Beneficial effects of rhenium additions on the cyclic-oxidation resistance of β-NiAl+α-Cr alloys

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 (without rhenium addition) exhibited poor cyclic-oxidation resistance, undergoing extensive scale spallation and internal oxidation. Additions of rhenium considerably improved the oxidation behavior, reducing the extent of both scale spallation and internal oxidation. These beneficial effects increased with increasing rhenium content. Rhenium additions improved cyclic-oxidation resistance by both decreasing the solubility of chromium in the beta phase and causing the interdendritic alpha-Cr precipitates in the alloy microstructure to become more spheroidized and disconnected. These effects aided in preventing both interdendritic attack and the dissolution of the alpha-Cr precipitates from the subsurface 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 between the alloy and the Al2O3-rich scale.