The oxidation of a Nd-Fe-B permanent magnet at 400 degrees C: a SEM, microhardness and Mossbauer study

Nd/sub 16/Fe/sub 76/B/sub 8/ sintered magnets were heated in an open furnace for times up to 29 days. The results of scanning electron microscope (SEM) observations, quantitative EDX/WDX analysis, microhardness measurements and Mossbauer spectroscopy were found to be consistent with the Nd/sub 2/Fe/sub 14/B phase having been disproportionated by oxygen entering via the surface or the cracks. The diffusion front between the oxidized region (oxygen amount approximately 35 at.%) and the unaffected region was observed moving through the grains, indicating that at 400 degrees C the corrosion behavior of the Nd/sub 2/Fe/sub 14/B phase is not intergranular in nature. This indicates that the corrosion resistance of Nd-Fe-B magnets at elevated temperatures could be enhanced by the presence of precipitates in the matrix.<
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