MICROSTRUCTURE EVOLUTION OF PRE-COMPRESSI- ON NICKEL-BASED SINGLE CRYSTAL SUPERALLOY DURING TENSILE CREEP

By means of pre-compression treatment,the cubicγ′phase in a single crystal nickel- based superalloy transformed into the P-type rafted structure and its evolution during tensile creep was investigated by measuring creep curve and microstructure observation.Results show that the P- type structure is transformed into the N-type structure in the initial stage of the tensile creep.Under the effect of the tensile stress at high temperature,the changes of the equilibrium concentration of elements in theγ′/γphases promoted the unhomogeneous coarsening of theγ′phase,and the further decomposition of the P-type raft induced the groove structure,in which the chemical potential of solute element enhanced,resulting in the directional diffusion of the elements and dissolution of the P-type raftedγ′phase,so the cubical-likeγ′phase formed.The shear stress induced the lattice constriction in the horizontal interfaces of the cubical-likeγ′phase,and the Al,Ta atoms with bigger radius are repelled out,while the tension stress induced the lattice expansion in the side interfaces of the cubical-likeγ′phase and the Al,Ta atoms with the bigger radius are trapped,which promotes the directional growing ofγ′phase into the N-type rafted structure.Thereinto,the change of the strain energy density in different interfaces of the cubical-likeγ′phase is also the driving force of the elements diffusion and theγ′phase directional growth.