Characterisation of oxidation products of modified Nd–Fe–B type magnets

In order to study the influence of Co addition on oxidation behaviour, magnets with the compositions Nd16Fe76B8, Nd16Co3Fe73.3B7.7, Nd16Co5Fe71.5B7.5, Nd16Co7Fe69.6B7.4, Nd16Co10Fe66.9B7.1, Nd14Dy3Co9.7Fe66.4B6.9, Nd13.9Dy3Co9.7Fe66.2B6.9Zr0.3 and Nd13.9Dy3Co9.7Fe66.2B6.9V0.3 in powder and solid pellet forms were oxidised at 400 °C for 96 h. The resultant phases were identified using X-ray diffraction (XRD) and Mössbauer spectroscopy. α-Fe2O3 is the main final oxidation product of the iron content of these magnets, while Fe3O4 is the intermediate phase of this oxidation procedure. In addition the presence of several Nd oxides, as well as Dy oxides in the compositions with Dy additions was confirmed in all samples. B seems to prefer to react with Nd with the formation of oxide phases, while a minority Fe2B phase is also detected in the powder samples. A delay of the oxidation process is found for the case of compact pelletised samples, and this is extended for those samples containing Dy in their structure.