A discontinuous precipitation reaction has been investigated in a high-refractory content nickel-base alloy. The reaction transforms the two-phase ␥-␥ ' parent microstructure into a three-phase cellular structure with a ␥' matrix containing Re-rich P-phase and agglomerated ␥ lamellae. The reaction has been studied in polycrystalline material and in bicrystals with varying degrees of boundary misorientation at temperatures in the range of T/T m ϭ 0.78 to 0.85. The early stages of the reaction are characterized by heterogeneous nucleation of P-phase precipitates and migration of the grain boundary. At low-angle, near-tilt boundaries misoriented by less than 10 deg, nucleation of P-phase particles was observed, but the cellular reaction did not occur, due to limited boundary mobility and diffusivity. The high degree of supersaturation of Re and W in the initial ␥-␥' alloy appears to be the primary driving force for the reaction. Small amounts of creep deformation did not significantly influence the extent of the transformation. The diffusivity of Re associated with the moving boundary was calculated to be 5 ϫ 10 Ϫ8 cm 2 s Ϫ1 at 1093 ЊC, which is approximately four orders of magnitude greater than the bulk lattice diffusivity of tungsten.

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