Hydrogenation behavior of rare earth metal, alloys, or intermetallic compounds and physicochemical properties of hydride have always been the important topic on applications of rare earth. Alloy ingot of rare earth permanent magnetic material also exhibits very strong hydrogenation tendency. Hydrogen atoms enter interstitial site between intermetallic compound main phase and rare earth-rich grain boundary phase and formed interstitial compound. Then interatomic distance increased and lattice volume expanded. The resulting internal stressing will produce grain boundary cracking (intergranular fracture), crystal fracture (transcrystalline fracture), or ductile fracture. These decrepitation comes with crackling and are therefore known as hydrogen decrepitation. Hydrogen decrepitation process of sintered Neodymium magnet is also referenced as HD process. Grain boundary cracking and crystal fracture generated in HD process made Nd-Fe-B course powder very fragile and highly advantageous to subsequent jet milling process. In addition to enhancing efficiency of jet milling process, HD process is also favourable to adjust the average powder size of fine powder.
