Permanent magnets by mechanical alloying (invited)

Mechanical alloying is applied to prepare Nd-Fe-B, Sm-Fe-TM type (TM: V, Ti, Zr), and interstitial nitride and carbide permanent magnets. Starting from elemental powders, the hard magnetic phases are formed by milling in a planetary ball mill and a following solid-state reaction at relatively low temperatures. For Nd-Fe-B, the magnetically isotropic particles are microcrystalline, show a high coercivity (up to 16 kA/cm for ternary alloys and above for Dy-substituted samples), and can be either used for making bonded magnets or compacted to dense isotropic magnets by hot uniaxial pressing. Magnetically anisotropic samples with a remanence up to 1.31 T and an energy product up to 326 kJ/m3 are formed by die upsetting. The mechanical alloying process has also been applied to prepare magnetic material of three new Sm-Fe-TM phases: Sm-Fe-V with the ThMn12 structure, Sm-Fe-Zr with the PuNi3 structure, and Sm-Fe-Ti with the A2 structure. They all show high or ultrahigh coercivities (up to 51.6 kA/cm for Sm-Fe-Ti), but their saturation magnetization is comparably low. More promising are the interstitial nitrides or carbides of the 2:17 or the 1:12 phases. Magnetically isotropic microcrystalline Sm2Fe17Nx samples with coercivities up to 24 kA/cm were prepared by mechanical alloying and a two-step heat treatment. Their remanence and energy product are equivalent to similarly prepared Nd-Fe-B samples, but their properties at elevated temperatures are superior because of the high Curie temperature of 470 °C and the large anisotropy field of 22 T. Also for Sm2Fe17Cy and 1:12-type Nd-Fe-V-N, coercivities of 18.5 or 6 kA/cm, respectively, were achieved.