The use of metal hydride powder blending in the production of NdFeB-type magnets

Abstract

The ability to vary the composition of NdFeB-type magnets, during or after the milling stage offers a number of advantages. To this end, powder blending processes have been developed. In the present work, the sintering characteristics of a Nd13Fe80.5B6.5 alloy have been improved by powder blending with neodymium hydride. A blending addition of 1 at% of neodymium in the form of Nd-hydride, consistent with a final composition of Nd14Fe79.6B6.4, was sufficient to reduce the required sintering temperature from 1130 to 1070°C, with a resultant improvement in the coercivity due to less grain growth and better grain isolation. The addition of dysprosium hydride to a Nd14Fe79B7 alloy by blending resulted in a further enhancement of the coercivity, a 2 at% addition yielding an approximate doubling in the value of intrinsic coercivity. Examination of the microstructure of dysprosium–hydride blended magnets showed the dysprosium to be concentrated in the outer regions of the matrix grains, with the centres being essentially dysprosium-free. Hydrogen has also been employed in improving the milling characteristics of the high melting point elements niobium and vanadium. Fine powders of niobium and vanadium hydride were prepared and were successfully blended into magnets with an even distribution throughout the microstructure. These studies have shown that powder blending of metal hydrides is an effective way of both promoting liquid-phase sintering in low rare earth composition magnets and of modifying the grain boundary phases and hence the magnetic properties.