ingle crystal powder particles after milling. (C) 1998 American Institute o f Physics. [S0021-8979(98)18911-9].