THE INFLUENCE OF THE REACTIVE ELEMENT YTTRIUM ON THE STRESS IN ALUMINA SCALES FORMED BY OXIDATION

The addition of reactive elements, such as yttrium, to high temperatur e alloys significantly improves the adherence of thermally-grown oxide scales but there is still no agreement as to the reason for this impr oved behavior. Some of the proposed mechanisms are based on the premis e that yttrium helps to decrease the residual compressive stress in th e oxide, thus decreasing the driving force for spalling of the oxide s cale. This investigation followed the oxide stress development in thre e alumina-forming alloy compositions, each with and without an additio n of yttrium over various oxidation times and temperatures. The stress in the oxide scales were measured at room temperature using a piezosp ectroscopic technique. The results clearly show that the addition of y ttrium does not decrease the residual stress in the scales but makes t hem more compressive, at least at short oxidation times. (C) 1997 Acta Metallurgica Inc.