On the Role of Superlattice Structures with Ni₃X-type Composition in Selected Ni-Based Superalloys with Commercial Grades

Close-packed superlattice derivatives of the face-centered cubic (fcc) structure with Ni₃X-type composition in some commercial Ni-based superalloys have been characterized, and their effect on tensile strength has been determined. Both the L1₂ and DO₂₂ superlattices can form directly from the fcc structure by minor atoms rearrangement. However, the two superlattices can act as transient metastable phases which transform into more stable superlattices (L1₂ → DO₂₄ and DO₂₂ → D0_a) with extended thermal exposure at elevated temperatures. It is shown that the stability and morphology of each superlattice are dependent upon the alloy composition. Although some superlattices produce adverse effects on tensile strength, others are found to have beneficial effects. The exact effect is correlated with the particular morphology and deformation mode. It is concluded that the DO₂₂ superlattice can form the basis for designing alloys deformable by twinning and combine high strength and high ductility.