The microstructure of the scale formed during the high temperature oxidation of a fecralloy steel

The protective scales formed on Fecralloy steel exposed to an oxidizing environment at temperatures of 1000, 1100 or 1200°C have been characterized with the aid of a range of microstructural techniques. The scales formed at the higher two temperatures are predominantly composed of alpha alumina. The alpha form of alumina has the desirable property of a low native defect concentration in addition to a low volatility and the surfaces of these scales are generally smooth. The scales formed at 1000°C on cold worked specimens include a substantial component of a transition alumina (probably theta alumina) which is structurally related to gamma alumina and is highly defective. This form of alumina grows as high aspect ratio whiskers at the outer surface of the scale and their appearance suggests a quite different growth mechanism from that of the alpha alumina in the scales. Once formed, the whiskers of defective alumina can be transformed to the alpha crystal structure, with no change in external morphology, by heating to 1200°C. The alpha alumina phase exhibits a much greater crystallite strain when it is accompanied in the scale by the transition alumina than when it is not. There is some yttrium present in the oxide layer, concentrated linearly near to where grain boundaries in the steel emerge at the oxidizing surface. The yttrium is present mainly as YA10 3 and Y 3AI 5O 12.