Atomic scale interfacial and compositional characteristics of the σ and γ phases of Ni-based single-crystal superalloys

High-resolution transmission electron microscopy and high-angle annular dark-field imaging were used to study differences in the interfacial and compositional characteristics between the plate-shaped σ and γ phases in a Ni-based single-crystal superalloy. An atomic structural model of the interface between the σ and γ phases that includes a step was proposed. The σ phase exhibits the following relationship with the matrix: [0 0 1] _γ//[112̄] _σ, (2̄20) _γ//(1̄10) _σ, (2̄2̄0) _γ//(1 1 1) _σ, [0 1 1] _γ//[1 1 0] _σ, (11̄1) _γ//(001̄) _σ. The compositional characteristics of the σ phase indicate that it is rich in high-Z elements (Z is the atomic number), especially the alloying element rhenium (Re). The impurity formation energies for the σ and γ phases doped with Re in different sublattices were investigated using the first-principles method based on the density functional theory. The bonding characteristics of the undoped and doped σ and γ phases were analyzed with the valence charge densities and the density of states. The results indicate that the alloying element Re, with a large atomic size, substitutes the W atom preferentially, owing to the nature of the bonding and the interstitial spaces in the crystal structure.