HIGH-TEMPERATURE OXIDATION BEHAVIOR OF IRIDIUM-BASED ALUMINA-FORMING TERNARY INTERMETALLICS

The oxidation behavior of iridium-based intermetallics in the ternary iridium-aluminum-silicon system has been investigated. Additions of up to 20at.% silicon to, iridium-aluminum binary alloys reduce the alumi num concentration necessary for the formation of a protective alumina scale from greater than 55 at.% in the absence of silicon to as low as 20 at.% when 20 at.% silicon, is present. Ternary iridium-aluminum-si licon alloys with 30-50at.% aluminum and 8-10at.% silicon exhibit para bolic oxidation behavior, and the rate constants are in good agreement with those previously determined for binary iridium-aluminum alloys. The oxidation rate of ternary iridium-aluminum-silicon alloys with 10a t.% silicon and 30-40 at.% aluminum deviates from the parabolic-oxidat ion behavior for alumina scale growth in the later oxidation stage, pr esumably due to the formation of a silica inner layer. Thermal cycling of the Ir-50 Al-8Si and Ir-60-Al samples does not result in scale spa llation due to the coefficient of thermal expansion match between the alumina scale and intermetallic substrate.