Development Of Sequential Multiscale Method And Synergistic Effect Of Elements In Ni-based Superalloys

Chemical compositions are critical to the mechanical properties of Ni-basedsingle-crystal superalloys. The strengthening mechanisms of multi-chemical alloy-ing elements, their interactions and synergistic efects are of great significance in bothscience and engineering application. By using sequential multiscale modeling, the re-lationship between the electronic structure of alloying element-fault complex and me-chanical properties of superalloys are studied. Besides, the synergistic efect of Re andRu in Ni-based single-crystal superalloys are explored via theoretical and experimentalmethod.The efects of alloying elements on stacking fault energy and difusion activationenergy of vacancy in Ni are calculated by first-principles. The results are used asthe input parameters for analytical expressions based on elastic dislocation theory toevaluate the efects of alloying elements on the velocity of dislocation climbing in γ-Niphase. The results suggest that the elements Re, Mo and W can efectively suppressthe velocity of dislocation climbing by raising both the jog formation energy and thedifusion barrier of the vacancy.First-principles electronic structure calculations are used to study the influences ofalloying elements on the planar fault energies and generalized stacking fault energies inNi3Al. In combination with dislocation theory, the evaluation of strengthening efectsof various alloying elements can be given. Re is found to be more efective than W,Ta, Ti and Ru in decreasing cross-slip activation enthalpy and increasing flow stressin γ-Ni3Al. Meanwhile, according to Rice-criterion, Re increases brittleness of Ni3Almost, which promotes the probability of cleavage fracture near crack tip.Four kinds of single-crystal model alloys are designed, including Ni-Al, Ni-Al-Re,Ni-Al-Ru and Ni-Al-Re-Ru alloys. Three-dimensional atom probe tomography andhigh-resolution scanning transmission electron microscope are used to attain atomicscale resolution and obtain direct experimental evidence of element partitioning and site preference. The results show that the addition of the element Ru drives Re torepartition to γ phase and form Re-Ru clusters. First-principles calculations indicatethat Ru and Re attract each other in the range of4thnearest neighbor (about5),the attraction becomes stronger as the distance between Re and Ru decreasing. Theanalyses of bonding strength shows that Re-Ru bonding is stronger than Re-Ni and Ru-Ni, the strong interaction between Re and Ru originates from d-d orbital hybridizations.