The oxidation morphology of SmCo alloys

Abstract

SmCo alloys form the basis of excellent permanent magnets with potential service applications up to 550 °C. It is suspected that their oxidation behaviour may limit their usefulness but this is a relatively unstudied subject. In this work, two grades of Sm 2(Co,Fe,Cu,Zr) 17-based alloys were oxidised in air at temperatures between 300 and 600 °C, and the oxidation morphology characterised by post-test SEM examination. Both alloys formed a thin external oxide scale composed of Cu and mixed Fe/Co oxides and an extensive internal oxidation layer consisting of Sm oxide within a (Co,Fe) solid solution. The depth of the internal oxidation zone (IOZ) was typically an order of magnitude thicker than the surface oxides and increased parabolically with time. The internal oxidation is an important degradation phenomenon because the transformation to (Co,Fe) causes a loss of magnetic properties proportional to the volume consumed. The morphologies of the oxidation zones, nature and mechanisms of various precipitated phases and the differences between the two alloys’ oxidation behaviour is discussed.