Hydrogen decrepitation of a permanent magnet Nd-Fe-B alloy

Abstract

The nature of hydrogen decrepitation when applied to a cast Nd-Fe-B permanent magnet alloy has been studied by following the microstructural changes on polished surfaces of the material exposed to hydrogen at a pressure of 4 bar. The milling of the material decrepitated at a pressure of 10 bar has also been studied by determining the particle size as a function of milling time. The milled particles have also been examined by scanning electron microscopy. It is concluded that preferential swelling of the neodymium-rich phase occurs and that this gives rise to 1. (i) shear stresses leading to fracture at the interface between the particles and the matrix, and 2. (ii) cracks in the iron-rich matrix phase which show 90° intersections characteristic of tensile stress relief.