Effects of rare earth yttrium on microstructure and properties of Ni-16Mo-7Cr-4Fe nickel-based superalloy

Effects of rare earth yttrium on microstructure of Ni-16Mo-7Cr-4Fe alloy were examined by optical microscopy, scanning electron microscopy, transmission electron microscopy and X-ray fluorescence. M6C phase was observed in the alloys with and without yttrium addition, and Ni17Y2 phase existed in the alloys containing 0.05-0.43 wt.% yttrium simultaneously. The amount of Ni17Y2 phase increased as yttrium concentration increased. When the concentration of yttrium increased to 0.43 wt.%, some multi-precipitated phase regions appeared in the form of large amount of coarse Ni3Y phase surrounded by M6C phase and gamma phase in the alloy. Influences of rare earth yttrium on high temperature static oxidation and mechanical properties of Ni-16Mo-7Cr-4Fe alloy were also investigated. The alloy containing 0.05 wt.% yttrium showed the best oxidation resistance and mechanical properties simultaneously. The adequate concentration of yttrium at grain boundary and in the solid-solution (gamma phase) and the adhesion enhancement of the outer oxidation scale to the substrate are key factors for the improvements in the mechanical properties and oxidation resistance respectively. (C) 2014 Elsevier Inc. All rights reserved.