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 temperature alloys significantly improves the adherence of thermally-grown oxide scales but there is still no agreement as to the reason for this improved behavior. Some of the proposed mechanisms are based on the premise that yttrium helps to decrease the residual compressive stress in the oxide, thus decreasing the driving force for spalling of the oxide scale. This investigation followed the oxide stress development in three alumina-forming alloy compositions, each with and without an addition of yttrium over various oxidation times and temperatures. The stress in the oxide scales were measured at room temperature using a piezospectroscopic technique. The results clearly show that the addition of yttrium does not decrease the residual stress in the scales but makes them more compressive, at least at short oxidation times. (C) 1997 Acta Metallurgica Inc.