The effects of Cr, Al, Ti, Mo, W, Ta, and Cb on the cyclic oxidation behavior of cast Ni-base superalloys at 1100 and 1150�C

Abstract

A series of cast nickel-base y/y′ superalloys as systematically varied at two nonzero levels of Cr, Al, Ti, Mo, Cb, W, and Ta with nominally fixed levels of 10 at. % Co, 0.06 at. % C, 0.05 at. % Zr, and 0.05% B. A 1/4-replicate 27-factorial statistical design supplemented by 18 additional alloys was used to estimate main effects and all two-factor interactions. The cyclic oxidation resistance was determined from specific weight change data as a function time for 1-hr cycles in static air at 1100 and 1150°C. A derived oxidation attack parameter, Ka, was fitted by multiple linear regression over the alloy sample space at each temperature. At the levels tested, Al was the most important alloy addition. The lowest Ka was achieved at the highest (13 at. %) test level of Al and the lowest (10 and 2 at. %, respectively) test levels of Cr and Ti. The results for the refractory metal additions were somewhat ambiguous, but the most oxidation-resistant composition was estimated to be 3 at. % Mo-1 at. % Cb-3 at. % W-3 at. % Ta for the compositions tested. X-ray diffraction analysis of the alloy scales indicated that at the lowest Ka value, most oxidation resistant alloys were associated with alumina-aluminate spinel formation at the high Al levels. Higher boron contents, which were unintentionally varied, were clearly detrimental even for the most resistant compositions. High boron contents apparently trigger nonprotective NiO formation. In addition, tri-rutile type oxides were found on all test alloys, but these were not considered to be harmful to oxidation resistance.