Rafting of γ′ precipitates in a Co-9Al-9W superalloy during compressive creep

The rafting of γ′ precipitates was investigated in a single crystal of Co-9Al-9W(at%) superalloy after compression creep tests at 850 °C. Scanning electron micrographs show that the γ′ precipitates raft perpendicular to the compressive stress axis, due to their positive lattice parameter misfit. The rafting of γ′ precipitates occurred after the minimum strain rate was reached. The dislocation structure and stacking faults were investigated by transmission electron microscopy. Dislocations preferentially moved in the horizontal γ matrix channels where they can relieve the coherency stress at horizontal γ/γ′ interfaces. After extended periods of creep, rafting of the γ′ precipitates perpendicular to the external compressive stress axis occurred. The merging of γ′ precipitates during rafting initiated at the precipitates corners, leaving pockets of matrix phase in the vertical γ channels between adjacent γ′ precipitates. The necessary diffusion of alloying elements during the rafting process between the two orientations of matrix channels should be decelerated as diffusion is slower in the ordered L1₂ γ′ phase than in the disordered γ phase.