Microstructure evolution of a pre-compression nickel-base single crystal superalloy during tensile creep

By pre-compressive creep treatment, the cubical γ′ phase in the nickel-base single crystal superalloy is transformed into the P-type rafted structure along the direction parallel to the applied stress axis. And the microstructure evolution of the P-type γ′ rafted alloy during tensile creep is investigated by means of the measurement of the creep curve and microstructure observation. Results show that the P-type γ′ rafted phase in the alloy is transformed into the N-type structure along the direction perpendicular to the applied stress axis in the initial stage of the tensile creep. In the role of the tensile stress at high temperature, the change of the element's equilibrium concentration in the different regions of P-type γ′ rafted phase occurs, which promotes the inhomogeneous coarsening of the P-type γ′ phase. And then, the decomposition of the P-type γ′ rafted phase in the alloy occurs to form the groove structure. As of result of the directional diffusion of the elements, the fact that the P-type γ′ rafted phase is decomposed to transform into the cubical-like structure is attributed to the increment of the solute elements M(Ta, Al) chemical potential in the groove regions. Further, the lattice constriction in the horizontal interfaces of the cubical-like γ′ phase may repel out the Al and Ta atoms with higher radius due to the role of the shearing stress, and the lattice expanding in the upright interfaces of the cubical-like γ′ phase, due to the role of the tension stress, may trap the Ta and Al atoms, which promotes the directional growing of γ′ phase into the N-type rafted structure. Therefore, the change of the strain energy density in different interfaces of the cubical-like γ′ phase is thought to be the driving force of the elements diffusing and the directional coarsening of γ′ phase.