BENEFICIAL-EFFECTS OF RHENIUM ADDITIONS ON THE CYCLIC-OXIDATION RESISTANCE OF BETA-NIAL-CR ALLOYS(ALPHA)

This study reports the effects of up to 4 at.% rhenium addition on the cyclic-oxidation behavior of beta-NiAl+alpha-Cr alloys having a base composition (in at.%) Ni-40Al-17Cr. Tests were conducted in still air at 1100 degrees C for up to 250 1-hr cycles. The ternary alloy (withou t rhenium addition) exhibited poor cyclic-oxidation resistance, underg oing extensive scale spallation and internal oxidation. Additions of r henium considerably improved the oxidation behavior, reducing the exte nt of both scale spallation and internal oxidation. These beneficial e ffects increased with increasing rhenium content. Rhenium additions im proved cyclic-oxidation resistance by both decreasing the solubility o f chromium in the beta phase and causing the interdendritic alpha-Cr p recipitates in the alloy microstructure to become more spheroidized an d disconnected. These effects aided in preventing both interdendritic attack and the dissolution of the alpha-Cr precipitates from the subsu rface region of the alloy. The maintenance of alpha-Cr precipitates at the alloy-scale interface decreased the extent of scale spallation by providing a lower coefficient of thermal-expansion (CTE) mismatch bet ween the alloy and the Al2O3-rich scale.