Abstract
The grain structure, precipitate size, morphology, volume fraction and distribution, coarsening kinetics, hardness and cyclic oxidation behavior of (γ + γ′) Ni-13Al-2Nb (at.%) and Ni-11Al-4Nb (at.%) superalloys, aged at 800 °C from 1 to 256 h, are investigated in detail. For the earliest aging times (1 and 4 h), the morphology of the γ′ precipitates are cuboidal. With increasing aging time, the γ′ precipitates coarsen, the average precipitate radius increases, and the morphology changes from cuboidal-to-parallelepipedic-to-irregular shape. The average precipitate size also increases with increasing Nb content from 2 to 4 at.%. At 4 h aging time, Ni-13Al-2Nb and Ni-11Al-4Nb superalloys exhibit peak microhardness values of 3.7 and 3.9 GPa, respectively. The results of cyclic oxidation tests show that important scale spallation is observed for the investigated samples. In addition, the surface scale product of both superalloys at all conditions is NiO.
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