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SmCo alloys form the basis of excellent permanent magnets with potential service applications to 500°C. It is suspected, however, that their oxidation behavior may limit their usefulness but this is a relatively unstudied subject. Two grades of Sm2(Co,Fe,Cu,Zr)17-based alloys were aged in air at temperatures between 300 and 600°C and the oxidation reaction characterized by post-test scanning electron microscopy 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-rich oxide within a CoFe matrix. The depth of the internal oxidation zone was typically an order of magnitude thicker than the surface oxides and increased parabolically with time. This internal oxidation is an important degradation phenomenon because the transformation to CoFe 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 behavior is also discussed.

ODIN is a 2-D finite-difference diffusion/interdiffusion computer model capable of predicting the solute depletion profiles evolved in binary and ternary alloys due to high temperature oxidation. The 2-D geometries that can be analysed include corners, rectangular edges and rib features. Empirical mass gain kinetics in the form of a general power law are employed as a boundary condition to define the rate of removal of solute at the oxidizing surfaces of the alloy. The diffusion equations are then solved over the bulk region to obtain the resulting concentration profile at a series of discrete time steps up to the desired exposure time t. The model has been validated for planar foil-type 1-D geometries using existing analytical models which consider parabolic oxidation kinetics. The code predictions have also been compared satisfactorily against direct measurements of aluminium depletion near the 2-D edges of FeCrAlY steel samples.

Localised loss of aluminium from the bond coat within thermal barrier coating (TBC) systems is a critical factor determining the lifetime of these coatings. In this paper, it will be demonstrated that electroplated MCrAlY bond coats with asperities of high aspect ratios can experience premature chemical failure during isothermal exposures at 1100 8C in air. This can result in the rapid conversion of the asperity into Co-and Cr-rich oxides. The enhanced aluminium depletion within these asperities, which leads to chemical failure, is a consequence of their increased surface area coupled with a limited physical access to the reservoir of aluminium in the bulk of the bond coat. This process has been demonstrated using the 2-D finitedifference model ODIN which predicts a depletion to less than 1 at% aluminium within a typical asperity after 1 h exposure at 1100 8C. The results also show that the aluminium content of the bulk of the bond coat is sufficient to enable the formation of a continuous protective alumina underneath these regions, as observed experimentally. It is suggested that the volume increase associated with the conversion of the bond coat asperities into breakaway oxide results in out-of-plane tensile stress development at the oxidation temperature. These stresses are anticipated to be at a maximum between the oxide asperities and are thought to nucleate small, sub-critical cracks at these locations at temperature. Further delamination will occur during cooling both by the extension of these cracks and by the growth of crack-like defects within the porous breakaway oxides.

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2-dimensional finite-difference calculations have been undertaken to examine the depletion of chromium at specimen edges and ribs in a 20Cr-25Ni-Nb steel which is being selectively oxidized. The calculations have been performed for parabolic oxidation kinetics and for temperatures in the range 800-1200°C. It is found that under re-entrant features, e.g. interior edges, the extent of depletion is less than under a planar surface but the converse holds for exterior edges. For this latter case, it is shown how the initial severe depletion can fill in with time by the supply of chromium from the bulk of the specimen. This process, however, results in the propagation of the ''edge effect'' away from the corner with kinetics that scale as (Dt) 1/2 . Rectangular ribs contain both types of geometric features but it is found that the two exterior edges forming the rib tip lead to extensive depletion in this region which gradually propagates along the length of the rib. Similar calculations for a trapezoidal rib are also given and, in this case, are compared with SEM/EDX chromium concentration measurements for samples oxidized at 1000°C in a CO 2 /1%CO environment.