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 γ 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 (γ 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.