Oxidation behaviour of Nd-Fe-B nanocomposite powders

The oxidation behaviour of several Nd-Fe-B based nanocomposite powders is investigated by Xray diffraction and Mossbauer spectrometry. The powders are nanocomposite two-phase Nd-2(Fe,Co,Al,Cr,Zr)(14)B / alpha -Fe powders with different volume fractions of alpha -Fe, and nanostructured single-phase Nd-2(Fe,Co,Al,V,Cr)(14)B powders, They were prepared by High Energy Ball Milling (HEBM) and annealed for 30 minutes at 630 degreesC. The microstructural investigations revealed that the powders are composed of particles with a very broad size distribution (1 to 50 mum) For the oxidation experiments, the powders were oxidized in ambient air at 300 degreesC for times up to 72 hours. Then, the oxidation kinetics were followed using room temperature Mossbauer spectrometry. The results showed that the oxidation rate of the Nd2Fe14B phase decreases as the volume fraction of alpha -Fe increases, in agreement with the fact that alpha -Fe is more corrosion resistant than Nd2Fe14B (as it contains no rare earth). A nanocomposite two-phase powder appears thus to be intrinsically more corrosion resistant than a single-phase powder. Furthermore, the positive influence of addition elements (Cr and V) was confirmed.