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

The rafting of gamma' precipitates was investigated in a single crystal of Co-9Al-9W(at%) superalloy after compression creep tests at 850 degrees C. Scanning electron micrographs show that the gamma' precipitates raft perpendicular to the compressive stress axis, due to their positive lattice parameter misfit. The rafting of gamma' 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 gamma matrix channels where they can relieve the coherency stress at horizontal gamma/gamma' interfaces. After extended periods of creep, rafting of the gamma' precipitates perpendicular to the external compressive stress axis occurred. The merging of gamma' precipitates during rafting initiated at the precipitates corners, leaving pockets of matrix phase in the vertical gamma channels between adjacent gamma' 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 L1(2) gamma' phase than in the disordered gamma phase.