On the Role of Superlattice Structures with Ni_3X-type Composition in Selected Ni-Based Superalloys with Commercial Grades

摘要

Close-packed superlattice derivatives of the face-centered cubic (fcc) structure with Ni_3X-type composition in some commercial Ni-based superalloys have been characterized, and their effect on tensile strength has been determined. Both the L1_2 and DO_(22) superlattices can form directly from the fcc structure by minor atoms rearrangement. However, the two superlat-tices can act as transient metastable phases which transform into more stable superlattices (Ll2->DO_(24) and DO_(22)->DO_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 DO_(22) superlattice can form the basis for designing alloys deformable by twinning and combine high strength and high ductility.