Formative and Controlled Mechanisms of Nano-Sized γ' Precipitates with Local Phase-Transition within Dislocation Networks of Nickel-Based Single Crystal Superalloys

Abstract

The local region of dislocation networks in a nickel-based single crystal superalloy crept under 1373K and 137MPa is spatially divided to two parts: gridlines which compose the arrangements of dislocation networks and mesh regions as intervals between gridlines. The formative and controlled mechanisms of nano-sized γ′ precipitates (γ′ n ) in mesh region are revealed. The enhanced segregation of Cr, Co and Re along gridlines with transformation of dislocation networks from transitional to equilibrium arrangements left the higher contents of γ′-rich elements such as Al in mesh region that facilitates the local phase transition and precipitation of γ′ n . However, the growth of γ′ n precipitates in mesh region, which requires great amounts of solutes diffusion of γ′-rich elements, is restricted by chemical barrier formed along gridlines of dislocation networks. Complete coherent interfaces between ordering mesh region and disordered gridlines could form γ/γ′ substructures to stabilize dislocation networks. In dendrite core, higher contents of Re, W and Mo effectively increase the lattice misfit to form denser dislocation networks. It can expect further subdivided γ/γ′ substructures in dendrite core increase the creep resistance to retard degradation of dislocation networks and topological inversion of rafted structures.